2014 Buick LaCrosse Hybrid with 66K miles

My mom purchased this car used from a private owner and has not had any major problems except for the following which started in 2019. I have had it checked out by a local mechanic, (2) Buick dealerships and a Ford shop (because we knew someone there). No one can find a problem and the problem will go away for 12-18 months before showing again. This has led to loss of confidence in the vehicle.

Problem: She used the remote start this morning and the car started without issue. She got caught up doing things in the house, by the time she made it outside the car had stopped running. She assumed the remote start timer had timed out and the car stopped. She used the key to start the car and without issue the car started up. The car is showing message 'Service Battery Charging System' on the dash. The battery light and check engine light will be on. The car will start and run. She was able to drive it to a local mechanic to have the battery tested (it tests ok). She stopped and started the car at the local mechanic's shop without issue. Since no issue was found, she drove it back home. On the way back home, she lowered the passenger side window. When she tried to raise the window, it would not raise. She tried to lower the driver side window and it would not lower. She made it home and turned off the engine. When she tried to restart the engine, she would only get a single click. After a couple of minutes she tried again and the car started without issue, she was able to raise and lower windows and the dash lights for battery and check engine and 'Service Battery Charging System' are no longer on.

This issue has presented itself two times previously, ~12-18 months apart. I have had the car towed into two different Buick dealerships to have the car checked out (along with hybrid battery system) and both places indicate the car checks out. When I had it towed last time, the car would not start for the tow company, but then started at the dealership.

I'm at a loss and about to trade this car in if I cannot find the problem. Any help is appreciated.

 

I would suspect a poor connection between the battery and the vehicle or parts of the vehicle..
Unfortunately, when the battery was checked , they stopped there. The connection could be anywhere between the battery and the rest of the vehicle and will require someone to do some voltage drop testing of the connections , ALL of them ! It could be involving either the ground side or the power supply side.

Because it is an intermittent problem it may take a little longer to pin it down. A good electrical shop should be able to find it without too much problem.