BATTERY BACK UP – THE RIGHT WAY

by W8CM

 Would you like to keep your ham FM radio on the air when the commercial AC power has been interrupted, so you can participate in emergency communications?  That, after all, is an essential essence of ham radio’s service to the community, not to mention your own piece of mind while sitting in a dark house with the wind whistling outside and you don’t know what’s going on.  While I have a great 7.5 KW AC generator at my house, it takes time to start up, stabilize, and connect in order to provide power.  I wanted another, instant changeover emergency power source for my radio so I could participate without interruption. 

 The key phrase is “without interruption”.  Sure, I could have a battery sitting around that could substitute for the trusty DC power supply, but that would take some time to change the power cord, and with my luck the battery would need charging. Let’s expand that demand: “without interruption, and with battery charging”.

 If you’re thinking you can do this by just hooking up a battery across the output of your DC power supply, DON’T DO IT! - lots of bad things can happen, including damage to the battery and/or the power supply by currents flowing in the wrong direction, etc. What is required is a “Battery Backup Module”, or BBM.  On my repeaters, I always use a BBM feature.  For general ham radio use, I have found two great stand-alone commercial products that can be integrated into a ham’s station, along with the existing radio and a battery, for a very nominal cost.  These units provide the function of controlling the DC current flow while preventing “reverse” currents that can damage the battery or power supply, and also provide a battery charge feature. Here is how the Battery Backup Module (BBM) is connected:

 What is inside the BBM?  Pretty simple, actually: a couple ‘big’ diodes and power resistor(s) with a good heat sink.  Schottky diodes are preferred because the voltage drop across them is only .4 volts and thus they operate cooler.  In this article I’ll discuss the BBM12100 model, a 100 amp unit by Samlex America. Here is what is inside the BBM12100 (a BBM1225  25 amp unit is also available from Samlex, as is a 40 amp unit by West Mountain Radio called the PowerGate 40 or PG40):

 As noted, the current capacities of the three units vary, and the charging current is nominal.  The main difference between the Samlex and West Mountain units is that the Samlex uses general purpose extra heavy duty screw terminals, while the West Mountain unit uses the Anderson PowerPole connectors.  The BBM12100 and PG40 cost about $60 for either model, so it’s your choice between 100 amps or 40 amps, and screw terminals or PowerPole connectors.  The BBM1225 25 amp unit costs a little less, but I suggest using the higher current model(s). 

Web sites: www.samlexamerica.com         and            www.westmountainradio.com

              SAMLEX BBM12100                                         POWERGATE P40