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kavakava 10-26-2007 05:38 PM

Sizing battery charger
I replaced my house bank of 2 4d's with 6 6v batteries that total about 750 amps, and continue to have one group 27 as a starting battery. My current charger is a 50 amp Promariner multistep. Is this large enough for my larger battery capacity? I also upgraded the engine alternator to a 120 amp Powerline with extenal "smart" regulator. Thanks.

sailingdog 10-26-2007 05:45 PM

Hmm... 6 T-105 Golf Cart batteries would be 675 Amp-Hours. IIRC, you need to size the charger amperage rate to approximately 10% of the amp-hour rating for it to charge the batteries properly... so you'd probably want at least a 70 Amp charger. Personally, I'd go with a 100 Amp charger... especially if you have AGM batteries, and use an Echo Charger to charge the starting battery.

camaraderie 10-26-2007 05:58 PM

Once you get up into that sort of capacity, the chargers themselves will run you 5-600 bucks. While you are at it, you might consider going with a charger /inverter like the Xantrex Freedom line which is only a couple of hundred more. This would save you the trouble of installing a separate inverter down the road and simplify the installation. With 750 a/h's you can go as high is 150 amps on the charging end with a minimum of about 75 amps as Dawg says.
Your 50amp will be fine if you just do weekending and remain plugged in most of the time but the above comments apply if you are out cruising or fitting out for it in the future. You have about 375 a/h's of WORKING capacity before you need to recharge your new bank so it is really a question of how long you have to do that.

sailingdog 10-26-2007 06:11 PM

Cam's got a good point... the price difference between an 100-amp charger, and an inverter/charger that includes an 100-amp charger as part of it is not that much.

kavakava 10-26-2007 07:34 PM

Thanks for the input. I already installed a 1000w Prosine inverter this year, for I prefer to seperate the charger from the inverter. I'll double check the amp hour capacity of the 6v batteries I purchased. The 10% rule is helpful, for then I can size the charger appropriately. Finally having some rain here on the Chesapeake!

btrayfors 10-26-2007 08:00 PM

You can't beat the Iota chargers for performance, reliability, and price. They're available in sizes up to 90A. The 75A or 90A versions, with the IQ-4 smart charger option either external or internal, would be appropriate for your battery bank.

You can even combine two identical models to double capacity, if you like.

Check them out online (Associated Industrial and other vendors) as well as at Jackrabbit Marine.


sailingdog 10-26-2007 08:10 PM

The 10% rule is a rough estimate... and generally, it is better to go up a bit, rather than round down. :D

Most Golf Cart batteries will range from 180–225 Amp hours @ 12 VDC per pair.... so six batteries will range from 540–675 Amp-hours as a rough estimate. :D

camaraderie 10-26-2007 08:30 PM

Bill...that is an excellent price for a high capacity smart charger.

dohenyboy 10-27-2007 01:27 AM

Okay here is illustrated a technical fact where, I believe, cruisers err.
If you have say 6 golf cart batteries (6v) with 100 amp hour ratings each and have them paired to produce 12v, at 12v usage the total amp hour capacity is only 300 amps. If you have a 6 volt system yes you would have 300 amp hours. Amps time volts equal wattage. Am I not correct?

Also. AGMs can support 30 to 40 percent charge rate, so if you want to minimize charging time you will go with the biggest charger you can with AGMs.

MidLandOne 10-27-2007 04:49 AM


Originally Posted by dohenyboy (Post 212898)
Am I not correct?

Yes, you are correct. As you say, it is a common mistake that is made.

With respect to the original question the size battery charger that suits ones particular needs depends so much on how much time is available to charge the batteries, how depleted they normally get between charges and what other DC loads one has running during charging (ie if one is drawing 20 A for other DC loads while charging then the charger will have to meet that demand too so reducing the rate of charge to the batteries) that it is not possible to give specific advice based just on battery capacity - except as others have done and suggest the 10% guideline.

If the battery capacity is actually 300 odd amp hours (ie there has been a mistake in working out the capacity of the series/paralleled batteries) then the existing 50 A charger will almost always be plenty. If the battery capacity is actually 600 odd amp hours then unless one has some particularly short charging time needs or a need to run other big DC loads while charging then I would be inclined to give the existing 50 A charger a trial to see if it meets your needs (assuming it is in ok condition and there is no other pressing need to replace it now).

We find that with 520 AHr of wet lead acid deep cycle batteries a 50 A charger is plenty for our particular needs (so meets the guideline of around 10% of capacity that others have mentioned) and still would be so with an over 600AHr bank of batteries. That given our own particular usage profile where most of our deep charging is done away from the dock and shore power, so main DC loads in the dock are the refrigerator, freezer, TV, lighting, etc rather than deep charging.

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