Ohm's Law and Battery Life

Remember Ohm's Law?  Tonight we are going to use it to learn a few things about battery operated radios and battery life.  In the following calculations no allowance is made for the power consumed by the radio itself.  All calculations assume the radio itself consumes no power and that all power used is converted into the transmitted radio signal.

You remember that P=V*I or Power equals voltage times current.  We will be solving for current, so the formula will become I=P/V.

First let's figure the power consumption for a radio producing 5 Watts with various batteries.

5 WATTS

VOLTS         AMPS
12V               0.42A
7.4V              0.68A
6.2V              0.81A
5.4V              0.923A

Actually, most hand held radios only produce 5 Watts when they have 12 volts coming in.  Many radios that are capable of 5 Watts and advertised as such produce only 4 Watts or less when the input voltage is less than 12 Volts.  Let's run the numbers for a 4 Watt radio.

4 WATTS

VOLTS     AMPS
12V           0.333A
7.4V          0.54A
6.2V          0.65A
5.4V          0.74A

Notice that for ANY amount of power output by the radio, the lower voltage batteries draw more current.  The lower the voltage, the greater the amps consumed.  Ohm's Law wins again!

What this means is that for any given radio at any given power setting, the lower voltage batteries are going to run out of "oomph" faster than the higher voltage batteries because of greater current drain.

Remember, a battery can sit around for weeks producing 12 volts at its terminals with no load attached.  Attach a load and the instant result is current drain.  It is that current drain that "exhausts" the battery and pulls it down into a state where it requires a recharge.

This is why you want hand held radios that have a jack on the side that will accept 12 volts.  Your 12 volt battery (with the exact same current or AmpHour rating) will last longer than the built in battery.  Add a 12 Volt battery with a much higher current rating, such as a 12 Volt 7 AmpHour Gel Cell, and you GREATLY increase your potential on air time!

How do you get this 12 volts?  Most modern radios have an optional battery replacement back that has a cord coming out of it with a cigarette lighter plug on the end.  Others have an actual 12 volt jack on the side.

If at all possible, get an emergency 12 volt source for your radio for use in emergencies.

Also, remember that your radio operates at THREE different power levels.  1) Power on but no audio, squelch is unbroken.  2) Power on and audio is being received.  3) Power on and the operator is transmitting (key down mode).  You can attach an external power source with a current meter in the circuit to find out the actual current draw for your particular radio.  It is educational.

N7OZH