What is battery drain?

[SJ]

> From: "Al Streicher" <streichea001 at hawaii.rr.com>
> Subject: RE: Too much Crankcase ventalation?
> > For some of us who don't understand how to measure "battery drain",
could
> someone please explain.
> Thanks
> Al
>
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Others have given How-To details regarding a battery drain problem. I'll
just provide a more general explanation for those unfamiliar with electrons.

Battery Drain = Current leaving the battery

All Batteries have a drain . .. even when sitting with nothing attached.
Internal leakage currents will produce small amounts of heat . .ergo, lose
energy. Over time the battery will go flat (no charge) - even with nothing
connected to the battery.

Car batteries have an engineered drain put on them even when car is not
running. Things like computers, clocks, alarm systems are always ON and
draining the battery. But these normal drain currents are pretty low and it
will take a long time to run down the battery. Typically the "normal" audi
drain current is about 30 to 50 ma., or 0.050 amps. Thats a small number. If
one amp is equated to one dollar is one amp, then 0.050 amps is equal to
Five Cents.

I'll provide an example to demonstrate the "size" of this current so you can
get your mind around the idea.
Car batteries are rated in Amp-Hours.
A car battery rated at 90 AH can handle a current drain of 90 amps for one
hour, or 1 amp for 90 hours. The battery still has charge left in it, but it
cannot supply the 1 amp into the same load. The current provided will be
less than one amp, and the current will start to sharply decline after the
original rated 90 hours.
If my math is right, a Battery Drain of 0.050 amps should last for 1800
hours . . .which equals 75 days.
So, for a typical audi, if you leave the car parked and not run it for 75
days, you may have an "issue" with starting it on day 76.

When you do have a problem with a battery running down over a day or two, or
overnight, the first thing to check is the actual battery drain. I'm
assuming that the alternator is working and you have about 13.7 volts at the
battery when engine/car is running.

Remember that Drain = Current. You are measuring current.
Instrument for measuring current is an ammeter, or amp meter.
Most Voltmeters sold have a connection for measuring amps.

The trick to measuring current is that you have to put the instrument in
SERIES with the current. Its not like a voltmeter where you simply probe the
electrical contact without taking anything apart.

For our example of a Battery Drain, the simplest method is to discconect the
ground from the battery and put the meter between the chassis ground and the
battery neg terminal.
And here is where a lot of meters get blown up -- Connected backwards, wrong
amp scale used, current is greater than what the meter can handle, etc etc
etc. But, this topic is for another lesson.

If you have more questions on the topic, just ask.

SJ