Wednesday, March 11, 2009

Battery backup for ham radio gear

The second issue I heard on last Monday's TechNet (a weekly net held by the club I'm a member of) was on the topic of battery backup for repeaters and other ham radio gear.  This issue was raised by Ryan (KC9OFF), and as it happens his specific situation is not going to be that interesting to hams as he is trying to provide power to an Icom FR3000 series repeater, which is not ham gear (it's business band gear) and that particular repeater already has a built-in battery controller so setting up a backup battery on it is just a matter of connecting a battery to the terminals on the repeater in the correct polarity.  (I really should charge him a professional fee for researching this for him, but hey, whatever.)  However, us hams tend to use gear with slightly fewer "creature comforts", which means we have to roll our own (or at least put together from components) power controllers for such thing.  Not that that's a problem: rolling our own  is part of what we do, right?

During the net I asserted that there was an article in November's QST on battery backup systems for ham shacks; fortunately, I was right: the article in question is on page 76 in the regular "Getting on the Air" column.  That column describes two approaches to battery backup.  The first approach is use an inline charger and a boost/buck regulator to provide regulated 13.8 volts to the radio no matter what voltage is provided by the charger/battery combination (which could be as high as 16 volts when the charger ramps up, or as low as 10 volts as the battery runs down).  The second is to use a switching power controller that has separate inputs for charging power and the battery; the difference is that the power controller will not draw from the battery unless the main power is down, while the inline design draws from the battery and the line power (charger) to the extent that each can provide power at any time.

In the inline design, you will likely draw from the battery while transmitting, because the charger probably only provides 5 amps at most and cannot provide the full 20 amps or more that a typical rig pulls while transmitting.  As long as your transmit duty cycle is low enough, the charger will have enough time to recharge the battery between transmissions.  The main problem with boost/buck regulators is that the majority of regulators out there generate "noisy" DC; virtually all of them use pulse-width modulation for power control, which generates RF noise and will require significant filtration to avoid RFI issues.  The switching power controller does not do this, but will present a variable voltage to the radio when switchover occurs and as the battery discharges (unless it also contains a boost regulator, in which case all the issues with those again arise).

Several people on the net noted the need to include protective hardware (mainly fuses and diodes) to prevent the battery from draining through the power supply and to protect against various sorts of possible failures that could lead to shorts.  The inadvisibility of using an ordinary car batteries (which are optimized for short high drains, not for extended low drains) was also mentioned.  Car batteries should never be used for backup power.  There are a variety of deep-cycle options for this purpose which are designed for long-term moderate-power draws.  Many are also engineered to avoid venting hydrogen gas, which would create an explosion risk when used in an enclosed space.

The solution I'm still looking for I haven't found yet in commercially available gear, and involves using solar as primary power and a battery rack as backup during the night, with line power being used to charge the batteries only if they drop below the point that they aren't expected to recharge in the sun the next day.  I think I've seen a controller that supports multiple power sources, so if I put the solar panel on one input and a line supply on another, but use a switch/relay to only provide AC to the line supply when the battery voltage is below the threshold, (with some hysteresis to avoid chugging) then the system would perform in the manner I desire.