# Maximum current draw from a battery

Discussion in 'Electronic Basics' started by [email protected], Sep 17, 2005.

1. ### Guest

I have 2 12v 12ah lead acid batteries from an old scooter. They say
they are 20 hour batteries on them. does this mean I can draw a maximum
of 6 amps from them?

2. ### JeffMGuest

I have 2 [each] 12v 12ah lead acid batteries...
The exact wording would be interesting to me.
..
..
If you need them to last for 6 hours, yes.
Otherwise, no.

3. ### ehsjrGuest

You can draw a huge amount of current for a brief time from
a good 12 volt 12 ah lead acid battery. It is impossible to
say, from the specs given, exactly how how huge, but *way*
more than 12 amps. As an example, an automobile battery
rated at say 60 ah at 12 volts can provide cranking current
of well over 400 amps.

Ed

4. ### Peter BennettGuest

The normal method of calculating the ampere-hour rating of a battery
is to determine the current which will fully discharge the battery in
20 hours - that is probably what the "20 hour" label on the battery
refers to.

Since the batteries are rated at 12 Ah, you can theoretically draw 0.6
amps for 20 hours, 1.2 amps for 10 hours, (or any combination of
current and time that equals 20) before they are fully discharged -
however, if you discharge a battery significantly faster than the 20
hour rate, you will get fewer total ampere-hours out of it than its
rating suggests.

The maximum current you can draw will depend on the internal
resistance of the battery, but will be much higher than the AH rating
might suggest. A car battery might be rated at 60 AH, but will easily
deliver a few hundred amps (for a short time) while starting the car.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
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