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The OEM battery in my 2008 Jetta SE 2.5L has been nearly fully discharged one month back (had insufficient surface charge after a prolonged use without the engine running, but leaving it idle for 20 minutes made the car start again), then worked fine for a few days afterwards (but without being subjected to much charge), but then was discharged again after only a quarter of the prior abuse (since it was probably not fully charged since the prior discharge, me underestimating the driving time it would take to bring the battery back to fuller life, plus possibly missing out on water, as I found out later).

Subsequently, on I-40 in Arizona in November, it has failed on me several times, with 7°C being the ambient temperature. In New Mexico, I added some water to it, 20ml to all cells for prophylactics, and 70ml to the nearest-plus cell to bring to meniscus, 30ml to second, none to the middle-ones, and 20ml to the nearest-minus one, but it still failed to contain much charge after some casual city driving. Subsequently, after a 12-hour drive at -4°C and a 3-hour drive at 10°C, it did let the car start two days after the last drive (yeay!), but still failed the next day afterwards (the starter would work just fine, but the voltage would drop to a level that the electronics seem to find unacceptable, so it couldn't start by itself).

Finally, the battery has been subjected to several days of charging at a stationary smart charger, with the ambient temperature being between 8°C and 25°C. It has then been driven for a couple of short trips, then sat idle for 5 days, and read 12.1V on the fifth day of being left undisturbed, and being able to start the car just fine.

It's still working fine a few days afterwards (but without being subjected to the extra abuse). The starter is not extra quick, but there are no signs that the power drops below the level required.

Has it been resurrected? Is measuring 12.1V after 5 days of disuse at 10°C to 25°C a good indication that it has at least some 40% (source 1: 12,0V @ 25%, 12,2V @ 50%) to 50+% (source 2: 40% is 11,96V, 50% is 12,10V, 60% is 12,24V) of its original capacity available for full use?

  • I'm not too sure about your source. A brand new fully charged battery will have 13.1vdc (or maybe a little more). If my car battery were at 12.7 fully charged, I'd be at least thinking about a new battery. It sounds to me like your battery only keeps a surface charge, which might allow you to start the vehicle, but ultimately fails when you need it most. – Pᴀᴜʟsᴛᴇʀ2 Dec 8 '14 at 22:23
  • @Paulster2, surface charge doesn't stay for 5 days at 20°C; on the other hand, your 13,1V mention looks more like a surface charge voltage to me -- do you have any reference? – cnst Dec 8 '14 at 22:36
  • Seven or eight years is a good life for a battery. If you're concerned about getting stranded or being late for work replace it. – Mysterfxit Mar 10 '16 at 3:33
  • @Mysterfxit that would depend on the conditions it is used in. Where I live 5 years for a battery is a really good result. No one is surprised if after 2 years a new one is needed. – I have no idea what I'm doing Mar 10 '16 at 8:33
  • for the curious -- the battery is still going strong. didn't require any jump starts in months, even with the winter weather here in central texas reaching the freezing temperatures overnight once or twice! – cnst Mar 10 '16 at 20:12
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The correct way to charge a battery is to apply a charging current of around 1.5-2 amps to a maximum electrolyte specific gravity, then to fully discharge the battery at a very low 1 or 2 amps, then to re-charge the battery at a low amperage again, then discharge at a very low amps again. This process should be repeated until the electrolyte specific gravity is at its maximum over three one hourly readings. Having been discharged and left discharged though, active material will be lost and fall to the bottom of the battery case and the battery will never again reach its peak performance. It is more the case of replacing the battery when weighed up in the light of time and costs.

  • A "controlled overcharge" (equalizing) can drive sulfation off the plates, but starting batteries can be too flimsy to stand for much of that. I know that if a car battery is discharged even once, a lot of its lifetime has been lost. Batteries are not that expensive, compared to a tow or other bad consequences, so I agree with you. – user15009 Apr 13 '16 at 20:29

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