[Software Glitch] Switching from Accessory/Service Mode to "Start" Bricks 12V System Causing 100A+ AGM Battery Drain

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Well-known member
Aug 20, 2020
Burnaby, BC
Hello, I'd like to share with you a rather alarming event that happened a few weeks ago.

After leaving the car in Accessory mode for my passenger (for HVAC and radio) to return some hardware at the hardware store (for maybe 5 minutes), I returned and "started" the car only to have the entire 12V system rapidly shut down. This includes the interior lights which dimmed considerably, the infotainment system, HUD, wipers, power windows, power locks, everything that draws from the 12V DC-DC converter and lead-acid cell ran sluggishly, as I know they were operating in an undervoltage state.

I was notably confused, and after attempting to cycle power using the start button, operate power windows and locks in the car, I started to realize that I was not going to be able to restore functionality in a normal way.

I keep a leatherman multitool in the car, and turned to it to loosen and disconnect the negative terminal from the car. A ratchet and socket set would have been preferable but the leatherman was all I had. The terminal post of the battery was extremely hot and the cable insulation was quite warm. This indicates some kind of anomalous short-circuit condition that dropped the lead-acid battery to low levels. Unfortunately, I didn't have a multimeter, clamp meter, or thermal camera on me at the time so I can only indirectly infer that the short-circuit current was around 80A-200A, battery post temperature felt like 70°C (158°F) and battery voltage under load probably less than 8V (enough to run computer and dimly illuminate stop/start button).

After disconnecting the battery, waiting 30 seconds, then reconnecting it, functionality returned to normal and there have been no issues since. With the help of other owners, I would like to ask if you are able to reproduce this issue.

Summary of the problem: When attempting to "start" the car while in Accessory Mode, a (2015) Spark EV becomes completely unresponsive to driver/user action and the 12V battery begins to rapidly drain itself. During this state, the car is unable to be driven along with the use of any of its features (OnStar not tested).

Expected result: Leaving Accessory Mode and entering Drive Mode (start the car).
Actual result: Total system failure. Control module relays unpowered (disconnected from high voltage battery pack), 12V system enters undervoltage state and short-circuit condition observed.

Steps to reproduce the problem:
  1. (may or may not be a factor)Have the car at 50% SOC or lower.
  2. Have the car in Accessory Mode (or Service Mode) by holding the Start/Stop button down for 10 seconds.
  3. (may or may not be a factor) have the car in this state for 0-5 minutes with radio on and some other 12V loads like heated seats and/or HVAC.
  4. Open and close the driver's side door twice (may or may not be a factor).
  5. Turn the car on (Drive Mode) by placing foot on brake pedal and pressing the Start/Stop button once.

Steps to resolve the problem:
  1. Release the hood of the car from its retainer by pulling on the lever between the driver's side footwell and the door.
  2. Locate the negative terminal of the battery (clearly visible under hood on driver's side).
  3. Using a socket set and ratchet, loosen the nut fastening the battery post to the cable clamp connector and disconnect the negative cable from its terminal.
  4. Wait at least 30 seconds for system voltage to drop. This is important to ensure the 12V system has been properly reset, otherwise the drain may resume in the next step.
  5. Reconnect the negative cable to the negative post of the battery and fasten the nut to secure it firmly in place.
  6. Turn the car on and test all system to see if they are in working order.
  7. Leave the car on for a few minutes to let the DC-DC converter charge the lead-acid cell. Depending on how long the drain persisted, more time may be required to restore lead-acid battery voltage.

Steps to avoid the issue:
  1. Have the car in Accessory Mode
  2. Press the Start/Stop button without touching the brake pedal
  3. Observe that the car is fully off
  4. Press down on the brake pedal and press the blue Start/Stop button to start the car (Drive Mode)

I've tried to reproduce the issue at least 20 times, but only in a single session at 100% SOC. I have not attempted a test at a lower SOC. At the time, the high voltage battery pack was under 50% SOC, which is why I state this as a suggestion in the reproduction steps.

If you own a multimeter, clamp meter, IR thermometer, thermocouple, or thermal camera, please reproduce this problem and take some readings. If you don't own any of these, simply reproducing this issue is of great value.

For those with these instruments, please measure the following:
  • Initial AGM Battery Voltage
  • Initial AGM Battery post temperature
  • Initial AGM Battery cable temperature
  • Initial AGM Battery cable current
  • Anomalous State AGM Battery Voltage
  • Anomalous State AGM Battery post temperature
  • Anomalous State AGM Battery cable temperature
  • Anomalous State AGM Battery cable current
  • Final AGM Battery Voltage
  • Final AGM Battery post temperature
  • Final AGM Battery cable temperature
  • Final AGM Battery cable current
Please also note the times of each approximate measurement with a stopwatch synchronized with the event.

For those with and without these instruments, please measure the following:
  • High Voltage Battery Pack percent state of charge, done by counting the number of battery bars (each bar represents 10%), or via the MyChevrolet smartphone app (must have an active subscription), or via the torque pro app realtime information (need OBD II accessory, don't need the Bolt EV PIDs for the app).
  • Ambient Temperature (found in infotainment screen under climate control)
  • Your model year Spark EV

Final remarks:
I don't actually know which component of the car is hanging and attempting to dissipate all the energy from the AGM, but it may be worth checking if you can pinpoint it easily with a thermal camera (assuming the cables leading to the device and the device itself give themselves away from the heat they generate).
It also may be worth connecting up an OBD II scanner to get realtime information from the computer if it is functional during the anomalous state. You could accurately get power consumption figures and temperatures from different modules if you have the Bolt EV PIDs. You can find a guide of how to get that set up here: https://www.mychevysparkev.com/forum/viewtopic.php?t=5204

Any similar experiences or attempts to reproduce the problem (even mentioning you couldn't) is valuable info and would be greatly appreciated! I'm sure more than a few people use Accessory Mode and would appreciate knowing what to expect, saving them a tow if the battery becomes completely discharged.
Smilies: Thank you for your super-useful post. Evident that you put plenty of work into documenting and organizing it. I don't use Accessory mode, but had the same symptoms of the 12V battery death after charging the car at home. This was last January 2021. I use a regular 120vac wall plug. AAA had replaced the 12V battery just a few weeks before, but had put in a non-AGM battery. (I didn't know any better at the time, but have since replaced the AAA battery with an AGM battery).
Anyway, this morning my normal 120vac charge didn't complete. I didn't know if I should even start the car. My new AGM battery is only 5 months old. I feared I would experience the same 12V battery "death" and not be able to move the car. After reading your post, I started it and am leaving it on for 30 min. or so. As your post instructs, leaving the car on in driving mode should allow the DC-DC converter to charge the 12V battery. It looks normal so far.
I will try driving it and update later. Still need to know why my 120vac charge failed to complete.
I should also mention I am in the Phoenix area, and we are in a rather catastrophic heat wave, with daily highs of 115-120 degrees, and night lows around 86.

I have had a problem with my 2014 Spark EV twice in the last two months that sort of (maybe) sounds similar. In the morning the car is dead. It can be jumped to start, then runs okay. When I got it to the mechanic after a 20 minute drive, it died again. Battery was reading 11.97 volts before being jumped.

I've just reviewed the manual and I see how to put the car into "service mode" which seems like you really have to do it on purpose, not likely a mistake. But it does say the 12v battery will discharge in service mode. The manual also mentions "ACC/ACCESSORY" as a mode the car will go into if power button is pressed but the car is still in gear. That is the only mention I find of "ACCESSORY" -- is that what you did?

I can imagine leaving it in ACC/ACCESORY by accident but we did own the car for 5+ years without this problem with the 12 volt battery.

You disconnected your battery, waited, and reconnected. Was your intent to re-set the car operating system or just stop the drain? If mine shows up dead again I may try that before jumping it.


I read through your post again. You asked for data on battery voltage levels, and car year: mine is 2015. It is an LT, having the fast-charge capacity, which I have used only a few times (it seemed to cause range loss). Note that I didn't have the car in Accessory mode or didn't know it if I did. My situation of "12V battery death" was caused, I assume, by having a non-AGM battery installed about 7 weeks previously by AAA.

1. The first indication I had a problem was getting the dashboard alarm "Propulsion Power is Reduced" while driving, and having the yellow "Service Car Soon" light come on and stay on. I would estimate my SOC to be 50%. I drove home and measured battery voltage at:
12.22V dc

2. I then tried to charge the car. It started charging, but after several minutes it stopped charging. I had the dashboard alarm "Not Able to Charge." At this point, the interior lights dimmed, the dash went almost completely dark. I measured the battery voltage at:
8.6V dc

3. The car sat for three days without my driving it. I then measured the battery voltage again at:
10.78V dc

4. I walked away confused. An hour later, I attempted to start the car. It started normally. I left it on in drive mode for 3 minutes, then turned it off and measured the battery voltage at:
12.49V dc.

Every time I was measuring the battery voltage, of course I was removing the negative cable from the battery, but putting it directly back on after 20-30 seconds. From reading your information I should probably have left it disconnected a longer time. I don't remember the black wire feeling hot. Your information explains why starting the car caused the 12V battery to come back to a normal voltage finally. I drove the car for a number of weeks while searching for the AGM battery that would fit the bracket in the car. It is AC Delco Part# LN1AGM. Another poster on this forum provided me that information (my service shop didn't know). I bought it at O'Reilly Auto Parts and installed in on March 11, 2021. Have had no problems at all up to this morning, when the charge would not complete.

I'm thinking about selling the Spark, by the way. It is a great car, but I worry about driving a car that hasn't been manufactured for years now. A car is always a "wasting asset," but perhaps the Spark EV is especially so.