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Old 20-06-2010, 04:48   #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.

s/v Skylark
Cal 34 Hull# 186
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Old 20-06-2010, 04:55   #2
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what size are the batterys .

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Old 20-06-2010, 04:59   #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, 05:43   #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.
The water is always bluer on the other side of the ocean.
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Old 20-06-2010, 09:52   #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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