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Old 20-06-2010, 05:48   #1
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Battery Charging, Charge Time and Discharge

Hi all,
I have 2 brand new house batteries in 1 bank and a older starting battery in another. Just recently, I noticed that the Guest 10 AMP Pro Charger is taking a long time 24+ hours to "finish" the charging process. I checked the batteries individually with an amp meter and they 13.5 when on the charger. I am almost thinking that there is some unknown draw that is keeping the charger in the finishing mode, but I don't know how to check for that.

As well, while out on the water last week, it seems that the batteries discharge way to quickly with the limited power I use. I was told that in an emergency, the deep cell batteries could be used all the way down to 10%, but after only reaching about 11.5 Amps, electronics stopped working. What I mean by that is, the GPS would not turn on or the VHF did not transmit.

What am I missing here? The charger is less than a year old and as mentioned above, the batteries are brand new.
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Old 20-06-2010, 05:55   #2
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what size are the batterys .
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Old 20-06-2010, 05:59   #3
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Hard to diagnose from the OP's description. Need more info...size of batteries, type, load on them, etc.

For starters, a 10A battery charger is WAY too small for what one assumes to be a house battery bank (two batteries) of around 200A. Should be 40-50A or more with flooded batteries, and twice that with AGMs.

The OP seems also to mix amps and volts. Which is is?

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Old 20-06-2010, 06:43   #4
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Hello Skylark,

First, big point, as btrayfors mentioned, 10 is a very small charger for most boats.

Then, you should understand that a 10 amp charger does not necessarily charge the batteries at 10 amps. For example, you have a 100 amp battery that is at 50% so will need 50 amps. A ten amp charger will not do that in 5 hours. Initially the charger will put 10 amps into the battery but as the battery charge level increases the amount of charge going in will decrease. So for a 10 amp charger to fully charge that battery might take 10 hours or more.

Finally, for most batteries it is not good at all to discharge them as low as 10%. Some types of batteries will take it but they are usually special, expensive batteries and probably not what you have.
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Old 20-06-2010, 10:52   #5
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From my experience a 25 charger will charge 160 of batteries run down to 50% in nearly 5 hours. Then it goes on float, but this means the charging is on.

As the batteries get older, the charger will not charge fully in one cycle (there is a time limiter in ours).

So my guess is that a 3 stage 10A charger is good for up to 60 of batteries, but not much beyond. And as the batteries age, it will take more than one cycle to charge fully.

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