Hi, The SOC is calculated, but the voltage should be measured. At no load there shouldn't be any measurable difference between the batteries that are all wired in parallel. I think something is likely wrong with the electronics in the battery with the 12.9 V. The other 2 are pretty close together (.02V). That's quite a bit different than .7 volts. Odd too that this is the battery with much lower charge. Just a guess, but .7V is close to one diode drop, which could mean that this battery has a blown protection diode in it and is therefore the primary soldier and the others are along for the ride until he is discharged.....