joining battery's

I HAVE A PLUG IN ON THE BATTERY BOX FOR THE FRIDGE.SO IM ONLY CONNECTING TO ONE BOX,,IN YOUR PIC FOR PARRAELL THE - AND THE + CABLE ........WHERE DO THEY GO??

CHEERS SKIPP

In the diagram as is shown, the (+) would be *coming from* the alternator/solar panel, and the (-) would be *going to* a good Earth on the vehicle.

Maîneÿ . . .

Cable variations

Cable area 6 mm ²
Diameter 2.7 mm
DC-resistance 0.0028 Ohm/m

+++++++++++++++++++++

Cable area 32 mm ²
Diameter 6.5 mm
DC-resistance 0.0005 Ohm/m

Maîneÿ . . .

LOL!

Maîneÿ - DC Resistance...!!

One of my (er..) "gurus" just stated on another site how he never uses wiring capacity tables.
He goes by resistance - ie, knowing the length, knowing the load, he decides an allowable voltage drop, and thus selects the cable (gauge).
He reckoned he sometimes "sanity checked" with tables.

(In person, he once stated how he could never understand the variations in the tables regarding current capacities. Or rather "how the heck do readers know what the capacity refers to?" - ie: loom, bundled, free-air transmission....?)

Nice to see someone quoting (IMHO) a "real" parameter.


I was also tempted to say the cable size should relate to the load (current), but 6mmsq etc is fine (some argue XXXmmsq for the 120AH battery capacity).

Besides, if matching interconnect resistances for the diagonal power tapping is important, and since each battery should have a fuse on it's +12V side of the interconnect - therefore the same for each -ve side, isn't a smaller fuse cheaper? (Or should I say, welcome to those that avoid paralleling batteries - unless perhaps they are side by side..?)

A little knowledge is dangerous.
A lot of knowledge becomes a pain in the butt. And head!

Onya Maîneÿ!

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Hello ChipPunk,

I'm curious as to why you think paralleling batteries (of equal charging voltage requirements) could be detrimental in any way?

And why lose sleep over a small ocv imbalance?

Let me know, thanks.

Best regards, Peter

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Peter, I'll simply refer you to here.

No need to repeat what OldSpark put so elegantly. (LOL!)


Pity about the moderation - I rather enjoyed the original comment about not worrying about a 90 degree rotation!
But I agree - it could have been seen as provocative....

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"Mike,
we are all talking about 12v *DC* battery cables

Definitely NOT "AC" which everyone else knows is 240v (Alternating Current)

if you have anything to say that makes any sense at all then that is all good,

Maîneÿ . . . "

- Cables are cables - they don't make DC cables and AC cables !

- AC can be any voltage, not just 240 volts

- fortunately most people here know who is making sense.

- the squareroot of Minus 1 is a concept essential to the understanding of AC.

Chill out Mike!

I was merely referring to the 90 degree operator aka "i" - or "j" as electronics/electrical people usually refer to it.

I thought resistance was DC. If it were AC is would be impedance but with nil j component.

And DC & AC cable... for most cases is in this forum it doesn't much matter - we only consider low frequencies and low currents.
Hence it's only insulation that matters.
As to switches, now that's a different ball game.

As to Anderson, my comments stand. I know the math. I know the issues. If you think it is ok to interrupt 20A then fine. Why would it be any different to 50A?
After all, DC and AC circuit breakers are the same since they react to the same heat (dc & RMS current(squared(t etc))).Well they would be the same if they weren't different.....


But I'm dropping this thread now.
I've spent enough time on an issue in which only "now" does someone admit to NOT understanding certain recommendations. (Opinions are fine, but to accuse others of avoiding issues..??? Is that what this site is about?)
And to start arguing what may be someone's humour gives me a zero-sequence reaction (that's a bit of 120° humour as opposed to rooting any negative ones).

Thanks for the link, ChipPunk

All the oldsparky has to offer against paralleling is 'reduced mean time between failure', and one battery losing a cell or two while the other one remains capable of pumping current through the defective battery, causing TR.

Let's look at this a bit closer:

The MTBF for every battery concerned remains the same. This means you're not going to lose your 12V supply sooner just because there are two or more batteries wired in parallel.
If one battery loses a cell by shorting out, the current from the other battery(ies) won't even be high enough to blow the fuse. Reason for this is that the remaining 5 cells can easily stand a voltage of 13V without going into TR for quite some time. And once the voltage has dropped to around 11.5V, the current would have reached the float charging rate of below C10/1000 which is <0.1 amps in case of a 100Ah AGM battery. And at the float charging level the issue of TR is moot.
Different situation of course if two cells go down at the same time (highly unlikely though): then the resulting high current through the remaining 4 cells would exceed the fuse rating, and the other battery(ies) will continue to function normally after blowing the fuse on the defective battery. Big deal.

Now what are the cons for big capacity batteries:

Shipping is expensive because lifting gear will be required, and some carriers most notably Australia Post won't even ship heavier than 20kg.
Installing a heavy chunk of lead in a vehicle requires counterbalancing which is easier done with two batteries.
Ironically, the MTBF for heavy batteries is reduced in mobile applications.
That's because vibrational forces causing material fatigue during normal battery aging increase proportionally with the mass involved. A battery with double the Ah rating will have roughly twice as heavy electrodes. But the interconnecting busbar on which the plates are suspended, and the grid structure itself aren't usually made twice as strong.
Then there is failure redundancy: if you lose the only big battery in the system, that's it.
If there were two or more batteries, you just lose a fraction of your total installed capacity (and get a warning to replace the one faulty battery).

Last but not least, smaller monoblocks in the 100Ah range are less than half the cost of a double sized battery because of economy of scale.
That's because for larger capacities individual cells are the mass produced battery of choice.

If you know of any other pros or cons pls let us know.

Best regards, Peter

Mike, we are only talking about 12v *DC* battery cables, nothing else.

I've posted the resistance in 2 different thickness battery cables, which are connecting 2 x 12v batteries, with no mention or thought of using 240v AC current in these 12v battery cables, for obvious reasons.
Even the quoted resistance numbers are stated as "DC - resistance"

Cable 6 mm²
Diameter 2.7 mm
DC-resistance 0.0028 Ohm/m
++++++++++++++
Cable 32 mm²
Diameter 6.5 mm
DC-resistance 0.0005 Ohm/m

Maîneÿ . . .

I suggest you reconsider your MTBF stuff.
As most know, if one battery goes down, so down the other. Hence one risk for new batteries (the bathtub curve).

But as OldSpark points out, 2 batteries in parallel (thus) have double the "unreliability" because unlike normal "parallel redundant systems", this is effectively a series system in terms of "reliability" analysis.
He qualifies what the "unreliability" refers to - but I understand what he means.
And by parallel, he does NOT mean when charging etc.
As I said above, if paralleling, the situation described is the pretty much ideal (it still double's the "falure rate"" though - not that this a large figure anyhow). But for those that consider that 2 identical batteries are matched - even with the "diagonal" loading - when one is in the boot, and one in the engine bay... Oh well, he tried!

It is intuitive to most that two batteries in parallel will last less time than if separated (again, one down, both down).

I'm not sure if was that forum where OldSpark took on one of Optima's Managers that made certain claims that Optima batteries were "fine to parallel". Although OS was discrete, it was obvious to others what he was doing and that Optima couldn't supply the data. In all fairness, the manager had only been with Optima for a few months. (He cited "conmpetitive" issues LOL!)
OldSpark on the other hand has probably purchased more batteries than we ever will. And he was well known several years ago for accurately calling/outing some major installation scams.

But I liked that thread. Though he didn't bother with full resources, I thought he quoted enough. Then again, I am familiar with those resources and OS's thinking.

I too think that the "Battery FAQ" series he links is one of the better battery resources around.

But for people that don't quite realise why AGMs don;t bother quoting "cranking currents", or whether certain companies are using recycled lead, it's probably a bit too complex.

Besides, as someone in this forum(?) wrote - they have hard-paralleled a pair (or more?) of AGM batteries for many years (6 or 8 years?). But they did point out the low discharge depth and detail to recharging.
I've seen some AGMs flame out (no pun) within months. (Not that TR is a serious hazard with 12V monoblocks in parallel.)
And a mate writes lovingly of a brick-wall situation involving a failed 3-month old Optima (Derek from ABS knows the type of story - "no, a big alternator (or battery) WILL NOT blow your battery (or load)" etc.)

But if 2 of OldSpark's described "unreliabilities" are the same as one..... (Not sure if that translates to MTBFs - it does, but in a misleading way.)

Like I say, I only get 6-8 years out of my paralleled flooded cells so what would I know. Maybe being paralleled doubles their 2 year warranty?
I'll ask OldSpark..... Or Derek?

ChipPunk,
I got 6 years out of 2 x Delkor DC27's (80ah DC) wired in parallel before I installed the 2 x AGM DC’s.
I only replaced the DC 27’s because I was going away one winter and I thought it was about that time, so I did not get a failure when away in the bush.

I then used one of the DC27's as my Cranking battery for a few months as the Yuasha cranking battery was showing signs of dieing anyway.

Maîneÿ . . .

Hello ChipPunk,

ok let's look at this MTBF stuff one more time:

....It is intuitive to most that two batteries in parallel will last less time than if separated (again, one down, both down)....

Wrong. One down (cell), the other batteries in the parallel configuration discharge with a decreasing rate in an exponential fashion. At 11.5V, the current through the defective battery has reached float charging level, thus can't be a threat to the other batteries any longer.
The remaining batteries survive thus the MTBF of the parallel configuration is not worse than for a single battery.


...As most know, if one battery goes down, so down the other. Hence one risk for new batteries (the bathtub curve).....
Again, no that's wrong. But my apologies, if I understand you wrong. By 'going down' I presume you mean 'losing a cell by shorting out'. In case of a short taking out a cell, the other battery merely gets discharged slowly to a benign voltage level.


I hope that explains it better.

Best regards, Peter

Peter (Battery Value P/L),

"At 11.5V, the current through the defective battery has reached float charging level, thus can't be a threat to the other batteries any longer."

Let's make sure I/we understand each other.

Yoy are saying that for 2 batteries that are connected in parallel, it does not matter if one loses a cell (or whatever) and hence is 11.5V?

Correct?

ChipPunk,

correct.

If one battery loses a cell due to an internal short, it receives current from the other battery - in other words the remaining 5 cells get charged by the other battery.
Because a cell has a nominal float charging voltage of 2.3V, the defective battery will exhibit a terminal voltage of 5*2.3V=11.5V while taking in a float charging current of typically less than C10/1000.
Since this current is now very low, it won't represent a danger for the good battery any longer and it'll just sit there at around 11.5V while getting discharged very slowly.
Note I've made no allowance for battery heating (due to the slight initial overcharge of 2.6V/cell) but it won't matter much because it'll only affect the float voltage by about 3.6mV/degree/cell.
Presuming there are 10Ah left in the good battery (until the voltage drops to 10.5V cutoff), there would be at least 100 hours left (if it was a linear function) to disconnect the two batteries in order to prevent the good battery from deeply discharging below 10.5V.
But because this current taper happens exponentially, there is a time constant involved, meaning the tail end of this process extends well beyond 100 hours.

Of course there is additional 'reaction time' because it'll also take several hours for the charge dropping sufficiently to a corresponding terminal voltage of 11.5V.

If I need to explain something in more detail, you're most welcome.

Best regards, Peter

OMG!
I do not know where to start!

So "two matched batteries" includes one ok battery and one with a collapsed cell?

And one ok battery with nominal full OCV of 12.7V with not be effected by a battery with collapsed cell connected in parallel?


Please tell me I am misunderstanding what you are saying?

PLEASE!

ChipPunk,

No. As soon as one battery loses a cell, the batteries aren't 'matched' any longer

And no. The 'ok' battery will be 'affected' by the defective battery in the parallel configuration in that it'll get discharged slowly.

Just to remind you of the original topic: it was about MTBF of a single battery versus MTBF of a parallel configuration.

Since the defective battery can't cause damage to the remaining battery(ies) within a time frame of at least 100 hours from when the failure occured, the MTBF rate of the parallel configuration is not worse than that of a single battery if:

The OP, within a week, will act and open the parallel interconnect to isolate the defective battery.

So as long as the OP observes the battery voltage on a weekly (or shorter) basis, the MTBF rate of the parallel configuration will not get worse.

Quite the opposite actually, if you take into account the failure redundancy of a parallel configuration.
As long as you replace the faulty battery in a timely fashion, chances are that you'll never be out of battery power.
This is especially true if more than two batteries are wired in parallel.

You can't expect this from a single battery.
If that one battery loses a cell, that's it.

Best regards, Peter

Oh praise Dog!

So, at what point do the two paralleled batteries previously known as "matched" become unmatched?

And how soon should you disconnect the now defective battery from the other good one?

And how do you monitor or determine the above mismatching?


Hence what do you advise people that are going to "hard parallel" 2 batteries?

Is it therefore ok to parallel batteries?

ChipPunk,

to your Q 1: by now, you should be able to figure this out yourself.

to Q 2: it may be necessary for you to read my previous post again, to be able to find a solution to this.

to Q 3: there are devices called voltmeter. You connect these across the battery terminals to give you a pretty good indication of the voltage. Many fixed installations include these nifty gadgets, so that the OP can have a look at them in a routinely fashion. More sophisticated installations include battery alarms which alert the OP if a certain 'set point' or 'threshold' gets crossed.

to Q 4: pls read my previous post again, and you may be able to find the answer.

to Q 5: yes absolutely, it's perfectly ok to parallel batteries.

Best regards, Peter

So as I have now in my vehicle, two batteries that are paralleled when the vehicle is charging.
The voltmeter connected across (whichever) battery reads 14.4V.

But one battery has a collapsed cell.
How will I know?
And which one?

If they are connected when NOT charging, how do I tell?
When I leave, the voltmeter says 12.7V (after I bleed off the surface charge)?

How long do I have before the collapsed cell battery degraded my good battery? 100 hours is okay?

Sorry, but I do not see the answer or mechanism in your above reply - unless you are saying disconnect the batteries every week and measure their OC voltage?

As to monitoring equipment, I presume you are talking impedance measurement & logging?
Otherwise what - where from and how much ($$)?


I know it is ok to parallel "matched" batteries? But you say it is not okay to parallel mismatched batteries(?)
Forgive my stupidity, but I don't know how to test for a mismatched battery when the two are connected together?
Nor do I know how often to do this test, or what equipment will do it for me.

I am very interested in a alarm system to alert me of battery problems.
What can you offer and at what price?


Thanks in advance.

ChipPunk,

during charging, a battery with a collapsed cell won't allow the voltage to rise to 14.4V without a significant increase in charging current.

If you'd monitor the charging current as well as the charging voltage, you'll certainly notice something is fishy because the 'normal' voltage/current relationship will be significantly out.

What's more, a 14.4V terminal voltage translates to 2.88V cell voltage in a 5-cell-battery, a severe over-voltage which may draw enough current to blow the battery fuse (makes trouble shooting easy).

Another indicator of course is battery heating. So if you notice something's not quite right, go and touch both batteries which will instantly reveal which one's defective.

While the batteries are not being charged, you'll notice that the voltage drops to below 12V within a few hours.
This should be enough cause for concern for the OP to open the battery interconnect and check both batteries with a voltmeter.
Observe the battery voltage for a few seconds. The voltage of the good battery will drift up, the bad one's will drift down.

You'll get 100 hours minimum (proportionally less, with DOD at the time of failure of course).
BTW, the bad battery doesn't 'degrade' your good battery right away. It's merely discharing it slowly until a voltage point is reached at which (more or less determined by laws of probability) the good battery could sustain some damage due to excessive sulphation which happens at an increased rate near or below the cutoff voltage.

In a nutshell:

cell collapses while charging: monitoring the charging voltage and total current (ideally individual battery temperature as well) will give you a good indication of what's going on.

If you happen to overlook the signs during charging, the problem will be more easily detectable during discharging:
Within a few hours, the battery voltage would come down to an unexpectedly low voltage of around 12V.

Even if you would not notice anything wrong at this stage, it'll definitely crop up come recharging time again.
The battery voltage will creep up slower than usual if at all (depends on a few variables).
I know in your example you're using an alternator (in which case a volt plus ammeter on the dash would be advantageous).
But if you were to use a a good mains powered intelligent charger (more details in my profile) it'll most certainly alert you about a collapsed cell in that it'll stop charging at 13V.
It does this majick trick by stopping the increase in terminal voltage during constant current mode, at the voltage of 13V. The charger then holds the 13V constant while monitoring the charging current. If all cells are intact, the current will taper off at some rate. Once the taper has reached a programmed threshold, the charger is satisfied and will continue with the bulk charging. A battery with one or more cells down, will not exhibit this current taper at 13V - pretty nifty hey.

Sorry, I didn't set this up to sell some alarm system to you, but since you mentioned this, how about a good charger ;)

I'll leave the alarm system to you. Somehow I think you should be capable of putting one together (just gut feeling).

Best regards, Peter

No problem with alarm systems - I've had access to them for years - the usual (minimum) 12-bit voltage monitors (for series strings), zero-impedance current montoring, and 1kHz or DSP impedance devices.


What I was trying to establish was what you considered reasonable to monitor or determine battery status in a hard-parallel connection. (Many say paralleling is ok, but fail to define exceptions or limitations etc. Some like jmelton in my link above claim "Not only is it done all of the time, with mismatched batteries alot of the time..." OldSpark provides interesting qualification...)


So as I understand it, either monitor the current into each battery & note any mismatching, or monitor overall voltage and note anything out of the ordinary.

And if the voltage drops to say 12V within a few hours (which may be ~ 50% discharged), then isolate each battery and check its voltage.

Or touch each battery and compare temperatures (in this case at least - not necessarily with an engine-bay & a boot battery etc).


Is that it in a nutshell?
(Specific to this installation - ie, two "matched" batteries in similar environment (not engine bay & boot or caravan etc) and (hence) with matched interconnection - ie, diagonal load tapping.)


Luckily I avoid all those issues since my batteries are only paralleled whilst charging - hence one doesn't effect the other.
I don't even have to worry about diagonal and matched interconnections.
If one battery is really bad, I will notice that on my dash voltmeter as you say (formerly a dash mounted ARB-Sidewider LCD unit; currently a blue-LED meter - both being connected across the main battery's terminals).
Otherwise I detect problems through other means (low voltage trip to the 2nd battery's loads; low voltage off the main battery (during cranking) etc.

I can parallel them if I want to - eg, for cranking or winching purposes.

I have no need for ammeters any of those situations - the voltmeter tells me all. (That's despite it being an old vehicle normally fitted with an ammeter. Newer vehicles have voltmeters instead, but like most oldies - I swapped my ammeter for a voltmeter. Piece of mind, and never looked back. And have avoided a few strandings!)


I do have a good charger - my alternator.
I'm happy with how it charges batteries. I think I mentioned how I considered 3 years ok for normal batteries but that I usually got more - and that was with my older "external regulator" alternators.

I only upgraded to newer integrated-regulator alternators (from the 1980s) about 2 years ago. My normal battery(s) are both 8 years old with lots of abuse - some overcharging; several flattenings (main beams on all day), etc.
I only recently replaced it/them with an old AGM.


My batteries charge at the maximum rate they and my system can handle - namely about 14.4V with no current limit.
I have yet to see the voltmeter dip below 14.4V after initial start-up with only the normal ignition etc and batteries connected.
(The battery isolator Amperage exceeds my alternator's output.)


Although I see the advantage to a true battery charging system in a vehicle, even for those that stay at 14.4V all day without dropping to 13.8V I think batteries last long enough (except for Fords with mid engine-bay mounted batteries LOL!)
I practice, IMO the desirable ~0.2V change over a ~50°C range seems not to make a big difference (especially if the common Bosch 14.2V setting etc).
Nevertheless, I see the merits for "real" chargers with independent remote monitoring of voltage and temperature. I just don't think many would consider it worthwhile - it's a bit like using distilled water on newer flooded cells - great for longetivity, but not economically worthwhile.


Thanks for clarifying that once a fault develops, the batteries are no longer matched, and also for giving an example as to how to determine whether a mismatch has occured.

I prefer to follow what IMO is typical expert advise and NOT keep batteries paralleled except when charging, and when required.

Hello ChipPunk,

it's good to see you try and get a handle on these things.
You still seem to struggle a bit though when it comes to the 'front end'.

Never mind, I'll give you some fresh ideas.

The task at hand is to get a quick indication of a shorted cell in a parallel battery configuration.
The fault may develop during charging or discharging and needs timely (within 100 hours or so) action by the OP to prevent further damage.
Looking at this failure mode, you quickly notice that the battery with the collapsed cell will draw significant current from the other battery.
So all you have to do is finding a way to monitor the inter-battery-current.

For this, remember what a Wheatstone bridge can do for you.
Then, look at the parallel configuration of the batteries with their associated line fuses which effectively form a bridge circuit.
Assuming equal voltage/current (thus resistance) balance of the batteries, and equal resistance of the (equally rated) line fuses, the bridge will be balanced. But if one battery loses a cell, the bridge will exhibit the character of an unbalanced 'quarter bridge'. The manifestation of an unbalanced bridge is of course the voltage it'll now develop.

So all you have to do is wire a voltmeter across the battery positives and you have a quick'n dirty early warning system for cell collapse.

Hope to have answered your many questions about how to go about all this?

Image Could Not Be Found

Best regards, Peter



If you realise that the battery configuration with the

No problem with alarm systems - I've had access to them for years - the usual (minimum) 12-bit voltage monitors (for series strings), zero-impedance current montoring, and 1kHz or DSP impedance devices.

>>>>>>>>>>>>>Good to see you seem to have a handle on these things.>>>>>>>>>>>>almost: to cut down on complexity, I suggest you monitor the total charging current (for the entire parallel configuration), and the voltage. If one cell dies, the total current will rise and stay up. The voltage reading will also be somewhat lower than normal due to small voltage drops in alternator output and in the wiring.
If you don't mind the added complexity it's certainly helpful (as you suggested above) to have the number of ammeters equalling the number of batteries in the parallel configuration. This way you can pinpoint any problems without much mucking around.
As an added bonus, individual ammeters provide an early indication/warning of developing battery faults because now you can do quick comparison checks at a glance.>>>>>>>>>>>>so now we are talking about manifestation of a dead cell during discharging (in contrast to the different indications during charging as described above):
If the failure develops during charging, and the OP won't notice it at this stage, then it'll become very obvious that something is wrong the moment charging stops. Instead of the batteries maintainingg their usual high end-of-charge-voltage for some time (load current dependent), the voltage will now initially drop rapidly and slowly begin to bottom out.
If it was for a mission critical application, I suggest to hard wire a voltmeter between each battery positive.
This would reveal any battery to battery currents by utilising the fuse resistances to become the shunt resistance for the voltmeter (hence the voltmeter then acts as a quick'n dirty inter-battery-ammeter):


Or touch each battery and compare temperatures (in this case at least - not necessarily with an engine-bay & a boot battery etc).


Is that it in a nutshell?
(Specific to this installation - ie, two "matched" batteries in similar environment (not engine bay & boot or caravan etc) and (hence) with matched interconnection - ie, diagonal load tapping.)


Luckily I avoid all those issues since my batteries are only paralleled whilst charging - hence one doesn't effect the other.
I don't even have to worry about diagonal and matched interconnections.
If one battery is really bad, I will notice that on my dash voltmeter as you say (formerly a dash mounted ARB-Sidewider LCD unit; currently a blue-LED meter - both being connected across the main battery's terminals).
Otherwise I detect problems through other means (low voltage trip to the 2nd battery's loads; low voltage off the main battery (during cranking) etc.

I can parallel them if I want to - eg, for cranking or winching purposes.

I have no need for ammeters any of those situations - the voltmeter tells me all. (That's despite it being an old vehicle normally fitted with an ammeter. Newer vehicles have voltmeters instead, but like most oldies - I swapped my ammeter for a voltmeter. Piece of mind, and never looked back. And have avoided a few strandings!)


I do have a good charger - my alternator.
I'm happy with how it charges batteries. I think I mentioned how I considered 3 years ok for normal batteries but that I usually got more - and that was with my older "external regulator" alternators.

I only upgraded to newer integrated-regulator alternators (from the 1980s) about 2 years ago. My normal battery(s) are both 8 years old with lots of abuse - some overcharging; several flattenings (main beams on all day), etc.
I only recently replaced it/them with an old AGM.


My batteries charge at the maximum rate they and my system can handle - namely about 14.4V with no current limit.
I have yet to see the voltmeter dip below 14.4V after initial start-up with only the normal ignition etc and batteries connected.
(The battery isolator Amperage exceeds my alternator's output.)


Although I see the advantage to a true battery charging system in a vehicle, even for those that stay at 14.4V all day without dropping to 13.8V I think batteries last long enough (except for Fords with mid engine-bay mounted batteries LOL!)
I practice, IMO the desirable ~0.2V change over a ~50°C range seems not to make a big difference (especially if the common Bosch 14.2V setting etc).
Nevertheless, I see the merits for "real" chargers with independent remote monitoring of voltage and temperature. I just don't think many would consider it worthwhile - it's a bit like using distilled water on newer flooded cells - great for longetivity, but not economically worthwhile.


Thanks for clarifying that once a fault develops, the batteries are no longer matched, and also for giving an example as to how to determine whether a mismatch has occured.

I prefer to follow what IMO is typical expert advise and NOT keep batteries paralleled except when charging, and when required.

sorry to have cut and pasted some stuff which is irrelevant below my signing off above.

Best, Peter

There you have it readers - just implement the Wheatstone bridge etc as per Peter's description, and get a voltmeter between the 2 batteries outputs as shown in his diagram.
(Yes, that does assume the fuses or circuit breakers have different voltage drops - that is what the voltmeter is measuring.)

You will then know IF a parallel battery is failing to you can disconnect it etc.


But get further details and pricing from Peter, or check the warranties that come with a paralleled battery system.


Like I said, I avoid all those issues.
[ FYI - I do use non-resistive ammeters in my vehicle (never resistive), but that is merely for experimental purposes. My $20 ARB-Sidewinder DVM provides an adequate condition report on my batteries. (I use a $2 switch so I can monitor either or none.) ]

ChipPunk,

No. You don't have to implement the bridge. It's already there in the form of the two batteries and the fuses.

You merely have to add the voltmeter.

And no. The fuses or breakers are to be of the same type i.e. should show the same resistance for the voltmeter reading to be zero if the bridge is balanced. Of course you can parallel a variable resistor to one of the fuses to trim out any residual voltages due to slight imbalances of the bridge.

Excuse me ChipPunk, I was merely answering your questions and came up with a viable and simple method for detecting a collapsed cell in a parallel battery configuration.

You seem to have an interest in planting something on me I sense.

But maybe I can learn from that and go and ask the modsters to let me mention my products with a $ figure in front of them, when posting in this forum.
If I don't get a favourable answer from them, I might have to come up with another (anonymous) nick and start promoting my own companies products?

Best regards, Peter

"You seem to have an interest in planting something on me..."

No - I was thinking the opposite.


I know the fuses etc are the same - I was merely mentioning their different voltages - otherwise the voltmeter would read zero.

IE - the interconnects are equibonded - the voltage along the top yellowish conductor is the same (around 12-14V), and the voltage along the bottom greenish conductor is the same (ie, around 0V/chassis).

If the batteries are different voltages, so too must the voltage across each fuse.

IE 11.5V + 0.2V = 11.6V + 0.1V = 11.7V; or 11.5 + 1.1V = 12.5V + 0.1V = 12.6V (between chassis and the +12V supply).

Agreed?
That's merely whatever law (Kirchoff's??).
(And although "buss bar" or conductor resistances are not shown, they cancel out - that's the point of the diagonal power taps and matched interconnects.)
Otherwise, where is the voltage drop that "drops" to the battery so that the voltmeter registers anything?

ChipPunk,

without going into too much detail, do the following:

Pull up the image above, I've included for illustrative purposes.
Take a deep breath, and relax, close your eyes for a few seconds (optional) and then look at the picture.

Ask yourself the question what the current path from the one battery to the other one looks like, and what are the resistances in this path.
You may find, there are 4 main resistive elements, the internal resistances of the two batteries and the ones of the two fuses.
Now you will notice that when a current (either clockwise or anticlockwise) flows through this arrangement, there will be voltage drop across the fuses.

And this is precisely what the voltmeter will display for you.

Note that this 'ring-current' will only flow at significant strength if one battery has lost a cell or has otherwise developed a significantly different ocv compared to the second battery.

I hope this simple visualisation experiment will make it click for you.

The need for equal fuse ratings (thus resistances) arises from the desire to keep the voltmeter readings close to zero while a load or a charging current passes through the (intact) batteries.

After all, we do hate false alarms don't we?

Best regards, Peter

So if it is a circulating current, the current through the left fuse and battery is the same as the right fuse and battery?

The fuses have the same resistance.

Ergo, from V=IR, the voltage drops across the fuses is the same.

If the batteries have different voltages at their terminals, where is the voltage drop?

Guys,

Can we look at it from another angle - simplification of thought?

Can we *assume* the 2 x batteries are identical, the 2 x cables are identical and the 2 x Fuses are identical?

If so, can we then assume everything will be ok ?

Yes, of course we will also have to assume the cables and fuses are adequately sized for the application as intended and are correctly attached etc.

Maîneÿ . . .

ChipPunk,

correct.

correct.

correct.

The moment you connect two batteries (with equal or different ocvs) in parallel both batteries will have equal terminal voltage. Looking at the voltage alone won't reveal the 'ring-current' inside them.
Only if you insert an ammeter into the current path between them, you'll get to see the problem which manifests itself as 'ring-current' (help me if I'm stretching my language skills...)

And that's precisely what the voltmeter does. It shows you the 'ring current' in this configuration (and not the voltage difference between the ocvs).
BTW, you may have to remember that an ammeter is just a voltmeter with a shunt across it.
So using the shunt action of the two fuses (which are in a series string for the ring current), you could interpret the voltage drop across them as a current reading.
It's just that the thing isn't calibrated to show the correct amount of 'ring current'. So it's better to just display the voltage drop across them and be done with it.
If you were serious about showing the accurate amount of 'ring current' you'd have to know the exact resistance of the two fuses (which changes in a non-linear fashion with the current through them) and then calibrate the meter accordingly (if that only would make sense because the thing would only be accurate for one particular amount of current, unless you had some number crunching happening in the background with the output then fed to the ammeter display).

To answer your last question, where is the voltage drop?

The voltage drop spreads itself proportionally over the 4 main resistances in the current path.
But it's easier to understand if you asked where is the voltage drop if you connect two batteries with different ocvs in parallel?
The drop will be split proportionally over the internal resistances of the two batteries.
But at this point you also have to remember that the internal resistance is not some fixed value like in an ordinary resistor. The battery resistance has some 'plasticity' to it which varies with the amount and duration of current through it and with the SOC of course.
That's the very reason why a collapsed cell in a parallel configuration won't lead to catastrophic failure of the other battery.
I.e. a simplified model would assume battery voltage differential divided by total internal resistance (e.g. 2V/10mOhms=200A). That current will only flow for a split second if the fault would develop just as quick. It'll then taper off rapidly to a much more benign current.

Can't be that hard can it?

Best regards, Peter

Maîneÿ - in this case, the batteries are no longer matched (as Peter BV puts it), hence the issue.

I come from the camp that recommends NOT paralleling batteries (unless monitored or whilst being charged or for specific reasons).
The earlier link is to an "OldSpark" that mentions the same in response to someone that wrote that there is nothing wrong with paralleling batteries - even if they are NOT matched.
OldSpark gives some references that supports that.
I know too that OldSpark responded to Optima's eCare Manager who wrote that Optima batteries could be paralleled, but couldn't supply data nor reasons to support that... (citing confidentialities etc - but OldSpark knows better...IMO of course!)

I know many have difficulty understanding the issues etc. Some find simple reliability diagrams & relationships too complex. (That's why many use sites such as this...)

I find that battery suppliers are keen to state that "hard wiring" batteries in parallel is ok, whereas others advise against it.
I know it is commonly done (eg trucks etc) and many do it with success as reported above.
And that for "matched batteries" it is - by definition - ok and quite reliable.
But that matching means a lot - including the diagonal connection and same temperature etc - the greater the deviation, the greater the liklihood failure.
Peter BV believes that a failed cell is not a problem - though when that occurs, the batteries are mismatched (hence I assume NOT ok to be paralleled?)

But I am not arguing that. I am merely trying to determine HOW someone with paralleled batteries can determine when one becomes "mismatched" so that they can isolated them BEFORE damage or mutual discharging occurs.


As I understand it, Peter BV reckons that the same current through the same resistance ie, (fuses) leads to a different voltage which can be measured.
I am still trying to figure that one out.

My experience is that V=IR and that if it is the same current I through the same (sized) resistance R, then the V will be the same. And V-V = 0, so the voltmeter shown will read 0V.
And I know the current doesn't go through the voltmeter (maybe 1uA at most!).
And a voltage drop across a resistance means a current flow - hence the "shunt" ammeter Peter BV mentioned - the current I through the resistance R is I = V/R.
I just can't understand how the Voltmeter as shown can read a voltage other than zero if the fuses shown do NOT have a different voltage drop across them.

Sorry Maîneÿ - I strongly suspect that I am teaching you the basics wrt to circuit analysis.
Battery dynamics - that's a different situation, and somewhat complex.
But that is why I avoid these issues and the need to match and balance systems cables and batteries by NOT permanently paralleling them.
I only parallel then whilst charging (additional cost in my case is merely a relay) and if needed (additional cost typically 2 relays and a switch).
Hence I don't even care about mis-matched parallel battery argument.
(In the linked post, the other dude reckons [B]battery(s) can be paralleled... "Not only is it done all of the time, with mismatched batteries a lot of the time"[/B] (not that he qualifies any impact/s). He seems supported by what Peter BV is saying....

I push this as a problem preventer.
In Peter BV's case, he has mentioned the feasibility of selling products to monitor this "problem". For him I push it as the Threat in his SWOT analysis - why go to all that trouble & expense when a battery isolator overcomes it?
Especially when that isolator is often a cheap relay. (My first was a spare relay. I only spent ~$15 when I wanted a 150A isolator.)

And I suspect that even those that think this is electrical degree stuff will register that for reasons unknown, others say NOT to parallel batteries (permanently). (After all, people sell isolators, though that might be for independence.....)
And if a solution is so cheap - and allows mismatched batteries - why risk it.


Sorry Peter, I'll address your circuit later (if I haven't above). (NB - the internal resistance voltage drop is INTERNAL - we are discussing terminal voltages...) But I reckon Maîneÿ can be quick to understand....

Guys,

Can we stop the bleep ing competition.

The number of complaints being recieved is mounting.

Any further comment will be DELETED.

Regards

The Modsquad

THANKS TO EVERYONE FOR YOUR ADVICE. SEEMS ALL TO CONFUSING FOR ME, I JUST LEAVE AS IS AND CHANGE OVER WHEN BEERS GET WARM

CHEERS SKIPP

skipp,
It is not too hard just do it.
"Beers get warm" is fine however what if it was "meat gets warm"

mandatory
1. Use big enough cable with + and - equal lengths
2. put big fuse at each positive less than cable rating

Optional
1. use a marine switch as recommened by Alistair
2. connect the fridge accross both batteries as recommeneded by a few.

Remember you can than also charge both together if you have enough input amps.

have fun

probably because 6 B&S cable *may* be considered too thin, it’s obviously not as efficient as thicker cable, it depends on various factors that I'm positive no-one wants to go into again and again and then again and again

Maîneÿ . . .

Hello olcoolone,

I don't know what all the fuss is about wiring batteries in parallel all of a sudden.

B&S6 has about 1.3mOhms per meter, so it's perfectly fine to use it for this.

Best regards, Peter

Mr Olcoolone,
I agree 100%, there's an aweful lot of people talking through their most fundamental orifice on this subject.

Do what works for you, protect the circuits as thoroughly as you can and avoid Internet experts like the plague, especially one's that quote figures they haven't got a clue how to explain!!

Hell, I can prove speaking English is a poor choice for your life expectancy!

The Japanese eat very little fat and suffer fewer heart attacks than the British or Americans.

On the other hand, the French eat a lot of fat and also suffer fewer heart attacks than the British or Americans.

The Japanese drink very little red wine and suffer fewer heart attacks than the British or Americans.

The Italians drink excessive amounts of red wine and also suffer fewer heart attacks than the British or Americans.

Conclusion: Eat and drink what you like. It's speaking English that kills you.

Alternate Conclusion: Parallel your batteries with an appropriate cable cross sectional area with well chosen circuit protection, perform dilligent maintenance and don't overly discharge.