Many thanks to Jim Cluett (W1PID) who has kindly allowed me to share this recent post from his website:

Field Day – 200 QSOs with a Flashlight Battery

June 2021

by Jim Cluett (W1PID)

Field Day was an experiment this year. I operated for nine hours using a 5V 18650 battery and made 200 QSOs before calling it quits.

I operated on the deck with an MTR 4-B designed by KD1JV. The antenna was an 88 foot doublet up about 45 feet. I used the ZM-2 tuner. For power I used the PowerFilm LightSaver. This is a 5 watt roll-up solar panel that charges a 3.7V 18650 battery rated at 3.2 Ahr. The battery inside the Powerfilm product is commonly used in flashlights. The combination of the rig and the power supply is crucial.

The MTR rigs will operate from 6 to 12 volts. The Powerfilm puts out 5V to a USB socket. The secret ingredient required to bring the USB voltage up to the operating voltage of the rig is a Baofeng USB charging dongle. This device takes a 5V input and outputs 10.3 volts… perfect for the MTR transceiver. With this voltage the MTR puts out a little less than 3 watts.

The PowerFilm LightSaver is designed to charge cell phones for hikers and campers. It weighs only about 5 oz. and rolls up into a tiny package. Any USB 5V cell phone charging battery could be used with the Baofeng dongle.

This year my whole station operated on 5V. I used a Samsung tablet for logging.

In New Hampshire it was cloudy for most of Field Day, but fortunately the amorphous solar panel provides some charging even when it’s cloudy. I’m guessing that after 9 hours of operating the battery was down to about half capacity. The beauty of this system is that one could operate indefinitely with moderate sunshine.

I’ve been experimenting with this setup during hikes and bike rides for the last couple of months with a view to using it for Field Day. This year’s emergency exercise proves that it is viable for an extended grid-down power outage.

Click here to check out Jim’s website!

What a brilliant challenge and test for Field Day, Jim! Thank you for allowing me to share it here on QRPer. Also, a hat tip to Eric (WD8RIF) for bringing this article to my attention!