On 22/04/2021 14:33, Dave Plowman (News) wrote:
In article ,
Fredxx wrote:
On 20/04/2021 20:02, Cliff Topp wrote:
All modern cars will have an amount of quiescent current draw to power
things like the alarm, the clock, the radio presets and so on when
the car is parked up and switched off. I've seen it written somewhere
that around 50mA can be considered 'normal'.

My question is - if the quiescent current draw is 50mA (0.05A), how do
I calculate voltage drop per hour?

For instance, if I park the car up at 10pm and the battery is showing
12.5V, with a 50mA draw overnight what will the voltage be at, say,
9am?

This is an article I feel might be useful for you.

https://batteryuniversity.com/learn/...tate_of_charge

Good article that, Fred. I knew by experience taking a spot reading of
lead acid voltage of little use. But didn't know the makers reckoned it
had to be left unused for 24 hours.

Thanks, I knew there was a relationship but thought it was flatter than
the graph in the link.

You can either leave it for a while or apply a load for a short time.

The best way of checking is to pulse a known current and measure
internal resistance. Though that tells you the stored charge rather than
state of charge.