Keep­ing things tip-topped up

Canal Boat - - Back Cabin - ANDY OX­LEY, via email TONY REPLIES…

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.

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