I have a 155 amp USBattery 12 volt battery. If your 6 volts are like my battery they take quite a few discharge/charge cycles to settle down and charge and measure accurately. Before that point mine was difficult to charge fully and each cell's specific gravity to measure consistently. Early on the cells would just bubble like crazy and not charge fully. Every chance you get hit the batts with some higher voltage charging and eventually you may see some better results from your alternator charging but it will never be enough to really get them close to full.