12 v Battery Drain

BiomedO1 · Dec 18, 2025 at 8:55 PM

pasadena_commut said: ↑

The battery I recently replaced had around 10 Ah of capacity left (measured at close to 25C). The electronic battery testers (mine and one in an Autozone) said it was still good.

It was replaced because, despite the good tests, there were still plausible scenarios where it might not start the car.

Parasitic load in this model is 20-25 mA. Using the larger value 10Ah/25mA is 400 hours or 16.7 days. While I'm not planning on parking the car for two weeks any time soon, it is a possibility (we might take a trip by plane, and we have had vacations that long) and the temperature conditions might not be ideal for battery longevity, it could be hotter or colder than is ideal. So, hypothetically, that battery could fail to start the car at the end of a trip. Even if the car did start, it would have discharged to a low SOC, and that is bad for the longevity of the 12V.

A battery with OEM capacity is 45Ah. 45Ah/25mA is 1800 hours or 75 days. A full capacity battery can sit for 14 days and it will only use 8.4Ah doing so, and only be down to 81% SOC. That isn't considered a damaging level of discharge on an AGM, although the most conservative suggestions for extending battery life sometimes put the cutoff as high as 80%.

Most of the mobility batteries people have installed have at least as much capacity as the OEM one, many claim to be 50Ah or 55Ah. On the other hand, probably none of those batteries can deliver a high discharge current for a load test.
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1. You used one of those random number generator battery tester; those numbers are derived from a 3-5W load resistor within the unit.
2. It showed you 10Ah and you seem to know what an OEM battery should be, so how did you reconcile that one and how did it fool you????
3. You didn't use a carbon-pile load tester - Even the cheapest carbon-pile load tester would've failed that battery. No calculations needed.

enough said; I'm done.....

BiomedO1 · Dec 18, 2025 at 8:58 PM

Tombukt2 said: ↑

Tell us what is weird tho I gotta HV battery showing 176V won't start a 07 Prius with all else in car perfect no. Lights etc . Put it in another gen2 starts rt up.
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loose connection or bad ground point somewhere??? Obviously not in the traction pack, since it worked in the other car. I'd be looking at the motor/transmission ground cables.

12 v Battery Drain

BiomedO1 Senior Member

BiomedO1 Senior Member

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12 v Battery Drain

BiomedO1 Senior Member

BiomedO1 Senior Member