Newer battery drained down to 9V - What should I expect.

Question

My kid took a lunch brake and ran the AC without starting the engine. After work, it (obviously) didn't crank the engine. My OBD dongle indicated 9V at the OBD port. It jump started easily BUT I am concerned about the long-term effect on the 8-month old Toyota True Start battery. I think it's made by Johnson Controls.

Anything I should look for on this young of a battery that took that type of abuse? We're in Southern California, near the ocean, so it's not going to see too many sub 40 degree nights...

***As a side note, I didn't know about the AC until I got on site, I was worried it was the newly installed Viofo front and back dashcam system that is set up for parking mode. The low battery had an interesting effect on the system and I had to disconnect the power so that it would start up again. The "mic" light was on steady, I guess to indicate that the low power protection kicked in...

Justin Shepherd · Accepted Answer

A single deep discharge that is corrected quickly may not hurt the battery long term, but it depends. When it's significantly discharged and left that way, the lead/ sulfuric acid reaction that produces electricity is difficult to "undo" because of the amount of sulfating that happens. It essentially crystalizes and does not revert back to lead and acid when you apply a current (alternator, etc). This is partly what causes batteries to wear out, especially in old peoples' cars who make a two mile grocery run once a week. You may want to take it out of the car and stick it on a battery charger to recharge it quickly as opposed to using your alternator after it was jumped. Most of those will tell you right away if the battery is bad or not. A battery normally is 12.6-12.7 volts fully charged, not 12 volts as the label says. A battery reading 12 V exactly is still significantly discharged, and will strain your alternator -it's designed to fix shallow discharges from starting and to run everything electronic, not recharge a deeply discharged battery and power everything.