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post #11 of 19 Old 03-23-2010
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Bill and I posted at the same time. Bill and I are in violent agreement that the simple solution here is that you need to buy new batteries.
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post #12 of 19 Old 03-23-2010
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Three guys in "violent agreement".....a pretty scary thought :-)

B.
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post #13 of 19 Old 03-23-2010
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Well, in that case, I'd consider the batteries TOAST too...

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Originally Posted by jjrunning View Post
I did check the electrolyte level and the batteries are full.

Doesn't a hydrometer just test the charge of the battery? What is the difference between testing the electrolyte fluid with a hydrometer and testing the voltage with a multimeter?

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post #14 of 19 Old 03-23-2010
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If you unplugged them and they were at 12.8 volts they are being undercharged. 12.9 is a fully charged battery and your charger should push about 13.6 volts to charge minimum, most alternators are in the 14V plus range unless regulated. The battery case will sometimes say the recommended float voltage.

If they were regularly undercharged they are most likely quite sulfated by now. The easiest way to tell sulfation is to fully charge them until they take no more amps then load them up, if the voltage drops off quick and you don't have much capacity as you think it's usually a safe bet. If these are wet cells you can try to do a equalizing charge and convert that hard sulfate crystals back into active material. Rolls (Surrette) batteries has a pretty good instruction guide on equalizing their batteries and it should fit most batteries.

Good luck sir!
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post #15 of 19 Old 03-23-2010
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Umm... A fully charged 12 VDC battery is 12.6-12.7 VDC, not 12.9... unless it has a surface charge from being charged.

I don't know what brand of batteries or where you got your information, but it's wrong. From the Trojan battery site.





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If you unplugged them and they were at 12.8 volts they are being undercharged. 12.9 is a fully charged battery and your charger should push about 13.6 volts to charge minimum, most alternators are in the 14V plus range unless regulated. The battery case will sometimes say the recommended float voltage.

If they were regularly undercharged they are most likely quite sulfated by now. The easiest way to tell sulfation is to fully charge them until they take no more amps then load them up, if the voltage drops off quick and you don't have much capacity as you think it's usually a safe bet. If these are wet cells you can try to do a equalizing charge and convert that hard sulfate crystals back into active material. Rolls (Surrette) batteries has a pretty good instruction guide on equalizing their batteries and it should fit most batteries.

Good luck sir!

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post #16 of 19 Old 03-23-2010
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Quote:
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Umm... A fully charged 12 VDC battery is 12.6-12.7 VDC, not 12.9... unless it has a surface charge from being charged.

I don't know what brand of batteries or where you got your information, but it's wrong. From the Trojan battery site.
You are indeed correct is it about 12.7 not 12.9 according the the trojan chart referenced. I look a quick peek at my link 20 to see what the voltage of my AGM bank was to get my 12.9. I charged it about a 2 weeks ago and it has been resting ever since. Of course I had to measure it with my fluke to make sure . I wonder if there is something different between wet cells and my AGMs to cause a full charge voltage difference. I have CSB brand VLRA High rate UPS batteries for reference.


Anyway back to topic, seeing 12.8 right off the charger does not sound like the charger is working as it should be.
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post #17 of 19 Old 03-23-2010
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AGMs and Gels typically both have higher voltages than flooded batteries, not only when fully charged but throughout most of the discharge curve.

Here's a comparison table of Open Circuit Voltages -- OCV (from East Penn):

Well, don't know why this **xx%%##*** Board won't let me attach a small file.


The 100% charged values for flooded, AGM, and gelled are 12.6, 12.80, and 12.85 respectively. "Or higher"
The 75% charge values are 12.4, 12.60, 12.65
The 50% charge values are 12.2, 12.30, 12.35
The 25% charge values are 12.0, 12.0, 12.0

Note that the true OCV can only be determined once the battery has been disconnected from the load -- charge or discharge -- for 24 hours.

Bill

Last edited by btrayfors; 03-23-2010 at 10:31 PM.
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post #18 of 19 Old 03-23-2010
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I'd point out that the OP's Stowaway Marine deep cycle batteries are wet cell battteries AFAIK, not gel or AGM.

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post #19 of 19 Old 03-24-2010
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Get new batteries.

Any batteries that self descharge below 12 volts in two days are cores.

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