On 23/04/2021 10:50, Ian Jackson wrote:
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
powerÂ* things like the alarm, the clock, the radio presets and so on
when theÂ* car is parked up and switched off. I've seen it written
somewhere thatÂ* around 50mA can be considered 'normal'.

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

For instance, if I park the car up at 10pm and the battery is
showingÂ* 12.5V, with a 50mA draw overnight what will the voltage be
at, say, 9am?
Â*Couldn't the manufacturers fit a separate small battery dedicated to
supplying the quiescent items? One that would last maybe ten days.
WithÂ* a user option to decide whether it should steal power from the
mainÂ* 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.

I empathise with your issue, but this is down to design. I am currently
using a microcontroller that takes sub microAmp when in sleep mode but
still providing a date and time function. It has to when working off a
lithium button battery.

I also know a acquaintance that had a similar issue to yours. It was a
Peugeot, and the discharge rate was solved after performing a firmware
update for his radio.

I'm more disheartened that your battery was allowed to go to 8V. Most
12V battery items I have ever come across tend to turn off at 11V to
save the battery from being destroyed. I guess there is the reduced
voltage during starting issue to contend with.

It's not a very good advert for Ford!