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Wife's 2018 Mercedes GLA250. Car is 3 Years and 4 months old. Just turned over 12k miles. The AGM battery is at 12.25Volts after sitting unused for a week. Everything I read says an AGM battery should have a residual voltage of 12.8v ~ 13.2v. Car fires up and runs just fine with an initial charging voltage of 15.2v which drops back to around 14.2v after twenty minutes of driving. A lead acid battery is only 50% charged at 12.25v but there is a bunch of contradictory numbers out there when you search for similar info on AGM. Would really welcome some comments from folks who can forget everything they know about lead acid batteries before answering.

One other thing. The car lives in high ambient temperatures (Palm Desert, CA) Thanks much. Kiwipop

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  • Welcome to Motor Vehicle Maintenance & Repair! I found a source which states what you are saying about the 12.24vdc being only 50% charge. I don't know the validity of it, though. I'd like to find something a little more "industry standard" which states this. I did notice on the page it also states this number is at 80°F and to reduce the voltage by 0.024vdc per 10° of temperature drop (ie: at 40°F, you should subtract 0.1vdc from the numbers given). Anyway, this just really does not seem correct to me. To me, if it is only 1/2 charged it would be 6vdc, but that's my linear thinking.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Nov 16, 2020 at 22:58

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An AGM battery is a lead acid battery, it's best to not go too far down the rabbit hole of comparing a regular liquid electrolyte battery to AGM, there's no practical difference when you are measuring voltage/state of charge. There is tons of misinformation floating around the web once you start reading about AGM's.

Measuring the voltage of a battery only gives you an approximation of the state of charge, and it is not reliable if the battery is under load. A battery in a modern car is always under fairly significant load, with the various computers and wireless systems on standby. This is really important, as long as you are measuring voltage while under load, you aren't getting an useful window into state of charge.

To get an accurate sense of your batteries condition, go for a decent drive (to fully charge the battery) then disconnect the negative terminal shortly after parking. Leave it disconnected for at least 20 minutes, preferably longer, then measure the voltage. It should be holding above 12.7 fully charged with no load. At this point you could compare your reading to charts that indicate state of charge by voltage, but ultimately if it's not holding a full charge without load, it's no longer in good condition.

Be sure to check your owners manual or otherwise read up on the ramifications of disconnecting your battery if you don't know them offhand. You will probably need one or more codes to re-enable your radio and other electronic in-dash, alarm, etc.

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  • Cameron, Thanks for your well written and thought out reply. Residual current draw is only .089Ma after all the computer systems etc. have settled down. The SAM's (Signal Acquisition Modules) take a very long time to go to sleep on these cars. Acceptable residual draw is less than .250Ma so .089Ma is well within specs. I am not real keen to open the circuit as this vehicle autolearns transmission shift points etc. But, that being said, I may well take your suggestion. KIWIPOP
    – KIWIPOP
    Nov 17, 2020 at 18:49
  • I'd be interested to hear your results if you do. Anecdotally I have seen voltage drops of 0.2v in my RV when running small loads like LED bulbs (~250ma), which surprised me. That still wouldn't pull you from full charge down to 12.25 of course. I'm not totally clear on how the size of the load impacts the voltage measurement. My 2002 Honda Odyssey was recently left to sit, it dropped from 12.7 to 12.4 after a week, but two weeks after that it was still sitting at 12.4 and started fine. You might want to look into "memory saver" options if you are going to disconnect the AGM.
    – Cameron Roberts
    Nov 17, 2020 at 22:03

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