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Reviving a Discharged AGM Gel Battery: From 8V to 13.6V and Back to 10V

porlock 22770 16
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Can a motorcycle AGM gel battery that fell to 8 V, charged to 13.6 V, then dropped back to 10 V be recovered, or should it be replaced?

The battery is probably beyond practical recovery and should likely be replaced, although a long low-current desulfation attempt may still work [#17257313][#17257523] One suggestion is to use a laboratory power supply in CC mode at about 50 mA and leave the battery connected for a few days, then discharge it with a small load like a 12 V 1.2 W bulb and recharge; after a few such cycles it may come back to life [#17258013] Another proposed method is to discharge it to about 9.5 V with a 10 W bulb, then charge it with current limited to 0.8 A up to 15.6 V and repeat the cycle [#17257659] However, if the battery has been sulfated for about six months, a few hours of charging will not desulfate it; the process takes much longer and too much voltage can just cause gassing in some areas [#17257937][#17258140]
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  • #1 17257305
    porlock
    Level 13  
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    Last year, at the end of the motorcycle season, I installed a new AGM gel battery, unfortunately, for personal reasons, the moto went under the tarpaulin so that I did not remove the same battery for the winter. Now I took it out, voltage 8V, connected to the rectifier, it charged to 13.6V, which took him 2 days, after disconnecting the rectifier, the voltage dropped to 10V within a few hours and I don't want to land any more :(
    Is there any more chance for him?
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    #2 17257313
    Zielonka
    Level 22  
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    Hello
    I will worry my friend. Battery to be replaced.
    Regards
    WB
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    #3 17257315
    pawel16150
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    Rather not, it remains to buy a new one.
  • #4 17257523
    _jta_
    Electronics specialist  
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    It looks like sulphation - probably half a year has been discharged, did you charge it before winter? You can try to connect to 13.5V for a month (you do not need a lot of current) - it may start working. But maybe there will be more trouble with it than it is worth. How about the warranty conditions?
  • #5 17257597
    porlock
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    Before the winter, it was charged because it was new, maybe I started the engine 10 times, and the warranty covers such cases?
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  • #6 17257626
    Anonymous
    Anonymous  
  • #7 17257631
    porlock
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    Exactly an automagic structure by Lidl, the capacity is not great because it is a battery from an 8Ah motorcycle, I have a meter as much as possible, as well as a laboratory power supply which in theory can pull up to 5A at 32V
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  • #8 17257659
    Anonymous
    Anonymous  
  • #9 17257727
    porlock
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    I will try, I have nothing to lose, how long the loading process to 15.6 may take, so more or less so that I do not get discouraged too soon :-) and he did not come to the conclusion that probably nothing of it, in other words, when I have to forgive myself, previously until 13.6 it took 2 days.
  • #10 17257741
    Anonymous
    Anonymous  
  • #11 17257746
    porlock
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    Ok, thanks, let me know how it went.
  • #12 17257937
    _jta_
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    If sulfation lasted six months, do not count on desulfurization for a dozen or so hours - too high voltage will result in gassing in some places, in other places it will leave sulfation.
  • #13 17258013
    kortyleski
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    If you have a laboratory power supply, set it in CC mode for about 50mA and leave the battery for a few days. Then discharge with a small current (e.g. a 12V1.2W bulb) and recharge. After a few such cycles, it should more or less come back to life
  • #14 17258050
    Anonymous
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  • #15 17258062
    kortyleski
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    Does not matter. It is about dissolving sulfur compounds in the electrolyte that have precipitated during deep discharge. Here you need a little electricity and a long time.
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  • #16 17258069
    Anonymous
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  • #17 17258140
    _jta_
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    Much more time - the process of dissolving the "insoluble" lead sulphate crystals takes a long time.

    Initially, the battery will accept a small charge - you can try to measure e.g. the charging time with a small (and always the same) current from e.g. 12.0V to 13.2V and check how this time changes - if it gets bigger, you can count on the fact that the battery takes more and more charge - its condition improves.
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Topic summary

✨ The discussion revolves around reviving a discharged AGM gel battery that dropped from 8V to 10V after charging to 13.6V. Users suggest that the battery may be sulfated due to prolonged discharge and recommend various methods for recovery. These include connecting the battery to a laboratory power supply set to a low current for an extended period, using a 10W bulb for controlled discharge, and gradually increasing the voltage to 15.6V while monitoring the current. The importance of patience and careful monitoring during the charging process is emphasized, as well as the potential limitations of recovery due to the extent of sulfation.
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FAQ

TL;DR: For an AGM that dropped to 8 V, a recovery attempt can use 0.8 A for "up to 14 hours"; if it stagnates, try gentle float or replace. [Elektroda, Anonymous, post #17257741]

Why it matters: This FAQ helps motorcycle owners revive or evaluate a deeply discharged AGM and decide when to stop trying.

Quick Facts

Can I revive an AGM battery that fell to 8 V over winter?

Yes, but it indicates sulfation. Connect it to about 13.5 V with low current and wait. Leave it connected for roughly a month. It may recover, though results vary. Consider the time and effort versus a replacement. Warranty terms might affect your decision. [Elektroda, jta, post #17257523]

Why did my AGM show 13.6 V after charging, then fall to ~10 V?

That pattern points to sulfation after long discharge. The charger restored surface charge to 13.6 V, but plates stayed sulfated. The open-circuit voltage then settled near 10 V within hours. This behavior matches a battery that cannot hold energy. Try slow recovery first. [Elektroda, jta, post #17257523]

What is a safe, gentle lab-supply recovery setup?

Do this:
  1. Set a lab PSU to constant current ~50 mA and connect for several days.
  2. Discharge using a small load (12 V, 1.2 W bulb).
  3. Recharge again at ~50 mA; repeat several cycles. This slow cycling aims to dissolve sulfates without heating. [Elektroda, kortyleski, post #17258013]

How long will a 0.8 A, 15.6 V charge take on an 8 Ah AGM?

Using a lab supply, increase voltage to keep 0.8 A until the battery reaches 15.6 V. Expect up to about 14 hours per cycle. This is an effort estimate, not a guarantee of full recovery. "Up to 14 hours" was reported for this approach. [Elektroda, Anonymous, post #17257741]

How do I run the 10 W bulb plus 15.6 V method?

Discharge the battery with a 10 W bulb down to 9.5 V. Then charge with a lab supply in current limit at 0.8 A. Raise the voltage until the terminal reaches 15.6 V. Repeat the discharge/charge cycle and recheck performance. [Elektroda, Anonymous, post #17257659]

Is pushing 15.6 V safe for AGM desulfation?

Use caution with high voltage. "Too high voltage will result in gassing in some places, in other places it will leave sulfation." Uneven reactions can harm AGM cells. Prefer long, low-current charging for uniform chemistry. Monitor closely if you try higher voltages. [Elektroda, jta, post #17257937]

How do I track whether desulfation is working?

Charge at a constant small current and time the rise from 12.0 V to 13.2 V. If that time increases over cycles, acceptance is improving. That signals gradual recovery. "The process of dissolving the 'insoluble' lead sulphate crystals takes a long time." [Elektroda, jta, post #17258140]

How much capacity does 50 mA add over five days?

At 50 mA for 5 days, the math is 0.05 A × 120 h ≈ 6 Ah. This illustrates why the gentle method needs time on an 8 Ah battery. Expect slow gains rather than instant recovery, even with continuous charging. [Elektroda, Anonymous, post #17258050]

Will an automatic charger (e.g., Lidl) recover a deeply discharged AGM?

It can raise voltage to about 13.6 V, but a sulfated AGM may drop back to ~10 V within hours. That shows surface charge without restored capacity. Manual, controlled methods are usually required for recovery. The automatic cycle alone was insufficient here. [Elektroda, porlock, post #17257305]

When should I stop and replace the battery?

If repeated recovery attempts show no improvement, replace it. Forum advice in this case was clear: "Battery to be replaced." Weigh time and reliability risks against the cost of a new AGM unit. [Elektroda, Zielonka, post #17257313]

What rest voltage after charging signals the battery is still bad?

If it falls to roughly 10 V within a few hours after charging, it remains unserviceable. That behavior indicates heavy sulfation and lost capacity. Attempt one recovery run; otherwise plan a replacement to avoid failures. [Elektroda, porlock, post #17257305]

What about warranty—does storage-related sulfation get covered?

The thread raised this question but provided no answer. Review your actual warranty and storage clauses with the seller. Provide proof of purchase and any maintenance you performed when you ask. [Elektroda, porlock, post #17257597]

What discharge threshold should I use when testing with a bulb?

For the aggressive cycle, discharge with a 10 W bulb to about 9.5 V. Then charge at 0.8 A up to 15.6 V. Repeat as needed before testing the battery under load in the motorcycle. [Elektroda, Anonymous, post #17257659]
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