Anchor Bay Amateur Radio Club

Elmer Night - September 17, 2008 - de W6WTI



External Power Sources for Your HT or Mobile Radio


How long will your HT radio operate on its batteries?

Some manuals provide estimated battery life on a fresh set of alkaline cells or on fully charged NiCad or NiMH battery packs of different capacities.  How long a battery pack will last depends on several things:

1)  The capacity of the battery ... AND its state of charge

2)  The duty cycle and the output power setting (Hi, Med, Low)

How much time in squelched receive (stand by) mode

How much time in receiving mode, and its volume setting

How much time in transmit mode


The Kenwood TH-K2 Manual on the Specifications page says:

For each minute the Duty Cycle is:

Transmit 6 seconds, Receive 6 seconds, and Standby 48 seconds.

Is this typical? If there is more Receive time or more Transmit time, the battery life will be shortened.


With a BT14 (9 volt battery case) and 6 fresh AA size batteries installed you should have:

4 hours on High power

8 hours on Med power

10.5 hours on Low power


How many extra batteries do you have on hand?

Using the example duty cycle on High Power how long would you be able to continuously operate if we had an earthquake and you had 6 sets of fresh AA cells for your BT-14 battery case?


6 cells/set x 6 sets = 36 AA cells… 6 sets x 4 hours/set = 24 hours

Then what? After 24 hours are you still on the air?


How about an External Source of Power for your HT ?

Your car battery?

RV/Marine Battery?

Such batteries are heavy, usually contain sulphuric acid as liquid or gel, and require attention from time to time, but contain a lot of electrical energy to run your radio(s).


Example: TH-K2A Duty Cycle:

10 min Standby at 0.10 amp = 1 amp-minute

40 min Receive at 0.50 amp = 20 amp-minute

10 min Transmit at 1.8 amp = 18 amp-minute

You would use 39 amp-minutes per hour of operation, or 0.65 amp-hours on High power and 15.6 amp hours for a 24 hour day. If your RV/Marine battery has 120 amp-hour capacity it’ll run the HT for more than 7.5 days at that duty cycle before needing to be recharged.


Deep Cycle/RV Battery Considerations


1.  Vehicle batteries are designed for heavy starting current ... but will power radios, too.

2.  Deep Cycle/Marine batteries are designed for lesser drains, over a longer period and deeper discharge.

3.  Keep the battery in an acid proof battery box for safety.

4.  Check electrolyte levels carefully, monthly or quarterly.

5.  Monitor the battery voltage and record values

6.  RV type batteries have wingnut / screw connections as well as cable terminals.

7.  Emergency recharging can be accomplished via generator/charger, or using jumper cables from your car.

8. Normally charge with a battery maintainer (not trickle charger)


High Current 12 VDC can be very dangerous. No rings, watches, bracelets or

other metal jewelry should be worn as you work with a charged battery. Even a

discharged’ battery can spark impressively. Watch your metal tools, they conduct!


Other Battery Packs?


The Power Station has a 6 amp hour 12 volt battery (360 amp minutes) - it’ll run the TH-K2A for 9 hours at that duty cycle ( 360 amp-minutes / 39 amp-minutes/hour = 9.2 hrs))

Designing emergency power sources to run your HT, mobile radios and other items is an area for a lot of creativity.


Designing a battery pack using ‘D’ Cells


The capacity of a D cell is substantially more than that of a AAA or AA cell. D cells deliver nominally 20.5 amp hour of service and AA cells deliver nominally 2.8 amp hour of service. 8 D cells in series would supply 12 volts ... but your radio needs AT LEAST 12 VDC to operate correctly ... so the life of the pack would be significantly shorter if designed to deliver 12 VDC, than one starting out at 13.5 VDC or 15 VDC.


Both battery sizes supply nominal 1.5 VDC per cell.

8 D Cells – 1.5 VDC batteries in series will provide 12 Volt DC

9 D Cells – 1.5 VDC batteries in series will provide 13.5 Volt DC

10 D cells – 1.5 VDC batteries in series will provide 15.0 Volt DC

The Kenwood TH K2A will accept up to 16 Volts. Starting out at 15 volts, the pack voltage will decline with use. As the battery pack drops below 12 volts, performance of the HT will become impaired.

Regardless of how many cells you use in series, a fresh D cell will provide 20.5 amp-hour or 1230 amp minutes of service. A Kenwood TH-K2A uses 39 amp-minutes / hour. So the D cell pack would be expected to run the radio at the design duty cycle for about

31 hours


Be Sure to Check Your Manual BEFORE You Start


Make sure you know the voltage requirements for your radio - check your Manual to be

sure of its requirements.


For example:

Kenwood TH-79 specifies:

External Power, DC jack 5.5V – 16 V (13.8 V)

Kenwood TH-KT2A specifies:

Operating Voltage, DC IN jack DC 12.0 – 16.0 V (13.8 nominal)


 Connections for External DC Considerations


1. There is often a coaxial outlet is on the HT (check size and polarity) most commonly

the center pin of the plug is + and the ring is -.

2. Connect to a regulated power source. (Batteries not connected to chargers

qualify if the voltage is within the specified range) e.g. Kenwood ‘s PG-2W DC Power Cable

3. Many manufacturers make cables to use auto cigarette lighter outlets. These

contain circuits that ensure voltage spikes or overvoltages are not applied to the

radio. e.g. Kenwood’s PG-3J Cigarette Outlet Power Cable

4. Check all disclaimers and warnings - the manufacturers are relatively conservative about their warnings regarding connecting HT’s to auto batteries installed in a vehicle.


About Connectors


If everyone utilized a similar connector for external power input for their radios, and if our emergency power sources used the same connector, then it would be easy to swap in fresh power sources for our radios ... or to change radios if someone wishes to take theirs with them when they go off shift or go to another assignment. Quick for station set up, take down as well.

Is there a power connector standard ? Yes, there is. Meet the Anderson Powerpole !!



ANDERSON POWERPOLE CONNECTORS are the standard for VHF Transceiver use.


Either the 15-ampere or 30-ampere sizes may be used, and both sizes mate with each other. The plastic parts are the same for both sizes. The connectors dovetail together as a compact unit. Identical connector halves are genderless—making assembly quick and easy and reducing the number of parts stocked.  When connections are disconnected, no metal parts are exposed. The 15-ampere contacts are designed for 16-20 AWG wire and the 30-ampere contacts are designed for 12-16 AWG wire.


The contacts can be soldered or crimped to wires. The contact lugs are designed to be crimped and non uniform crimping can distort the barrel fitting so that the contact will not fit in the housing however soldering can be done if  the solder is applied sparingly.


There are a variety of devices on the market using Powerpole connectors ... e.g. RigRunner. The wire from your battery or power supply to a RigRunner or ‘Y connector’ power distribution network should be heavier gauge and use the 30 A connectors.



OK ... How About a Mobile Radio ???


Example: A Kenwood TM-731... an elderly dual band mobile transceiver.


Power required: 13.8 VDC +/- 15% (calculated: 11.7 to 15.8 VDC)

Current Drain High Power < 11 amps

Receive w/ no signal ~0.6 amp

Output 2 Meters: High 50 Watts, Low 5 Watts

Output 440:  High 35 Watts, Low 5 Watts

Fuse 15 amp


A Kenwood TM-271 - a current model 2 meter mobile transceiver


Power required: 13.8 V DC ±15% (11.7 ~ 15.8 V)

Current Drain High Power 13 amps or less

Receive: 2 Watt output - less than 1 amp.

Output 2 Meters: High Power 60 Watts

Low Power: 25 Watts

Fuse 15

In mobile installations Kenwood’s recommendation is to connect the radio directly to a nominal 12 volt vehicle battery using the supplied wire (#16 or larger gauge) with fuses on both positive and negative sides. Red wire connected to positive, and black wire to negative.

The same 120 amp-hour battery will run one of these radios at the 40/10/10 duty cycle for about 40 hours. (Assumes 13 amp drain in transmit, and 1 amp drain for standby and receive.) There is no apparent warning about overvoltage conditions for connection to the vehicle battery in the manual for either of these mobile radios.


Grabbing power from other places?


What to do? ... ‘my batteries are all flat and I ran out of replacements and no chargers are running’ With a power cord with heavy battery clips on one end and Anderson Powerpoles on the other, you can connect to any 12 VDC auto battery that you can borrow.  Caution: you must watch polarity.


Brush or wipe down ‘borrowed’ batteries so that chemical crusts or drips don’t come into

contact with your skin or sensitive components. (A box of sodium bicarbonate can

neutralize any small spills.) This solution also allows quick connections to power supplies that may be available when your wrench, screwdrivers, or other tools aren’t at hand. Clip onto the terminals, plug in the Powerpoles and you’re in business. These are a bit more vulnerable to jostling which may cause short circuits or other interruptions. Position them safely out of harms way.




Further Reading


The ARRL Handbook for Radio Communications has information about discharge planning, battery charging, sample projects and more in Chapter 17.