How to choose a Battery Amp Hour for QRP

Many QRP operators that are adding battery packs or solar power panels to the grab and go kit find it a challenge to understand how big a battery they require. Let’s take a look ate several examples:

The important thing to know is we calculate the receive consumption and also the transmit consumption to come up with a total. I will look at three common QRP radios.

The Norcal 40 – Discrete components Wayne N6KR

This 40m rig puts out about 2 watts and is designed for low power consumption and we will use a 50:50 receive to transmit ratio and calculate in Ampere-Hours or AH.

  • Receive: (0.020 A) X (0.5 hour) = 0.010 AH
  • Transmit: (0.200 A) X (0.5 Hour) = 0.100 AH
  • Total: (0.010 AH rx ) + (0.100 AH tx ) = 0.110 AH
  • Thus, a 1-Amp-hour battery will operate the Norcal 40 for 1AH/0.110 H = 9 hours. Ideal for a weekend doing SOTA work

Yaesu FT-817 transceiver, operating CW with 5 watts output. This Swiss army knife of radios is the most popular QRP radio

  • Receive: (0.40 A) X (0.5 hour) = 0.200 AH
  • Transmit: (2.00 A) X (0.5 Hour) = 1.000 AH
  • Total: (0.20 AH rx ) + (1.0 AH tx ) = 1.200 AH
  • Thus, a 5-Amp-hour battery will operate the 817 for 1AH/1.2 H = 4.167 hours. Ideal for day in the park

Yaesu FT-857D transceiver that runs 100 watts

Another popular mobile radio that does hf/VHF/UHF and is the big brother to the FT-817. The FT-897D tabletop radio is similar.

  • Receive: (1.00 A) X (0.5 hour) = 0.500 AH
  • Transmit: (22.00 A) X (0.5 Hour) = 11.000 AH
  • Total: (1.50 AH rx ) + (11.0 AH tx ) = 11.50 AH
  • Thus, a 35-Amp-hour battery will operate the 857 for 35 AH/11.5 H = 3 Hours

A 12v 35AH SLA battery will last 3 hours. Ideal for day in the park at High Power. If I run the radio at 20 watts, then I can run the radio for almost 8 hours

 

If we look at the FT-817 and factor in run time (as some people like to do) at a 25:75 transmit to receive ratio the numbers come out to

0.4 A on receive X .75 H = 0.3 AH

2.0 A on transmit x .25 H = 0.5 AH

Total is 0.8 AH so using a 5AH Lion battery gives 6 Hours of use

I use the previous conservative method to allow some head room for error and to keep some juice in the battery and not drain it completely. Thus I can be safe to say i can run for 4 hours out in the field

So choose your batteries wisely and calculate the run time for the power being used and you will have success when operating off batteries.

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