The bat­tery’s a bit weak

Canal Boat - - Back Cabin: Experts - ROB MARKHAM, via email

QAfter a day’s trav­el­ling my leisure batteries are at 14.8v, but the volt­age drops im­me­di­ately I turn the en­gine off, even though I’m not us­ing any power apart from the in­ven­tor. The in­di­vid­ual batteries are 12.6v with one at 9.5v. I have no­ticed a ‘rot­ten eggs’ smell since plug­ging in the bat­tery charger, and two batteries seem to be bub­bling. Do I need to change all four leisure batteries?

ATONY REPLIES: The smell, 9.5v read­ing, bub­bling, and any tar­nished bright metal, higher than nor­mal charg­ing cur­rents or a charger that re­fuses to step down to the float volt­age all in­di­cate in­ter­nal short­ing cells, so one or more batteries need re­plac­ing. They will be the ones with cells bub­bling far more than the oth­ers.

What’s hap­pen­ing is that the charg­ing cur­rent through a bat­tery is pro­por­tional to the dif­fer­ence be­tween the bat­tery volt­age and the charg­ing volt­age, so the bat­tery show­ing 9.5v draws a far greater cur­rent than the oth­ers. This reduces the cur­rent avail­able to the other batteries and prob­a­bly reduces the charg­ing volt­age, which ‘fools’ the charger into think­ing the batteries are not fully charged so it never steps down to its re­duced float volt­age.

Then when charg­ing stops, the other batteries will try to charge the 9.5 volt bat­tery so they dis­charge faster than nor­mal – hence the drop to 12.6, which would con­tinue drop­ping faster than nor­mal un­til the bank was flat.

The deeper you dis­charge your batteries, the more life you use up. So your bat­tery prob­lem is mak­ing things worse for the other batteries, be­cause you never fully charge the bank and they are dis­charg­ing them­selves to an un­ac­cept­able de­gree.

Best com­mer­cial prac­tice is to say yes, re­place all four; but if you un­der­stand what is go­ing on and the po­ten­tial for fur­ther prob­lems, and are happy to test your batteries reg­u­larly, you can just re­place batteries that are bub­bling or show a low volt­age af­ter be­ing dis­con­nected from the bank overnight.

Once you re­place the short­ing batteries things should go back to nor­mal. But we can be fairly sure the re­main­ing two old batteries will fail sooner than the new ones. When one fails, it will start to dis­charge the new batteries lead­ing to the same prob­lem, so it should be re­moved im­me­di­ately.

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