[Elecraft] Keeping the battery charged

[Bob Lewis (AA4PB)]

> In this configuration, I don't have a power connector on the lid,
except for
> the internal battery connection that plugs into the main board.

What you need to "float" the battery is 13.8 Volts at the connector
that normally goes to the radio. The current limiting resistor is in
the lid so you are protected from excessive charge current when the
battery is fully discharged. The only potential problem with directly
connecting a 13.8 volt power supply is that if the AC power fails, the
battery could possibly discharge back thru the resistor and the
supply. You can guard against this by putting a diode in series with
the positive supply lead but then you need to increase the supply
voltage by the diode drop. If you get one of the diodes that Elecraft
uses (0.2 volts as I recall) you will need to adjust the supply to 14
volts. If you use something like a 1N4007 (0.7 volt drop) you'll need
to set the supply to 14.5 volts.

Another option is to get an automatic charger such as the one offered
by PowerSonic or A&A Engineering and connect it directly to the
battery. Just be sure the maximum charge rate for the charger is less
than about 600 mA. Since the battery is exposed with the lid off the
radio just unhook the elecraft cables and clip the charger cables to
the battery.