This battery is used on a racing sailboat to power sailing instrumentation and lights. No provision is made for on-board charging and the battery is removed to charge. At the end of last season the battery was removed and tested (method not divulged) and the owner informed that this battery, less than two years old, was on its last legs.
The battery is possibly a deep cycle battery; however this designation does not appear on the case. When first checked the open circuit voltage was 8.89 volts and the specific gravity in each cell was 1.10 or less. The estimated capacity remaining would
be below 10%.
No battery should be discharged to this low level; this is damaging to the battery and shortens its life. The recovery process used was to treat the battery as a deep cycle and charge in stages, with 12 hours on charge and 12 hours off charge, until such time as the battery had returned to full charge. At each stage the specific gravity increased and after some 70 charging hours the specific gravity had reached 1.285 to 1.290 in each cell. During this extended charge, the charge rate held at about 2.5 amps and 14.43 volts.The starboard photo was taken several hours after the charge was complete and the open circuit voltage was 12.79 volts and the average specific gravity 1.286. This translates into an estimated capacity of 95% and a reasonable recovery from the “dead” state at the start.