Electric boat conversion - the solar top-up cells

Ideally, I would have loved to charge and re-charge the drive batteries via solar power.

Unfortunately, to have a re-charge current of magnitude anything near that available from the generator would, I calculate, need an area approximately five times that of our boat's cabin roof area and at the present time (August 2008) the units would cost approx. £45,000(!)

So... I opted for solar power merely to do final topping-up of the drive batteries and to maintain them during the ages the boat, sadly, is idle on its mooring.

To use anything over 5W (solar) cells would require a (n expensive) regulator since in bright sunlight the solar cells can produce 16.9V - and the batteries must not have more than 15V across each 12V unit or they will gas and deteriorate. But unless the batteries are extremely fully recharged, the solar cells are effectively "loaded" and output limited to less than 15v. Just.

Well, that was my information! However, it turned out that if the boat was on the mooring for long stretches of bright weather the battery plate voltage rose (slightly) above 15V. I considered fitting a low power regulator to avoid this problem - but such a regulator would necessarily lose about 1.2V (minimum) of potential charging voltage, which would slow re-charging at other times when the batteries aren't almost maximally re-charged....

One could switch between regulated and non-regulated modes, of course... but a much simpler solution is to simply run the drive motor for a few minutes (perhaps 10) once a week if this "over-voltage" situation is clearly about to happen... and it'll take a week before the re-charge gets that complete again!

Obviously, this sytem wouldn't do for anybody living remote from their mooring... but in our case I always can and do visit the boat at least once a week to check mooring ropes, etc., so if a quick check of battery plate voltage reveals it's getting close to 90V (thanks to the solar input) I simply run the drive motor a bit... which I usually want to do anyway to check all is "ready to go"!