Earlier this evening, I was leaving for work when I encountered a completely dead 2014 BMW i3. I know from reading the i3 forum that it HAD to be the 12v battery. I called for roadside assistance and listed my car as a “hybrid“ because I do have the gas-powered range extender. While I waited for the tech to arrive, I disengaged the high voltage battery, as I had read about this multiple times on the i3 forum. When the tech got to my house, I told him that I had made the disconnect, and he said that we needed to reconnect it, or the charge wouldn’t “work.” I was skeptical, but I agreed, and we reconnected the 12v to the HV. Prior to the “jump,” my HV battery was charged to 100%. After he hit the 12v with his battery pack, my HV percentage wouldn’t even show up on the screen. My Rex percentage showed up, but not the HV. Is it possible that the ultra high voltage from his battery pack knocked the charge out of my HV? Could it have done more extensive damage? (Dear God, I hope not!) The car would only stay on for a few seconds, and then shut off again. So, I plugged the car back in to charge the HV, and I ordered a trickle charger to put on the 12v again tomorrow.
I’m just looking for someone to reassure me that there is some sort of safewall that would not allow my HV to be “overcharged“. The 12v doesn’t feed the HV directly, correct? Like, that massive current wouldn’t travel from the 12v through to the HV? This is a 2014, so my highest charging level is two. I should have insisted that we at least try the jump without the batteries connected, but I didn’t want to be contrary.
Please tell me I didn’t kill my perfect car….
Let me start by saying I’m not an expert on the specifics of your vehicle. But Your HV was most likely not engaged. EVs default in a state of HV open circuit (contactors open) and it’s extremely difficult to force it to close the circuit with all of the software safeguards in place. It seems like there’s something wrong on your 12v system which is sending error messages preventing the car from starting. That being said, it’s extremely foolish to jump an EV 12v battery rather than replace it. it’s function is akin to a backup battery in a PC or those old school calculators with a AA plus coin battery, you know? If it were dying, you’d probably notice behavior earlier. They aren’t used to start cars any more, but to keep them alive. Anyway, take it to an expert. Not a general tech or roadside service. Either the OEM service center or somebody with proven success on your vehicle. EVs are not yet at a place where the common Joe schmo can try to service themselves. This is proven by your experience and the lack of understanding in the tech. Good luck!
Have had to jump an EV several times what with the Kia I had around COVID time mixed with not driving anywhere leading to frequent flat 12v battery. Jumping a EV is both normal and described in the manual, and a flat 12v as a one off does not indicate it needs replacing.