My XJR6 battery died the other day. It’s the end of summer here. My car is a daily driver.
I was overseas in November for 6 days and when I got home the battery was flat. I jump-started it, then properly charged it overnight with a charger. Behaved all summer.
Then last weekend I drove the Daimler for 3 days for a change, and the XJR was totally dead when I got back in it after 72 hours. So I bought a new battery, a big sucker as we all know that’s necessary. I replaced the old 780CCA battery with the same. The old one was only 5 years old.
Anyway, it’s all good again, but I have been watching the voltmeter and cross-checking it with the ECU volt measurement using my Kiwi3 OBDII reader.
The dashboard voltmeter is surprisingly accurate. Mine over-reads by about 0.2V compared to the ECU readout. It will typically charge at a couple of different voltages, 14.0, 14.5, 14.8
So I think it was just a 5 year old battery that was the issue. I think the alternator is fine.

A sign, in retrospect, that the battery was failing is this: for the past few months I’ve noticed that once the engine has started, the dashboard voltmeter slowly rose to about 13-14 V over about 4 seconds. Now it has a new battery, it swings up to 14V + in about 1 second.


ECU reads 14.6V when the gauge says this...