For tonight's training we will review the calculation used to determine how long you could stay on the air with your existing equipment while using battery power with a base station, or with a handheld radio. We will start with a mobile radio in a base station application.
First, review the user manual for the radio that will be powered by the battery. Considering the possibility it will be necessary to operate on a simplex frequency to communicate with distant stations, I suggest using the specifications for the maximum transmit power when you make this calculation. For my Kenwood TMD700, the spec sheet indicates a draw of 11.5 amps while transmitting at 50 watts in the 2 meter band, with a 1 amp draw when receiving in either band.
Next, estimate the duty cycle you could expect if you were operating in an emergency situation. This relates to the time spent transmitting, versus time spent listening. If you are acting as net control, for example, would you expect to transmit at least 50% of each hour, or maybe more? If you are a participant in the net would you transmit 20% of each hour, or more or less?
Using the specs for the Kenwood TMD700 with an estimated duty cycle of 20% transmit and 80% receive, I would multiply the 11.5 amps draw during transmit by 0.2, for the 20% of the hour spent transmitting, resulting in a sum of 2.3 amps. Then I multiply the 1 amp draw while receiving by 0.8, representing the 80% of the hour spent receiving, resulting in a sum of 0.8 amps used while listening. Add the two sums together and the total draw is 3.1 amps over the course of 1 hour at a 20/80 duty cycle.
If I had a 12 volt battery with a known amp hour rating, I would divide the amp rating of the battery by the 3.1 amps used per hour to determine how long I could stay on the air with the particular battery and duty cycle. If my battery was rated at 10 amp hours, the expected duration of operation at a 20/80 duty cycle would be just over 3 hours. With this information I can determine if I need to consider a battery with a higher amp rating, or to add a solar charging system as a viable way to increase operating time with the existing battery.
If I do not have a battery, but want to determine the amp hours I would need for a specific operational period at the same 20/80 duty cycle, such as 8 hours, I can multiply the 3.1 amps used per hour by 8 to find the minimum amp hour rating needed to operate for that amount of time at that duty cycle.
**Note: There are other things to consider, such as the ability of the battery to deliver usable voltage while discharging. For example, Sealed Lead Acid batteries do not maintain a usable voltage as long into the discharge cycle as a Lithium battery will.
The calculation process is the same for a handheld radio. Consult the manual to determine the amp consumption while receiving, as well as when transmitting. Most handhelds have a high and low transmit power setting. As in the previous scenario, I would suggest using the amp consumption rating of the highest transmit power setting for this calculation.
In the case of a handheld radio, to extend your operating time you must have additional batteries on hand, and a means to recharge spent batteries that does not require commercial power.
There are a variety of low cost handheld radios on the market with USB-C charging capable batteries, as well as a variety of solar charging portable power banks, also USB compatible. It’s still a good idea to pick up a spare battery for whatever handheld you choose, so one battery can be recharging on the portable power bank while the other battery is in use.