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Yesterday I bought a new Varta Powersports 508 012 008 AGM 12V battery. Assuming it was mostly charged I immediately hooked it up to my generators starter motor and it started it without problem. After using it for a couple of seconds in total (starting a couple of times) I decided to hit it with the multimeter to see if it was still fully charged or not - i.e do I keep it hooked up or do I bring it to full charge first. To my surprise it read at 10.8V only. This had me worried that the battery is actually already broken when I bought it.

I used my smart battery charger which also would warn me if the battery is damaged. I set it to AGM mode and it was running for a couple of seconds at 0.2A charge current and then indicated that the battery is fully charged still with a voltage of 10.8V.

Now I was really confused and started googling and the information is inconclusive. It seems pretty consistent that the fully charged voltage of a 12V AGM battery should be between 12.7 and 13.0V max. It is also mentioned there is a difference between under load and open circuit voltage, but nothing that would say that 10.8V is a good voltage.

My battery charger has never steered me wrong in the past and has always done its job so I dont really have any reason to distrust it saying the battery is full. The battery is just barely strong enough to start my generator according to its specs, so I would assume if it were well below 0% charge (which 10.8V would indicate according to all the charts I found) there would be no chance that it would start the generator, but it did and still does...

Does anyone know what is going on here? Batteries are always a bit of a mystery to me :(

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    If you are reading unexpected voltages with a multimeter, sometimes it is worth checking the batteries in the multimeter. Jul 3 at 12:50

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I've had problems charging AGM batteries in the past. Just for this purpose, I keep a small 12v lead acid battery around and put it in parallel with the AGM battery, then apply the charger. This tricks the charger into looking at the voltage of the lead acid battery instead of the AGM and charges accordingly. This should allow it to charge to full capacity. If this doesn't work, I'd suggest the battery you have may be faulty.

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  • Interesting, I do have a (large) lead acid battery sitting around, I might give it a try. You mean like this, right: qph.cf2.quoracdn.net/main-qimg-60716050894f39c35d77c57aa7e5b56e . Does this mean the lead acid battery would charge the AGM battery and lose voltage and the charger would replace that in the lead acid battery? Jul 2 at 11:18
  • @YanickSalzmann - Your diagram is accurate. That is what I'm talking about. As far as charging, I'm not sure exactly how it works, but basically (the way I see it), the charger "sees" the lead acid battery and charges it, though the AGM will bring the overall charge of the "system" (the two batteries as a system) down. The charger will continue charging until it seems the system as being fully charged. (Again, I don't know if this analogy is completely accurate, but it is how I see it in my head.) Jul 2 at 12:28
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    I am not exactly sure what happened, but before doing that I hooked it up to the generator again and did a few more starts, proabaly a minute or two in total. Eventually the battery was down to almost 9V but it would still crank normally. I then added it to the charger again and the charger showed 40% empty and charged it back to 100% over 2-3 hours. Its been resting for a couple of hours now and staying strong at 12.9V. Running it more has kind of "unkinked" it. Jul 2 at 18:24
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    Also now it starts the generator like there is no tomorrow. How weird… Jul 2 at 18:37
  • @YanickSalzmann I'd guess it was a combination of the battery sitting on the shelf at the shop for months and they didn't do any topup charges, AND you cranked the engine over on the battery. A smart charger should continue to trickle charge or "maintain" the battery if left connected overnight.
    – Criggie
    Jul 3 at 1:54
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Generally, a 12V lead acid battery going straight from around 10.8V to "fully charged" is indicative of one of the 6 cells being completely dead and acting as a near-short with whatever its internal resistance is. Then the charger's logic sees the remaining 5-cells-in-series reaction to further charging attempts as them being all the way charged - often the dead one will momentarily also take a charge, giving the charger a phantom 13+ volts that drain immediately back to 10.8 as soon as a load is put on the battery or even after just sitting a few minutes.

You should insist the seller take back the battery as defective and get you a new one.

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    He's talking about an AGM battery, not a lead acid battery. The design of AGM batteries does not lend to a short like you are describing. Jul 3 at 11:06
  • @Pᴀᴜʟsᴛᴇʀ2: AFAICT AGM is lead-acid, just a specific type of lead acid. Jul 3 at 14:49
  • They do use lead and acid, but they are built and put together completely different. Jul 3 at 15:47
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    It's true that AGM batteries don't usually develop shorted cells by themselves, like normal lead-acid batteries do. But a manufacturing defect can still result in a short.
    – jpa
    Jul 3 at 17:16
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You are measuring the voltage incorrectly.

The proper way to measure voltage is not to apply a huge current (starting a generator), for multiple times, and measure the voltage immediately after that.

The proper way to measure voltage is to let a battery rest several hours without applying any load during that time, and then measure the open circuit voltage at rest.

If the battery reads 10.8 volts at rest, it doesn't have a good charge. But only if it reads that at rest, if it reads that after a huge current, and then waiting only a minute, then it's perfectly possible the reading you will get is skewed.

A second problem might be that the battery is quite small. Only 8 amp hours. A car battery is probably around 50 amp hours. If you apply a car battery charger to it, the car battery charger could have too high current at which it transitions from absorption charge to float charge. Ideally battery chargers would let you specify these four parameters:

  • Bulk charge current
  • Absorption charge voltage
  • Current at which you transition from absorption charge to float charge
  • Float charge voltage

...but unfortunately even in best cases you may be able to set just the voltages not currents, and in worst case there will be few preset settings and you can't freely determine the voltages even.

If the charger read 0.2 amperes, and then immediately concluded the battery is full, it sounds like the charger concluded the battery is full due to low current. The current may be low due to two reasons: (1) the battery may be old even though it was sold as new, it may be partially sulfated, reversing that requires little current but a lot of time, (2) the battery is small, so small it wouldn't let much current through even if in optimal shape.

Have you measured the voltage of the charger after the charger concluded the battery is full? Some chargers have a float charge, others don't, if yours has a float charge, then it may just take keeping the battery for few days in that float mode. If the charger is such that it makes several "pulses" after concluding the battery is full to keep the battery full, you're out of luck as the pulses may be optimized for larger good batteries, and a small battery that hasn't seen a charge in last 6 months wouldn't take so much current in a pulse to make the charger conclude the pulse is still ongoing.

If you have access to a lab power supply, you could set it at 13.6 - 13.7 volts and leave the battery attached to the power supply for multiple days.

It isn't impossible the battery is bad, but you don't yet have enough evidence to claim it as bad.

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