Connecting batteries in parallel long distance

[whodunit]

Hey everyone, I'm working on a conversion van that already had a house battery under the hood that's currently only charged by the alternator. This is my first time working on any type of vehicle electrical system. My plan is to add solar and, with it, another battery or two. There's a few parts to my dilemma. 1st is that I'm interested in taking advantage of the space under the hood to store one of my batteries but I wonder if it's practical to have such long wires going from the battery under the hood to another battery behind the drivers seat where I'd plan on placing the second battery. Is this a bad idea? If not then I'd assume I'd need some extremely thick gauge wire? 

The 2nd part of the question is that I just got a great deal on a 79Ah deep cycle AGM battery and I have a chance of scoring 2 more cheap 75Ah deep cycle AGM batteries but I have read people strictly say to not combine different Ah capacity batteries in parallel. The difference doesnt seem significant enough to me but maybe people here could chime in with their opinions?

If it helps at all I'm planning on putting two 100w panels on the roof and the only things I'm powering are some LED lights, some fans, water pump, and an old norcold top loading compressor fridge. I'm not sure if three batteries is overkill for that. I'm kind of just seeking some advice from someone who knows more about these things than me. Thanks so much for reading this far! Love the forum!

 

[RvDreams]

Welcome whodunit to the CRVL forums! To help you learn the ins and outs of these forums, this "Tips, Tricks and Rules" post lists some helpful information to get you started.

Most of our rules boil down to two simple over-riding principles: 1) What you post should provide good information (like your introductory post), and 2) Any response to someone else's post should make them feel glad they are part of this forum community.

We look forward to hearing more from you.

 

[B and C]

Welcome.

Batteries should be the same age, type and capacity. If the batteries are close in age and usage, the Ah difference is minimal. You sure don't want to mix newer and older together. The old will suck from the new. Wiring will need to be from one end of the parallel circuit string negative and the positive needs to be wired to the other end of the string positive. Attaching both feeder and charging cables in the middle or one end of the string is a no go.

Best practice is for the bank to be together but with large cables tying the bank together, it should be OK. 10 Ga wire is not large, more like 4 or better yet 2 ga to lower the voltage drop over distance.

You need buffer space in your battery storage capacity as there are always rainy days or heavy cloud cover to reduce solar charging.

 

[whodunit]

 Attaching both feeder and charging cables in the middle or one end of the string is a no go.

So by this do you mean I should attach the feeder cables to one battery and the charging cables to the other battery? This is relevant info for my situation because you're saying if I have one battery under the hood and the rest inside the van then my solar charge controller is going to have to connect to that battery as well since its already connected to the alternator thus being the charging end of my battery bank, correct? Thanks for the response!

 

[B and C]

Let's see if I can be clearer. The battery under the hood is connected to the alternator and grounded to the frame. Remove the frame ground and run a new cable to you new banks negative post at one end of the new bank. Run a new positive cable from your existing bank to the new bank.

You will have a red wire from the alternator connected to the battery under the hood. This battery will have a red cable connected to the remote battery positive and to the next battery.

Remove the ground cable from your underhood battery and run a new black cable from the underhood battery to the first remote battery and onto the next remote battery.

All negatives connected together and all positives connected together (parallel). From the last battery in the house string, run a ground cable to the frame. You now have one big battery by combining the three (one underhood and two remote).

This has the charging working right. The positive side of any load should be connected to the underhood battery. Your ground connection is already to the frame so any good ground for the load will work except for the underhood battery post and the middle negative battery post.

Frame ground is important, do not use sheet metal for these high loads.

 

[abnorm]

Is "one bank" what the OP asked for?

I "thought" it said........... Start Battery stays the same.....

House bank will have two new bats behind the seat and a third currently under-the-hood from the upfitter

 

[B and C]

What I understood was one house bank, starter separate. The upfitter put a house battery under the hood charged by the alternator.

 

[whodunit]

Is "one bank" what the OP asked for?

The van currently has a starter battery and one deep cycle house battery on separate sides of the engine bay. I believe the house battery is connected to the alternator via a dual battery isolator. I just need one bank for the house stuff. I was thinking of just spreading it across the van to use the space under the hood but now I'm kind of leaning towards just removing the battery from under the hood and putting two or 3 batteries right next to each other inside the van and using the battery compartment under the hood to store something else to simplify the wiring.

 

[DLTooley]

I’d do a variation of that strategy, keep the existing house battery as a totally separate circuit and set up a new bank for totally separate loads.

You should be able to wire to use either as a **temporary** backup.

Likely you’d put smaller loads on the existing battery, which is likely older.