Batteries

Battery power is critical for emcomm (emergency communication) operations. AC power cannot usually be relied upon for any purpose and portable operation for extended periouds is common.

Batteries must be chosen to match the maximum load of equipment, and length of time that operation must continue before they can be recharge. It is advisable to use "Sealed Lead Acid" (SLA) battery.

SLA batteries should never be deeply discharge for example a 12Volt SLA battery will be damage if allowed to drop below 10.5 Volts. Exessive heat or cold can damage SLA batteries too.

How to calculate battery Ah ?

Estimate 24hrs power for base station operation on VHF transceiver.

Received Current 1 amp x 24hrs = 24

Transmit Current  5 amp x 6 hrs  = 30

Total AH                                         54Ah

54Ah Actual consumption ...

Actual battery choice  54 x 1.2 = 64.8Ah

The nearest choice is 60Ah battery which is available

in the market.

Battery Chargers

You should have two or more batteries. So that, one can be charging while another is in use. It is best to slow charge all batteries since this helps avoid over heating and extends their over all life span. In general automative and deep cycle batteries can be charged with an automobile and jumper cables or any constant voltage source. If proper battery charger is not available  any DC power supply of suitable voltage can be used but a heavy duty isolation diode must be conected between the power supply and the battery. This is important since some power supply have a “crowbar” over voltage circuit which short circuits the output if the voltage exceeds a certain limit. The output voltage of the supply must be increased to composate for the diode voltage drop. 

The charging voltage must be kept between 13.8 and 14.5 volts. A good rule of thumb is to keep the charging current level to no more the 1/3 its rated capacity. For example if you have 7Ah battery you should charge it at no more than 2 amps. The time it takes for SLA battery to recharge completely will depend on the amount of charge remaining in the battery. If the battery is only 25% discharge than it may recharge in a few hours. If the battery in discharge 50% or more than it will take between 18 – 24 hours for charging.

DC to AC Inverters

While direct dc power is more efficient and should be used whenever possible, inverters can be used for equipment that cannot be directly powered with 12VDC.

Not all inverters are suitable for use with radio, computers or certain types of battery chargers the best inverters are those with “true sine wave” output. Inverters with a modified sine-wave output may not operate certain small battery chargers and other wave form sensitive equipment. In addition all “high frequency convertion” inverter generate significant RF noise if they are not filtered both radiated and on ac output. Test your inverter with your radios power supply and accessories (even those operating nearby on DC) inverters should be grounded when in operation, both for safety and reduce radiated RF noise.