TL;DR: Is it ok to leave the blower wiring intact/connected and splice a temporary short from PWM to NEG? Does this power it from the 12V battery, and should I connect a trickle charger to the 12V to maintain it during the charge if this is the case? OR is it better to disconnect the fan and connect the 12V rail from a PC power supply separately (assuming it can provide 100W on the 12V rail)? Either way when charging in-vehicle is there any other component that needs to be disconnected?

Just some history: Just coming back to this after about 5 months. Over the first week I did notice some slight degradation, but assumed that was because the battery was never back to 100% actual charge, only the 80% upper limit. The battery has been performing well with the noted exception of when this last week we did a lot of ~5 mph driving which depleted the battery to <10% SoC. Since we weren't just idling or above 15 mph, it never triggered a forced recharge while the SoC was so low. Soon after it threw a P1433 code and IMA light came on. I was not really paying much attention to the SoC until the light popped up, but its pretty clear what caused it. At such a long time at a depleted state some of the cells were probably a bit lower than others. Hopefully no significant permanent damage caused. After a longer drive at highway speeds the SoC is back to full and the IMA light is off but the P1433 is still there (haven't tried to clear it yet). Needless to say I probably have less than optimal balancing right now, so I want to setup an in-vehicle grid charge since I don't have nearly the time as I did last time.

Within this forum post 'SoC' seems to be a reference the dash gauge (~20-80%) rather than actual charge (0-100%). This is how I used it here, please correct me if I am wrong.

 

FWIW, SoC is an absolute term even to the car. The car tracks/report (through OBDII, CAN bus or proprietary means) true SoC, but it doesn't display it. The battery gauge on all cars is nothing more than an approximation of where the car thinks the SoC is relative to the usable capacity, not absolute capacity.


Many cheap 8A+ 12V power supplies on Amazon for ~$20.


IMHO, optimal would be to splice in a connector routed to the trunk. The connector would not alter the circuit. The PSU would have a connector that has (-) to both (-) and PWM lines with (+) to (+), so that when the PSU is connected, the PWM is grounded. When the PSU is not connected, the circuit is unaltered.


Schools of thought vary, but I recommend disconnecting the 12V battery entirely to wipe codes and de-electrify the entire car.