If my group 48/H6 70AH 120RC Led Acid car battery is at 11.8v in the morning and 12.5v after a 20 minute drive is it bad? My van and battery are only 10 months old but I do frequently use the battery down to 11.5v with some devices. I've never had a problem starting the van.

I'd like to upgrade to AGM with more Reserve Capacity, maybe 160 or even 200, but there isn't much in that group 48/H6 size. I've seen some good Dual purpose batteries in a near size with 210 R.C. but not sure i can manage to fit it.

  • Are you asking if the battery itself is bad, or if it is a bad practice to frequently drain the battery using devices? – GdD Sep 5 '19 at 13:58
  • if it's bad? I know i'm pushing my luck, but i have no choice. Wish i could run a dual purpose deep cycle. – Hell.Bent Sep 5 '19 at 14:47
  • I'd say if your devices are have enough power to leave the battery as it is and get an portable Li-Ion jump starter as a backup, in case your battery is depleted – Martin Sep 5 '19 at 15:02
  • It may make more sense to have a portable power bank for the high draw devices if they are usb charged. – GdD Sep 6 '19 at 7:53
  • This sounds perfectly normal..as mentioned a battery platous at a charge level and depletes very slowly after..11.8v is ample to start..11.2-4v is the floor for a vehicle to start..jump pack or leads are still good to carry..they are very helpful.. – Postaltrash Oct 7 '19 at 20:59

Automotive batteries are designed for infrequent, brief high amp output with minimal parasitic load. Regular use as a power supply battery providing sustained low amp load will lead to a reduced service life of the battery. A better solution is an isolation circuit and a second, power supply type battery such as a marine or wheelchair battery.

The voltage is dependent on the charging rate of the vehicle's charging system and the capacity of the battery, so the minutes required to charge will be a function of these. If your battery goes extended periods without being fully charged, the battery will fail sooner from hard sulfate formation.


I see 2 possible scenarios for the battery voltage.

  1. You have some accessory that is pulling a small draw consistently. 0.7v is not a huge amount, but that not very little either. So it is possible that something is drinking all night and bleeding that voltage off.
  2. Depending on how you're getting the 12.5v reading, that may not be where the battery is stable, i.e. the battery may not be holding all the juice needed, and may be shedding the amount it can't store. If one cell is not working properly, that could account for the loss of storage.

In my experience, once you remove a battery from a charge cycle it will gradually reduce voltage until it hits a stable point. So it's possible the 12.5v is before it has stabilized, in which case you battery could be a little weak. But it's also possible there is some parasitic drain happening. Acceptable drain would be at max 0.5mAh (from what I've found) which isn't much. So unless you have some accessory that is sucking juice all the time, I'd say it's likely (not guaranteed) something is wrong with the battery.

You should definitely check for parasitic drain to make sure (rather than jus buying stuff to throw at the problem). Make sure you do this 1 hour or more after you've stopped running the vehicle. (From what I understand some systems will continue to operate for a bit afterward, so if you check too quickly after driving you will likely get an inaccurate result.)

Final possibility (I think), if the battery is reading 12.5v while running with minimal accessories then it could be a bad/weak alternator. Generally voltage at the battery (after 10-20 min of alternator charging) should be around 14.1~14.5v. If it's more than 1-2v volts lower, then probably bad alternator function (which can be caused by a loose or deteriorated belt - may not need a new alternator) or some hefty accessory usage. If it's more than 14.5~15v while idling then there is a strong possibility the alternator is outputting too much voltage and adversely damaging the battery. In this case you'd likely need a new alternator or a rebuild because I believe the voltage regulator is usually inside the alternator for fullsize vehicles. (Note: Vehicles that have been outfitted for high accessory loud output, whether OEM or aftermarket, may have larger alternators that output more voltage.)

As far as the practice of running it down to 11.5v, like Weather Vane said, there shouldn't be anything wrong with that as long as the process of charge/discharge is gradual.

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