Keeping things tip-topped up
QI go to my boat most weekends and use my battery charger to charge the (3 x 110Ah) batteries. I have been given a 2.4W solar panel that the instructions say will keep a 12v battery charged: will it keep mine topped-up between visits?
AIf it’s the panel I’m thinking of, the rated output is 0.138mA – but that’s probably at the equator, with the panel perpendicular to the sun’s rays… In the UK it will produce half that or less, so a true output of 0.07mA.
They are sold as ‘battery maintainers’, aimed at cars, and can probably keep up with the selfdischarge of a 30 to 40Ah car battery, not boats with 330Ah.
Modern lead calcium batteries lose about 0.5% of their charge per day: about 1.5Ah for your 330 Ah bank. It would take around 21 hours to make that up with a constant 0.07mA – but you will not get that, because in winter there are well over 12 hours of darkness and clouds often obscure the sun. So long as it has diodes, it will do no harm – but I suspect little good either. It would be far better to leave your proper multi-stage marine charger running (using an isolation transformer or galvanic isolator) when not on the boat – unless you have any inkling that a battery cell might be failing.