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I got a Stanley FATMAX RE-CHAGEiT 8AMP Automatic Battery Charger & Maintainer from Walmart for 29,97 USD, which appears to have three states of charging the battery -- detecting the voltage, rapid charging at supposedly 8A with the yellow LED, and then maintaining the charge at supposedly 2A with the green LED.

I've tested the voltage with the in-car 12V socket tester in my 2008 Jetta, which did read between around 13.7 and 14.2V when the car was running, but it seems to be only testing around 12.5V when the charger is attempting to charge the battery now (it started with mid 13V when I just got the charger, but after several days of use, never seems to get above 12.7V anymore, even if I turn it off, discharge the battery for a while, then turn it back on). This appears to be the case both when the rapid charge is supposed to be taking place, as well as when the maintenance charge.

Is this normal? Would a sulphated old battery (which only appears to work fine when the overnight weather doesn't get too close to freezing) accept any charge to improve its condition when the charger must only be supplying something like 13,0V at most? Should I get a different charger, one that would supply closer to the gassing voltage of maybe 14,5V?

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You should be checking the voltage directly at the battery. Battery voltage measurements need to be precise to tell if the battery is good.

Open circuit voltage is as follows.

  • 12.66 = 100% state of charge
  • 12.45 = 75% state of charge
  • 12.24 = 50% state of charge
  • 12.06 = 25% state of charge
  • 11.89 = 0% state of charge
  • 10.45 - 10.65 = bad cell, battery should be replaced

It looks like that's a smart charger, if so Voltage measurements won't tell you if the battery charger is working correctly. It may help recover the battery depending on the method the charger "maintains" the charge. There are pulse chargers that are suppose to help with a sulfated batteries, and some of the smart chargers use a similar method when maintaining the battery, some go as far as cycling between charging and discharging once it's in the maintenance mode.

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  • Thanks! Yeah, the latest voodoo science I've applied is added a bit more water to the front cell (somehow it seems like only the front cell gets very dry!), and also drove in Texas hill country -- Austin -- and I haven't had any issues for several weeks now! I'm quite surprised, but I think driving in the hills (to avoid acid stratification) might be one of the things that was overlooked in my prior experiments.
    – cnst
    Commented Dec 30, 2014 at 5:16
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While a normal battery should be around 13.1vdc fully charged, I wouldn't consider 12.5vdc too much to worry about. If you are having issues where the starter is dragging or just plain not getting the car started, then I'd worry about it. I doubt your battery is sulfated. If it is having problems starting your vehicle, then get a new battery, not a new charger. The charger you have will work just fine. Since your charging system (in the car) seems to be working fine, I don't think you'll have any issues there, either. Also, how do you know the charger is only producing the 13vdc you are suggesting? This seems more like a guess on your part. Remember, the lower amperage is where you should be charging your battery at anyway ... charging too fast is bad for your battery.

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    sigh. you didn't read the question, did you? the voltage measured was with the charger connected.
    – cnst
    Commented Nov 30, 2014 at 3:20
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    Not familiar with this charger, but, battery charges fall into two main catagories: Constant voltage or constant current. Later chargers are thought of as 'smart' chargers and act as both by using a conductive test on the battery. Sounds like you have a modern smart charger, in this case the charger is doing its job. Constant voltage chargers operate at a set voltage and the current drops from a high to a low on charging. Constant current chargers charge at a set rate and the voltage drops off. Smart chargers check the conductivity of the battery and adjust to best for that battery.
    – Allan Osborne
    Commented Nov 30, 2014 at 10:45
  • @AllanOsborne ... great followup and addition to this answer. Thanks!
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Nov 30, 2014 at 21:38

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