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I have a GLP 120-12 AGM battery, this thing to be exact:

https://glpower.eu/produkt/glp-120-12/

It's a 120 amps battery, and it is about 26 months old. All of a sudden, I noticed that the battery is holding only about 10-15% of it's designed capacity. The voltage drops from 12.8 to 10.5 within half an hour when I'm draining about 100 W of power out of it, and after that, it also rises back quickly to 12.8 V, it looks more or less like this:

Draining (100 W):
Start => 12.8 V
5 minutes => 12.5 V (and it stays that way for another 20 minutes or so)
25 minutes => 12.4 ... 12.2 ... 11.9 all down to 10.5 in a minute or two

Charging (10 Amps):
Start => 11.5 V (after my inverter disconnects at 10.5 V, the battery gets back to about 11.5 V, with no load)
30 minutes => The charger stops, and it reads 12.8 V (or slightly more)

Right now, I measured the voltage (fully charged a week ago and disconnected from the inverter,)) and it reads 12.8 V as if fully charged.

I'm wondering if I could somehow revive it? Are there any chargers or other devices that could somehow restore at least part of the missing capacity? Or is it a lost cause?

EDIT:

Just to let those that may be reading this know...

I was able to recover some of the lost juice (20-30%) by using a Novitec 'Megapulse' desulfator (cost ~ 70 euro). It made the battery somewhat usable again after about three weeks.

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    \$\begingroup\$ This is due to sulphation. What was the storage/op. temperature and time spent below 10V? Brand datasheet? I would inject current pulses with flyback at 20kHz and series diode while charging at regular charger over battery with fastest current rise time but low average ripple current every time to restore and prevent drawing energy from battery while being charged or V>=13V. I have seen this work on recently sulphate batteries and heard it works >80% of dead batteries \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 30 '19 at 15:27
  • \$\begingroup\$ @SunnyskyguyEE75 The batter was always connected to my Inverter/UPS and stored in my room. The ambient temperature was between 18-30 degrees Celsius. The battery was always fully charged, it was never fully discharged. Brand datasheet? This is probably the closest one: glpower.eu/wp-content/uploads/2015/07/GLP_100-12-spec_EN.pdf It's for 100 amp battery but from the same manufacturer. "I would inject current pulses with flyback at 20kHz[...]" Um... any automatic charger that can do something like that? \$\endgroup\$ – user2384366 Jun 30 '19 at 17:23
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    \$\begingroup\$ No , search for Battery rejuvenator designs. my former employer built them then Owner of patent sold to US company and others I know have made Blowtorch versions for restoring dead car batteries in Jamaica controlled by high power pulses and PIC uC. I tested it on Motive Power batteries and found it restored and balanced pH content in acid or specific gravity, the “acid test” for flooded cells. some designs are good, others not \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 30 '19 at 17:35
  • \$\begingroup\$ What is the charging voltage immediately before the charger stops? \$\endgroup\$ – RoyC Jul 1 '19 at 1:37
  • \$\begingroup\$ @RoyC I'm measuring the voltage of the battery, using an automatic car charger (if you don't choose program it just displays voltage while idle). I'm not sure if I'm thinking correctly but it shows 13.6 V when my UPS is charging the battery... So maybe 13.6 V is the answer? I'm sorry, I'm pretty green to all this... \$\endgroup\$ – user2384366 Jul 2 '19 at 22:05

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