On Saturday, April 18, 2020 at 10:00:06 AM UTC+1, John wrote:
....before it becomes unusable. It was flat last weekend after 5 weeks
self isolation. I bought a compact booster and started it on thursday, ran
it for about 20 minutes, but today (Saturday morning) it is flat again.
It's a relatively new car 2019 Golf R.

A knackered lead acid battery will say 13 volts when charged,
but as soon as you turn on a light
it will dip to 10.5 volts if one cell is broken,
or 9 volts if 2 are dead

[g]