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Charging batteries

I have a 45amp alternator on a BMC 1.5 engine and a domestic battery set up of 3x110amphour batteries. Now Im spending longer on the boat and less time running the engine the batteries are not being fully recharged.Ive been told that maybe I should uprate the alternator to a 70amp model or install an external voltage regulator to make the existing system more efficient. Which would be the best option or isit a combination of both

Asked by: David Hooper  | 12.35pm, Monday 29 June


WW says:

The best answer is a combination of both! Your 45 amp alternator is probably only outputting around 15 to 20 amps at just above tickover, so upgrading to a larger unit would give you twice the current output.
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An external voltage regulator will pull down the apparent voltage at the alternator, so fooling the alternator into producing maximum current for a given speed. However, this does not mean that iyour current alternator will produce 45 amps at tickover....
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Fitting both an alternator controller and a larger alternator will give a much faster recharge (and to a greater level of recharge) though you will need to keep more of an eye on the water levels in the battery cells, as the higher voltage can make gassing (and so water loss) more prevalent.
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There is another suggestion, which would require more work. Some alternator controllers, such as the STerling alternator to battery charger, do not require rewiring of the alternator, and can take two inputs (two alternators), and convert them to charge both a domestic bank and a starter battery, via a multistage charging routing (like a good quality mains powerered charger).
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So you could fit a second alternator (Calcutt Boats are probably your best place to give you information as to the suitability of your engine to do this) which would require some alterations to pulleys, etc.- however it would giev you both redundancy (if one alternator fails) and potentially three times the charge rate of your current set up. It may be worth looking to do this, providing it can be easily achieved.

Mark Langley  | 1.56PM, Monday 29 June

I should also add, having two alternators running hard (as the external control would make them do) should also help with additional load on the engine, if you are running the engine just to charge batteries. I would, though, check that you have good ventilation to the engine room, as if the alternators work hard, they get very warm- and if there is insufficient cooling air, it can lead to premature failure of the alternator and/or, particularly, the drive belts.

Mark Langley  | 1.58PM, Monday 29 June

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