Did the DC Solar engineers make a rookie mistake paralleling the batteries?

[Consumerbot3418]

When I first started dipping my toes in the water with solar, I had a 3000W Windy Nation inverter w/ a Battle Born 100ah 12V battery. Later, I picked up a 2nd battery, and that's when I learned that The Right Way™ to connect a pair of parallel batteries is with the POS lead to one, the NEG lead to the other, and a jumper connecting both POS together, and both NEG together. Like this:


Now, I've been struggling with a pair of lead acid forklift batteries on a DC Solar trailer that aren't balanced well--when I'm discharging them, I can see a substantial imbalance in supplied current with my clamp meter. As it happens, they ran a pair of 4/0 wires to a point between the batteries, then used an Anderson wye to split and run POS and NEG to each battery. (The charge controller had its own smaller conductor run directly to just one of the batteries)

What I don't know is how much of a difference it really makes. I suppose doing it correctly (as above) means a loose connection or a poor crimp will equally impact both batteries, but that in a perfect world, running equal length identical conductors to both batteries should work equally well. Too bad we don't live in that perfect world!

 

[fromport]

As long as the cables connecting the batteries have the same length (aka resitance) to where they are paralled it is okay.

I have inspected many DC solar trailers and I have not seen 1 setup that had both batteries at > 80% of nominal capacity.

 

[GXMnow]

Now, I've been struggling with a pair of lead acid forklift batteries on a DC Solar trailer that aren't balanced well--when I'm discharging them, I can see a substantial imbalance in supplied current with my clamp meter. As it happens, they ran a pair of 4/0 wires to a point between the batteries, then used an Anderson wye to split and run POS and NEG to each battery. (The charge controller had its own smaller conductor run directly to just one of the batteries)

My first question is about the batteries and their "health". Are they the same model, age, and use? If the system was running on just one battery for a year, then the second was added, the newer one may well have more amp hours of capacity and it will pull more of the load. The batteries may also need maintenance, such as checking/filling with water and running an equalize charge to remove sulfate build up.

Assuming the batteries match near perfectly... From your description, it is very possible that the battery where the solar charge controller is directly connected is getting more of the charge power. Is that the same battery that is supplying more current?

If that is the case, I would first move the solar charge controller to the other battery, and watch it for a few days. If the charge difference was causing the problem, it should slowly shift to both batteries being equal, but then the other battery may start supplying more current. If that turns out to be the case, I see 2 easy fixes.

The first easy one is to just move one of the charge controller leads so the negative is on one battery an the positive is on the other. This makes it charge like your diagram above.

The load connection using the "Y" should be fine. And that also gives us the other fairly easy option. Move the charge controller to the common output of the "Y" connection. Then both batteries again have the same resistance to the charging energy source.