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Old 11-05-2019, 10:20   #1
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How to test for over-charging

My sailboat has been tied to the dock for the winter. The only DC being used has been for the cabin lights and the water pump. I've a BMV-702 installed and it doesn't show anything out of the ordinary. Yet it seems that I"m having to top off the batteries a lot.

The ProSport 12 charger I have came with the boat and I'm not sure how old it is. How do I determine if it's overcharging my batteries?
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Old 11-05-2019, 10:32   #2
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Re: How to test for over-charging

You give very little information...

What do you mean when you say you have to "to top off the batteries a lot."

Do you normally leave the batteries on a float charge? If so what is the voltage they float at?

How old are the batteries? What kind?

If you have FLA batteries, do you add water? How often?
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Old 11-05-2019, 10:35   #3
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Re: How to test for over-charging

I have to add water to the batteries and given that they are under very light use I think I'm having to add water too frequently. I have 2 Group-31 flooded batteries. I've added 2 gallons of distilled water over a 2 month period.
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Old 11-05-2019, 10:41   #4
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Re: How to test for over-charging

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I have to add water to the batteries and given that they are under very light use I think I'm having to add water too frequently. I have 2 Group-31 flooded batteries. I've added 2 gallons of distilled water over a 2 month period.
Wow.... that's a lot... a lot, lot! But to help I really need to know the answers to the other questions...

Are the batteries on float charge all the time? (I assume "yes") What voltage do they float at? (A vital piece of information....)

Do you have to add water to every cell about equally, or are some "greedier" than others? (Also really important to know...)
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Old 11-05-2019, 10:44   #5
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Re: How to test for over-charging

What’s “too frequently” and how much water

But you can “test” if you are “overcharging” just by looking on your battery monitor. If the batteries are at absorption (14.6-14.9) and amps in are real low (way less than 0.5%C) the charger should have switched to float and you are “overcharging”
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Old 11-05-2019, 10:46   #6
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Re: How to test for over-charging

I"m not on the boat right now, so I can't say what voltage they currently show. They are on the charger all the time. All cells seem to be at the same levels.
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Old 11-05-2019, 10:49   #7
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Re: How to test for over-charging

2-gallons!!!!!!!!

That’s more water than my 4 batteries use in 2 years
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Old 11-05-2019, 11:02   #8
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Re: How to test for over-charging

We go through about two gallons a year with 4 golf cart batts!!!!!

Those little bass boat chargers (the little potted electronics) are bad about not switching to float.
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Old 11-05-2019, 11:07   #9
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Re: How to test for over-charging

It is a good idea to never just trust any charge source, even very expensive sophisticated ones.

Yes yours are getting way overcharged, just from how thirsty they are.

But that situation should be prevented from the beginning.

You don't "test" after the fact you watch the instantaneous volts and amps behaviour "live" with known-good meters, or look at charts / logs if you have gear to do that, and

adjust the charge regulation

to comply with the bank mfg specs, each battery will have different needs.

If those were GEL they'd have been murdered in short order.
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Old 11-05-2019, 11:11   #10
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Re: How to test for over-charging

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I"m not on the boat right now, so I can't say what voltage they currently show. They are on the charger all the time. All cells seem to be at the same levels.
There are two possibilities I can think of. One is your charger is either not set correctly, or is broken, and is overcharging your batteries. The first of these is more likely than the second. Check your manual for how to set up the charger. If the charger is holding the batteries at a voltage higher than 13.5 or so, something is wrong. Before deciding that the charger is bad, do the next test to be sure it's not the batteries....

The other possible cause is that you have one (or more) shorted cells in the batteries. Disconnect the charger, Disconnect both batteries completely, and let the batteries rest for an hour or so. Then measure the terminal voltage. If one (or both!) batteries are closer to 10.8 volts than 12.5 that battery is toast because of a shorted cell. This can be a serious problem and can cause the batteries to dangerously overheat if the charger keeps pumping power into them. If you find this to be the problem DO NOT put the bad battery back into service! Melt down, fire, or even explosion is possible.
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Old 11-05-2019, 11:25   #11
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Re: How to test for over-charging

Thanks for all the great input. I'll soon be back aboard and will make the checks suggested. I suspect it's time for a new charger.
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Old 14-05-2019, 00:21   #12
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Re: How to test for over-charging

Solar panels are safer than a shore charger because they turn off daily when the sun goes down.
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Old 14-05-2019, 02:10   #13
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Re: How to test for over-charging

Proper measuring and adjustment is needed in either case anyway.

Undercharging is by far the more common problem.

Solar needs to be well regulated, adjusted away from the defaults to get there even assuming there's enough total energy per day to get back to 100% Full - which often there isn't.

At least with Shore power you know you have high current in effect unlimited and available all night.
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Old 14-05-2019, 02:28   #14
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Re: How to test for over-charging

The recommended float voltage of most flooded lead acid batteries is 2.25V to 2.27V/cell.
Larger stationary batteries at 25C (77F) typically float at 2.25V/cell, or 13.5V for a nominal 12V battery.
Manufacturers recommend lowering the float charge when the ambient temperature rises above 29C (85F).
If your charger stays on bulk/saturation charge, and does not drop below 2.30V/cell, remove the charger after 48 hours of charging. Recharge every 6 months while in storage.
Over-saturation, by not switching to float charge, causes grid corrosion on the positive plate. This also leads to gassing and water-loss.
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Old 14-05-2019, 04:50   #15
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Re: How to test for over-charging

Or if you don't trust your setup while away,

as long as you've verified that trailing amps dropped to the Full point, say 2A or less at Absorb voltage,

just isolate the batteries if there are no loads to carry.

Check on them at least every few weeks, hit them with a short charge

safer than leaving them hooked up Floating IMO
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