[Elecraft] Field Day experience with K3

[Edward R. Cole]

Look at energy instead of power.  You have a 120AH solar panel to a 
110AH battery.  What is your load?  Two 100w transmitters running 12v 
at 17a dc load (204w load to the battery for each radio).  Your Tx/Rx 
duty cycle is probably = 30% during FD (are you calling CQ FD CQ FD a 
lot?).  So the load is 204w x2 x 0.30 = 122w which after an hour has 
totally discharged the 110AH battery if were not being charged by the 
solar panel.  With solar charging at 120AH you still have a negative 
energy equation (so maybe it takes a couple hours operation to 
discharge the battery).

It sounds as only one battery was used for two radios.  A better 
solution would be separate batteries very close to the radios.  Still 
the 120AH solar charging system is undersized to maintain the 
batteries very long.

So lower RF power to 50w (as has been suggested) to lower dc 
load.  Also increase dc wiring size to lower ohmic losses.  Battery 
boosters will give a little more voltage at the end of battery life, 
but at the expense of battery current (no free lunch).

I ran 20w psk-31 one FD using a single 60w solar panel and a 100AH 
diehard marine battery and was able to run about 6-hours.  Of course 
psk-31 is keydown in transmit.  The radio was a FT-847 so I do not 
know its efficiency running at 20w RF.  The Rx and digital ckts 
probably consumed 3-4 amps continuously, and transmitter probably 50w 
at 50% efficiency for another 4 amps.  So say it was 7 amps in 
transmit (7x12= 84w).  I did not call CQ extensively but instead 
searched and pounced so most of the time was Rx so Tx/Rx duty cycle 
was probably 10%.  Overall the load was probably 48w per hour so the 
60w solar panel should hold the battery charge long-term.  Things 
rarely run exactly according to theory.

In my former job I maintained two remote repeater sites that were run 
on solar-charged batteries in summer and on alkaline batteries in 
winter (system auto-switched when solar battery voltage dropped to 
10.5v).  The solar system was two 60w solar panels feeding two 100AH 
deep-cycle batteries; winter was a 10,800 AH air-activated alkaline 
battery bank (90 1.5v cells in 10cell banks).  Each 1.5v battery was 
rated at 1200AH.  The site was operated in a stby status 99% of the 
time with only the UHF control radios activated full-time.  We got 
three years life between battery replacements (helicopter only 
access).  With new batteries the site had a 30-day operational 
status.  Repeaters were 30w and there were more than one at each site.


73, Ed - KL7UW, WD2XSH/45
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