>There is a simple way to have a reliable backup battery system without a
>relay. Try this:
>
>Connect your fused repeater power cable directly to the battery terminals,
>without any intervening devices. Connect your power supply to the battery
>through a single Schottky diode. Adjust the power supply voltage so that the
>battery is floated at its optimum voltage, to account for the small forward
>voltage drop across the Schottky diode. I use a Schottky diode because its
>forward voltage drop under load is much smaller than that of a silicon diode
>and is stable.
>
This is the way I have operated several repeaters for over 5 years. The only
difference is the charger for the battery. I use a standard transformer
rectifier charger, with no filtering. The battery provides all the filter
required. The main advantage in using the battery charger is that when the
battery goes completely down, it will not destroy the power supply trying to
recharge the dead battery. The battery charger is designed for this type
service and will bring a dead battery back up to voltage without a charger
failure. The diodes in the charger serve to disconnect the charger from the
battery load when the 115 VAC is not present, so no external diode is necessary.
As Eric points out above, connect the charger and load directly to the battery
terminals. I actually connect the load to the wing nut terminals and the
charger to the battery posts to keep the impedance between the charger and the
load to a minimum and have no common path for load and charger except for the
battery itself.
>Unless the repeater operates almost continuously, the power supply does not
>need to match the current draw of the radio during transmit. I have a 50 watt
>base station set up this way that has a 26 Ah VRSLA battery floated by an
>Astron RS-10 power supply, and it has been 100% reliable through many power
>outages.
>
I use a charger rated at 10 amps for a load that peaks at 20 amps. The battery
supplies the excess current when the repeater is key down and is recharged when
the repeater is key up. The filtering effect of the battery keeps the DC with
minumum ripple even under load.
The Schulembarger (SP?) transformer/diode chargers I use do not provide the
13.5 VDC float voltage I want with a standard setting on the charger. The 2
amp setting provides too low a voltage and the 10 amp setting provides too high
a voltage. With 115 VAC on the primary I put the 6 volt secondary of a 115:6
volt transformer in series with the input voltage to the charger to either add
or buck the voltage to the charger. I can set the battery voltage to precisely
13.5 VDC with the proper combination of add/buck.
With no other electronics in the charger besides the diodes, there is a minimum
possibility of a problem in the power system, and absolutely no possibility of
an overvoltage to the equipment should there be a failure.
I take the buck transformer out of the circuit every few months to 'stir' the
electrolite in the battery to reduce the sulfation of the plates. I use a
stardard type 27 deep cycle battery from Wall Mart for the backup and have had
excellent service for over 5 years in the several repeaters using this scheme.
73 - Jim - W5ZIT
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