[KYHAM] KEN TRAINING for April 22: A 12 volt stand-by power system

[Ron Dodson]

A 12 volt stand-by power system
Anthony Whobrey KC4JTV

This simple system will provide a back up source of 12-volt
power for your station. It has no moving parts, and is
silent while operating.

The major components are a regulated 12-volt power supply of
6 to 20 amp rating, and a 12-volt automotive battery. Many
people would use the power supply as a float charger across
this battery to produce a simple source of continuous power.
This approach is easy, but can be improved with at minimal
cost and with little effort.

Place your battery in a safe well-ventilated location.
Nearly all batteries produce explosive gas while charging;
it is best to have your battery in an area separate from
your operating position. My battery box is in a garage
adjacent to my shop, with 8-gauge wire running about 15 feet
to my radio. I used a plastic box designed to house a
trolling motor battery; they are not too expensive.

Always place a fuse at the positive battery terminal, I use
a large paper cartridge fuse (non-30). This fuse is
available at any electrical supply store. All equipment
except the power supply should be connected though this
fuse. Use a quality fuse holder and wire of adequate gauge
for the fuse rating. Remember that the battery can supply
hundreds of amperes for a considerable time, don�t cut
corners here.

Connect your power supply through a suitable fuse and a
large rectifier to the positive terminal of your battery.
Place the fuse near the power supply output and the
rectifier at or near the positive battery terminal, in this
way the wire is protected from a short circuit from both
ends. Most power supplies will overcharge a battery; the
voltage is usually a bit high. Use of a rectifier will drop
the power supply voltage by .7 volt or so and also protect
the battery from discharge through the supply during a power
outage. Check with the manufacturer of your battery to
determine the recommended float charge voltage (generally
12.8 to 13.2 volts). It may be necessary to adjust the power
supply voltage slightly, or alternatively, use two
rectifiers in series to further reduce the charging voltage.

While running the wiring, it isn�t a bad idea to find a
place for a 12-volt light. Use an automotive type dome light
to supply a handy source of emergency illumination. A couple
of sources offer a 12-volt �trouble light� with a magnetic
base and retractable cord; they are nice if your budget
isn�t too tight.

After an outage of a few hours duration, your power supply
will recharge the battery within a day or so, depending on
its current capacity. In the event of a prolonged outage,
consider other means of recharging the battery to near full
capacity, before turning on your power supply. Most power
supplies in common use are not current limited, and the load
presented by a deeply discharged battery may blow the output
fuse. If your battery becomes deeply discharged, use an
automotive type charger (or your automobile) to bring it to
nearly full charge before placing your power supply back in
service.

This system is not too expensive to build, and offers
instant transfer to back up power; it will switch to or from
battery power while your radio is in operation without the
slightest interruption. Many commercial and public safety
base stations use battery and charger systems, the same
concept is also used in telecommunications offices. This
system, while on a somewhat smaller scale, will work well
for the average emergency operator.