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CD28 04-28-2012 09:13 AM

Charger Sizing... (too big?)
I've been digging through the tons of great info on the subject of chargers and find a lot advise about installing too large or too small of a charger. 'Size it to fit your bank(s) and use requirements.' (10% rule) The issue I have is that the new(old) boat has an ancient ProMariner charger that I'd like to upgrade to a modern multi-stage charger (ASAP). However, within the year I will also increase the size of the house bank. Currently have a group 24 starter battery and 4 golf carts (200Ah) on the house side. Longer term plans include upgrading to 400+ on the house side (possible AGM) as well as solar and wind-gen. But while the big refit is underway, she'll be on a dock with shore power. Will the modern multi-stage chargers like the ProNautic 1250P (50A) toast the current battery bank? Do I need to overhaul the battery bank and the charger at the same time?
Thanks for any and all input.

btrayfors 04-28-2012 10:17 AM

Re: Charger Sizing... (too big?)

You can't have a battery charger which is "too big". The batteries will take what they're going to take in the way of amperage and no more, provided that the charging voltage is correct.

AGMs, of course, can take a LOT of charging least as much as their AH rating. You're not likely to be able to fit a charger which can provide all that amperage :-)

At dockside, of course, it really doesn't matter very much. You just need a charger sufficient to counteract self-discharge of the batteries, and handle expected loads. But, when you're cruising, charging capacity is important unless you plan to run a generator for very long periods of time.

Which battery charger to fit now depends on your plans and your pocketbook. The best current charger out there is the Sterling AKA ProMariner Ultra Pro series, which come in several sizes up to 60 amps. These can be combined if you need extra charging capacity. But...they're pretty expensive. Figure $500 or more for the largest model.

The most economical good charger out there is the Iota/DLS series with the IQ-4 smart-charge option. These come in sizes from 15A to 90A and provide a lot of charging capacity for the money. While they're not "marine rated", they are used on many boats (including my own and many of my client's) and do a very good job. A good source for these is Northern Arizona Wind and Sun.

There are other good chargers on the market, too, including Victron's (top-notch but expensive), Mastervolts (ditto), and others.


hellosailor 04-28-2012 11:28 AM

Re: Charger Sizing... (too big?)
Something Bill skipped over, is that all else being equal, a charger with an excess capacity rating will run cooler than one that is "just" enough. And by running cooler it should also last longer. Electronics are often derated at 1/2 life for every 10 degrees of temperature increase, so running cooler can make a big difference in the long term.

Bottom line, buy the capacity you will need, shoot a little over that, and you'll be fine. Any decent charger will handle "smaller" battery banks without a problem. Worry more about the installation (proper ventilation and cooling, keeping water away, proper cabling) itself.

mitiempo 04-28-2012 12:31 PM

Re: Charger Sizing... (too big?)

Originally Posted by CD28 (Post 864407)
...within the year I will also increase the size of the house bank. Currently have a group 24 starter battery and 4 golf carts (200Ah) on the house side. Longer term plans include upgrading to 400+ on the house side (possible AGM)


You already have over 400 AH in your house bank if you have 4 golf carts. Golf cart batteries are between 210 and 240 AH each. 4 in series/parallel will give you between 420 and 480 AH.

And the others are right - the charger can't be too big.

CD28 04-30-2012 07:23 AM

Thanks everyone. Very much appreciate the input.

denverd0n 04-30-2012 11:29 AM

Re: Charger Sizing... (too big?)
Just to expand a little... A charger can't be "too big" in electrical terms. But it can be "too big" in monetary terms.

That is, no point paying for a 100 amp charger if you're never going to use it to charge more than a single, group 31 battery. That would just be a silly waste of money. You wouldn't hurt the battery or the charger doing that, but you would beat the heck out of your wallet for no good reason!

Therefore, buy a charger that has the capacity for a battery bank somewhat larger than the biggest you ever expect it to have to charge. Don't go out and spend a boat-load of money, however, on the biggest charger you can find, just because "a charger can't be too big."

Stu Jackson 05-03-2012 01:43 AM

Re: Charger Sizing... (too big?)
You should also get rid of that old charger, asap!!!


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