What does it mean when a battery does this...

[DXHum]

I have a fully charged Lead Acid battery. With the battery tender on it, it reads 12.9 volts.

When I take the tender off, it drops to 12.7 volts. And after 15 minutes, it dropped to 11.7 volts.

But now it sits at 11.7 volts for the last 4 hours without the tender.

Why would it make such a quick drop in voltage and then stabilize at 11.7 volts?

 

[Hedges]

Any load?
If not, it is not fully charged. Or, has a bad cell.
A "tender" isn't a charger. Might get there eventually. See what manual says regarding charge voltage and absorption time, for cyclical and for standby use.

Charging is typically more like 14.5V, and temperature compensated.

Here's a charger I use. Has AGM and FLA settings (if I remember correctly) and I think the "*" icon means colder weather (two settings for temperature rather than an actual sensor.)

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4A is not ideal for your 65 Ah FLA battery. I think about 10A is more appropriate.

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[jberger]

It's just reached the end of life, the 12.9 volts is just a surface charge that drops off rapidly when you take it off the charger.
Could be the plates are sulfated, could be the electrolyte has gotten weak or contaminated, etc.

 

[Rocketman]

Can you fill with distilled water - is the water low?

Probably a dead battery.

 

[Brett V]

Be more specific than "lead acid battery". If it's an AGM, it's probably sulfated and good for scrap value. If it's an FLA, you need to make sure the cells are topped off with distilled water and do a good equalize charge.

 

[Hedges]

But first, "charge".
A battery tender would not deliver proper voltage to fully charge, and maybe not enough current to keep FLA happily at float.
Jumper cables to your car battery and operate for a while. Or cigarette lighter adapter or other wires and charge it up in the trunk (secure upright if FLA.)

 

[DXHum]

Any load?
If not, it is not fully charged. Or, has a bad cell.
A "tender" isn't a charger. Might get there eventually. See what manual says regarding charge voltage and absorption time, for cyclical and for standby use.

Charging is typically more like 14.5V, and temperature compensated.

Here's a charger I use. Has AGM and FLA settings (if I remember correctly) and I think the "*" icon means colder weather (two settings for temperature rather than an actual sensor.)

4Amp Fully Automatic Microprocessor-Controlled Battery Charger/Maintainer

Amazing deals on this 4Amp 12V High Freq Battery Charger/Maintainer at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

4A is not ideal for your 65 Ah FLA battery. I think about 10A is more appropriate.

2/8/15 Amp Automatic Microprocessor Controlled Battery Charger

Amazing deals on this 2/8/15 Amp 12V Automatic Charger Starter at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

It was fully charged with a 10 amp NoCo. Then I put the 1.25 amp tender on it. It was when I took the tender off and just let it sit with a meter on it, that I saw the voltage drop I described.

 

[Hedges]

OK, not sure if the 10A fully charged or not. What voltage does that reach? How long is it held at that voltage?

Are these the FLA you show in another thread?
If FLA, you can remove caps (wear goggles and old clothes). Use a hygrometer to measure specific gravity, see if it varies too much between cells.
Add water just to cover plates, not full, before charging more. If large variation between cells, perform an equalizing charge (higher voltage for extended time.)