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An Idea How to Measure the 12V Battery Capacity

SageBrush

REJECT Fascism
May 7, 2015
12,241
15,164
New Mexico
My Model 3 LR 12V is about 2.5 years old, and I know from monitoring the voltage that less than a day passes when the car is sitting at home before the battery drops to ~ 12.6v, which leads to a charging session. I suspect that my relatively high vampire drain is related to these 12v battery charging sessions.

I don't feel like swapping out the 12v battery blindly so I've been thinking about a way to check its capacity. The method follows. I have not tried it yet, so this thread is meant to elicit comment and advice.

Tools
Scan my Tesla (SMT) App. It reports current flow into or out of the 12v
Battery Monitor. 12v voltage monitor, logged every 2 minutes

Method
Catch the 12v when it approaches 12.6v
Use SMT to log current during the charging session
Read Battery Monitor to know how long it took to reach 13.1volts
Calculate Wh added to go from 12.6 to 13.1v, which I presume is ~ 50% of total capacity

I'm OK with +/- 5% inaccuracy. Will it work ?
 

Tz00

3 LR AWD
Jan 5, 2019
277
528
NB, Canada
Read Battery Monitor to know how long it took to reach 13.1volts

It will reach 13.1V within a few seconds, as it is charging, and the voltage is always higher. Here's a graph showing the voltage when the car is woken up. Sample rate of 5s.

Just take the battery out and put it on a carbon pile load tester.


Screenshot_20201115-232454.png
 

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SageBrush

REJECT Fascism
May 7, 2015
12,241
15,164
New Mexico
It will reach 13.1V within a few seconds, as it is charging, and the voltage is always higher.
Yep. I expect the end of charging will be evident by either the SMT logging shutting off, or the Battery Monitor showing the voltage start to drop
 

cdiggs

Member
Jun 21, 2019
7
3
VA
As to measuring the battery's capacity, that can be done with a multimeter or a car battery tester. Some of the big auto parts stores have car battery testers in their free loan a tool program. An Internet search for "test a car battery" will tell you how to use any of these devices and how to tell if your battery can't hold a full charge anymore.

Because third party apps have been reported to have their own vampire drain problems from time to time, may I suggest you start by turning off that app for a day or two and see if the battery capacity drops at the end of that test period. That would help isolate a vampire drain problem to one not caused by a third party app (and would be covered under warranty) or one caused by the SMT app.
 

SageBrush

REJECT Fascism
May 7, 2015
12,241
15,164
New Mexico
Hi cdiggs,

My Battery tester draws 3 mA, and the vampire drain has been going on way before I started monitoring.
I'll look into the store offers. The last time I checked, they measured volt and amps, but this is only an immediate reading and cannot be used to measure capacity
 

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