Paul was thinking very hard :
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?

Part of the problem with batteries, is determining what is going on,
and exactly how healthy the car battery is.

I have a lot of trouble with that.

For example, I've had many batteries, where I get out in the
drive, hook up the dumb charger, and after two minutes the stupid thing
measures 18V. And you just know that's wrong, the battery is not
"stiff enough", it's only got a fraction of the normal ampere-hour (Ah)
capacity. Yet, for maybe several weeks, it continues to turn over the
car. But you know its days are numbered. And how do you tell
one 18V charging session from another 18V charging session
(where it's failed and won't start the car) ?

There are a ton of articles to entertain you.

https://batteryuniversity.com/index....attery_runtime

https://batteryuniversity.com/learn/..._is_the_c_rate

OK, so the second article says the end-of-discharge voltage
is 1.75V. Six times that is 10.5V .

https://batteryuniversity.com/learn/...d_acid_battery

A cell voltage of 2.10V at room temperature
reveals a charge of about 90 percent.

Six times that is 12.6V and that's after you've disconnected the charger
and let it stabilize for 12+ hours or so. Right after it is charged, it
reads higher than that, and taking an open circuit voltage reading
is then "not representative" of battery state. Your smart charger
does its best to terminate charge at the right condition ("topping"),
but the actual working voltage is not visible until the electrolyte has
settled and equilibrated (sulphuric acid the same composition, right at the
plate
surface, as 5mm away from it). Using Specific Gravity (SG) with a
liquid sampling device, is about as useful as a voltage reading,
and even SG can be deceiving if only half of the plate (down deep)
is working.

For practical discharge ("running a golf cart"), you limit the
depth of discharge to about 25% on lead acid. The 10.5V value,
would be if you were designing a "cutoff circuit", to disconnect
the battery from the load and stop all additional discharge.
25% depth of discharge is preferred, to maximize the number of charge
cycles you can get from it.

Lead Acid batteries are inefficient at high load. This
is captured in some of the discharge curves shown for
UPS (uninterruptable power supplies, sealed lead acid SLA).
If you run a UPS at the rated load, maybe the battery lasts
for 2 minutes. But the relationship is non-linear, and
if drawing 50mA, then you get the ampere-hour rating of
the battery back.

50Ah is 50000 mAh, divided by 50mA is 1000 hours.
Whereas if you drew 50000mA, you might be expecting
1 hour (60 minutes), but the stupid lead acid battery only lasts
for 20 minutes.

Summary: At the low load you describe, if the battery
is brand new, mint condition, fully charged, it might last
1000 hours before it hits 10.5V (and can't crank the car).

*******

To crank a car, some percentage of charge is needed, because
the battery impedance matters and it gets "weak in the knees"
at low charge. For example, I did a test using two metering
devices here. A clamp-on DC ammeter with peak hold. A voltmeter
with peak hold. And when the car starts, the values I got
were 9V @ 150A . The battery voltage then, was 12.6 internally,
but Ohms law saw to it that 3.6V dropped across the "resistor"
inside the battery. 3.6V/150A is 24 milliohms. When drawing
50mA, there's practically no voltage drop at all, from the
ideal internal value.

I would shoot for 25% discharge, so if the battery is
60Ah, that gives you 15Ah to work with, 50mA load is
300 hours or 12.5 days. And then, perhaps the car will
still start.

There are some examples here of "pulse-challenging" a battery.
And starting a car is a rather large pulse. Car batteries
are designed to provide CCA, they're not a stationary
storage system, like keeping your house warm for 10 days.
They're actually intended for short term use, cranking the
starter. Whereas a golf cart battery is "motive storage"
and could consist of many batteries and many amp-hours.
And it might take all day, to make a dent in the bank of
batteries.

https://batteryuniversity.com/learn/...ct_performance

*******

Getting back to the voltage, here are some values.

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

SG
100% 1.265 12.65V === 26°C (78°F) after a 24h rest
75% 1.225 12.45V Voltage is a strong function of temperature,
50% 1.190 12.24V and temperature compensation is required
25% 1.155 12.06V in the real world, like when it is -20C in the
drive.
0% 1.120 11.89V

10.5V is cutoff (hard on the battery, could reverse bias something).
10.5V is not a useful value for our home maintenance plan.

We're more interested in voltages that will start a car.

11.9 or 12.0V is about as low as you should go, before
turning the key and starting. The battery needs some juice
in it, to be useful. You can run a 20mA LED all the way down
to 10.5V because the battery impedance doesn't matter for
ridiculously low loads. We need that "stiff 24 milliohms"
value, when pumping 2 horsepower into the starter motor.

When I had a car battery fully charged, and a suspected starter
problem (on a four cylinder car), the measurements during starting
were 9V @ 150A. A four cylinder starter should probably have been
drawing 100A, in which case the battery voltage would not have
dropped so low. That's at the point where the starter is heating
significantly. And you'd give it a lot of time between "trials" :-)
Cars which start easily on the fourth crank, might not be so bad.

For max life then, I could run the battery, day after day,
from 100% full to 75% full. This would (perhaps) maximize the
number of charge cycles. Like, if I was running a golf cart,
that would be a useful operating range.

If I run the battery from 100% ro 25%, that's 75% charge difference,
if the battery is 50Ah times 0.75, I have 37.5Ah or 37500mAh,
divided by 50mA, 750 hours. Round and call it a month. That means,
if I do that for a month, the car will barely start, the starter
will be getting a bit warmer than normal.

But then, real batteries (and you probably have some idea how
poorly the current one is doing), If I was connecting my charger
and seeing 18V while it charged, I would be out of my fricken
mind to be waiting a month to test. I'd be lucky to get three
days our of it at 50mA. When they're on their last legs,
the vampire load really hurts them.

Ya see, it's like predicting when a relative is going to die :-)
You're hoping hoping... and before you know it, they're 92 :-)
You can sorta tell from some of the symptoms, that the battery
"is not well", but it's damn hard to tell "only three days left"
or "won't be able to crank the car a second time". That's why
we don't try to run them right down to the wire.

You can see in my worked results, I'm mostly interested in
the ampere-hours and what percentage of the battery
I've kicked the snot out of. While the voltage gives
some indication of the remaining charge, you must allow
the battery to stabilize before reading it. Whether
charging or discharging, that disturbs the acid and plate
surface conditions.

The 11.9V or 12.0V minimum starting voltages, that is at 25C,
and temperature corrections must be applied at temperatures
other than the "room temperature". If you don't apply
temperature correction, you can be off by 50% of battery
capacity! (Mistake a reading of 100% full, for actual
50% full.) Good smart chargers have a thermistor, and
measure temperature and attempt to do at least a little
bit of correction when charging. Equipment at 5X to 10X the
price, comes with a thermistor on a cable, to be clamped
to one of the electrodes in an attempt to measure temperature.

Even hydrometers, for measuring SG, you need to run your
finger across the chart, for the right temperature.

Paul

Wow! Thanks for all that Paul, that'll take me a bit to digest lol