I did eventually figure out one or more of the batteries are dead.  What I did was I used a nitecore charger that is designed to get the chemistry going again in dead batteries and it got up to 98%. I charged them in fours. So I had to do this three times.  As far as replacing the batteries yourself, the problem is that they’re welded, and doing spot welding on 18650 batteries with nickel tabs at home isn’t exactly ideal, especially if you don’t know what you’re doing.  They (Sanho) offered to sell me a new battery, like JUST the battery, not the case and the controller board, but the price they wanted was obscene.  I opted not to go that route.  I was trying to figure out some other way of connecting the batteries to eachother but I came up empty unless you get batteries with the nickel tabs already welded onto them, and then you can just solder wires to the tabs, but even then, that has to be done very carefully, and the batteries have to be discharged first.  You can get those “project tab” batteries on...