Emergency Power Supply for Amateur Radio Stations
N4FMN - Sep 2005
When a natural disaster strikes your community, and you have lost power from the
friendly power company, there is nothing like a backup power source for your amateur
radio station.
Batteries and generators can supply backup power. Each power source has its
maintenance and operating issues. A small (less than 5 KW) generator has the benefit of
being able to supply AC power for small appliances, and it will run as long as you can
supply the unit fuel. With the rising cost of fuel, you may consider alternative sources.
A generator system should be permanently installed and it should be installed were there
is adequate ventilation for the generator and where the exhaust can be adequately
disperse without poisoning the household. Consider where you will have to store the
fuel. And if the fuel is stored for any length of time, it should be rotated out, to keep a
fresh supply on hand. Also consider that the oil and oil filters, air filters had to be
periodically changed and the unit is subject to mechanical breakdown if not maintained.
This paper deals with battery backup systems. The heart of the system is the battery.
You can use several types of lead acid batteries, but I will recommend a high capacity
deep discharge sealed battery for our applications. If you buy a brand new battery, it may
cost you. If you ask around, you may be able to locate deep discharge sealed batteries
that are cycled out of telecommunications centers and public facilities such as hospitals.
A lot of these industries change out batteries at a fairly frequent rate as they are looking
for minimum maintenance and high performance use in a short limited time period.
These types of batteries may be available for the asking. I will point out, if you volunteer
to move them, come prepared. High capacity batteries are 70 pounds and up. You will
need all the volunteers you can find, a heavy-duty hand truck, and gloves and suitable
work clothes. Consider the vehicle, which you will use to haul these batteries. If you get
20 each 70 pound batteries, your hauling 1400 pounds, which is over the weight limit for
small pickup trucks.
I highly recommend the seal lead acid type of battery. It is made to operate in confined
atmospheres and it will only vent if a fault is applied to the battery. This is a good type
battery to maintain indoors, which is typical of an amateur radio station. If you use
vented lead acid batteries, you need to provide air circulation to exhaust the fumes. And
remember this is acid that is released, so there is a possibility of corrosion, if the fumes
are not exhausted. The fumes are acid and hydrogen. If allowed to build up without
ventilation, at some point the hydrogen will spontaneous combust!  Also, if a device that
may cause a spark, should not be operated in this type of atmoshepere. No smoking.
Batteries are rated in amperes per hours. This specification is rated let's say 100
ampere/hours. This spec means the battery can put 100 amperes of current in 1 hour. Or,
10 amperes in 10 hours, or 1000 amperes in 1/10th of an hour.

To find the capacity of the battery you need, you will have to know how much your radio
and its support systems draw in amperes. Estimate the time you need to operate, taking
in consideration of the duty factor of operating your station. Most of us receive all the
time and talk very little. But when you talk, your transmitter draws maximum rated
current. Also consider if you can operate with lower RF power, as you will draw less
battery power.
Lets say you have a 100-watt HF radio, you plan to operate. At full power the radio
probably draws around 20 to 25 amps. Lets say you talk about 10 minutes in one hour.
That's a 16 percent duty factor. Lets say you listen for the remaining time in that hour.
Your radio probably draws under 2 amps in receive. Lets say you plan to operate for 6
hours. This is one hour of transmit time, so you need 25 ampere hours of capacity. For
receive, we will leave it at 2-ampere hours, times 6 hours, or a capacity of 12-ampere
hours. 12-ampere hours of receive plus 25 ampere hours of transmit, requires a total
capacity of 37-ampere hours to operate that six-hour period. If you want to be able to
operate for 3 days on battery power, it will take a battery rated at 111 ampere hours,
minimum. You also need to de-rate a little bit, as you need to consider the state of the
charge in the battery and the lifetime of the battery. If you operate with minimum RF
power, your system will be available a lot longer.
Another important specification of batteries is the end of discharge cell voltage. Battery
cell voltage for lead acid batteries are 2.1 volts fully charged and standing. End of
discharge is usually 1.75 to 1.94 volts per cell (10.5 volts to 11.64 volts for a 12 volt
battery). Most radios will not operate well at lower voltages - they will not develop full
power and they may FM the RF output, on AM and SSB.
Proper charging of the battery is important and directly affects the lifetime of the battery.
Nominal battery charging is performed at 1/10th of the ampere-hour rating or less. The
object is not to overheat the battery. If a battery heats up, its making gas, and it has to
be
vented at some point due to pressure. Your system should be designed to limit the
current of the charger to a safe value.
Lead acid batteries require around 13.8 volts to charge. A sealed lead acid battery
requires a little bit less, usually 13.62 volts. The best battery charging system will use a
current limited charge on start of charge and voltage limited at the end of the charge.
BATTERIES MUST BE FUSED. You can start a fire, or blow acid all over they place if
you do not have some system in place to limit energy during a fault condition. I use a
GNB Marathon industrial sealed lead acid battery for my radio shack. This battery is
model M12V90; it's rated 90-ampere hours. This battery has an internal resistance of 3.7
milli-ohms. It will develop 3365 short circuit amperes! I use to test batteries by using a
piece of #12 house wire and just racking it across the terminals. If you got sparks, I
considered the battery OK. I did this with a Marathon battery and it burnt up the #12
wire, while it was in my hand. I use a meter now, lesson well learned.
Features to look for in a good sealed acid battery are reinforced and thick cases. Also
consider you need to mount the battery in a case that will contain the contents, in case the
battery cracks open. The case should have baking soda in it or other neutralizing
chemicals. The battery should use flame retardant casing. It should have a built in flash
arrestor in the venting system. The battery should be a high-tin, calcium, silver, lead
positive plate designed for maximum service float design, and say a 10-year life,
minimum. The venting system should be a one-way system that self seals after venting.
If I were designing battery systems for a telecommunications center, it would be designed
around figure 1. A power supply is sized to carry the load of the equipment. The battery
charger is designed to only supply 1/10th of the ampere-hour capacity (or less) of the
battery system. Diodes are used to direct the flow of current towards the load - the radios
in our case. When the power goes off, a power supply can began to look like a load to
the battery system. Some power supplies will fail upon the application of reverse current.
Using a high current diode to isolate and steer the current, will resolve the problem. The
diodes must be rated at the maximum load, and for safety reasons, you should double the
required rating minimum for gross fault conditions. Also the power supply must be
operated at a voltage higher than the battery charger. If the voltage in the charger and the
power supply were the same, it would attempt to share the load. But, the charger is
typically rated considerably less than the power supply - and cannot carry the load by
itself. So you turn the power supply voltage up, just above the charger voltage. Also the
diodes will drop 7/10th of a volt and that has to be considered. So if the charger is
running at 14.5 volts, the power supply should be adjusted to say 14.7 volts.
A typical diode to use is a 1N1183, it's a DO-5 stud mounted diode. Two of these units
on a heat sink designed for two TO-3 transistors should provide adequate heat sinking.
You can pick up these diodes fairly cheap at a ham-fest. If you purchase new, expect to
pay about 6 dollars each. This diode is rated 35 amps RMS at 50 PIV.
Your power supplies and chargers should have metering circuits. This will allow you to
monitor the system for state of charge and by a monitoring, you can determine when the
battery needs replacing. Batteries usually failed by shorting a cell out, so a 12 volt
battery now becomes a 10 volt battery - the charging circuit will show a hi rate charge
that does not come down.
The power buss in figure 1, should be a very low resistance conductor, such as 1/2 wide by
1/8-inch thick copper. A buss bar system should be covered/shielded so no conductors or
tools can fall across it.
In figure 1, a typical ham station setup (for say a 100 watt HF rig and a 25 watt 2M rig)
would consist of a 35-amp power supply. F5 would be a 35-amp fuse. D1 should be two
1N1183s in parallel. The charger would be a 10-ampere current limited charger. F3
would be around 15 amps. The battery would be a 90 to 110 ampere hour unit. F4 would
be 40 amps. D2 would be two 1N1183s in parallel. F6 would be about 25 amps for a HF
rig; F7 would be around 8-10 amps for a 25 watt 2M rig.
As most hams do not want to spend a lot of money on the hobby, you may consider using
a single power supply to run the shack and charge the battery. Refer to figure 2. The
power supply should be designed to current limit its output. If we have a operational
load of say 25 amps, we need a power supply that can supply about 35 amps continuous,
and it should current limit to around 35 or 37 amps maximum. D1 should be two
1N1183s in parallel. F1 should be 40 amps. F2 should be 40 amps. As the diode is
ahead of the battery, the power supply will have to be adjusted to supply 13.7 volts at the
battery or 14.4 volts at the power supply terminals.
I cannot over-emphasis, safety first, and use a fully fused system. It can save you life.
Also consider making a fuse panel that is easily accessible. The fuses should be rated at
25 volts or higher. Use fast blow fuses for the lower power accessories, and slow blow
fuses for the radios. Use a fast blow fuses for the battery output, and slow blow fuses for
the battery input.
When you mount your battery in a case, tape you battery receipt to the side of the case.
This will come in handy, if you have a warranty problem. Also, you may be amazed at
how long your battery may last, by having it's in service date.
Batteries should also have a maintenance charged placed on them once or twice a year.
This charge is called an equalizing charge, it is a charge at a higher than nominal
operating voltage, usually for 24 hours. You need to keep records of this, your station log
book is a good place to enter this data. I equalize my Marathon battery once a year, and I
run it at 14.1 volts for 24 hours.
I would like to point out, that I cannot be liable for the use of this information, but I
have
tried to present it, in a useful, accurate and safe manner. You will ultimately be
responsible for your actions. If you are uncomfortable with designing a system, consider
retaining professional or other qualified personnel to assist you. I cannot stress to
consider safety in every aspect of a battery operated power supply system.