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Does my 12V battery need to be replaced?


Active Member
Oct 22, 2013
McKinney, TX
About 10 days or so ago we removed our MCU so that we could send the Terga board off to be rebuilt with a better eMMC chip. During this process we had the MCU out for about three or four hours and the car was closed up, with only the frunk open.

When we reinstalled the MCU that same afternoon, the 12V battery was completely dead. We had to put a battery tender on it and "jump" th e car so that we could open the charge port and plug the car in. (For those new to the process, the car will operate with a dysfunctional MCU in it, albeit with no "smart" functions like GPS, HVAC control, nav, etc. The reason for putting the MCU back in even though it's missing parts is so that the car will operate as a "dumb" car and will also recharge the 12V battery.)

The car worked fine all week, with the expected limitations of the crippled MCU, of course.

Today, when we removed the MCU again to put the repaired Terga board back in and then reinstalled the MCU, a process that took less than an hour, the 12V battery was completely dead again. Once more I had to attach the battery tender to the front terminals so that the car would awaken enough for me to plug it in for the HV battery to charge the 12V battery.

This afternoon and evening I've taken the newly repaired car out a few times and it has performed perfectly. Never during any of this have I received any warnings about the 12V battery but I'm now concerned that it dying in less than an hour means that it's about to fail completely and I should replace it.

What's the consensus? Does a 12V battery dying completely when the MCU is out for less than an hour mean that it's time for a new 12V battery?

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