[aprssig] Re: Charge controllers

Stephen H. Smith wa8lmf2 at aol.com
Fri Apr 29 18:22:34 EDT 2005

ad6nh at arrl.net wrote:

> Well, speaking of charge controllers (I think we were speaking of them 
> here a few days ago), does anyone have a source for a simple charge 
> controller that I can use between an Astron P/S and a battery?  I'm 
> looking for an affordable backup power option - the West Mountain 
> Radio PwrGate is nice, but it's a bit pricey.  Many of the charge 
> controllers I'm finding on Google are of the variety for solar panel 
> charging, and are very expensive.  This is for a digipeater site that 
> is subject to power outages and needs battery backup.  The idea was to 
> have the Astron go through the charge controller to the battery, and 
> the digi hooks direct to the battery.  Thanks!
As long as the power supply is well regulated, you don't need a charge 
controller at all.   Use a good digital voltmeter to set the supply 
output at about 13.2 volts and connect both the battery and the load 
directly across the supply.  At 13.2 volts, the battery terminal voltage 
will rise to exactly match the supply voltage when the battery reaches 
full charge.  With the supply and battery terminal voltages equal, the 
battery will stop drawing current and can be floated indefinitely.      
You DO want some sort of maximum current limiting in the supply, so that 
when the AC power comes on after the battery has been partially 
discharged, you don't get an infinite inrush current that will overheat 
the supply or blow a fuse.  Further, you may want a low-voltage 
disconnect controller to prevent the batteries from discharging during a 
power outage  to such a low voltage that they get permanently damaged.

You really only need charge controllers when the charging source can 
potentially produce a voltage well above the battery voltage (such as a 
solar panel or simple transformer/rectifier battery charger) that can 
continue to force high current into the battery, even after it has 
reached full charge.   That is, if the charge voltage is high enough 
that the battery voltage can never rise to match it, the voltage 
difference will cause a large current to flow into the batteries 

This floating of batteries directly across the load with a 
WELL-REGULATED precisely-set power supply is done all the time in 
telecomm sites. 

Stephen H. Smith             wa8lmf (at) aol.com
Home Page:                   http://wa8lmf.com

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