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Battery Backup Information

Written by Mike Morris WA6ILQ
Maintained by Robert Meister WA1MIK
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If you inspect the circuit diagram you will see that the BB option is simple - it consists of a pair high current diodes wired cathode to cathode, with one anode going to the supply, and the other to the battery bank, and the cathodes feeding the load. This configuration allows either the power supply or the battery to feed the load. In the low current supplies they use half of a bridge rectifier, in the high current supplies they use two separate diodes in parallel for each current path. The charging circuit is not well designed as it is just a single resistor from the battery to the output of the supply (look for R103 in the schematic for the RS-12-BB and RM-35A-BB)... and this overly simple charger circuit can boil your battery dry! Since this design uses the supply voltage as the charging voltage you must have the supply voltage set EXACTLY to the voltage that is the sum of the diode voltage drop plus the battery float voltage... regardless of what the voltage your load wants to see (and the voltage drop across a diode varies somewhat from low-load to full-load). Overly simple charging circuits ALWAYS leave lots of room for improvement.

The rule of thumb on maximum charging current for nickel-cadmium or lead-acid batteries is no more than 1/10 of the amp-hour capacity of the battery. Therefore a 20 amp-hour battery has a maximum charging current of 2 amps.

There are two major flaws with the Astron charger design:

Now in real life, the battery voltage is not going to be zero, so the difference voltage is not going to be the full Astron output of 14 volts and the charging current is going to be less than 8 amps, but still... there are better charging circuits out there than a 14 Volt 50 Amp power supply and a single undersized current limiting resistor. Especially with the extra requirement that the supply voltage has to be set to the battery plus the series diode voltage drop (no matter what voltage the load - your repeater system - wants to see).

If you need a battery backup system for your repeater, I would do one of two things:

I've seen an Astron where the owner mounted a DC toggle switch (I'd use a toggle-type-circuit breaker) of the appropriate amps rating on the front panel of the supply near the diodes, wired it in series with the R103 charge current control resistor and labeled it as "Internal Charger Enable / Disable".

If you are going to "roll your own" high current BB conversion, and you don't mind a "non-pretty" implementation, then a part of the diode array from a high-current alternator will work just fine. The normal "12 volt" automotive alternator diode array has six diodes in it (or sometimes 12 in 6 groups of 2 in parallel or 18 in 6 groups of 3 in parallel), and the metal plates with the diodes mounted in them are usually tossed in the trash if one diode is dead. A long time ago I needed to swap out the alternator in my old 1971 Dodge Dart DEA undercover chase car... (that was a FUN car to drive...) I used the visit to the alternator rebuilder to pay $5 for a three-diode common-cathode array out of a 120 Amp alternator that had one bad diode that was on the end of the plate.

When I got home I mounted the diode plate in a bench vise and used a drill and hacksaw to remove the bad diode out of the end piece. This gave me two good diodes and enough space around the diodes to allow drilling a couple of mounting holes. Using that diode plate mounted on standoff insulators I ended up with the same circuit as the Astron BB option at a cost of $5 plus the standoffs... and in the proof-of-concept prototype the plate was held to two pieces of wood by some drywall screws. The final version used polystyrene rod for the insulators. All of this was done a couple of years before Astron came out with their BB option. Nowadays it may be easier to just buy the BB version from Astron - but sometimes it's more fun (and more educational) to roll your own.

Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.

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