The OEM start battery on my new van is 70AH, 120RC.

I have a 12v freezer that consumes about 3amps @ 12v. It takes the freezer about 6 hours to freeze a gallon of water.

My battery is not a deep cycle battery. but I wonder about what safe and responsible consumption of that 120 minute @25a reserve really is? In theory that means I have about 50AH available before my battery is done.. but I need my battery to start and I also would like to have my battery last 3+ years at least.

anyways... freezer 3amp for 6 hours is about 18AH.

Question 1. Safe to run the freezer for 6 hours and expect to start and not hurt my battery too much?

Question 2. I ran the freezer for an hour and my battery dropped from 12.7v to 12.3v.. but then I ran my engine for 5 minutes and my battery read 12.7v again. My alternator is 160a (2.4L gas 4cylinder engine). Possible my alternator recovered that hour so quickly? If so wonder about the strategy of just checking in on my battery with some 15 minute engine runs a few times during the 6 hours? Note the freezer has a low battery 11v disconnect feature.

Question 3. Wondering about dual purpose batteries rated at 110AH, 205RC with plenty of cranking amps (like 900CCA)… if this fits, any reason not to upgrade to that next time? I've heard of others doing this.


1 Answer 1


Make certain you turn the headlights on for two minutes after "recharging" and before you take a voltage reading. The 12.7 you see immediately after engine shutdown may represent a "surface charge" created by tiny bubbles on the plates due to charging. It's a phantom voltage, and doesn't really represent the state-of-charge (SOC) of the battery. Once the bubbles break up they will not create this false reading.

A fully charged battery should be 12.7-12.8 volts after it sits (surface charge dispelled).

It's difficult to predict if running your freezer six hours will damage the battery. As you mentioned, automotive batteries are not typically designed to deep cycle.

Venturing below 12.0 volts is certain to cause damage. Most of this can be mitigated by immediate recharging. However, extended rest below 12 volts will cause sulphation and chemical migration that cannot be reversed. Below about 8 volts and serious damage will occur. While you may be able to jump start or recharge the battery back to a level that starts the vehicle, you have damaged the battery capacity and lifespan.

Obviously a larger capacity battery would be a good thought, if you can find one that fits. Also consider a spiral-cell gel "Absorbed Glass Mat - AGM) battery for automotive use - Perhaps an Optima brand red or yellow top, although there are many brands. This type of battery, although not a true "deep cycle", will take a lot more discharging abuse without damage. The convenience comes at a cost, however.


Here's an article and a diagram (although generic) that might help:

Measuring State of Charge

Voltage vs State-of-Charge

Note that you can multiply the Ah rating of the battery by the SOC % to get a feel of how much "freezer time" you can safely use.

  • Thanks. Okay, I will retest now with your suggestion about surface charge.... Last night three time from 5pm to midnight I was walking over every few hours and the freezer was drawing around 3a and the battery was dropping to around 12.1v and running the engine for just 5 minutes appeared to be taking it back to 12.7v If true I would find it strange that only a 3a load is REALLY consuming 75% of a battery in 2 hours and that my alternator could take it back to 100% in 5 minutes.
    – Hell.Bent
    Commented Mar 2, 2019 at 12:47
  • So this morning walked out to the van and resting voltage was 12.67v .. took a short drive and when I came back was 12.83v.. plugged in the freezer for about 1.5h and it had dropped to 12.32v, ran the van for 10minutes and initially it was 12.87v. Ran the freezer and lights for 2 minutes with engine off and it was 12.65v... Ran the freezer for about another 40 minutes and it read 12.52v. It's a brand new van and brand new battery. Need help making sense of the data and calculating REAL and SAFE available AH in this battery. In my last van had 90AH 140RC battery and was never careful with AH
    – Hell.Bent
    Commented Mar 2, 2019 at 14:51
  • Starting to think that maybe one of those overpriced 200WH lithium power packs might outperform my Automotive starter battery for this application. Ideally I'm looking to Freeze a 1 gallon jug of water and run a 12v 2 to 3a fan. I have 200w 18v 11a solar panel that returns about 5a with really good sun.
    – Hell.Bent
    Commented Mar 2, 2019 at 15:05

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