Thread: Charging third battery via starter battery

Originally Posted by drivesafe

Hi Aidan, and I’m taking a guess here but when your father-in-law got home, he probably took a while to unpack the D4 and as such, had doors open and there would have been a fair bit of cranking battery capacity used while the doors were open.

As a result, the next morning the D4’s BMS, after monitoring the battery once the motor was stated, set the voltage at a higher level, that the BMS deemed would replace the used capacity as required.

First off, thanks for sticking with me on this journey! I do appreciate it.

That could be the case, that the starter was drained, I'd have to ask him. However when we were camping he had the back open a lot (the fridge was in the back of the car) and I even commented that the cabin lights seemed to be on a fair bit. This didn't seem to make the alternator voltage increase.

One thing I guess he could keep an eye on is the battery voltage before he starts the car, to see if a low initial charge on the starter leads to a high alternator voltage once the car is started.

Originally Posted by drivesafe

If your father-in-law wants to continue his experiments and get even higher voltage readings, put a small load on the cranking battery while the motor is off, like a compressor fridge.

Leave the load on the battery till the voltage is around 12.2v and then go for a drive.

Don’t be surprised if the readings go up over 15v.

Good idea. I'll see if he wants to experiment more!

He just got the D4 serviced, and asked about the low(ish) alternator output. Their only response was that as long as it was 13.5V or above there was no problem.

Thanks again