Simple Battery Back-up for Radio Equipment

Many of us may have experienced a power outage during the recent wind storm.  If you didn't have an HT or an emergency battery to run your equipment, you were out of luck.

No power = no radio = no communications

There is a solution for this dilemma. It is called a Battery Back-up system. A 12V sealed lead acid battery can provide a good bit of operating time based on the power demands of your radio and the Ampere-hour rating of the battery.

A 7AH battery can be obtained for around $20 and can provide a number of hours of useful operating time. Larger capacity batteries are available depending upon how big a bankroll you have to invest.

But these batteries must be charged and kept charged. There are commercial units that not only charge the battery but trickle/float charge to keep the battery "topped-off".

If you don't want to invest in a commercial charger, you can build your own charger with a few parts and a 13.8VDC power supply.

Batteries should be charged at about 0.10 times their AH rating. So a 7AH battery will require about 700 milliamps of charging current. The diagram below shows how to construct a charger for your battery. It will perform both the charge as well as the float charge functions. It will allow your equipment to automatically switch to battery power when the AC Power Lines go off and recharge the battery when the power is restored. 

The current limiting resistor (lamp) will limit the charging current to about 750 ma and keep the power supply from tripping off during initial recharging of the battery after a power outage. Sealed batteries should be charge-limited to 0.10 x AH rating. A battery of 7 AH or larger can safely be charged with the resistor (lamp) shown. Depending on the size of the battery, it may take an extended period of time to bring the battery back to float voltage. It is desirable to recharge the battery off line with a standard battery charger after an extended power outage.

For larger loads, the current capacity of the diodes will need to be increased. One method would be to use one half of a full-wave bridge rectifier with a heat sink. Bridge rectifiers up to 25 amps can be obtained from Radio Shack. A power resistor could be substituted for the lamp, selecting it to limit the charging current to something less than the rating of the power supply.

NOTE: See the article titled "An Uninterruptible Power System for 24/7 Operation" in the September 2006 issue of QST Magazine on Page 33.

John Fletcher, KB7FND