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Posted: Tue Sep 13, 2011 3:37pm

Post Subject: Battery Life

What is the best way to look after batteries, keep them on float charge all the time (for months)when the boat is not being used, or leave them fully charged with every thing switched off (for months) ? My mains charging is via a Mastervolt Mass Combi.


Posted: Tue Sep 13, 2011 4:08pm

Post Subject: Battery Life

There is no straight forward answer to this. It depends upon circumstances and I see you refer to "months on end". This implies that you are not able to visit the boat every few weeks so limits your options. Depending upon technology, condition, temperature and other things lead acid batteries self discharge at anywhere between 2% and 40% a month. One with partial internal short circuits will discharge more than this. The battery materials that are discharged will, over time, change their state so it becomes all but impossible to recharge them. This means that you are trying to make up for the self discharge before the discharged materials have had a chance to change their state. If you can get to the boat every three or four weeks charging for the first period and then leaving the battery isolated might minimise the risks of leaving it on charge all the time but if you are not able to do that then given a few caveats I would leave them on charge all the time. The first caveat is that the charger must be a proper multi-stage one, designed for marine use that has a float charge voltage properly matched to your battery type. If the float volatge is too high it might cause gassing, loss of electrolyte and possible erosion inside the battery. The second is that if a battery goes faulty and gasses a cell's electrolyte away in might cause a battery explosion. Both occurrences are rare with decent quality equipment. I would also point out that no mains device should be in an under deck engine bay because of damp. This means your combi-unit is probably inside the cabin so will slightly warm the boat whilst it is in use. This might be an advantage in the winter. Tony Brooks

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