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Discussion Starter #1 (Edited)
In the continuing saga of my Bolt seemingly randomly refusing to start (details in an earlier thread (Bolt 12 volt system troubles) I would like to see what the typical resting voltage people measure across the battery. Resting voltage means when the Bolt is off and you haven’t had the door open for a few minutes. When you open the door, typical the Bolt momentarily comes to life, turns on some lights, then if you close up it shuts down. If the Bolt is on, the voltage can be as high as 14.8 because the traction battery/charging system is charging the 12 V battery.

Mine seems to typically read 12.20 to 12.50V with some drifting around. Today, I put on a small AGM (Adsorbed Glass Matt) charger on the battery and it showed a low charge state. After a few hours of charging at a max of 2 A, the battery read about 13.6 V just after removing the charger. About 1.5 hours later, it was still reading 12.83 V. Never seen that high on the Bolt in a resting state.

I suspect the Bolt charging system is not working properly. During the last failure to start, the Service Battery Charging System error came up along with the yellow vehicle symbol. In addition the 5 or so failures of this system may have greatly weakened the battery.

In any case, if anyone has data on the resting voltage for the 12 V system, please post it. I will be driving to Fremont Chevrolet on Tuesday, for their third try to fix this.
:nerd:
 

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Yes, I saw 12.2 today and didn't like it. I thought that was too low. I have been measuring 12.4 to 12.6 but nothing higher. I've noted when I put the leads of my VOM on the terminals that the voltage begins increasing by 1/100th. Withing a few seconds the voltage will increase by a 1/10. I am assuming that there's some sensor picking this up and trickle charging while I am testing the voltage.

As noted on a couple other threads here, I measured 3.7 V when the car was dead, dead, dead and I had it towed. They didn't find anything wrong. It worked fine at the dealer. . .

Paul
 

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OK I am a EE and happy to add some data. Car has not been plugged in for about a week. ambient temp 27F garage temp 40F. drove about 10 miles today
just measured 12.49Vdc The Bolt has 5200 miles and was put in service June 5th. No abnormal issues ever observed, appears to be working as designed.
 

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Are you sure your voltmeter is accurate?

I have a license-plate-size solar panel maintaining a second farm tractor that doesn't get much use. A HF free voltmeter, the recent red model, shows 12.56 volts on the battery. Ok, all is well. However: I also have four of HF's earlier yellow freebie voltmeters. All brought home at different times over a few years. One with a fresh battery inside, two have replacement batteries. All four show 12.35 volts measuring the same tractor battery.

I need to step up to better measurement tools.
 

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Discussion Starter #6
OK I am a EE and happy to add some data. Car has not been plugged in for about a week. ambient temp 27F garage temp 40F. drove about 10 miles today
just measured 12.49Vdc The Bolt has 5200 miles and was put in service June 5th. No abnormal issues ever observed, appears to be working as designed.
These numbers are close to what I measure, so chances are the charging system is working in mine as designed, at least most of the time. It may be the battery has been degraded by the earlier problem of the transmission module running it down several times and now as all these automatic actions the Battery Control Module and Hybrid Power Control Module (BCM and HPCM2) are doing it drops below 11 V and all **** breaks loose. Who knows? Or from time to time some module puts a draw on the battery. My car ran without incident from mid January to mid August, when it failed to start and had to be jumped. That time it got down to at least 9 V. Anyway, in principle, all this should be figured out by the technicians, but since they have "fixed" it twice, I am not confident they will get it right. Not to mention the closest dealership that is responsive is a 40 mile trip one way. Luckily, I am retired so I can find time to travel there.
My next step is to connect my DVM to the 12 V auxillary 12 V port inside the car. While it doesn't show the voltage with the car off, it is a direct measure on the 12 V system when the car is on. Just have to make some cables to connect up.:nerd:
 

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Discussion Starter #7
I was reviewed the description of the BCM and charging system supplied earlier by drdiesel and I looks like if the draw on the 12 V battery is enough to pull it down in two days, while the car of OFF and not being charged, this can result dipping into Batter Saver Mode Active and all the problems that follow. I have never had this problem within a few days of when it has been plugged in, probably because in that state the control system is checking on the 12 V system and boosting it up, if needed. If it is off and not plugged in, it can go 2 or 3 days with being checked. Hmmm. The battery may be a 50 amp-hour new, but guessing mine is not that anymore.

"When the vehicle cord is plugged in
  • The Hybrid/EV Powertrain control module (HPCM2) will check the 12V battery every 6 hours if the ignition is off. If the voltage is below a temperature dependent threshold ranging from 12.1 (cold) to 12.4 (warm)V, the 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 2-3 hours. If the Ignition is ON, the APM will cycle on as needed to maintain the 12V SOC.
When the vehicle cord is not plugged in
  • 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..
 

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Discussion Starter #8
Are you sure your voltmeter is accurate?

I have a license-plate-size solar panel maintaining a second farm tractor that doesn't get much use. A HF free voltmeter, the recent red model, shows 12.56 volts on the battery. Ok, all is well. However: I also have four of HF's earlier yellow freebie voltmeters. All brought home at different times over a few years. One with a fresh battery inside, two have replacement batteries. All four show 12.35 volts measuring the same tractor battery.

I need to step up to better measurement tools.
I haven't calibrated my DVM, but it does seem to read correctly on new 9V batteries and also it shows the Bolt charging system goes up to 14.8 V as expected.
 

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Discussion Starter #9
OK I am a EE and happy to add some data. Car has not been plugged in for about a week. ambient temp 27F garage temp 40F. drove about 10 miles today
just measured 12.49Vdc The Bolt has 5200 miles and was put in service June 5th. No abnormal issues ever observed, appears to be working as designed.
Thanks for the information.

The only "external" device I have had plugged in is a old iPhone to the USB port, which I typically leave connected. I assume the USB port is live with the car off, but not sure. I can't imagine its draw is enough to affect the 12 V battery lifetime over a two day period of not driving the car, but maybe if the battery is on the edge. According to the service manual, if the car isn't used, the 12 V system is not checked for two to three days. :nerd:
 

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You need to check the 12V batteries SOC without disturbing it. That means, no key FOB to
active the BCM. Not opening any doors. Also be advised, the hood latch is monitored.
The best way to check 12V SOC would be to drive the car. Park it and pop the hood. Close
the hood latch with a screwdriver and put a rag into the latch and let the hood sit on it without
closing it. Let the car sit for about 6 hours. Leave the FOB in the house and lift the hood.

Hookup the DVOM and see what the 12V batteries SOC really is :nerd:

If the car dies over two or three days, test it as is without activating anything over those
two to three days and see where the SOC goes. If it's keeps dropping off, you have a
parasitic load and it's needs to be diagnosed by someone that's qualified to know what
they're doing. Otherwise, you'll be back again and again >:)
 

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You can also measure the parasitic load with a DVOM, but it needs to have the M/A setting and the 10 amp, amperage port to set it up inline with the 12V battery. Fluke DVOM's work best for this type of testing. Most new cars have a 35m/a load limit and some cars can take 1 hour to go into sleep mode.

You CAN NOT disconnect the 12V battery to do this test. If you do, you run the risk of losing the draw due to control module reset. You'll need to be very careful when attempting this test.
I clamp my negative cable on the cable and slide it upwards from the post without losing contact and slip the other clamp onto the batteries post.

Make sure the meter is setup properly before hooking it up, or you'll lose the connection and might never get it to show up. 12V testing is not for amateurs. Most screw things up for us and then complain because we can't fix their car.
 

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Does this mean I don’t need to top off the 12V battery once in a while when I get my Bolt?

For my regular gas cars I top off the 12V battery with a tender once a week. Just trying to be in the safe side since I drive very short distances and don’t give the alternator time to charge the battery properly.
 

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You need to check the 12V batteries SOC without disturbing it. That means, no key FOB to
active the BCM. Not opening any doors. Also be advised, the hood latch is monitored.
The best way to check 12V SOC would be to drive the car. Park it and pop the hood. Close
the hood latch with a screwdriver and put a rag into the latch and let the hood sit on it without
closing it. Let the car sit for about 6 hours. Leave the FOB in the house and lift the hood.

Hookup the DVOM and see what the 12V batteries SOC really is :nerd:

If the car dies over two or three days, test it as is without activating anything over those
two to three days and see where the SOC goes. If it's keeps dropping off, you have a
parasitic load and it's needs to be diagnosed by someone that's qualified to know what
they're doing. Otherwise, you'll be back again and again >:)
Wow! Thanks for that detail. That's what we need because the service departments are not finding any problem--if there is a problem. ;)

Paul
 

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Does this mean I don’t need to top off the 12V battery once in a while when I get my Bolt?

For my regular gas cars I top off the 12V battery with a tender once a week. Just trying to be in the safe side since I drive very short distances and don’t give the alternator time to charge the battery properly.

I have always done my ICE cars every six months. My bolt will be a year old in Jan.
I might check the SOC and consider a balance charge then :nerd:
 

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I'm a long way from being an EE, but I've always heard that once a lead acid battery's SOC hits 11V, it has been damaged. You said yours was at 9V at one time. Is it possible that your battery is now pretty much toast?

Of course replacing it without finding out why it wasn't maintained at the proper SOC will not be a permanent fix, but I think along with all the good suggestions above, you may want to have a knowledgeable service person stress test your 12V battery and replace it if necessary as they are nailing down the rest of the diagnosis.
 

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I'm a long way from being an EE, but I've always heard that once a lead acid battery's SOC hits 11V, it has been damaged. You said yours was at 9V at one time. Is it possible that your battery is now pretty much toast?

Of course replacing it without finding out why it wasn't maintained at the proper SOC will not be a permanent fix, but I think along with all the good suggestions above, you may want to have a knowledgeable service person stress test your 12V battery and replace it if necessary as they are nailing down the rest of the diagnosis.
The battery was tested by the dealer. It's a mandatory prerequisite before replacing any
battery with GM. They use a GM specific charger/tester that spits out the results.
If it fails, they get a code and submit it with the warranty repair order. If it passes,
they can not replace the battery under warranty. These batteries are not standard lead acid.
They're an AGM battery. Still acid, just not the old plate saturation design.
 

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Discussion Starter #18
You need to check the 12V batteries SOC without disturbing it. That means, no key FOB to
active the BCM. Not opening any doors. Also be advised, the hood latch is monitored.
The best way to check 12V SOC would be to drive the car. Park it and pop the hood. Close
the hood latch with a screwdriver and put a rag into the latch and let the hood sit on it without
closing it. Let the car sit for about 6 hours. Leave the FOB in the house and lift the hood.

Hookup the DVOM and see what the 12V batteries SOC really is :nerd:


If the car dies over two or three days, test it as is without activating anything over those
two to three days and see where the SOC goes. If it's keeps dropping off, you have a
parasitic load and it's needs to be diagnosed by someone that's qualified to know what
they're doing. Otherwise, you'll be back again and again >:)
Thanks, drdiesel.
Certainly possible that it is a parasitic load, although seems more random than that. (could be an intermittent parasitic load.) The only consistent behavior is hasn't happened within 3 days of being on the EVSE. I will give the techs one more shot at this on Tuesday, and if they can't find the problem and won't replace the battery, it will be back to square one and I will do a 3 day test of the SOC. After I used an external AGM (2 A max charger), for a few hours the SOC was above 12.6 (T~ 65 F) and even 24 hours later was 12.56, higher than the Bolt ever charges it to. Seems like the GM strategy is to charge the 12 V just enough, but not to give it much margin for error. I drove it around, monitoring the 12 V signal on the interior port and after a short period up to 14.8 V, it came down to 12.68 to 12.72, which is probably too little to put any real charge into the battery. Of course, since I had charged it a day before that was not a good test. I will monitor what it does during the 40 mile trip to Fremont. The last time there, they found the transmission module was not telling the BCM that it was in the driving mode, and the BCM was in the service mode, which doesn't charge the 12 V. That was corrected. On the other hand, I suspect the several severe discharges of the battery have weakened it.
 

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Discussion Starter #19
You can also measure the parasitic load with a DVOM, but it needs to have the M/A setting and the 10 amp, amperage port to set it up inline with the 12V battery. Fluke DVOM's work best for this type of testing. Most new cars have a 35m/a load limit and some cars can take 1 hour to go into sleep mode.

You CAN NOT disconnect the 12V battery to do this test. If you do, you run the risk of losing the draw due to control module reset. You'll need to be very careful when attempting this test.
I clamp my negative cable on the cable and slide it upwards from the post without losing contact and slip the other clamp onto the batteries post.

Make sure the meter is setup properly before hooking it up, or you'll lose the connection and might never get it to show up. 12V testing is not for amateurs. Most screw things up for us and then complain because we can't fix their car.
While I understand to concept, I will refrain from the experiment. Not enough upside, to risk the the downside.:nerd:
 

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Discussion Starter #20 (Edited)
I have set up to measure to SOC with the hood closed. When driving I can monitor the voltage through the 12 accessory port. Basics. The quiescent SOC seems to stay around 12.6 volts over more than one day, indicating no slow drain. The charging cycle when driving is consistent. At turn on goes to about 14.75 V for about 2 minutes, then drifts down reaching about 12.5 and then goes to around 12.7 V at least for some miles. ( I haven't tested it over a long trip). Today, I watched the charging on a 3 mile trip. Everything normal. I left the car for roughly 2 hours and when I returned it wouldn't start. Battery voltage read about 9.6 V, which is what has happened six times now. I had forgotten my phone and was in a parking lot at the Berkeley Marina. Fortunately I had my jumper cables and a nice guy in a pickup helped me jump it. As soon as the voltage gets to 11 or 12, I can "start" the car, the traction battery takes over and runs the voltage up to about 14.6 V. It stayed there for about 4 minutes then came down to 12.74 for the remaining 10 minutes of the trip. I set up my dvm with the hood closed and 3.5 hours later the voltage is 12.60 V at about 50 degrees F. So what is going on.
1. SOC stays steady so no low level drain on the battery
2. The charging system appears to work as designed.
These sudden failures seem random. I am not driving a lot of miles, but if that was the problem I would expect to SOC to drop day to day. The time between the last EVSE charge and a failure has been at least 5 days and other times 7, 9, 10, 13 days. I suppose if it was on the EVSE charger every day, I wouldn't have had the problem, but my predictive powers are suspect at this point. I think this one may even stump Drdiesel1.

Do I have a trick battery??? Taking it to the dealer for try three this coming Tuesday. The last time they said the transmission module was tell the charging system it was in service mode rather than in gear, but they fixed that.
 
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