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Posted: Sun Nov 11, 2012 9:38am

Post Subject: Solar panel battery charge

I recently fitted a 100w solar panel to my narrowboat, wired directly to the domestic batteries. It works well. The boat has twin alternators with separate start battery circuit. I find when leaving the boat for a while the domestic batteries are fully charged, 13-14v. but was surprised to find the start battery almost as well charged and holding well over 13v. This is inspire of me leaving the boat with all electrics switched off and both main isolator keys removed. How is it managing to do this?


Posted: Sun Nov 11, 2012 11:02am

Post Subject: Solar panel battery charge

Assuming no one in the past has attempted to parallel the alternators by fitting a voltage sensitive split charge relay. Although the starting current is very high at the start of starting and dropping to perhaps 200 amps within seconds the time the starter is in use on a well maintained engine means that only a few amp hours (amount of stored electricity) has been taken from the battery so it is normally fully charged in well under an hour of starting so when you leave the boat the start battery is very well charged. With nothing connected and drawing current you are left with self discharge so over several months you will see the battery flatten but not over days/a few weeks. As the battery ages the self discharge is likley to increase but when that happens and when it causes a problem just pop a jump lead between the engine and domestic battery positives. Then the panel will charge both batteries. However do not do this if there is any question about the condition of any cells because that could lead to all batteries being discharged. Also you must make sure the lead does not short out. Tony Brooks

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