KC8WVG , Bill wrote:


I'm trying to locate an emergemcy source of power to operate my 2meter
base rig in the event of a power outage. My HTX-242 manual states the
current drain of the transmitter as 8A @ 45W and 4A @ 10W. Would a 17aH
jump start battery work? Or is there something better, yet not to costly?

This transceiver is being used for voice, right?

Figure worst case you'll be transmitting 20 percent of the time at 45
Watts, receiving (with squelch open) 75 percent of the time, and in
standby (with squelch closed) 5 percent. Standby current is 0.5 A;
let's assume receive current as 1.2 A. At 45 Watts, your average
transmit current will be 0.2*8, or 1.6 A. Your average receive current
will be 1.2*.75, or 0.9 A. Your average standby current will be 0.5*0.05,
or 0.025 A. Add them all up, and you get 2.525 Amps as an average. This
is approximately 5.5 hours from your prototypical 17 AH "jump start battery",
based on the most common rate for calculation AH capacity being one that
discharges the battery over 20 hours and the fact that your average current
draw is a little more than three times that rate. The figure of 5.5 hours
does include a fudge factor as well.

Best case (you're not NCS, just calling in and responding when NCS calls
you), figure 10 percent transmit at 10 Watts, 10 percent receive, and 80
percent standby. Your average transmit current becomes 0.1*4, or 0.4.
Your average receive current becomes 0.1*1.2, or 0.12. Your average
standby current becomes 0.5*0.8, or 0.4. Average current is now 0.4 +
0.12 + 0.4, or 0.92 Amps. A straight-line approximation shows a
discharge time of 18.48 hours under this load. You could reasonably
expect 18 hours under this best-case scenario.

disclaimer
If this is a critical application you might want to consult with someone
who does these calculations for profit. I just do them for fun.
/disclaimer

Bottom line is if you expect power to be out more than 5.5 hours and to
impose heavy loads on your backup battery then you'll need either more
battery or a photovoltaic system to help keep it topped up under load,
while if you expect power to be out less than 18 hours and to impose
only light loads on your backup battery then your 17 AH jump start
battery should be adequate.

HTH
--
To design the perfect anti-Unix, write an operating system that thinks
it knows what you're doing better than you do. And then adds injury to
insult by getting it wrong.
- esr