Question about wiring accessories to dual battery setup

[Jeepin Jason]

I have two blue top batteries set up in parallel. The charger is set up with positive on one battery and negative off the other battery. Do I need to set up the inverter and accessories the same way or can I take the them off the positive and negative on the same battery?

 

[Perfect Switch]

If the batteries are in parallel, it doesn't matter. Note that the gentleman in the next post suggests that it does matter due to resistance between the batteries and under high current discharge conditions, it will create an imbalance. I suspect there would be a discharging imbalance no matter how good the connections are as no two batteries are going to behave identically. Assuming the two batteries in parallel are right next to each other and electrically bonded in parallel with large cabling or buss bars, I don't know if connecting positive to one battery and negative to the other battery would actually make a significant difference. In all fairness, I've never connected a load where positive was connected to one battery and negative to a different battery in parallel, but I don't think the additional resistance between the batteries would cause significant drama.

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[4x4junkie]

Not correct.
Pulling current off only one battery of a parallel setup puts the resistance of the interconnecting wiring between it and the rest of the batteries, which at very high current load causes unequal power sharing among the batteries.

Always connect between the positive terminal at one end of a bank of parallel batteries, and the negative terminal at the opposite end of the bank.

 

[Jeepin Jason]

Not correct.
Pulling current off only one battery of a parallel setup puts the resistance of the interconnecting wiring between it and the rest of the batteries, which at very high current load causes unequal power sharing among the batteries.

Always connect between the positive terminal at one end of a bank of parallel batteries, and the negative terminal at the opposite end of the bank.

I was thinking that this was probably the best way to hook it up. It is a bit more difficult as I will be loading up just the two terminals.

 

[pugslyyy]

If the batteries are in parallel, it doesn't matter.

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This.

Unless you have some sort of battery isolation device, it doesn't matter which battery you are making the mechanical connections to - there's a big honkin' cable between the positive terminals of both batteries, and the negative terminals of both batteries.

An exception might be if you are using a shunt to measure power consumption, in which case the shunt would need to be between the batteries negative terminals and anything else.

 

[pugslyyy]

In the nitpick world of battery connections, it does make a difference.
Each connection at a battery has a miniscule resistance (we assume the connections are 100% mechanically superduper clean & tight they still have a tiny resistance. )
Drawing power from opposite ends of a battery bank averages this added resistance of each connection into the load.

Throwing my prior ************** troll question into the mix,
The center battery discharges at a different rate vs the end batteries.

Well every wire has resistance per unit length as well. Why not connect to the closest connection and save some money? That 0000 cable is expensive.

If you want to nitpick, if your vehicle is charged by an alternator it really becomes an impedance issue not a resistance issue anyways... but unless you work for NASA and are planning spacecraft, it really all just gets lost in the noise. Do whatever is easiest for you.

 

[patoz]

The information in post #3 is correct! This explains it pretty well, and Method 2 is what you would use with just two batteries in parallel.

How to correctly interconnect multiple batteries to form one larger bank

http://www.smartgauge.co.uk/batt_con.html

Also, the batteries need to be identical in type, size, capacity, age, and condition, otherwise one will drag the other one down.

 

[Jeepin Jason]

The information in post #3 is correct! This explains it pretty well, and Method 2 is what you would use with just two batteries in parallel.

How to correctly interconnect multiple batteries to form one larger bank

http://www.smartgauge.co.uk/batt_con.html

Also, the batteries need to be identical in type, size, capacity, age, and condition, otherwise one will drag the other one down.

Thanks for the link. It looks like I will hook everything up to the opposite leads. The blue tops are exactly the same so this should work well.

 

[patoz]

Thanks for the link. It looks like I will hook everything up to the opposite leads. The blue tops are exactly the same so this should work well.

No problem, I hope it helps!

Until I came across that, I thought it didn't matter either, and always just hooked them up in parallel and then came off the first or last battery with my supply leads and accessories. Not trusting just one source, I started doing more research of my own and found other sources which stated the same thing.

Here are several more links with very good information about battery systems in general:

http://www.batteryfaq.org/

http://www.expeditionportal.com/forum/threads/79600-preliminary-electrical-drawing-for-scrutiney

http://www.expeditionportal.com/forum/threads/122111-which-two-6-volt-or-one-12volt-battery