Proper resting voltage for a AGM

[menancyandsam]

So I bought a AGM group 49 Napa Legend battery Jan. 2020. My understanding is that AGM's like a higher voltage then flooded batteries.
When I haven't driven for a while it discharges to about 12.6V. No problem starting but accordingly to this chart, my 8 month old battery is at about 85%. Does this indicate a bad battery or not.?

 

[ramangain]

No

 

[menancyandsam]

Thanks for the fast reply, so 12.6V on a AGM is normal State of Charge.

 

[ramangain]

All rechargeable batteries lose capacity over time. The time to get concerned is when the starter isn't as vigorous when engaged. We can revisit this thread in, say, 2+ years

 

[midniteRNR]

You should check SoC a few hours after driving, not a few days. Our vehicles will consume some level of power when sitting around, due to all the electronics.

 

[MJK]

Short version: No, at 8 months old your battery is almost certainly fine.

Long version: It is difficult to separate the battery performance from the charging system without doing more homework. It is also normal for batteries to discharge due to small parasitic draws when the car is off. So which is it? Alternator didn't charge it to full voltage, or parasitic draw pulled it down while the car was off? Almost certainly the latter in your case, but set that aside for a moment.

Pretend your battery was removed from the car, and properly topped up with a smart charger or similar. Voltage (in your diagram) is still only part of the picture. You also need to know how much current it will discharge quickly at that voltage to consider it 'good' or not, for example to your starter.

Real life example: Yesterday my wife's car wouldn't start after she left the hazard lights on for 20 mins. (Don't ask) Got it home, removed it, charged it, and voltage was good. Car starts, yay. Is the battery good? As it turns out, no. Even fully topped up it was only putting out 340CCAs compared to its rating of 750CCA. So, only a matter of time before she was stranded again. Time for a new battery.

If voltage AND current test OK with a fully charged battery, then the battery is good. Voltage is easy to check. To test CCAs, you either need a tester or you can approximate by checking voltage under load. I have an Auto AL539B for this, but an extra set of hands and a little creativity or a trip to your local auto parts store works too.

 

[e9999]

The resting voltage is also a function of the temperature.

 

[wapitimud]

You are using this AGM for accessory electronics, with a master disconnect switch? Rather than a vented conventional battery that you use for starting? A two-battery setup?

 

[menancyandsam]

I have only the one battery. But thinking hard on a two battery set up to run a frig. And maybe a winch.

 

[wapitimud]

I see the value for an AGM or deep cycle for deep discharges for your fridge or stereo or whatever. For a starting battery, you may have better lifespan with a flooded, vented maintenance type battery for that purpose in your two battery system.

That said, you might not have enough very cold starts there for it to matter either way.