I have a AGM deep-cycle battery, 150 Ah, 12V: Banner "Stand By Bull" SBV12-150. 12V150AhC20.

Up until recently it was working quite OK, I was able many times, for many months, to drain about 170 mA from the battery for a month before recharging it (such a drain should not deplete the battery below 20% full).

But recently the performance suddenly dropped, the last time I tried to charge it, here is what happens: the voltage on the battery without any load before starting to charge was 9.69 V. I don't know why it was so low, because the battery was charged previously only 10 days ago with 16 hours of about 5A charge, at the end of which the voltage on the battery (while still charged) was 14.7 V, and the specifications of CCCV charging of an AGM battery say that an AGM battery is at least 80% full when the voltage reaches 14.1 V, so 14.7 V should be really full. And then the battery was used for 10 days with a load of about 170 mA, so no more then 41 Ah should have been depleted from the battery.

This time (after these 10 days of 170 mA load), I did 20 hours of 2A charge (which should put about 33 Ah back into the battery). The voltage on the battery at the end of charging was 14.0V. Then I disconnected the charger, and the voltage measured on the battery just after disconnecting the charger was 13.0V.

And then I placed a 60 mA load on the battery, which is a very tiny load for such a big battery, and it should go for about 40 days like this on an 80% full AGM deep-cycle battery before the voltage should drop below 12V. But in my case, the voltage dropped below 12V after only 10 hours - this is about 100 times less than what it should be.

Is there anything I can do to try to revive this battery? Is there something like a desulfation or equalization charge that I could try to restore this battery, and how to do it to have the highest chance of success?

  • \$\begingroup\$ Did you 'fix' the battery? What happened? \$\endgroup\$
    – Russell McMahon
    Feb 20, 2020 at 6:10

4 Answers 4


I found an article on deep-cycle batteries, and they comment that if a battery goes below 10.5V, it's "dead" due to the specific gravity of the acid being too low to be useful any more.

It appears that you may need a new battery. : (


A battery with 12.7 volts is fully charged, 12.5 volts is 90% charged. If the battery drops below 10.5 volts after the floating surface charge is removed (wait three hours after disconnecting charger), you have a shorted out cell (electric short between plates). You can remove the caps and measure each cell's voltage with a tester.

Shorted out cells in sealed case batteries cannot be repaired. Replace the battery.

I do hundreds of deep cycle gel and AGM batteries a year. I use $14,000 battery regenerators / desulfators. Most electronic charger/desulfators are a scam. A good machine warms up the battery in the process because of the voltage and current required. Little 'advanced electronic chargers' costing a hundred dollars aren't worth their claims. Buy a Schumacher SC8 from Walmart for $80! It has an automatic desulfator cycle when necessary for almost dead batteries.

AGM (AcidGlassMat) batteries are 'dry', with all the electrolyte absorbed into the mat, but may have dried out the mat too much from overcharging. Adding 1 ounce of distilled water to each cell may help. Do not fill with water or electrolyte above what can be readily absorbed by the mat.

  • 1
    \$\begingroup\$ AGM batteries are sealed by definition. Suggesting someone add water or electrolyte makes the quality of this answer rather suspect. \$\endgroup\$ Apr 21, 2017 at 19:50
  • 2
    \$\begingroup\$ @patricktokeeffe they are not sealed that hard. Look here youtu.be/oDzaFnpzfRE \$\endgroup\$ May 9, 2021 at 20:52

Added 2020: I have edited this post, removed some unrelated material and somewhat improved the formating. BECAUSE: It is interesting and may be useful but should be treated with care. It contains substantial non standard advice, quite a lot of which goes against standard practice. However - The OP claims that it works (for him at least) and it seems worth looking at. - RM

Sorry, I know this is old question and late answer...

Like all lead/acid batteries (AGM,deep cycle wet, automotive starting batteries,whatever) they want to be kept near full charge and you should know the longer they are not at peak charge means the electrolyte is weaker than normal and that means the sulfuric acid is converted to a lower concentration and is weaker and is now more water and less sulfuric.

So you are telling us you leave the battery this way for a month, but like we said, all lead acid batts need to be near full charge and recharged immediately for best results and lifespan.
So how many times(hours/days/months) were you "actually" leaving the battery one quarter empty, half empty,three quarter empty,etc??,see??
Even though it was "in use" it was technically/basically only putting out the equivalent of a self discharge current.

Best to of had a tiny solar panel helping it along if possible.
I could be wrong but I think you would of been better off using a smaller batt and charge more often ..hope you have had goodluck in the long time since your original question friend


Sorry, i re-read your question and you asked how to possibly revive the battery.
What i do (and have had success doing for 7 years to my four 12ah batteries in my 48volt electric scooter, yes 7 years on original AGM batts that I drain almost dead every ride!!) - what i do is open the batt and open the 6 cells and slightly re-wet the fiberglass mats..do not soak them!! This helps tremendously because even AGM batts try to outgas when recharging.
Obviously this is "wetter" than factory and will bubble more vigorously so I like a knife slot into the rubber valve cap to stop excessive internal pressure and when the batts seem to "run dry", I just add a tiny bit more and I guess I have been lucky to add just the same amount that outgassing each year because it worked for 7years, but cutting the slit makes the outgassing easier so feel free to take a chance not cutting a slit or poking a pin hole in the pressure valve.
I did have one 20ah batt on a different ebike pop a seam on its side due to a valve not releasing pressure so now I make sure to cross slit all my batts caps and i just re-water more often.
I take the battery down to 10.8volts and then wet the cells slightly more than factory, then charge at a slightly higher than suggested amps to re-convert any stubborn sulfation, then i drain the batt at one quarter of its rated Ah and recharge and repeat a couple times. Each time you keeps track of the time it takes to calculate its new Ah rating, which should improve. But most people nowadays don't have the patience

I use a 12volt 300watt xantrex brand inverter and a few incandescent lightbulbs plugged into it to drain the battery at the wattage and amps that I want, and I have 2/8/12amp automatic, intelligent chip, smart desulfate CanadianTire brand charger. I even can fool the smart chip into desulphate mode if I want to with by using a dual element automotive brake bulb and some alligator clips that i wire in series with the charger to increase the perceived internal resistance of the battery. I can use the dim setting (high resistance) or I can use the brightest mode which means low resistance to current and higher amps). I prefer adding the least resistance (higher wattage) bulb filament. :-) not only that but if I feel like having the battery charge longer than the charger "thinks" it should on its own, I can use the 2 different wattages of the bulb in parallel connection to the batt terminals and the charger will sit at top voltage longer by perceiving the battery is still accepting a higher amperage when in fact it is the bulb and battery in parallel. This allows me to boil off excess water if I added too much.

The default shutoff in the charger is for 0.3amps at the 2amp manual setting, 0.8amps at the 8amp manual setting, and 1.3amp shutoff at the 12amp setting.
Each manual setting tells the charger what approximate size Ah battery you are charging and the charger shuts off to respect your wishes. It assumes you know what you are doing. It correctly assumes that a huge battery is finished charging when it only takes 1.3 amps at top voltage, and that a small battery is done charging when it takes 0.3amps at full top voltage. Or you can let the smart charger constantly monitor the internal resistance of the batt on automatic setting and hope for the best.

  • \$\begingroup\$ Hi Chris, welcome to EE.SE. Could you please add some formatting to your answer, please? It's just a wall of text, currently. \$\endgroup\$
    – Ariser
    Sep 10, 2019 at 14:50

If you can get the battery to 13V 24 hours after the charger has been disconnected, no cell are probably dead.

I would use a desulphator for several months on it as it seems to be a large battery and the rule of thumb for the desulphating time is one day per pound of battery weight.

  • \$\begingroup\$ After doing some searching, this appears to be a questionable technology. \$\endgroup\$
    – gbarry
    Dec 12, 2012 at 20:17
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    \$\begingroup\$ I have tried it with several batteries and it does usually work. I would recommend to read about it here: leadacidbatterydesulfation.yuku.com \$\endgroup\$
    – Gunnish
    Dec 12, 2012 at 20:20

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