What our team has done in the past (lacking any charger better than the KOP one, which has a dubiously effective indicator light) was simply to line the batteries up in a row on the pit floor. We'd add a strip of masking tape, label it "charging... charging... charged" and always add batteries to the left and remove them from the right. A very imprecise system to be sure, but a simple and fairly reliable one, at least in our experience. If your batteries are all pretty much equally healthy, then 9 times out of 10 the one that's been charging longest is the most charged.
For this season, however, we purchased an
Interacter X5 with multiple indicator lights for showing the charge state of each battery, so it remains to be seen if we will use the old system. We've never tested our batteries and tried to rank them, although it'd certainly be a good idea.