Electrical Parasites?
The owner of a 46-foot power catamaran called after finally getting back to the boat from an extended delay of six weeks. The AC shore power circuit breaker was tripped at the marina dock, and the boat’s DC voltmeter showed 7 volts. The boat was dark.
After the dock circuit breaker was turned back on and AC power was reestablished, the batteries still wouldn’t charge. With more than 800 amp-hours in the house battery bank, the owner wondered how the batteries could be dead after such a short amount of time.
We could answer that question, but first, we had to get the batteries charged back up.
Back from the Dead
Many smart, electronically controlled battery chargers need a minimum DC voltage (usually 8 to 10 volts) in order to turn on. Ironically, the dead batteries prevented the not-so-smart charger from charging.
To solve this dilemma, we had the owner parallel his isolated and charged start batteries with the house batteries (the boat has a paralleling battery switch). This allowed the charger to sense a higher voltage and begin charging.
These AGM batteries were only a year old. A deep discharge like this was bad for them and potentially removed some life span. How much life span is debatable, and their longevity will depend on how they are treated in the
