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Battery charger 101

Hi all,

Finding a bit of confusion regarding the selection of a battery charger. Trying to keep the budget low on this, as the batteries are used and have other areas that need more serious money thrown at them. Would like to get a 'fancy' charger when I replace the batteries. But for now...

- I've heard that the charger should be at least 10 amps. True?
- The charger should be 'automatic'. Does that mean it completely shuts down when the batteries are charged, or is switching to 'trickle mode' sufficient?
- Are there any types or brands to specifically avoid?
- Recommended inexpensive chargers?

Thanks for the help in advance.
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post #2 of 9 Old 06-24-2009
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Corny—

The battery charger is usually sized according to the type and size of the battery bank it is charging. Having too small a battery charger is as bad as having too large a one. Buying a "cheap" charger is generally a really bad idea, since it will often shorten the life of the batteries.

What you need is a good three-stage smart charger. This means that the charger will go through the bulk, absorption and float phases of charging and step the voltage and amperage up/down as the charge level on the battery changes.

You don't say how many amp-hours your battery bank is or what chemistry they are. As a general rule, the battery charger should be 20-25% in amperage capacity of the 20-hour amp-hour rating of the battery bank. For instance, if you have a house bank of two Trojan T105 batteries, that would have a capacity of 225 amp-hours, you'd want a 40-55 amp charger or so.

However, AGM batteries usually have a much higher current acceptance rate, especially some of the newer, more modern technology batteries, like the Optima Spiral cell batteries. In the case of these batteries, you might want a larger charger, due to their higher current acceptance rate in the bulk phase of charging.

You also want a battery charger that can be set for the voltage requirements of the battery chemistry that you're using—wet cell, AGM or gel. Not doing so can drastically shorten the life of the batteries as well.

A good brand of "smart" battery chargers are the Iota brand. They're quite reasonably priced and given their price, it doesn't really make much sense to get a "cheap" 10-amp charger. The 30 amp charger is about $130 and their 40 amp unit is only $145. See this website for more information regarding them.

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post #3 of 9 Old 06-24-2009 Thread Starter
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Thanks. I have been doing some research since my initial post and the two brands that seemed to strike a chord with me as far as value:quality were Iota and Guest.

I had discovered on my own that Schumacher's claim to a 'smart' charger is complete hooey and that they are...well, you get what you pay for there.

Our batteries are pretty much garden variety...your basic farm and barn deep cycle for 'everything else' and a more or less standard starting battery.

My goal is to get thru this season with these batteries and look into quality replacements for next. "Season" here ends around October.

The Guest marine chargers seem similarly priced with the Iota's. Any comparisons to be made there?

Since the boat is a couple hours away, I can't provide data on amp hours of the batteries. I'll get that info next week. I'm looking at 10 amp chargers from Iota and Guest right now...any other suggestions?
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Unless you're not planning on having more than one battery for the house bank, I'd go with at least a 20 amp charger. A 20 amp charger is appropriate for a 80-100 amp-hour bank, which is usually ONE group 24, 27 or 31 battery. If you're planning on using more than one group 24/27/31 as a house bank, get at least a 40 amp charger.

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Can a Iota charger be used at home? I do not have shore power (on a mooring) and bring my battery home with me every time. I assume I would just need to add a standard ac plug and two battery cables.
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post #6 of 9 Old 06-24-2009
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Chargers

I don't know much about the Iota but I've had a Guest smart charger on board for 10 years and couldn't be happier with it. I have a dual output version that connects to my two house banks and a echo charger that tops up my engine battery. I leave them connected at all times at the dock and my house batteries are going on 6 years old this summer. When I checked them recently with a a battery tester they still checked out as good. So given that I'd certainly recommend the Guest as a viable option.

Oh BTW I use golf cart batteries as my house batteries.


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post #7 of 9 Old 06-24-2009 Thread Starter
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Without getting too far off the original topic, I eventually would like to augment with solar. I do have some concerns about weight, size and the ability for wind to catch the panels.

The boat is a Ranger R-28 sailboat. It's mostly used for day trips and occasional overnighters on Lake Superior. It's currently equipped with one deep cycle and one starting battery. That isn't to say that it's always going to be that way, that's just what it is right now.

I have a lot of experience with solar, having a cabin on 20 acres that is entirely solar powered with 2 95 watt panels and a charge controller. I use "Farm & Barn" deep cycle batteries - nothing fancy, and get about 4 years of service out of them. This is with the batteries sitting out in the wind, the subzero (-30F at times), etc in only plastic battery boxes. However, such a system would seem enormous on our sailboat. Is there such a thing as a 'sailboat friendly' panel design, that won't catch the wind, that won't take up useable space and get in the way of rigging?

Glad to hear a vote of confidence in the Guest. The Iotas sound good too, but I read somewhere that they're not 'marine chargers'. What exactly does that mean? Some 'marine' electronics, such as stereo units, seem to be no more than a white case vs. a black one.
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Quote:
Originally Posted by sailingdog View Post
Unless you're not planning on having more than one battery for the house bank, I'd go with at least a 20 amp charger. A 20 amp charger is appropriate for a 80-100 amp-hour bank, which is usually ONE group 24, 27 or 31 battery. If you're planning on using more than one group 24/27/31 as a house bank, get at least a 40 amp charger.
SD, I see here that you are qualifying the charger sizing to the house bank only. What if I had 1 house battery and 1 starter battery. Taking your advice about not mixing battery technologies, they would each be a 90 hmp hr AGM battery (because these fit physically).

If I had a battery charger that had 3 outputs, do I hook one battery up to each output, and not need the echo charger?

If I do that, would I want 25% or 90 amp hr or would I want 25% or 90 + 90 amp hr?

If I hook the engine to the starter battery, does this mean the engine no longer charges the house battery?
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post #9 of 9 Old 06-25-2009
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Quote:
Originally Posted by jarcher View Post
SD, I see here that you are qualifying the charger sizing to the house bank only. What if I had 1 house battery and 1 starter battery. Taking your advice about not mixing battery technologies, they would each be a 90 hmp hr AGM battery (because these fit physically).
The reason for basing it on the house bank size, is that the starting bank is generally almost fully charged under normal conditions and requires very little charging. Starting an engine uses a miniscule amount of energy.

Quote:
If I had a battery charger that had 3 outputs, do I hook one battery up to each output, and not need the echo charger?

If I do that, would I want 25% or 90 amp hr or would I want 25% or 90 + 90 amp hr?

If I hook the engine to the starter battery, does this mean the engine no longer charges the house battery?
I would connect all the charging sources to the house bank and use an echo charger/battery combiner, since that simplifies what you have to do. Usually, th e house bank is significantly larger than the starting bank, and the battery charger and other charging sources should be based on the size of the house bank. YMMV.

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her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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