Parellel batteries of different size only. Yes or no?

[brianjwilson]

I hate that I'm starting a new topic for this question, but please hear me out.

The reason for this example is because a vehicle only has room for a small starter battery under the hood, and it may not be as cost effective to use 3 of them as house batteries. Additionally the house battery will be difficult to access, and extra connections just add to the cost and complexity (and things to fail).

First off I'm talking about two brand new batteries, same type, just different size. For example only, let's use a brand new lifeline group 24 AGM (starter), and a brand new lifeline 8D (house battery). Different amp hour ratings but same chemistry, same manufacturer voltage recommendations for charging.

This example vehicle would use a 225-250 amp alternator, large wiring (TBD), and a blue sea 7622 ACR connecting/isolation the batteries. 200-400 watts of solar with mppt charge controller set to manufacturer's recommended charging profiles connected to house battery.

Yes I understand the batteries will have different internal resistance and discharge at different rates. When the ACR connects the batteries the voltage will attempt to "equalize" (I'm sure that's the incorrect term) between the two, at a rate determined by resistance of wiring, batteries etc. When discharging they will be isolated.


Does anyone see a real issue with this? Am I misunderstanding anything? Is there a better way to do this, that will still allow a high amp charge to get to the house battery with the engine running (to power an inverter and microwave or induction cooktop)? Suck it up and buy a bunch of small, matching batteries?


I know the conventional answer is to not mix different size/type/age batteries. However if they have the same composition and charge profiles, is it really any issue? Using the ACR the batteries aren't really discharging together after the voltage drops below a float, they are simply charging together.

Before it's said, yes I've searched. For every website that says it's ok, there are two more websites with a generic "don't mix different batteries" hard stance on the subject. I'm hoping some of the resident electrical experts will weigh in.

 

[brianjwilson]

I have looked at the ctek chargers/isolator and smart pass, but their descriptions were confusing in some aspects and vague in others. Mostly it just looked liken something expensive and complex, something to fail. I like the relative simplicity of the ACR.

 

[Ducky's Dad]

I have been running mixed batteries for years in two trucks. Sometimes with one Optima Red and the other an Optima Yellow. Sometimes with one small Odyssey and the other an Optima, Red or Yellow. Seven years with a big Odyssey and a small Odyssey. Currently have a big Odyssey and two smaller Optima Blues. All were/are running through Painless Performance switched solenoids, nothing automatic, with alternators ranging from 105amp to 270amp. Other than the normal charging issues with the Odyssey 2150/DieHard PM1, no problems with the mixing.

 

[DiploStrat]

Basically ...

The electrical/battery engineers will groan, but, to the best of my knowledge, belief, and three years of real world use:

For Lead Acid batteries. (Not/Not Lithium)

When CHARGING:

-- You want batteries that call for grossly similar voltages. Typically 14.2 - 14.8v at around 20C.

-- The internal composition of the batteries, i.e. starter vs. deep cycle (camper/house) does not matter.

When DISCHARGING:

-- You want the batteries separated so that you do not damage your starter battery by subjecting it to a deep discharge.

-- You want to use deep cycle batteries for your house/camper so that they can withstand the abuse of deep discharges.

Thus for typical RV/boat/overlanding use, you WANT batteries that are dissimilar in size and construction.

Assuming that the charge profiles of both batteries are basically the same AND that your vehicle produces the correct charging profile - basically the correct voltage, then a relay based system, whether key controlled or intelligent, with properly sized wiring, is ideal. This is what I have and it works very well. I have no need for any kind of secondary controller; just big wires.

BUT, if your vehicle's charging profile/voltage is inadequate (voltage too low), as, for example, the case of some Toyotas (and Sprinters?) trying to charge a deep cycle camper battery, then you may be much better off with some form of battery to battery charger, such as those from CTEK or Sterling. These will have relatively low amperage output, but they will boost the voltage to assure that the deep cycle camper batteries are fully charged.

The CTEK D250S, when combined with the CTEK SmartPass (an intelligent relay) is a very nice combination as it allows higher amperage, if lower voltage at the start of the charge and then, once the amp flow drops to about 20A, it opens the relay and continues with the boosted voltage from the D250S.

 

[robgendreau]

I agree with DiploStrat. Lotsa RVs just use dumb charging for house and vehicle batteries, with an isolator. You never get as good a charge on the house battery as you would with smart charging though. You notice the difference in these RVs when you charge the house batteries through shore power through the converter, which often has smart charging, or even when using a trickle charger at home.

 

[Chris Boyd]

Their are also specialty chargers for marine and emergency vehicles that have multiple circuits for different batteries that isolate the charge legs. That would be the best option because then the smaller battery doesn't cause the aggregate voltage to rise as it fills and drop the amps going into the second battery. Check Kussmaul for the vehicle application. They have Multiple types and functions for various applications.


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[Ducky's Dad]

Thus for typical RV/boat/overlanding use, you WANT batteries that are dissimilar in size and construction.

Well, not necessarily. If you select your batteries carefully, you can use the same type of battery in different size and construction. Example: All Odyssey AGMs are deep cycle, dual purpose batteries by design, so you can use a Grp 65 as your starting battery and a Grp 31 as the house battery. Another: All Optima Yellows are dual purpose, so you can mix those as starting and house batteries. Optima Blues come in both starting and dual purpose versions, but if you use dual purpose Blues for both applications, you will be fine. Or mix dual purpose Blues with Yellows. I think all Northstar AGMs are dual purpose, so you can mix and match those as needed. Just need to understand what you are using. As noted above, I have mixed Odyssey/Optima and Optima Red/Yellow eith no problems. Currently running Odyssey 2150 as starting battery and a pair of Optima Blue DPs as house batteries. Seems to be working just fine.

 

[DiploStrat]

... then the smaller battery doesn't cause the aggregate voltage to rise as it fills and drop the amps going into the second battery.

That will only happen if the cabling between the starter battery and the camper battery is too small. (Which, to be fair, it is on many vehicles.) I do not/not see this on my truck, but then I have large, 100mm2, cables.

 

[brianjwilson]

Ok thanks guys. On my Tacoma I use three dual purpose AGM batteries (1 starter, 2 aux for camper), all same size with the blue sea ACR. The solar controller tops off with a multi stage charge, as does the converter. I just wanted to make sure there was no real issue doing this with different sizes.

The ctek d250s and smart pass looks like it is a max of 80amps through the smart pass, 20 amps otherwise. It just looks complex in operation. Price is similar to using a blue sea ACR and separate solar controller, but I guess I like the familiarity. I'm thinking I could use a xantrex inverter with multistage charger and this would cover all bases.

 

[DiploStrat]

If you have a Toyota, check the charging voltage. If over 14v, all is fine. If below 14v, you may find this to be worth the money. Friend of my uses a variation of this on his Tacoma and then uses a Blue Sea ACR. The voltages on my Chevrolet run all the way to 15.5v at 0C, so I have no need for any form of voltage boost.

http://www.mechman.com/accessories/...e-voltage-boost-module-for-oval-3-pin-toyota/

As I say, for many of us, the voltage of the stock alternator is plenty high enough and there is no reason to pay the extra money for a battery to battery charger.

In any case, the real secret to achieving a really full charge is some form of solar (or shore) power charger. Most of us never drive long enough to fully charge the camper batteries.

For a deep dive into this, start reading here: http://www.smartgauge.co.uk/facts.html

 

[KMG]

I have the CTEC 250s with the smart pass. Works great with multiple charging sources. I wanted to charge the house batteries while driving and with my solar panel to allow complete charging. I've also used their battery charger for years with no issues. For me, the CTEC is a simple cost effective solution!

 

[brianjwilson]

I have the CTEC 250s with the smart pass. Works great with multiple charging sources. I wanted to charge the house batteries while driving and with my solar panel to allow complete charging. I've also used their battery charger for years with no issues. For me, the CTEC is a simple cost effective solution!

Good to hear! Really the only negative thing I see that the max charging is 80amps with the smart pass. Likely not an issue in the real world, but would be nice to be able to match the draw from an inverter and induction cook top or similar.

Another idea I could entertain would be to mount a second alternator (it is a factory option on this particular vehicle). Run it completely independent of the vehicle, solely to charge the house battery. No worries about mixing batteries at all. Sterling power makes a multistage charger just for this purpose. I may dig around more and see who makes multi-stage stand alone regulators as well.

Still early in the planning stages for the next rig, trying to think out ideas.

 

[DiploStrat]

A user on this forum, "Skygear" runs a Blue Sea ACR on his Tacoma with a voltage adjustment.

I have never heard anything bad about CTEK stuff. Audi and several other brands recommend CTEK shore chargers. I know of at least one Tiger owner who is running an all CTEK setup.

As far as using an induction cooktop with the CTEK gear, there is no problem. The induction cooktop is going to draw from the camper battery, through the inverter. If the engine is running, it will attempt to recharge the battery as quickly as possible. Depending on the state of charge of the camper battery, this can run from 50 to 150A, depending you your wiring, etc.

The simple fact is that unless you have really hammered your camper battery (which you shouldn't), you are not going to see sustained charge rates over 100A. Even with my wiring, I rarely see more than 150A and never for more than about 30 minutes. So, in the real world, the limitations of the CTEK SmartPass are not that significant and, if your alternator runs at less than 14v (and your wiring is a bit on the small side) the voltage boost function of the D250S could make all the difference in the world. And if your solar kit is small enough to be handled by the D250S as well, then there may be a cost savings.

 

[brianjwilson]

I think you may be right. An aftermarket sorce of the second alternator mount and 105 amp alternator lists at $400. Not to mention charger controller/regulator. A larger single alternator and ctek may be more cost effective anyway.

 

[KMG]

Good luck. Let me know if you go with the CTEC system. I'd like other users feedback. Thanks, Kevin