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Posted by David Sherman [24.32.202.83] on Friday, April 02, 2010 at 01:44:23 :
In Reply to: OT electrical question need experts! posted by David [76.103.64.254] on Thursday, April 01, 2010 at 23:52:12 :
There are several possibilities, with varying degrees of complexity, but if everything else on the boat, including the starter and generator, is 12 volts, you really need to exchange the 24 volt pump for a 12 volt. There's no inherent reason why a 12 volt one can't have just as high a capacity.
Some big trucks have a system where the starter is 24 volt, in order to keep the wire size manageable, but everything else is 12 volts. The way they do that is they have a set of 4 high-power relays that disconnects one of the 12 volt batteries from everything else and connects it in series with the grounded one, and connects the combination to the starter just for cranking purposes. Once the engine is running, the relays connect that battery back in parallel with the always-grounded one for charging purposes. Assuming you have a 12 volt starter, you really don't want to get into the mess of trying to make one 24 volt item work with a 12 volt system. You'll have to deal with how to charge both batteries while the bilge pump is running, how to run everything else while the bilge pump is going, etc. Every idea I can think of is a mess. Just to check them off the list, other options are: A) a complicated relay box or "frankenstein switch" and the requirement to switch the "high" battery back and forth between "charging" and "bilge pump running", B) a 12 volt to 24 volt step-up inverter (they are made), C) a separate battery and generator/alternator to charge it, just to run the bilge pump. All of these ideas are bad, though each is bad for a different reason.
If you were building this from scratch, and if you needed 24 volts for the starter or for some other non-negotiable purpose, then we could talk about how to keep the two batteries in series, charge them with a 24 volt alternator/generator, tap off 12 volts from the "low" battery to run your 12 volt systems, and add some charge equalization electronics to keep from overcharging the "top" battery. But since you don't need 24 volts for anything else, far and away the best solution is to just take the 24 volt pump back and get a 12 volt one.
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