Bob Kenyon – K8LJ
We
all know the importance of battery power for emergency communication needs and
have been encouraged to have a ready supply of alkaline cells and other batteries
on hand for that purpose. But what happens if that is insufficient during some
particular event?
That
got me to thinking about other potential sources of battery power and hence the
title “Creative Emergency Battery Power”, to deal with situations where
additional power sources need to be acquired quickly. An inventory of our home,
garage and barn revealed the following (neglecting the small things, of
course): Three cordless drills, two cordless screwdrivers, a weed-whacker (nice
gel-cell), a socket wrench, home alarm system (nice gel-cell) and a small
vacuum cleaner. The batteries of any these could probably be pressed into
service, one way or another in an emergency, perhaps with some difficulty.
But
what caught my eye was one of the drills, which came with two 14.4 volt battery
packs. What intrigued me was the voltage and the fact the battery packs had
stiff blades for contacts, which easily mated with common spade lugs, making it
very easy to connect to them - spade lugs on one end of a cable and Anderson
Power-poles on the other.
The
14.4 volt rating gives a good bit of “head room” above a conventional 12 volt
source (actually nominally 13.8 volts) and allows the radio to operate for a
longer period of time as the battery discharges. A concern, however, is that
the 14.4 volt rating (twelve 1.2 volt cells in series) is its nominal voltage
rating at full load. Fully charged and unloaded, the voltage will soar somewhat
above 16 volts, which could damage some radios (see your manual for voltage specs.).
The
solution is quite simple – place 2 or 3 forward biased silicon diodes in series
with the positive lead to lower the voltage the radio sees. The voltage drop
across each diode will be about .7 volts and, unlike a series resistance, is
independent of current flow and the power sacrificed is very low. For a 1 or 2
watt HT, diodes rated at a few hundred ma. in the forward direction will
suffice. For even greater utility, place the diodes in a small container, with
mini switches across each, that allow you to selectively short them or leave
them open and in the circuit. This will yield a greater duration of operation:
when the battery is fully charged, have the diodes un-shorted to prevent
over-voltage to the radio and as the battery voltage falls over time, short the
diodes one-by-one to bring the voltage seen by the radio up in .7 volt
increments.
Some
radios have a built in voltage monitor which makes this quite easy to do. If
yours doesn’t, you’ll either have to monitor it with an external meter (a
hassle) or react when the microprocessor/phased-locked-loop starts to
malfunction as the voltage drops too low. Look at your radio specs or try it
out on a bench supply and determine at what voltage level this occurs.
The
purpose of this training tip was to get all of us to “ think outside the box” a
bit concerning potential sources of
emergency battery power. I’ve just touched the surface. Don’t rush out and buy
a 14.4 volt drill unless you need one for other reasons. These days there are
also 19 and 20+ volt drills available. They’re great for their intended
purpose, but the voltage level is so high you would need a voltage regulator
circuit for the uses described here and that’s much more of a hassle than a
couple of series diodes. Let’s all
think about ways to come up with power sources that might not be so obvious at
first glance but might be of great value when needed.
The
bottom line is - do it now and don’t wait until disaster strikes.