I have posted how to calculate battery life here before. John gave a pretty
good reply, but there are a couple of other points that should be made.
1. A lead acid battery is rated assuming the current is drawn at the 20 hour
rate. If you draw more current than this, you will get less capacity from
the battery. At the 10 hour rate, the capacity is diminished by about 10%,
at the 5 hour rate it is diminished by 25%, and at the 1 hour rate it is
diminished by 40%.
2. It is difficult to fully charge a lead-acid battery by trickle charging
it at a constant voltage. Trickle charging at a constant voltage will only
restore about 85% of the battery's capacity. To do better than this you need
to use one of the "smart" chargers or triple mode charges such as those
based on the Unitrode (now TI?) 3906.
If you do not realize this you can get substantially less life from the
battery than you anticipate.
My K-1 draws about 70 mA on receive with the headphones and volume set for
comfortable listening in the field. The transmitter draws about 1.1 A on
transmit. With half of the time spent listening, and half of the time spent
transmitting with a 40% duty cycle I get an average current draw of 0.276 A.
If you constant voltage charged the battery, you will have a 20 hour
capacity of 3.4 AH. At a current draw of 276 mA, I am pulling current out of
the battery at a 12 hour rate. Since this is greater than the 20 hour rate
our capacity will be diminished by about 10%. Reducing the capacity by this
amount leaves 3.06 AH capacity. Dividing by the average current draw,
0.276 A, we get 11 hours capacity.
Of course, in actual operation the average current draw can vary quite a
bit, usually on the down side (we forgot to include bathroom time and
emergency antenna repairs in the above calculations) so it is best to only
use these numbers as a guide and not to really expect to get that last hour
or half hour out of the battery.
I hope that this helps. - Dr. Megacycle KK6MC/5
______________________
James R. Duffey KK6MC/5
Cedar Crest NM 87008 DM65
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