Mirai weak battery flickering lights

I had a similar issue with my 2016 Volvo XC90. The symptoms sound very familiar. 'Weak battery performance when starting' means the battery isn't holding its charge as it should, possibly due to sulfation or internal damage, but in your case, a faulty sensor could be giving a false reading. Similarly, 'flickering lights' often indicates an unstable voltage supply. Low voltage or fluctuating power can cause lights to flicker. Was the voltage regulator ruled out? Before anything else, you should check the battery voltage with a multimeter. What voltage are you getting when the car is off versus when it's running?

Voltage off is normal, around 12.6V. When running, it fluctuates, dipping as low as 11V at times. Does that match your Volvo experience?

Yes, those voltage fluctuations sound exactly like what I experienced. That definitely points to either a failing battery or, more likely in your case given your suspicion, a defective voltage sensor that's misreporting the battery's health. Since the voltage drops significantly when the car is running, the alternator or regenerative braking system might not be charging the battery correctly due to bad sensor data. I recommend taking it to a trusted workshop for a diagnostic check. They can accurately determine if it's the sensor and prevent further problems.

Thanks. Defective battery sensor confirmed by the shop. Repair cost 315€. Replaced and all good now.