Dual <span class="highlight">battery</span> systems -mixing <span class="highlight">battery</span> types? (long)

Submitted: Saturday, Jan 20, 2007 at 09:34

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Justin

I have done quite a bit of research, here and elsewhere but am unable to come with satisfactory answers to some questions I have.

I have a simple dual battery set-up in my car, with a ‘dumb’ solenoid to isolate one of the batteries when the ignition is turned off. (will be running a 32L Engle for 2-3days using the isolated battery).

I am looking at battery options for the second battery, and by all accounts an AGM battery is the best way to go.

I spoke to a proprietor of one of the large 4wd accessory chains the other day and he said that I can not install a AGM with a traditional wet cell lead acid battery as my primary – at least not without installing a ‘smart’ charger which simply senses voltage levels to determine which battery to charge. He explained that if he sold me the AGM, he would not offer a warranty if installed in a dual battery system where the other battery is a traditional wet cell. My main battery was manufactured less than year ago and I don’t really want to shell out on buying two AGMs!

When asked why the proprietor then started to mumble stuff about differing internal resistance (true), but said nothing that really explained why mixing batteries will lead to premature failure of the AGM. He was happy to sell me either two AGMs, a marine battery, or a single AGM with smart (dumb?) charger.

Anyway, why are we told not to mix battery types and sizes?

I accept that mixing battery types is bad if and when one fails, it can cause the other one to fail prematurely. By keeping the batteries of the same type and age, you reduce the risk of a dud battery ruining the other good battery. But until the first failure, why would a dual battery system accelerate the failure of the first battery?

Another argument commonly put forward is that the alternator does not correctly charge both batteries, particularly if they are of different kinds. I am not sure about this, and this is why…

As far as I understand, an alternator (in most cars) does not really care, or know as such, what battery, or how many batteries are connected. All it really cares about is trying to maintain its preset voltage (about 13.9 -14.0V in my car), regardless of current draw - within the capacity limits of the alternator. It is the batteries own internal resistance and any other electrical accessories that determine how much current the alternator will put out. I understand that 3 stage battery chargers vary the current and voltage, but am not aware of any alternator car charging systems that do this.

So why would any one battery not be charged (as much as it can be at the preset voltage of the alternator) in a dual battery system, provided the alternator was operating correctly and of sufficent capacity?? (assuming the cabling is good, and with minimal voltage drop between the two batteries).

Of course, odd batteries will charge at different rates, and some may take longer, but this would be the case regardless of how many batteries are connected. I also understand that some battery types require a different charging profile (different voltage?) and this can cause problems if its ideal charging voltage is higher than what the alternator provides. However, AGMS are supposedly able to charge quite well at relatively low voltage so this should not be a problem on my car.

Can someone give me a logical reason as to why installing a AGM and a traditional battery in a dual battery set up will result in one battery (particularly the AGM?) dying prematurely?

And if I install a AGM in parallel with a traditional battery, can the alternator fully charge both batteries?

The car is a 1995 Disco TDi, with a standard 100 or 120Amp alternator (can’t remember which).

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Reply By: Robin - Saturday, Jan 20, 2007 at 09:45

Saturday, Jan 20, 2007 at 09:45

The different batterys simply have different voltages at full charge.

This leads to 1 or other begin charged and the other not and one
constantly delivering more current than the other etc etc.

Hence your battery supplier is trying to steer you towards
an independant system.

Never bothered with two batterys here , just use large enough
Spiral wound AGM and that solves it all in one.

Robin Miller

AnswerID: 216596

Follow Up By: hl - Saturday, Jan 20, 2007 at 10:14

Saturday, Jan 20, 2007 at 10:14

Hi,
I can't see a problem connecting the 2 the way you suggest. They will have different charging requirements to be sure, however, the alternator being usually set at somewhere between 14 and 14.4V actually won't charge the normal starting battery optimally either.
So, you may get 80% in one and 85 in the other, big deal!
Cheers

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Follow Up By: Kiwi Kia - Saturday, Jan 20, 2007 at 11:34

Saturday, Jan 20, 2007 at 11:34

Hi Justin, I am with you and hl. I agree completely, most of the stories you get are self serving or just repeating what someone's brother told to them etc... The level of charge in each battery may not be optimum but what's the difference as each individual cell in each battery has it's own characteristics anyway.

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Follow Up By: Robin - Saturday, Jan 20, 2007 at 13:31

Saturday, Jan 20, 2007 at 13:31

Hi Hl

Unfortunately the state of charge difference is 20% not 5% and this causes power flows to the square. 40% or so.

There are two views of the problem in Justins post -> 1 is from supplier who has to support the result pre-mature failures.
You need go no further than to look down this post to see that ABR has been working with enigneers for a work around to this problem, to see why dissimlar batteries should not be directly paralled.

The 2nd point of view is from the consumer - > You can plug them together and they do charge - but who cares or even really knows that the get less life , often the cars been traded in by then anyway !

Robin Miller

FollowupID: 477049

Follow Up By: hl - Saturday, Jan 20, 2007 at 16:40

Saturday, Jan 20, 2007 at 16:40

Hi Robin,
I don't know why it would cause power flows to the square and even if it did, the currents would be quite minor. Assuming the end voltage of different types of lead acid based batteries is no more than .5V.
I am not aware of any dual battery isolator that actually steps up the voltage to the aux battery to ensure close to 100% charge. With the conditions such as they exist under the bonnet where most 2nd batteries are installed, it would be futile to attempt building a charger that would treat a battery as it could be done in stationary standby/emergency supplies. The constant temperature changes, prolonged heat, vibrations etc all contribute more to the early demise of a battery than "crude" charging setups. I usually buy an Exide Extreme, I know they will last 2-4 years and cost less than 150 bucks if you buy them at the right place.
Cheers

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Follow Up By: Robin - Monday, Jan 22, 2007 at 15:47

Monday, Jan 22, 2007 at 15:47

Hi Hl

The exide extreme as a second is a good cost effective way to go.

One of the reasons for not paralleling an AGM as a second to the starter battery
is that its ineffective.
Not only doesn't it charge as fully as normal for previous reasons, its
typical installation under a car bonnet generally sees it hotter, and
hence charges even less.
And batteries left in a lower state of charge die earlier.

I suspect your extreme would actually hold more on average.

A step up voltage charger (Dc/DC convertor) for second batt is not un-common at all.
Arrid are one supplier, and allows effective use of AGM as 2nd.
Good where battery is a long way away e.g. in Camper Trailer etc, as can
adjust volts to make up for cable loss.

Robin Miller

FollowupID: 477481

Reply By: Member - andrew B (Kununurra) - Saturday, Jan 20, 2007 at 10:03

Saturday, Jan 20, 2007 at 10:03

Gday Justin

I agree with most of your assumptions above, and it sounds a bit like the bloke wants to maximise his sale.....although those who know much more than me will let us know the problems, there is a wealth of knowlege here.

In my case, I have gone with 2 dual purpose batteries (exide extreme, cranking and some cycling) I get 2 days runnng an 80l waeco in the tropics, with the aux still above 12v. These batteries are relatively cheap and have a 2 yr warrenty, so I reeally don't care if I wreck 1 early. I may have investigated the AGM/deep cycle if we camped withot starting the car for more than a couple of days, but generaly we move on or go home after 2 nights. The exides charge pretty quickly, so an extended stay may require me to start the car and run ir on high idle for half an hour, but as yet I haven't had to. Having the 2 in parallell (similar dumb solenoid to you) also gives me greater winching power when req. and I don't know how well a deep cycle would go with this....I think some AGM's can handle both cranking and deep cycle type loads, but thy'd want to be good for the price.

I had an interesting chat with a rep that come to the local ag show promoting the optima battery range. He was promoting how good their value was as they would last 5 to 8 years. They only had a 1 year warrenty, I asked him to put his money whee his mouth was, give me a 3 year warrenty and I'd buy 2.....he wouldn't come to the party.

My opinion is to keep it simple. to go from my safe 48 hours + to a set up that would last say 5 days is a large step, requireing large expensive batteries, maybe solar etc. In my case, which seems similar to yours, I'd be going with a simple dual purpose set up, but you can spend an ungodly amount on this!

Not really answereing your question, but hopefully a bit of helpful info

Cheers Andrew

AnswerID: 216600

Reply By: Member - Norm C (QLD) - Saturday, Jan 20, 2007 at 10:24

Saturday, Jan 20, 2007 at 10:24

Justin, from your post, I think you have a pretty good understanding of the issues and risks.

I don't think there is any real problem with mixing AGMs with normal wet cell batteries. I've been doing it for two years now and the system works fine. I have a normal cranking battery (the one that came with the vehilce). In the engine bay and isolated by a 'dumb' solenoid, I have a 105 AH Federal wet cell deep cycle battery. In the back of the Hilux. I have a 120 AH AGM that is charged in parrallel with the deep cycle in the engine bay. I can plug / unplug this connection at will via an Anderson plug. I also have a 120 AH AGM in the CT which is connected in parrallel with the engine bay deep cycle whenever the CT is connected. Occasionally all three deep cycles (one wet cell and two AGMs) are all in parrallel, then in parrallel with the cranking battery when the engine is running.

I've had this set up for just under two years now and it works well.

I can identify ony two potential problems with my set up. One is as you state, one stuffed battery could lead to many. That's a risk I take along with most others who parrallel batteries.

The other is that AGMs have a much lower internal resistance to normal wet cells. They therefore accept a higher charge current. There is the potential that if an AGM is drawn down, then charged in parrallel with another battery, the AGM may take most of the charge current at the expense of the other battery (including the starter). This is only a short term problem. Once the AGM builds some charge and it's internal resistance increases, charge to the other battery will increase. I have not noticed this problem at all and it is only likely to be an issue if you do lots of short runs after running down the AGM. If it is a worry, you can install a voltage sensing isolator like a Redarc or similar.

I don't claim to have all the answers (and may in fact be missing something important), but my first trade was Electronics Technician (Army trained but long before AGMs were invented) through which I learned the basics. Like you I've spent a bit of time on research - and found that many people claim to be experts in 12V systems, but are not. Most importantly, I've set up my own system and it has worked well for two years so far. Hope I can say the same thing when it is 4 or 5 years.

AnswerID: 216602

Reply By: Derek from Affordable Batteries & Radiators - Saturday, Jan 20, 2007 at 10:43

Saturday, Jan 20, 2007 at 10:43

Hi Justin

We have been working on this small problem. It is not a big issue and 90% of people won't notice the how the battery capacity and life is effected by parallel charging.

We have been working with engineers and electronic manufacturers and now have Bi-solators on the production line and due to land here in 2 months.

These will eliminate all the current issues of charging battery banks from alternators and solar or mains chargers. 150A rated and 800cca.

0.04 Volts drop through the device.

Site Link

Regards

Derek.

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Follow Up By: Andrew from Vivid Adventures - Saturday, Jan 20, 2007 at 11:42

Saturday, Jan 20, 2007 at 11:42

Hi Derek,

Interesting post.

I am running mixed setup now and have no evidence of particular issues at a practical level.

I'd guess your new Bisolator will cost more than your isolator which is working very well in my situation ... we are talking about the issues that are probably in the noise ... after all, the effects on battery life of extreme under-bonnet temperatures, and vibration and shock from typical 4WD activities is probably as much to worry about as the issues of differential charging rates/voltages etc.

If you want someone to test out your new product, however, drop me a line ...

Cheers
Andrew.

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Follow Up By: Derek from Affordable Batteries & Radiators - Saturday, Jan 20, 2007 at 16:15

Saturday, Jan 20, 2007 at 16:15

Hi Andrew

Thanks for the reply. Yes most systems work perfectly and our new one is just the top of the range model. We have 3 types on the way.

We are going to make it a more than affordable option.

I will drop you a line when we unpack the container. It is one of 3 containers and is full of 'GOODIES'

Regards

Derek.

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Follow Up By: Thylacine - Saturday, Jan 20, 2007 at 20:41

Saturday, Jan 20, 2007 at 20:41

Derek, your diagram shows a diode-based isolator. How do you avoid voltage drop with such?

ed

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Follow Up By: Derek from Affordable Batteries & Radiators - Saturday, Jan 20, 2007 at 21:08

Saturday, Jan 20, 2007 at 21:08

We have incorporated solenoids into the Bisolator.

Thank You for the good question though.

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Follow Up By: Thylacine - Saturday, Jan 20, 2007 at 23:05

Saturday, Jan 20, 2007 at 23:05

Doesn't that defeat the "true" isolation of a diode system? And basically make the diodes redundant anyway? And why not in the circuit diagram above?
Also I don't understand the "150A rated and 800cca" statement.
Not trying to poke holes in your product, in fact I'm very interested in a device that meets your claims if it provides a true battery isolation.
I spend a lot of time solo with one-vehicle (60 series) in remote locations, so currently run a solenoid based system with simple relay-logic controlling operation (ie batteries paralleled when alternator and oil-pressure are good), with Cole-Hersey for manual override when winching. I'm qualified and competent to fault-find electronics, but prefer the modularity of the system I currently use ($30 of spares provides a replacement for any failure with a simple swap-over).
The problem (for me) is that the batteries are separated rather than isolated.
As stated above I'd be very interested in your product if it meets specs and appreciate any comments.
cheers

ed

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Follow Up By: Derek from Affordable Batteries & Radiators - Saturday, Jan 20, 2007 at 23:33

Saturday, Jan 20, 2007 at 23:33

Hi again Ed

Yes, our new Bi-solator is all I claim it to be. It has a bit more electronics than simple diodes and solenoids inside it.

We have combined a rotronics type system with a marine isolator and battery management electronics.

Why not send me your email address and when they arrive you can test one for a few weeks. I plan to have test modules for people to try out and then return if not happy.

Regards

Derek.

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Follow Up By: Sam from Weipa Auto Electrics - Sunday, Jan 21, 2007 at 11:27

Sunday, Jan 21, 2007 at 11:27

hmm this looks like an interesting piece of equipment. basicaly its an inofiecient didode isolater double as a normal isolater. How do you propose it knows when to switch?? and whilst the vehicle is running will the isolaters be closed or will the be charging through the diodes?? how much will it cost and what brand is it??

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Follow Up By: Sam from Weipa Auto Electrics - Sunday, Jan 21, 2007 at 11:34

Sunday, Jan 21, 2007 at 11:34

Does the isolater part of it switch the batteries seprately or do they switch together??
it doesnt make sense its still no better than a redarc voltage sensing unit??? whenever the car is running its going to be charging both batteries no matter what??
it seems overly complicated. Sorry if I sound like I'm knocking holes it you're system its not my aim cheers sam.

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Follow Up By: Derek from Affordable Batteries & Radiators - Sunday, Jan 21, 2007 at 13:39

Sunday, Jan 21, 2007 at 13:39

It is fitted with twin solenoids. It has a voltage sensing circuit on the each solenoid that will engage the battery that requires the most charge. It has diodes yes but the solenoids override them when needed. Each solenoid is rated at 150amps continuous and has a surge rating of 800amp. There is nothing like this on the market that I know of.

We have 2 models. 100 amp and 150 amp

Pricing will be under $150.00 for the 80 amp and under $200.00 for the 150 amp.

One of the engineers I was working with said our current systems are primitive.

Regards

Derek.

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Follow Up By: Sam from Weipa Auto Electrics - Sunday, Jan 21, 2007 at 13:45

Sunday, Jan 21, 2007 at 13:45

I'll be honest I'm not so sure I have been working with and fitting battery system into vehicles for years and last 3yrs of that on the cape (were they get used very regulary). And I still beleive a basic system is adequate. If battery failier was a big issue I would be looking for a better solution but its not and with the primitive systems you speak off they work well at least the redarc (For the note I have no affliation with redarc they just have a good product). Allright maybe what I am saying is what are the big "advantage's of you're system?)

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Follow Up By: Derek from Affordable Batteries & Radiators - Sunday, Jan 21, 2007 at 14:00

Sunday, Jan 21, 2007 at 14:00

Hi again Sam

Yes I know current systems work very well and will do the job.

I sell Redarc and love the product. We were looking to produce our own isolator so we were not a 'middle man' and hence this product.

It is better because it is marine grade and fully sealed in resin. It has a heat sink and is fully automatic in all respects. It even starts the car off both batteries if needed automatically.

It allows for solar input and manual override.

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Follow Up By: Thylacine - Monday, Jan 22, 2007 at 00:34

Monday, Jan 22, 2007 at 00:34

"Why not send me your email address and when they arrive you can test one for a few weeks. I plan to have test modules for people to try out and then return if not happy."

Thanks for the offer Derek, but no thanks, for several reasons.
Firstly I can't see the point in a few-week test, or a few-month for that matter.
Secondly, If it has more electronic components than diodes, bush fault-finding becomes difficult. And looking at your pic of the unit it's set in resin so irreparable anyway. I'd have to know it's principle of operation (and be happy with it) before thinking of trusting it.
This is the reason I stick with a simpler system (and drive an old landcruiser) - I can fix most things that break in the bush.
cheers,

ed

FollowupID: 477389

Follow Up By: Sam from Weipa Auto Electrics - Monday, Jan 22, 2007 at 08:28

Monday, Jan 22, 2007 at 08:28

I suppose cutting out the middle man is a good thing anyway lets us know how the product goes some more details on how it works would be good?

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Follow Up By: Derek from Affordable Batteries & Radiators - Monday, Jan 22, 2007 at 09:02

Monday, Jan 22, 2007 at 09:02

Sure, I will let you know when they arrive.

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Reply By: Sand Man (SA) - Saturday, Jan 20, 2007 at 12:45

Saturday, Jan 20, 2007 at 12:45

Justin,

IMHO your local Proprietor gave you sound advice, based on the specific "dumb" isolator configuration you have and the type of batteries you wish to run.

He was not only protecting the image of his own business, but trying to protect you from any unsound investment.

There is sufficient information in past Posts on this issue for me not to elaborate further.

In short mate, "you get what you pay for".

Bill

I'm diagonally parked in a parallel Universe!

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Reply By: Member - John and Val W (ACT) - Saturday, Jan 20, 2007 at 13:29

Saturday, Jan 20, 2007 at 13:29

Hi Justin,

You have raised some interesting points. From my (incomplete) understanding, your analysis is spot on. I've done some delving and I'm convinced that there are grounds for concern though not as extreme as your supplier suggests.

Some of the fundamentals:

Charge requirements are temperature dependent.

Ideally a battery should be charged initially at constant current at a current of up to 25% of its Ah rating (eg 25 amps for a 100 AmpHour battery).

Constant current charging should be maintained until the voltage rises to 14.2V for lead acid, 14.1 for a gel type, 14.4 for AGM and 15V for a calcium/lead battery.

Once the terminal voltage reaches this level, charging should continue at this constant voltage, during which the charging current will slowly decline to a low level.

After this a float charge may be maintained, 13.4V for lead acid, 13.3 for gel and AGM, and 13.8 for calcium/lead.

Because of the different charging requirements, putting two different types in parallel will necessarily not be optimum for both. In practice of course, the alternator is basicly supplying about 14.2 volts without much current limiting, which works ok for the lead acid battery fitted to the vehicle for cranking. On the basis of these figures, that will not be enough to fully charge an AGM, and even less so a calcium/lead battery. I suspect that your battery supplier is right to the extent that if both main and aux batteries were the same type, the voltage regulator could be set appropriately. I reckon he's wrong though in thinking that you'll damage the AGM unless you buy 2 from him.

Fact is, our batteries are never well treated, never charged under ideal conditions, yet seem to survive quite well. I think we probably don't get full capacity out of them, especially an AGM or calcium/lead one charged as if it is a lead/acid type.
Consequently we don't get a long life from them.

I've always used lead/acid types, so can't speak from experience of mixing battery types. From what I've been learning recently though, reckon I'll stay with lead/acid. I had contemplated once fitting a second alternator and regulator just to handle the auxilliary battery/s, (and provide a spare alternator should one die in a remote area) but that seemed a bit of overkill.

Not sure if I've shed any light, or just confused the picture even further. Someone here recently commented how discussion of 12V brings out lots of pseudo-experts - perhaps he was right, so (disclaimer) I'm no expert, even pseudo!!!

HTH

John

J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein

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Reply By: Crackles - Saturday, Jan 20, 2007 at 14:24

Saturday, Jan 20, 2007 at 14:24

Justin, I have been mixing starting & deep cycle batteries for over 20 years & will be the first to admit it's not the ideal situation. On average I would get around 3 years, an acceptable age but nothing stunning. It's always difficult to asses whether it was mixing the batts that led to a relitively average life or if it was the rough driving, vibrations, heat or deep discharging down to 10%. The reason I stuck with deep cycles was to get a long run time without the need of generators or solar panels, something 2 starting batts aren't as good at.
Of course it's near impossible to tell exactly if a battery is 100% charged but I never found the capacity of either battery to be noticably low after a days drive, if they weren't fully charged they were close to it.
Cheers Craig...........
Twin 115AH deep cycle Trojan batts.

AnswerID: 216646

Reply By: Justin - Saturday, Jan 20, 2007 at 17:19

Saturday, Jan 20, 2007 at 17:19

Thanks for all the great responses guys.

I think the answer I will take, is that yes, it is ok to mix battery types. It is a compromise, but an acceptable one. As far as I can tell, in terms of charging batteries, the alternator, and its lack of intelligent charging profiles is the main culprit in not fully charging batteries, but as others have said we seem to mostly get by on our partially charged batteries.

I will get some final advice from my mechanic, but am leaning to the AGM. I will probably only use the dual system for the trip (Perth - Kimberlies, Cape Leveque - Sydney and other bits in between, :o), and revert to a single system when I get back - and donate the old battery to the Hillman Minx, which is due for a new battery!! No point driving around town with the extra mass of a 2nd battery when it is not needed.

I understand why the 4wd shop said what he did, pitty he could not explain himslef more clearly, and I think he is being over-cautious - but then I am not the one who has to pay out on warrenty claims!! He won't be getting my buisness this time.

Derek, your device looks interesting. If I was travelling full time I might consider it, but for now I will stick with KISS.

On the voltage output of my alternator, yes it is a little low (13.9 - 14.0V), and I am not sure why. I will get the mechanic to check it out, although I have read elsewhere the diesel Disco alternators run at lower voltage to the petrol Disco - supposedly as diesels have lower power requirements.

AnswerID: 216678