[Elecraft] BATTERY SIZING for portable operation

[Windy Dankoff]

This started as reply to Ken Kirkley <no4d at bellsouth.net>
>
> 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