I recently purchased a 2012 Volvo S60 T5. When I shut the car off and left the radio on, after about 30 seconds I got a low battery warning and the system shut down in 2 minutes.
I removed the battery and installed a battery from another car. The battery is the same size (H6, group 48), and is less than a month old. After installation, I left the radio on for 5 minutes and thought the problem was solved. After driving the car for 30 minutes and then shutting it off, I got the low battery warning again after about 30 seconds. I drove the car a few more times and got the battery warning every time I shut the car off and waited 30 seconds.
I purchased a new larger, AGM battery (size H8, group 49). Just like with the other battery, there was no battery warning until I drove the car, then the problem returned.
I checked the voltage with the car running and followed the voltage drop after turning the car off with the radio on. While running, the alternator supplies 14.6-14.8 volts. After turning off the car, the low battery warning came on when the voltage dropped to 12.6. If I disconnect the negative terminal, then reconnect it, I don’t get the low battery warning until I drive the car and then shut it off.
Does anyone have an idea what would cause this?