Power fluctuation and SOC weirdness

I’ve been having this issue recently where my state of charge doesn’t seem to be accurate, and the car’s power fluctuates strangely. For the past year or two, the power of the car seems to be significantly lower than it used to be, especially when climbing hills. I’ve done all the standard troubleshooting of cleaning the MAF and throttle body, replacing sparkplugs, 12v battery, making sure the cat isn’t clogged, but to no avail. I just assumed it was due to the car getting old (~300,000 miles), until the last couple weeks when I started having the power fluctuation issues. What happens is when the SOC is nearing empty (2 bars), my car’s acceleration will suddenly drop for a second or two, then pick back up for a second or two, then drop, then pick back up. It will fluctuate like this until the SOC is empty, at which point it drives relatively normally again (although with less power of course, since the battery is empty). It drives fairly normally when the battery is above two bars too. It just does this fluctuating power thing when it’s approaching empty. Note that I’m keeping the accelerator steady and the rpm’s don’t seem to fluctuate when this happens.

Also, I’ve noticed that the SOC reading seems to be off or lag sometimes. I will be going up a short hill and level out at the top of the hill, and my SOC shows over half full, then the SOC will keep dropping without me actually using any battery power. Sometimes it will keep dropping even when the display shows that the battery is being charged.

Also, a couple weeks ago I ran out of gas and couldn’t get the car to turn off (at the time my dash instrument cluster wasn’t working. It’s been replaced since), so I had to disconnect the 12v battery. When I disconnected it the SOC showed empty, but as soon as it was reconnected 10 minutes later it was ¾ full, even though the engine had been off the whole time.

The HV battery was replaced by the dealer roughly 3 years ago, so I would hope that it’s not the issue. I wonder if it could be something to do with one of the ECU’s and maybe how the SOC is being read. There’s been no codes yet, but the problem does seem to be getting worse. Any insight would be greatly appreciated.

 

I’ve done some more testing and it looks like my HV battery is dying. A few of the modules intermittently show big differences in voltage, and when the car is doing the power fluctuation thing it seems to be the worst (the voltage difference between max and min is 1-2v). The battery was a new OEM installed by the dealer, but I do a lot of driving up and down really steep hills, and I've put ~150,000 miles on so I wonder if it was an overheating issue that lead to the failure. The cells in the middle of the pack (#7-11) seem to be the worst, which seems to me like overheating since temp is highest in the middle.

I plan on replacing the battery with a new OEM, but I’ve heard of some people using gen 3 or gen 4 cells in their gen 2’s. I’m wondering if using the newer cells might mitigate the overheating problem of the stressful mountain roads around here since the newer cells have lower series resistance and therefore generate less heat. Does that sound reasonable?

Also, you mentioned that the SOC should never drop down to 1 bar, but from what I can remember, the car has always had moments where it dropped down that low, especially when going up mountain passes. Is that not normal? Could that indicate that something is wrong with an ECU too?

I’ve just been using a bluetooth scantool with the Torque pro app (no techstream).

Thanks for your help!