In message , Fredxx
writes
On 22/04/2021 19:41, williamwright 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?
Couldn't the manufacturers fit a separate small battery dedicated to
supplying the quiescent items? One that would last maybe ten days.
With a user option to decide whether it should steal power from the
main battery when it became depleted?

At 50mA that would be less than the self discharge rate of a lead acid
battery.

I therefore don't see the point, it would also make a car even more
complex than it they are already. And you've have to replace two
batteries rather than the one. For some cars a battery change is
already a dealer operation.

Last year, during the first lockdown, after my new
full-of-eltronic-gizzmos Fiesta had been left undriven for four weeks, I
had great trouble with its battery (which eventually found had fallen to
8V). Essentially, the car was dead. The doors would not unlock, and I
had to consult then manual to find where the hidden door-handle keyhole
was. When I got it open, the alarm went off, and wouldn't stop.

I only have 'simple' battery chargers, and even with a homemade beast
that can deliver over 20A I couldn't get the voltage to rise. I guess
that at least two - and maybe three - cells had gone reverse-charged.
After many hours of charging, and a rather too-hot charger, I did get
the voltage up about 9.5V, after which I was able to start the car - and
the on-charge voltage rapidly rose to around 14V.

In contrast, I had absolutely no trouble with my wife's Citroen C1
which, being essentially a 2CV in a modern body, has none of the
electronic fripperies of my Fiesta. The battery was around 12V, and it
started instantly.

So I guess that the Fiesta's battery was discharging at a lot more than
the C1's. So yes - it certainly would be a good design feature if a
car's electronics were powered from a separate battery, so that the main
battery could always be available to start the car.
--
Ian