I have a GEL battery 225Ah. After fully charging it, it holds a 13.1 - 13.0 volts after 24 hours. I tried to put it on load (20 watts light) for 2 hours and voltage droped to 12.5 volts. I know that battery is bad. but after an hour without load and without charging the battery voltage rised to 12.8 volts. Any idea on what is happening here?

Note: the battery was part of my battery bank which was 6 x 225 Ahr on 24 volts system. Three batteries went dead as a sudden and dropped voltage to 11 while the others were 12.8 (at 6 am). I tried to revive one of them and the result was the first situation.


  • Batteries are 4 years old.
  • bat. temp between 20 - 25 celsius
  • I use an MPPT charger giving a 28.8 volts on bulk stage, and 27 volts on float stage.
  • I do use it while charging but My MPPT calculats the battery current and I never let it goes to negative while charging.
  • 1
    \$\begingroup\$ It's not uncommon (I don't know about GEL batteries, specifically) that the open terminal voltage rises as a normal part of recovery after being loaded. It's visible over periods of minutes even and measuring after an hour I'd expect some kind of rise to show up. \$\endgroup\$
    – jonk
    Mar 29, 2019 at 18:43
  • \$\begingroup\$ I see. so is that a sign that the battery will recover and be able to use it. because the other 2 batteries goes to 11 volts after load and never rise. \$\endgroup\$
    – Jaz
    Mar 29, 2019 at 18:49
  • \$\begingroup\$ What is the battery age (cycles , years) and operating temp? What were your charger settings? Do you draw load during charge, under what conditions? V, W, time? It might be de-sulph. treated with high current pulse charge while keeping mean voltage 13.5/bat using a load. The absorption dielectric is your battery memory but the bulk dielectric has lost capacity. give any details in question. All it takes is 1 bad cell. \$\endgroup\$ Mar 29, 2019 at 18:50
  • \$\begingroup\$ @Jaz Even bad batteries will show a rise. There's usually (for most chemistries at this level of amp-hour) a set of several measurement kinds involved in determining whether a battery is good or bad. For example, if while the battery voltage is rising during the minutes afterwards it is resting and also the internal series resistance isn't changing much (rapid differential measurements) then the battery is usually "okay" (assuming the terminal voltage is within the right ranges too, of course.) And some battery chemistries are easier to check than others. Study up, I'd say, on GEL. \$\endgroup\$
    – jonk
    Mar 29, 2019 at 18:54
  • \$\begingroup\$ It would be quite beneficial to start from BatteryUniversity.com before expressing these concerns \$\endgroup\$ Mar 29, 2019 at 18:56

1 Answer 1


enter image description here

Some OEM GEL suppliers suggest; After charging all batteries, if any battery voltage is still greater than 0.3V compared to any other battery in the set, you may have a failed battery.


100% State of Charge (SoC) =( equiv. to )= 0% Depth of Discharge (DoD)

Wh Energy Capacity = Vavg * Ah = 2790 Wh = 12.4 * 225Ah new @ 25'C

Your tests

  • 20 watts + inverter loss?, 2 hours 12.5 volts
    • Bulb energy= 40 Wh = 1.43% of rated new capacity
    • 12.5V = 60% SoC= 40% x 2790Wh=1116Wh
    • thus Used/rated = 40+? / 1116 * 100% = <4% of expected capacity. really dead

There are too many contaminants that affect battery aging, but most common is lead sulphate crystals (rust) that can be ablated and settle to the bottom of the case. Some products are good and many poor that apply high-frequency (> 20kHz) high-current but narrow (<1us) current spikes while drawing from power on float charge.

Probability of success varies greatly with chemistry, pulse device, time to recovery and duration of condition. If 50% in capacity if detected early success rate can be > 50% but after 1 yr, unlikely.

My experience is the best cure is a preventive maintenance device that operates continuously using < 5W while on float charge only. ( Auto-sense)

My simple electrical equivalent circuit of a battery is below.


simulate this circuit – Schematic created using CircuitLab

Sulphation causes ESR1 to rise ( and also drained battery)
C2 has higher ESR that takes longer to charge which completes after CV is current reduces to cutoff.
C2 has the capacity to restore C1 voltage and is the "memory effect" in all batteries. ( some more than others)
But ESR2 is too high to support higher currents.

enter image description here

Trojan AGM REF


Never attempt absorption charge levels ( overvoltage) on AGM batteries.

  • 1
    \$\begingroup\$ I just notice about "Never attempt absorption charge levels ( overvoltage) on AGM batteries" and I found that I am setting a absorption stage on my MPPT for 2 hours with a voltage of 28.8 volts. and btw, you gave a really helpful information. \$\endgroup\$
    – Jaz
    Mar 31, 2019 at 10:34
  • \$\begingroup\$ Glad someone found it useful. \$\endgroup\$ Mar 31, 2019 at 16:25

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