Chevy Bolt EV Forum banner
  • Hey Guest, welcome to We encourage you to register to engage in conversations about your Bolt.

Best way to check 12V Battery?

12592 Views 53 Replies 17 Participants Last post by  240vPlug
It's becoming apparent from posts on this forum and others that the 12V battery is somewhat of an Achilles heel for the Bolt. I know that lead-acid batteries have a finite life, and it seems that when they give up the ghost on Bolts they can create a variety of problems.

My question: What's the best way to monitor the battery status besides waiting for a problem to occur (which of course, will be at an inconvenient time). A routine check with a multimeter? I know that the Bolt system tries to keep the battery charged, so a good reading may be inaccurate on a weak battery. I could "load test" the battery, but would probably have to disconnect it to do that. Your informed opinion would be appreciated.
  • Like
Reactions: 1
1 - 9 of 54 Posts
This chart shows SOC open circuit voltages for a typical AGM 12 volt battery. A tenth ofa volt is a big deal. A 12 volt accessory battery sitting at 12 volts for any length of time is losing capacity due to sulfation. Unlike lithium ion batteries, lead acid batteries must be maintained at full charge to maintain capacity.


See less See more
  • Like
Reactions: 1
This image shows how the Bolt maintains the 12 Volt accessory battery when left idle for 3 days or more and unplugged. This is equivalent to no 12 Volt accessory battery maintenance at all. Allowing the battery to fall to 12 volts with no attention is a formula for Sulfation and lost capacity. Also Charging for 15 to 90 minutes from that Level of depth of discharge is most likely inadequate. For those who drive and charge daily this will not be a problem.


See less See more
Don't forget this part: As long as the HVB is at 40% SOC or higher.
As stated in the image. In this case no action is taken for the 12 Volt accessory battery. It is left to discharge Endlessly. This is an even worse case than the minimum attention given to the 12 Volt battery when the traction battery is above 40%.

This is an out take from my post (#16) You should read all of it.
I don't think you're testing it properly because you're trying to tell
me something that's not possible, unless the car has an issue.

The Hybrid/EV Powertrain control module (HPCM2) will check the 12V battery every 4 days (2.5 to 3 days) and if the voltage is below a threshold of 12.0 may activate battery maintenance. If the high voltage battery state of charge is greater than 40% and the propulsion system is not active, Hybrid/EV Powertrain control module (HPCM2) will send the voltage set point to the engine control module (ECM). The engine control module (ECM) will send this to the 14V Power Module. Battery maintenance mode will charge the battery for 45-90 minutes..
What have stated in my comment that is not possible?
What do you understand will be the OC voltage and the state of charge of the 12 volt accessory battery in the Bolt if left unplugged for a week?

My understanding is that regardless of the SOC of the traction battery, that given enough time, the OC voltage of the 12 volt battery will drop to 12 volts and a 30% SOC before remedial action is taken.
So you don't drive the car ? Unhook it and charge it with your battery tender.
I don't understand why you would even own a car if you never drive it.
You say "not possible" without saying what is possible. Answer the question!

What do you understand will be the OC voltage and the state of charge of the 12 volt accessory battery in the Bolt if left unplugged for a week?

If you agree, say so. To disparage, discredit and be evasive regarding facts does not serve the interest of the group.
The 12 volt accessory battery is problematic in all modern automobiles whether electric, hybrid or piston. The Bolt is no exception. A modern car parked at the airport or elsewhere for days at a time will experience 12 volt battery discharge due to paracidic loads.
12.4 to 12.6 if you start with a 12.6 or better SOC. I don't think you're testing your battery properly to get the reading you claim.
Good guess for moderate temperature where thermal management is not in play. Even so you ignored the SOC part of the question.

At 12.4 - 12.6 volts the 12 volt accessory battery is at 50% -75% SOC. Sulfates at this level will harden and crystalize. Once crystalized, no amount of charging caan reverse the process. Capacity will be lost.

FYI, I use a Fluke with two decimal digit of precision to test OC voltage after 8 hours of rest.
1 - 9 of 54 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.