Do I need the same size batteries for dual system?

Hey guys been doing lots of research here and on other sights about dual battery systems. I am getting conflicting info on same or different size batteries on a dual system. I have a 2010 CM Platinum so I have a 27f as my main and it looks like I can only fit a 35 on the other side where the windshield reservoir is now. I like the 27f size and was wondering do I have to keep the same group size on both? I will have only a bed mounted winch and inverter hooked to the aux battery and everything else to the main. I will be using an isolator hooked to the system so I could really use some help with your guys expertise. I’m in a quandary, so waiting on your knowledge. Thank you

 

I don't run duals on the Tundra but have about 12 years of experience in other vehicles. My take:

When charging and when isolated different size batteries aren't an issue as long as they are both of the same type (AGM etc). They both like the same voltage so both will be happy and only accept the charge they can accept.

The problem is more if you leave them connected in parallel (linked) for an extended period. As you draw the connected batteries down, the larger battery will eventually attempt to charge the smaller one. This condition can cause premature failures.

If I was in your case, I would run a controller that unlinks/isolates the batteries automatically X minutes after shutting off the engine. Becomes virtually worry free. A smart controller manages this well - I still love the IBS system.

 

Pete, thank you for the input on this. I just can’t seem to get a good answer on this. I will look into the IBS system.

 

I have extensive experience in DC circuitry, 22 years as a Caterpillar Technician. Here is what I know. to answer your question simply - YES you need the same size batteries. If you are putting batteries in a parallel circuit, your batteries need to be of the same (AH) Amp hour rating - or simply, of the same group size. Why you may ask, here is the answer. It's all about charging. If your batteries are wired in a parallel circuit then they will charge equally as long as they are of the same AH Rating. If your batteries are of different AH ratings then issues will occur when the batteries are charging. It will not happen quickly but over time it will happen. And like @mountainpete said, the larger AH rated battery will force a charge into the smaller AH Rated battery. You can think of it this way, the smaller AH Rated battery will drain the charger from the larger AH Rated battery in the attempt to charge the smaller battery. Different rated AH batteries will fight for charging rights. It's all about the electrons and protons that flow in the charging process. I hope this helps

 

Anytime I can be of assistance, i like sharing my knowledge

 

Greg that was explained so my simple mind could understand it. I thank you very much. It looks like they will be the same group then. Thank you again.

 

If you use a smart isolator or charge controller you won't have any issues with different sizes of batteries with the same chemistry.

 

That is my experience too, but I differ to expertise vs my personal experience.

 

You would be correct. However that just adds more potential issues with the circuit because there are more components in the circuit. It will take a simple charging circuit and turn it into a complex charging system. In my experience this can lead to potential issues and difficulty in troubleshooting if something in the system was to go wrong. It just adds to many what if's to a simple charging circuit. For some individuals with troubleshooting experience in DC circuitry I would say go for it, but for others that don't have that experience, simple is better.

 

Redarc BCDC chargers are purpose built, simple, allows for solar input, and is waterproof. Below is an example of a wiring diagram where the 2nd battery is isolated from the primary battery and has a solar panel input and a momentary button for jump starting the truck if ever needed.