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Old 05-27-2015, 08:13 PM   #1
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Battery Charger

Having a problem with the charger on the boat we just purchased. Want to replace it but not sure how to size it amp wise. Only have four group 24 batteries,two house two starting. It has a 10 amp charger on it now but seems to be under sized. ??
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Old 05-27-2015, 08:16 PM   #2
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You might want to consider an inverter that charges when shore power is connected.
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Old 05-27-2015, 09:20 PM   #3
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Would there be a need for an inverter if there are no AC circuits onboard? I've got several AC outlets onboard and the air compressor and hot-water heater use AC. If not for those, an engine-powered alternator and a shore-powered AC-to-DC battery charger would seem to be sufficient.
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Old 05-27-2015, 09:34 PM   #4
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Tell us more. Are those 4 G24s all start batts or are 2 of them deep cycle? If they're DC, they'll have a 20-hr AH rating on them. If they're start batts, they'll have CCA or other cranking amp specs.

What's your typical usage pattern? How much juice do you need each day at anchor? How will you power this charger? Shore power only? Generator?

It takes a long time to charge a decent size bank with a 10A charger, but if your bank is small, 10A might not be as undersized as you think.

Lots of info needed to help answer your question.
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Old 05-28-2015, 12:20 AM   #5
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Rule of thumb is charger amps output should be at least 10% of battery size. So 100Ah batt needs 10A or more. Sounds like you have 2 banks, a charger that can charge 2 banks separately and simultaneously would be good.
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Old 05-28-2015, 07:01 AM   #6
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My rule of thumb. If you basically spend your time in a marina, charger should be 10% or more of battery capacity. If you regularly spend time at anchor using a generator to charge the batteries the charger(s) should be 20% + of battery capacity.

When in the marina or the boat yard I use a 20 amp charger on a 1300 amp bank. At anchor I use two inverter chargers for 200 amps, plus two solar panels and a wind generator.
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Old 05-28-2015, 04:20 PM   #7
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And a 10/10/10 amp charger may work fine, as your alternator should keep up with the start batteries while underway (supposing the start batteries are isolated from the house and not used during anchoring) and a smart charging 10/10/10 will usually put much of the full 30 amps to the bank most in need, in this case, the house.
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Old 05-28-2015, 05:13 PM   #8
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All of what has been said is good, particularly the 10/20% rule. So how do you apply it.

Your starting batteries will need very little shore power charging. Your car can sit for a couple of months (with good batteries) and still start the next time and so can your boat engine.

Your two G24 house batteries are what really determines the charging output that is required. A G24 has about 60-70 amp hours of capacity. So two is 140 AH or less. So a 20 amp charger would be fine and since most have dual isolated outputs, connect the second output to the starting batteries to keep them topped up.

But realistically even a 10 amp charger will work. It will recharge those 140 AH house batteries easily within 24 hours.

The only reason to have a big charger is if you want to charge with a genset while on the hook. You want to limit genset running time so a big charger (up to 25% of the AH house capacity if flooded cells, more if AGMs) is needed.

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Old 05-28-2015, 05:54 PM   #9
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Quote:
Originally Posted by djmarchand View Post
The only reason to have a big charger is if you want to charge with a genset while on the hook. You want to limit genset running time so a big charger (up to 25% of the AH house capacity if flooded cells, more if AGMs) is needed.

David
When I replaced my shore charger, I wanted the largest charger my Honda generator could comfortably power without the need for a full RPM generator operation. My loads run 150-180 AH per day on a 660 AH house bank. I settled on a 55A charger.

The generator can handle 13.3A and the peak load for this charger is 13A but I knew most charging would be well below this level so it seemed like a good fit. The charge level is less than 10% but in the real world, after an hour or so, the charge current drops below 50A for my lead acid batts so any extra capacity would be unused from this point on in the charging cycle. When the charge load drops to about 15A +/- (approx 80% battery charge), I stop the recharging.

Without an engine run, I find that I run the Honda 1.5-2 hrs in the morning and evening...breakfast and dinner...to maintain the house bank between 60% and 80%. If I run the engines for a couple of hours repositioning the boat, I generally won't need a generator top off charge.

One big savings on my battery bank has been a revised operation for me. I no longer routinely run my microwave on my inverter. If I need to conserve electrons, I start up the Honda 2000 for the microwave operation. This puts the load on the generator and provides some battery charging at the same time. It works well for me 80% of the time. If I'm underway, I just run the microwave from the inverter.
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