Wiring batteries in parrallel

Hi Peter
Distance isn't a problem as I will be able to keep the 2 batteries next to each other.
Have had no problem with the 120 Watts (3 x 40W panels) of solar keeping up with the 2 Engel's (given sun shine). We only use the fridges as fridges not freezers and cold enough to keep the beer at a good temperature. On a typical day the battery will get up to "float" charge within 4 - 6 hours
Cheers
Daz

Hi Peter
Thanks for that. I think I even understand most of it. The test will be when I try to explain it to my wife. I assume that I don't need to put a fuse between the 2 batteries?
Cheers
Daz

no worries Daz,

no need to get the missus involved, secret men's business.
No fuse required in your situation if batteries sitting next to each other, so the whole issue of balancing the currents/voltages is moot.

cheers, Peter

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You mean Willem Pedro???????

is that normal-to use a fuse as a resistor?

Hello fisho,

fuses do have electrical resistance otherwise they wouldn't get hot as the current through them increases, and ultimately the metal inside them melts and the fuse blows.
A typical 25A blade type (fast blow) fuse shows a voltage drop of about 0.12V at 12.5A, so the resistance is about 10milliOhms at this current. The heating power at this current is 1.5W which makes the fuse hot to the touch, even at half of its rated current (no resistance would mean no heating).

But no, it's not 'normal' to use a fuse as a resistor, mainly due to its nonlinearity which makes it undesirable for this.
Most of the time fuse resistance isn't wanted/needed in the circuit, but it's there nevertheless.

One interesting thing to note is that the fuse resistance can be in the same ballpark as the internal resistance of lead acid batteries.
That's why the placement and rating of fuses in a circuit with more than one battery influences the current distribution between the batteries.

cheers, Peter

No Fisho, it certainly is not normal to utilise a fuse as a resistive element in a power supply circuit. Nor would it be desirable. The primary aim in a 12 volt power circuit is to introduce as little resistance as possible, hence the importance of using large cross-section cable.

If batteries are connected in parallel with low resistance cable, both batteries will be presented with identical voltage and the current flow in each battery will be proportional to the state of charge of the battery and its internal resistance. The internal resistance is a function of several factors, largely the battery size, type and its condition.

In a situation where battery 'B' was connected with cable introducing voltage drop it would be presented with a lower voltage and lag somewhat in receiving a charge. However as the charging proceeds, the current to battery 'A' will taper off and battery 'B' will eventually reach the same state of charge. This is not an ideal arrangement but is acceptable provided that the wiring to the second battery is not of an extremely small size. This is the common situation where the first battery is located close to the alternator whilst the second is located in the rear of the vehicle with several metres of cable introducing some (acceptable) voltage drop. To intentionally introduce any further resistance into that circuit would be foolhardy.

Hi Allan,

well explained, but there's something which doesn't receive a lot of attention.

In your example of batteries A/B in parallel, in real life battery A (starter battery) is nearly fully charged while B can have a very low state of charge.
If the wiring between A and B doesn't have sufficient resistance, the alternator is capable of pushing extremely high charging current through B, sometimes exceeding the max rated by a factor of 3 or more.

This is detrimental to battery health, especially if B is mounted under the bonnet where it is subject to the double whammy of high temperatures in connection with little voltage drop due to short wires.

It's a lot better for the auxiliary battery (which gets discharged a lot deeper) to connect it in a way which guarantees sufficient voltage drop while the battery's hungry for current.

What's really foolhardy is, to aim for minimum voltage drop in the wires for reasons given above.

And as the battery reaches 80~85% of charge at a typical alternator voltage of 13.8V, the battery reduces its current intake which makes the wire resistance insignificant.

So the battery gets to its max possible state of charge regardless of wire resistance, since the limit is governed by the alternator voltage and not by the wire gauge.

8B&S (or sometimes 6B&S) wire should do the trick nicely since it provides good resistance, while not creating too much voltage drop caused by concurrent loads like a compressor fridge.

cheers, Peter

Well actually Peter we were discussing the situation of Daza's original post where he wished to connect two auxiliary batteries in parallel, not a Starter and an Auxiliary. He would already have that connection.

Nevertheless your comments re SOC imbalance and charge current distribution are interesting and I concede that a little impedance between the starter battery and auxiliary battery (or batteries) may be desirable.

---------------------"You mean Willem Pedro??????? "------------------

Certainly NOT Willem, Bonz ...................

I've seen his Travels map and read his posts . . . . . . . that man is truly a LEGEND.
I'm apaulled you would think of him that way . . . . . . . . !!!

no thats what I thought Allan.
Using a blade fuse to introduce resistance is like using a vicegrip instead of a socket set.
It works but doesnt seem like a good idea to me.

What Id more like to know thoughis-
why doesnt he use 2 identical batteries?

Very interesting discussion guys, I have followed it with great interest.

Fisho, The concept of using only identical batteries when paralleling would appear logical although I have never seen credible argument to support it. For my own part I have used non-identical batteries and not experienced any problem. They have been of the same 'chemistry' however. I have even seen recommendations to use only batteries of the same 'age'. Again that may seem sensible but is not supported by reason.

Hi Fisho. Daryl could have one of a dozed reasons for not using identical batteries. I find this discussion interesting because it indicates that battery capacity can be increased incrementally rather than needing to be doubled without the sky falling in. A good discussion chaps!
Kevin

yes it certainly is interesting.
heres another question (hope it isnt considered a "hijack")

My boat is fitted with dual batteries, (identical) and a BEP VSR battery isolator.
If I replace 1 battery with a deep cycle as house battery, should I only charge it on one or the other or can I let the VSR do its work?

@Allan,

...Well actually Peter we were discussing the situation of Daza's original post where he wished to connect two auxiliary batteries in parallel, not a Starter and an Auxiliary....

Duh...if you had read the whole thread you would have noticed I was already aware of Daz's wiring.
My reply was more in response to your...This is the common situation where the first battery is located close to the alternator whilst the second is located in the rear of the vehicle...
So you brought this starter/auxiliary thing up, not me.

@fisho,

...Using a blade fuse to introduce resistance is like using a vicegrip instead of a socket set. It works but doesnt seem like a good idea to me.....

Oh really, so what type of fuse do you recommend then, one which is supra conductive?
Let me give you more information:
A typical fast blowing glass fuse (3AG) rated 15A still has 6milliOhms cold resistance, more if some current flows through it.
And industrial quality DC circuit breakers aren't better in this department, with around 7milliOhms for a 20A rated unit.

So no matter what type of fuse you select, the added resistance can significantly alter the current distribution between batteries in a parallel configuration.
The only way to prevent this is to mount the batteries close to each other so that only one fuse is required (which seems to be the case with the OP's system).

About your other query about paralleling a charged battery with a discharged one for charging (this is what happens in any setup with an isolation switch) is a good recipe for either overcharging the full one, or undercharging the discharged one.
So charging them individually is the best way for maximum battery life.

cheers, Peter

ok-Im sure you reply wasnt intended to sound a little narky.
But a fuse is intended for protection.
Your suggestion seemed to be solely as a means of introducing resistance into the circuit.
I may have got this impression as you said that a fuse wouldnt be required if the batteries were closer together?

Anyhow I appreciate you answering my query, thanks.

fisho,

you're welcome.

About the added fuse:

you're certainly aware that basic safety requirements call for a fuse close to the battery's positive terminal.

As the OP intends to wire another battery in parallel to the existing one, it could have meant the batteries are going to be mounted away from each other in which case a second fuse would have been the go.
This triggered my original response.
Then, after it became clear the OP's going to wire the batteries side by side, I concluded that only one fuse will do the trick, doing away with any imbalances introduced by a second fuse.

No, I didn't recommend the additional fuse for the purpose of introducing resistance, as you can see if you read everything above carefully.
The resistance is just there, vicegrip or not, and if you want to create a carefully designed system you take this into account in the circuit layout.

cheers, Peter

@ Battery Value Peter: I certainly did read the whole thread!

I assumed that Daz intended to connect a second auxiliary battery in parallel with an existing auxiliary battery, and still believe that. No reference was made by either him or myself to a 'starter' battery. I also allowed that these two auxiliary batteries could be adjacent or distant from each other. Saying that one battery is "located close to the alternator" does not make it the starter battery. I referred to the two batteries as 'A' and 'B'. The first reference to a 'starter battery was made by yourself in Follow Up 8.

This is the actual situation in my vehicle with the first auxiliary battery adjacent to the starter battery and with the second auxiliary located toward the rear of the vehicle close to the fridge. These two auxiliary batteries are interconnected with 10mm2 cable and if there were excessive resistance between them the rear battery near the fridge would supply a majority of current to the fridge yet be deprived of charging energy.

The issue under discussion was the means of interconnecting two auxiliary batteries and the desirability, or otherwise, of impedance between them. Not limitation of charging current to the auxiliary battery or batteries. That is another subject altogether.

I stand by my original expression that paralleled batteries should not have intentionally introduced resistance between them whether by cable resistance, fuses or other means. Certainly it may be necessary to insert fuses between them if not adjacent but these fuses only need to protect against cable short circuit and should be sized appropriately with minimal resistance.

Hello Allan,

....The first reference to a 'starter battery was made by yourself in Follow Up 8.

Oh?
And what about the followup before this, No7 by you? No starter battery/auxiliary been mentioned :>

Anyway, I don't think we've to go on like this.
I think the OP has got more than enough to sift through by now.
No more input from me in this instance.

cheers, Peter

Peter, what on earth are you on about? The word "starter" does NOT APPEAR in Follow-Up 7, unless your copy differs from mine!

Even if you cannot see the point I'm sure others can, so yes I'll leave it also.

And impedance is an AC term, for DC it is resistance.

Hello Allan,

sorry, I got mislead when you wrote...This is the common situation where the first battery is located close to the alternator whilst the second is located in the rear of the vehicle ...

In most cases (dual battery setups), the term first battery refers to the starter and second battery to the aux.

If you had called them 'second' and 'third', it would have been crystal clear (even to me) that you meant two auxiliary batteries, one located near the alternator, and one some distance away in the back.

Everything makes sense now.

Please accept my apologies for messing this up on an otherwise perfect Sunday :)

cheers, Peter

No worries Peter. I think we are in step now.

Hi Ted,

Re: "And impedance is an AC term, for DC it is resistance."

Not entirely true. Although more commonly applied to AC circuits it is the correct term for some DC circuits.

It is also used as an Electrical Engineering generalised expression to indicate the opposition to current flow from any cause in either AC or DC circuits without needing to specify the nature of the impedance. That is how I used it here. In such a situation it has no dimension.

Good discussion all. Didn't realise what I thought was a simple question would create such debate.

Fisho's asked the question about why not use a second battery identical to the existing 130ah battery. My thoughts are that to run my existing 2 fridges increasing my capacity to 260ah would be good but possibly overkill and also expensive. My current TROJAN battery set me back $450. However adding another 100ah would increase my capacity to 230ah which should be adequate and less expensive
Cheers
Daz

And bear in mind Daz that no matter how large the battery capacity, it is still necessary to be able to replenish the consumed energy eventually. So oversized batteries will only assist with periods of no, or low, input charging but not help if the charging system is inadequate.

Just when I thought I had my batteries sorted out. I went to purchase a 100 ah battery. Explained what I was going to do and the gentleman I was dealing with suggested that as the 2 batteries were of different amps that the bigger battery would never fully charge. Is this right?
Cheers
Daz

It's absolute rubbish Daz. Some people who work and sell electrical merchandise including batteries have no adequate training in electrical matters but would have you believe that they are experts!

Consider this: You have two water tanks of different size (but the same height) connected together with a pipe with a tee in it being fed from a pump. Both tanks will receive the water and both will rise at identical rate and height. The larger tank will require and take more water quantity (Amps) and finish up containing more litres but the pressure at the bottom of each tank will be the same (Volts). Water/electrical analogy is not always perfect but in this situation is quite accurate.

Certainly the charging current to the two (or more) batteries will not be equal but will apportion as per the internal impedance (or resistance!) of the batteries and generally will be in proportion to the size of the batteries, and both batteries will arrive at the fully charged state at about the same time.

Have no fear Daz, I and many others have had unequal batteries in parallel for years without problems.