Battery Backup or
Backup Battery
What size battery do you need to
power your radios?
Here is a simple formula :
[Total Receive Amperage Draw +
{(Transmit Amperage Draw X Mode Duty Factor) X Percentage of Transmit
Time}] X Hours of
Operation = Amp Hours
OK, how to "plug in the numbers:
Start by looking in your manuals. To figure you amperage draw, you need
to figure all connected equipment. You need to know what each piece of
equipment draws on receive, and what it draws on transmit. You also
need to use a duty factor for the mode you are operating. Here is a
good guide to those factors:
Mode 
Duty Factor 
(CW)

40%

SSB

20%

FM

100%

Digital

100%

AM

100%

To get to the "Total
Receive Amperage Draw" add all the amperage figures for your
connected equipment on receive. For instance:
VHF Radio = 3 amps
TNC = 1 amp
Total = 4 amps
Then figure your "Transmit
Amperage Draw":
VHF Radio = 7 amps
TNC  2 amps
Total = 9 amps
Now apply the "Transmit Mode
Duty Factor":
9 amp X 100% = 9 amps (duh!)
Now, multiply the "Transmit
Amperage Draw X Percentage of Transmit Time". Say you figure you
will be keyed 50% of the time...
9 amps x 50% = 4.5 amps
Add that to the "Total
Receive Amperage Draw" to the "Total Transmit Amperage Draw"...
4.5 amps + 4 amps = 8.5 amps
Multiply that figure by the "Hours
of Operation", say 5 hours...
8.5 amps x 5 hours = 42.5 "Amp
Hours"
So you need a battery rated at 42.5 amp hours or greater to operate 5
hours without a recharge.
SLA type computer UPS battteries are good choice for the
station backup
supply. Some power supplies can be used to charge a battery, some
cannot. A better solution is a 3 stage charger, with the lowest stage
being a "trickle" or "maintenance" charge.
WARNING:
All the above data is to be used at your own risk!