To give you some background I bought this car in June 2021, and the car had 118,00 miles on it. I bought the car for $4200 and the guy that I bought it from told me it was a city service car for the city of Austin. The car was in great shape and there weren't any obvious issues with the car. I took it for a test drive and everything was running perfectly well. I figured that since it was an Austin service vehicle they maintained the car very well. When I took it to get inspected they told me that the only issue with the car was the tires (Easy fix). I even drove it round trip from Austin to the south padre, then Austin to Lubbock (Around a 2,000-mile journey in total) with no issues. Then 1 month later while I was driving the car the red triangle symbol and service soon symbol came up and I had to pull over. Eventually, I was able to get the car back home by jumpstarting the hybrid battery, but the problem was still present. The car was able to drive for a short period before it would break down and I would have to jump-start the hybrid battery. I bought a new 12v battery for the car and that seemed to solve the problem and I was able to drive it with no issue for the next 6 months. Starting in March of this year I noticed that if I leave the car off for 2 to 3 days, I would have to jumpstart the hybrid battery. Keep in mind that it didn't affect how the car drove. As long as I drove it every day (which I was to get to school) the car had no issues starting, driving, etc. It was only when I left it off for 2 to 3 days that I would have to jumpstart it. Then as time went on it went from 2 to 3 days of being off, to 1 to 2 days, then to a day and a half. Now in June of this year, it can't even hold a charge overnight. I have also noticed that the hybrid battery won't even engage when it's supposed to if I'm driving in the city. However, when I'm driving for a long period on the highway, the hybrid battery will work normally until I get off, then it will go back to not engaging at all. I took it to Autozone to test the battery thinking it was the same problem that I had last year, but they told me it wasn't the battery that was going bad but the alternator that was going bad. I know that FEHs don't use alternators but rather DC-DC converters, but I'm not sure if that's the issue. Sorry if this is a lot of information but I just want you guys to understand everything from my perspective from when I bought the car until now. If anyone can guide me in the right direction on how to trouble this issue it would be greatly appreciated.

Also, the MPG has been going down as well. It went from 28 MPG to 23.5 MPG.

 

You need to replace/rebuild the hybrid battery. Classic symptoms.

 

Recommend you get Forscan. DEL_MOD_V value will confirm a failed battery.

These symptoms can also be caused by cooling issues - A/C not working perfectly as well as blend door and fan failures. The battery is essentially disabled beyond starting the vehicle if it exceeds 100°F, and without a perfectly working cooling system, 100°F happens fast. The battery SoC will drift ever lower as it's not charged when hot. Park an old, hot battery at a low state of charge, and it's very common for it to drop too low to start in just a few days.

 

I'll go ahead and test it to make sure. I really hope that's not the case I'm a broke college kid.
Thank you so much!

 

If it's not your A/C improperly cooling as SKeith says, I'm sorry to tell you this but you're describing an HV battery pack that is no longer working properly and needs to be replaced, as others have noted. It's a good deal on the car and the basic vehicle will last a long time but your HV battery has probably bought the dust. Scope out the cooling system first. It's **CRUCIAL**

 

How many volts should each battery cell have? I'm reading around 5 v to 5.5v

 

"around 5V to 5.5V" is horrific.

They should all be the same.

A dead stick, assuming you're measuring all 5 cells, would be 6V.

Given the low voltages, I would assume you're probing through the heat shrink and are only picking up 4 cells. Dead would be 4.8V, low SoC would be about 5.15. High SoC would be about 5.5.

Regardless of the actual measurement, I would not expect more than about 0.05V variation.

What did Forscan say?

 

Wow okay so I measured all of the sticks in one post (26 sticks) and they all read about the same reading 4.6v (with a 0.05V variation).

I used a Forscan before I took apart the battery and it sent me codes about a disabled battery and the battery temp. When I first used the forscan I did a cold start on the car and it was about 106 degrees outside when I did it so I figured that's what was messing with the temperature.

I actually looked at the live reading for the SoC and it started at 30% and it slowly was increasing as I left the car running. It reached 40% after about 10 minutes and thats when I turned the car off.

Lastly I checked the cooling fans and both of them were working.

 

Any error codes? Did you really start disassembly before troubleshooting?

Was your blend door operational?

Is your A/C working perfectly?

 

Yea if you are lucky, you just can't charge because the battery A/C is out. When mine goes out it stops charging or running in electric after the batteries reach temperature. Since you are in TX I bet that happens faster. It could just need a recharge. Mine went for about a year before the level dropped enough again to fill it once more.

Since you have dual A/C you have to fill it, shut off the cabin a/c and then make sure the battery a/c isn't toggling the clutch on and off. It will try to run it for the battery even when "off". Otherwise it needs more freon. The fans by themselves don't do enough. Notice that as it got hotter, you got less and less use of the battery.