I have a secondary (cheap chinese) battery hooked up in parallel to one of my ebikes for extended range. It is perhaps a year and a half old. Recently, the range of this bike has become drastically reduced. When I disconnected the batteries from one another, the voltage of the main battery that came with the bike was several volts lower (~46v) than the extended battery (~50v). The bike’s display read 46v as well, so I initially assumed the secondary battery just became disconnected during a ride. I was able to ensure that all connections were fine, the batteries are connected via their outputs and are charged from one of either battery’s charging port. I charged the main battery up to 50v, hooked them back together and continued to charge to 54v … Now, when I disconnect the batteries from one another, the main battery reads 54.1v and the secondary battery reads 52.3v. How is this possible? I thought two batteries connected in parallel would equalize each other out, and with a 2v disparity, shouldn’t something bad be happening?
On the other ebike, which is configured exactly the same, voltage always reads exactly the same on both batteries, regardless of where they are sitting in the charge cycle.
I am hoping someone can tell me if I perhaps have some bad cells, and based on the math how many? The battery is 48v (in marketing speak) and is composed of 18650’s arranged in (I think?) 13s7p.
Here is the battery: