I have 4 SOK marine batteries in my boat. Yesterday we motored for about 4 hours and my monitoring system showed the battery level to be good as expected. This morning the battery levels looked good, but I had reason to remove one of the batteries for about 2 hours to get at something underneath its location. Since we were at anchor all night and usng power this morning and during the time the one battery was removed, I wanted to check that the battery bank level wasn’t significantly lower than the single battery that I had removed. (It is my understanding that it’s not good to connect multiple batteries in parallel that have significantly different voltages). But when I checked the voltage of the single battery, to my surprise to measured 10 volts! When I isolated the other batteries, 2 of them measured 13.3 volts, but another one measured 10 volts also.
I have tried connecting one of the batteries to the other 2 while charging with the alternator, but after 15 minutes, after disconnecting it again, it was at 10 volts still.
What is wrong? How can I get these 2 batteries that are at 10 volts to start charging? Is there a problem with the BMS?