On 21/04/2021 11:19, Dave Plowman (News) wrote:
In article ,
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?

Just what a battery reads voltage wise (to decimal points) has a lot to do
with the individual battery. You can work out how long the battery will
last by its capacity in amp.hrs. And knowing the quiescent load. Using
that, discharging by half seems to be the maximum that will allow the car
to start OK. As a very rough guide.

But the capacity of a battery deteriorates with age.

All true, and it also depends on how the voltage is measured, and the
type of battery.

My car has automatic engine stop when the footbrake is on and the car is
stationary. My understanding is that the battery is specifically
designed to cope with repeated starts. There were a number of conditions
where that would not apply (such as A/C running), but also where the
battery capacity and/or voltage was not deemed sufficient (by software?)
to restart the engine when the brake was released.

Over the last year the "battery insufficient" symbol has been appearing
more regularly, and now the automatic engine stop will not work at all.
The car (and battery) is 5.5 years old, so I guess it's battery age
which is preventing the autostop working. The car always starts
perfectly first time, even when it hasn't been used for a week or so.
It would be interesting to see what parameters are required to be met
for the "battery insufficient" symbol to not appear.

--

Jeff