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

Posted: Tue Feb 16, 2021 12:44 am
by Infinion
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: ... 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.