Our message board recently had a great discussion about how to properly size a battery when operating remote.
Robert W9RKK asked:
I’m looking to buy a deep cycle battery for SARC in the Park. What kind of ampere hour rating would be sufficient? I have an Elecraft K3.
Rob N9MVO replied:
The question is: How much power do you want to run? A good way to start is to assume <50% efficiency, so double the transmit power, and add some to it. Also, assume 100% transmit time. Since you don’t actually transmit that much, you actually have more running time than you expect. Finally, recognize that you cannot suck the battery dry. Add 30% to the battery capacity to be able to operate without completely exhausting the battery.
If you are going to run low power, e.g. 25 W, you can expect the radio to draw about 5 A, key down. If you are there from 8:00 ’till noon, you need 20 AH. Add 30%, and you could use a 26 AH battery. If you want to run 100 W, the rig will draw about 20 A. For that, you should have a 100 AH battery. While it will be heavy, it will give you enough power for SARC in the Park or emergency use. That 100 AH battery will run your 25 W field day station all night. 24 hours at 100 W on battery is not practical. The batteries would be excessively large and VERY HEAVY.
