This started as reply to Ken Kirkley <[EMAIL PROTECTED]>
Subject: [Elecraft] K3 & Portable Operation
I got carried away, so I'm making it a new subject.
It answers your question Ken -- YES, you can operate a 100W model at
QRP levels OR at full power, given sufficient battery capacity.
--------------------------------
HOW TO ESTIMATE OPERATING TIME FROM A STORAGE BATTERY
Let's calculate how long a transceiver can operate on a battery, for
example a 100AH (amp-hour) unit (a typical RV/marine deep-cycle
battery). You can substitute numbers for other battery sizes, and
other loads -- QRP, multiple rigs, etc.
First, you need to know the AVERAGE current draw of the transceiver
during "average" operation. It depends on transmit duty cycle so we
must make some assumptions. I'll base this example on casual operation
of a K3/100.
K3 draws ~1A on receive.
On transmit, CW or SSB, I estimate the AVERAGE current is about 10A
(using full power).
Assuming you receive 90% of the time, transmit 10%:
AVERAGE OPERATING CURRENT = (1A X .90) + (10A X .10) = 1.9A Round
that up to 2A.
Time available from a battery of 100AH capacity would be:
100AH / 2A = 50 hours.
HOWEVER:
USABLE battery capacity is less. If the battery is colder than
standard temp. (77°F) and new (not broken in) or well-used, it will
have less capacity than its factory rating. It would be safe to de-
rate the battery to 75%. So:
Real time available = 50 hours X .75 = 37.5 hours.
This will discharge the battery to roughly 11V under moderate load.
You don't want to do this habitually to a lead-acid battery -- it's
the maximum.
USING A VEHICLE BATTERY
Are you car-camping or hill-topping? You can use the vehicle battery
if you don't discharge it below about 12.0V. That represents roughly
40% discharge. In cool or warm weather, it should still start the
vehicle and is not too abusive to the (non-deep-cycle) battery. It's
hard to get true AH ratings for a vehicle battery, but a typical one
in a pickup truck is at least 60AH. It may not be in great condition,
so let's be conservative and consider only 70% of its capacity.
Therefore:
Usable battery capacity = 60AH X .70 = 42AH
Using 40% of that (so you can safely start the vehicle):
Available battery capacity = 42AH X .40 = 16.8AH round it to 17AH
Time available from battery capacity of 17AH:
17AH / 2A = 8.5 hours
So, you should be able to operate a 100 watt transceiver all day from
your vehicle battery if you aren't too long-winded.
CAUTION: Watch battery voltage and don't drop below about 12.0 (under
load). Measure battery voltage close to the battery terminals so it
ignores voltage drop in your load wiring.
Next, I'll advise on sizing a solar panel (or array) to extend the
running time. I was a solar-electric system designer, supplier, and
educator for 25 years.
Windy Dankoff KM5Q
K3 #? due in late March
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