[aprssig] backup pwr systems

Joel Maslak jmaslak-aprs at antelope.net
Sat Dec 9 23:02:41 CST 2006

On Dec 9, 2006, at 9:38 PM, Tad Burnett wrote:

> The secret is to not let the battery voltage go above 14.2 volts...
> 13.8 will not fully charge a battery so you do need to charge up to  
> 14.2...


I might add that every automobile I've worked on has a charger  
(alternator/generator) continuously running, connected to the battery  
with no diodes, electronic switches, complex charging circuits, etc.   
They do use a relatively simple voltage regulator to control the  
field current, which in turn keeps the stator voltage where it needs  
to be (also reducing the need for the regulator to handle high  
voltages, since it only has to regulate the relatively small current  
field, not the high current stator current).  In effect, this means  
there is a steady voltage power supply in an automobile, wired IN  
PARALLEL with a battery.

Long road trips don't hurt car batteries at all.  And it's not  
unusual to get 5 or more years out of a car battery even when  
environmentals are poor for the battery (hot and cold temperatures).

If you play around with some electrical theory and find out the power  
supply voltage and battery voltage are the exact same, you'll find  
there is no current in a circuit with a power supply and battery  
wired in parallel.

That said, it's good to not charge batteries too quickly.

(nor do I recommend using a car battery - they don't do deep  
discharge well)

So...in essence, you don't need a good charger at all.  You need a  
power supply that never supplies too much voltage.  A properly sized  
diode and resistor in parallel on the positive lead of the battery  
will manage this quite well (diode to let the battery discharge at  
full current, and resistor to ensure that you never are charging too  
quickly).  Don't forget fuses - your power supply's internal fuse  
won't keep your house from burning down when the rig's power cable  
shorts out - you need a fuse on the battery's positive lead rated for  
maximum load of everything hooked up, and a fuse on every wire in the  
system that can't carry the maximum load of the previously mentioned  
fuse (for instance, if you have a 12 gauge wire going to your TNC,  
you probably don't want your 50 amp fuse on your battery to allow 50  
amps through that 12 gauge wire - and even 12 gauge wires can start a  

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