BATTERIES, CHARGING AND ISOLATING

Any boat bigger than a small runabout will probably carry multiple batteries, with one or more batteries dedicated to starting the engine, and a second unit to run the onboard electrics.

This installation prevents cell damage, as there are two types of lead-acid batteries: starting, and deepcycle. Starting batteries deliver a large current for a short period and cannot withstand repeated discharge. A deep-cycle battery provides sustained power over an extended period without being damaged.

For two different battery types to co-exist happily in the boat, a voltage sensing relay (VSR) is used. This ensures the starting battery is charged up first by the engine’s alternator, and only once it’s fully topped-up is the house battery charged. It also prevents the house circuit from draining power from the starting battery – so the starting battery remains fully charged.

Added to this complexity is new battery technology, which requires higher charging voltages than in the past. Most engine alternators deliver a peak of around 14.2 volts, but modern AGM and calcium batteries may require 14.4 to 14.6 volts to charge properly.

Douglas recommends replacing the built-in regulator with an external regulator to get optimum charging and extend battery life. He says this will save money in the long term.