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Old 16-07-2009, 00:56   #1
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Charger/invertor question

A question for those of you who are expert in the black art of electricity -- you know who you are.

The boat I am still struggling to buy has got four battery banks: two 24 volt service banks (one for house, one for winches, windlass, thrusters), and 12 volt starting batteries for each of the main engine and generator.

The boat was used as a marina queen/gin palace and is poorly equipped and will require a lot of work to make her ready for roaming the wide blue. Battery capacity seems ok (and all the batteries are brand new) but the boat entirely lacks an invertor, and has wimpy charging capacity -- a 35 amp Newman charging both 24 volt banks through a splitter diode and a 10 amp charger on the main engine start battery. No charger at all on the generator start battery. And no emergency interlink.

This is fine for a marina queen where you can leave the chargers on overnight, but this will suck when you're making passage or spending a week or so on the hook in some desolate place -- because you won't be able to charge the batteries with the generator in a reasonable amount of time. I'd like to be able to run the generator for no more than a couple of hours in the morning to make water and charge batteries and then live in peace and quiet afterwards (the whole point of being on the hook, isn't it). So I think I need more charging capacity.

The existing 35 amp charger is a good one, so it seems a shame to throw it away. Here's my question: could I buy an additional charger of greater capacity, maybe even better an invertor/charger, ditch the diode splitter, and use it together with the old charger? I would leave the old charger dedicated solely to the service bank, and put the new one -- maybe 60 or 70 amps -- onto the house bank. Is there any electrical reason why this would not work? Will they interfere with each other somehow? The generator is a 6.5kVA unit which ought to be happy with the load -- maximum 70 * 24v plus 30 * 24v plus 10 * 12v = about 2.5kW maximum theoretical load from the chargers, right?

The second question concerns inverters. I have never been on a larger cruising boat without an inverter, but the guys trying to sell me the boat say there's no good reason to have one unless I want to run an electric teakettle or something without turning the gas on (which does not seem like reason enough to have one to me). They say audiovisual equipment and computers can be run with their own dedicated inverters and that I shouldn't bother. Besides computers and A/V, I can imagine wanting to run power tools, but I guess if you're drilling or sawing away at something you won't mind the generator running. What do you guys think about the idea of going inverter-less?

Last question: is there any inherent disadvantage to combined charger/inverters, compared to separate ones?

Thanks in advance for all your words of wisdom.

Cheers, Dockhead

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Old 16-07-2009, 03:34   #2
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Yes, you can install a second charger serving a second battery bank, provided the banks remain isolated from each other when chargers connected.
For the stated reasons, I agree with the POs, that you donít need an inverter.
A 2.5 kW load is a little light for a 6.5 kW generator, which would prefer to work harder - I recommend 50 - 80% loading, if possible (refrigeration, hot water heating, water-making, Air-Con, etc).
Mornings may not be the best time to run your Gen.
The ideal time, from a mechanical perspective, is late afternoon early evening, when your refer needs cooling down, cabin needs A/C, & dinner dishes want hot water. Of course, this may disturb the peace, so compromises may be in order.

Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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Old 16-07-2009, 03:46   #3
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Thanks. We don't have aircon (yet), but the watermaker (which we will have sooner than aircon) might use in the range of a 1.5kW, so with refrigeration and hot water heating that setup will load up the genset pretty good.

As to when to run the genset -- I am new to this, since this will be my first boat with a generator. We are used to running the main engine for an hour or two in the morning to charge batteries, when we're on the hook (which I know is awful for the engine, but with no genset you don't have any choice). Maybe our habits will be different with the genset. Putting it on for a while in the evening while we're washing up dinner dishes and settling in sounds good. Maytbe in addition to a morning session to spread loads out. It will be less disruptive than running the main engine anyway, because the genset is fully encapsulated and really very quiet (it's a 1500 RPM Kohler unit).

Proper loading of the genset, by the way, seems to me another argument in favor of having an inverter on board. It allows you to avoid putting on the generator when you don't have a lot of loads for it.
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Old 16-07-2009, 11:41   #4
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See the latter parts of this thread for more enlightenment on inverter/chargers and gensets:
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