I've got two identical 100ah batteries connected in parallel, but they were bought three weeks apart. Recently, I set up a small solar system in my shed to help charge my smaller solar generators. I've noticed something odd: the batteries charge and discharge at different speeds. One will hit 4% charge before the other even starts to charge. Is this normal behavior? Everything else about the batteries seems equal except for their charging and discharging rates. I understand my setup is small, but I'm curious why this happens.
4 Answers
Make sure that the wiring for both batteries is the same length. Sometimes, differences in wire length can cause one battery to charge or discharge faster than the other.
If the charge difference is just around 4%, I wouldn't stress too much. Just remember to avoid frequently discharging your batteries all the way to 0%. That can lead to problems, especially with DIY setups.
I used a similar parallel setup and noticed imbalances. It seems the wiring paths can cause one battery to discharge more quickly. Make sure all the connections go directly to the bus bar to improve efficiency and balance between the batteries.
To clarify, both batteries have BMS, and I haven't drained them below 0 too often. They generally stay within that 4% range, so I'm not seeing major shifts.