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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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Old 04-08-2010
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Question Matching charger to battery

I have to replace my batteries. I have an older Guest Pro Charger 2608 that says its automatic. Apparently chargers can only handle a certain size battery. Specifications on new model 2608a Guest chargers say the maximum size battery is 72 amp hours with a maximum charge time of 12 hours. I'm thinking of buying an Optima D31m deep cycle battery. It is rated at 75 amp hours and appears to have charger requirements similar to the 2608.
Can anyone tell me how crucial this matching of charger and battery size is? I don't want to buy a new charger unless I have to.
Thanks for anything that would help shed some light on this situation.
Bill
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Basically, battery chargers should be sized according to the size and type of the batteries they're going to be used with.

Wet cell batteries should be used with a charger that has an amperage rating equal to about 20-25% of the bank size in amp-hours at the 20 hour rate. Ex: Two T-105s make up a 225 Amp-hour battery bank, and the charger should be about 45-55 amps in capacity.

AGM batteries have a higher acceptance rate, and can be used with chargers rated at up to about 50% of the bank's amp-hour capacity in amps.

At a minimum, the smallest battery charger you should use should be rated at about 10% of the battery bank's size in amp-hours @ the 20-hour rate. Ex: the two T-105 battery bank should have a 20 amp charger at a minimum.

The Guest Pro 2608 you have is a 6-amp charger, and is pretty undersized, especially if you consider that the Optima batteries have a pretty high charge acceptance rate, being a form of AGM battery.
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I am pretty sure they will still charge with the 6a charger. They do not self discharge at 80 watts. It will just take a long time to get them charged. I have a 6a charger on a bank that is probably around 175ah. They charge, it just takes a long while.
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I run the same Optima 31 (blue top) battery that you are suggesting. I also run the CTEK 3300 charger to charge it. The charger puts out ~3 amps and charges the Optima very well. The charger has an "Optima" setting on it (charges at/to 14.7 for the Optima). CTEK rates this charger for batteries from 2 to 90 Amp hours. I monitor this with an "E" meter and can verify the results, and I'm quite happy with it so far. I keep my boat in a moorage with shore power to keep the battery topped up. I run a depth sounder,VHF, Nav lights, stereo, and interior lights. No LED lighting. After a full day of sailing, using everything that I want/need, the charger usually kicks into float/maintaince before I'm done putting away the gear and securing the boat for the night.
Best of luck
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Smile Great Big Thank you

Thanks Kary,
for that information. You describe my situation almost exactly. I have wondered if the Optima 31 would be enough. My load is very similar to yours with the exception that I have changed all the lights, Nav and cabin, to LED's. I'm in a slip with shorepower which is exactly how I plan to keep the battery topped off. The FM stereo radio was the only thing I thought might draw it down. I just didn't have any experience with this to make a judgement and you now given me peace of mind about this arrangement.
Bill
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