Basic dual battery question

[Martinjmpr]

Sorry if this has been asked but I couldn't find it in a search. I've ordered the hardware for my dual battery setup (2004 GMT800 Suburban 1500, 5.3) but I'm wondering something:
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If my "main" battery is my "starter" battery (that is, the one that is in there now) and my 2nd battery is the "house" battery, is there any need or reason for the 2nd battery to be of the same type as the main battery? What I'm wondering specifically is that if my 2nd battery will be isolated when not powered (using a Stinger 200a isolator) then can my 2nd battery be a deep cycle marine/RV battery? I'm just thinking that if I can get a 100ah deep cycle battery that would be better for running the fridge and other accessories than just putting another 55ah starter battery in there.
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Also since my "main" battery is an AGM does my 2nd battery also have to be an AGM or does that not matter as long as they're isolated from each other?

 

[IdaSHO]

Generally if it is isolated, you can go for a different type of battery.

Id certainly opt for a deep cycle if I were you :ylsmoke:

 

[dwh]

That's how it's done. Cranking battery for cranking. Deep cycle for house loads.

Don't mix gel with agm or flooded, but you can use agm and flooded together.

 

[Martinjmpr]

That's how it's done. Cranking battery for cranking. Deep cycle for house loads.

Don't mix gel with agm or flooded, but you can use agm and flooded together.

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Thanks, that makes sense. Does anyone know of a Group 34 (side terminal) or 78 (top terminal) sized deep cycle? I'm not finding anything on a search. Or if not something close that will fit? The battery tray is for a 34 or 34/78 but I don't know how much leeway it has in terms of size.

 

[tdesanto]

Martin,

Will the batteries always be isolated. That is, will your isolator combine them while the vehicle is running? If so, that's where I'd think it could matter. I'm no expert on this, but I think that's the real question that needs to be answered if your isolator works like mine (separate when vehicle is off, combined when running and charging).

I do know that each battery type can have a bit different charging requirements. E.g., AGM has different voltage requirements than wet cell. So, if you want further confirmation, I recommend looking up some information from the AGM manufacturers Deka, Odyssey, Optima, etc.

Also, I've also always wondered about combining two different sized batteries, even if they are the same type. I would think the charging system would see it as just one battery and would charge until the voltage reaches the appropriate state and step down accordingly. That makes sense, but what happens in the interim when the batteries are first combined and are in different levels of charging state. E.g., house battery has been used for a while and is at 12.2V (50%) and the starting battery is at 12.58 (90%)? I would think eventually they reach a state of equilibrium (equal voltage) but not for some period of time. But again, this is just my limited understand and am no expert, but I think these are questions that would need to be answered by one before assuming it'll be OK.

I found this article that discusses some of cons of this very topic. Hope this helps. The scenario they lay out isn't exactly the same, as the batteries in the scenario would always be in a "bank" wired together. In your scenario (if my assumptions about how your isolator works are correct), only the discussion points related to charging would apply.

 

[dwh]

Doesn't matter if only tied during charging.

Also different battery recommended charge profile is irrelevant - the vehicle's charging system won't be dialed in precisely for either one.

There won't be a drain from one to the other - the alternator, not the high battery, will supply what the low battery is drawing.

 

[tdesanto]

Well, I guess it all depends on how the isolator works. Mine combines the batteries while charging. My understanding, in that particular scenario, is that the battery with the higher voltage will supply current to the lower battery. You might be right about the vehicles charging system not being precise enough to matter.

 

[Ducky's Dad]

OP, don't overthink this. Get a pair of 34/78 Optima Yellows or Odyssey 1500s and you are done. Both are deep cycle and both are suitable for starting duty. And you can mix Odyssey and Optima in the same vehicle, even if they are different sizes. My GMC has a pair of Optima Yellows right now, with a simple toggle isolator, and has in the past had various combinations of Optima Red and Yellow and Odyssey 1500s, mixed and matched with no problems. My Dodge currently has an Odyssey 31 starting battery and a pair of Optima 34 Blue dual purpose as house batteries, same simple toggle isolator. Dodge has had various combinations of Odyssey and Optima in the past with no issues other than the usual Odyssey Grp 31 charging/maintenance PITA. My batteries are ALWAYS isolated when the trucks are turned off. A true deep cycle will give you more capacity for the fridge, and can jump the starting battery in a pinch.

 

[Tazman]

I just did the same project with my GMC. I had a red top Optima so I added a blue top for my aux loads. I wanted to keep the charging characteristics and battery chemistry the same. I have used Optima for years and have always been pleased. For a bigger battery, look at Lifeline. I use them for my sailboat with great success. They have various sizes including the big D batteries. One thing that I found helpful is to Google battery size and look at the chart. The sizes are industry standard. Once you find the size that fits, shop the battery. I would keep the chemistry the same.

 

[dwh]

Well, I guess it all depends on how the isolator works. Mine combines the batteries while charging.

Mine too.

My understanding, in that particular scenario, is that the battery with the higher voltage will supply current to the lower battery.

It would if the alternator wasn't running.

 

[Ducky's Dad]

No offense to the man who suggested Optimas, but IMHO Optimas just aren't the quality that they used to be.

I am not an Optima fan-boy, but I have been using Optimas since about 2000. They used to be great batteries, then they turned to crap when production moved to Mexico, and now they are OK again. One of my trucks has Optimas, one has a combo of Odyssey and Optima, one has Northstar, and the other two are Odyssey. The current generation Optimas are fine, but size for size, they don't have quite the capacity of Odyssey or Northstar. I am currently running four Optimas.

 

[dwh]

At that point current will pass through that battery when charging and probably damage the other over time.

No it won't. Alternator to battery to ground and back to alternator. Times two. Two seperate paths. They never go "through" each other unless tied with engine off.

And technically, current is always passing through when charging...

Just be sure that your batteries are compatible (AGMs take a little different voltage to charge) ... so use similar types of battery and you should be ok. Just my 2 cents.

Again, it doesnt matter. The voltage regulator holds a preset voltage. It doesn't change the preset voltage just because you stick an agm in the truck. It has no idea what kind of battery is connected.