[mythtv-users] Power outages and UPSs

David Brodbeck gull at gull.us
Mon Jul 30 16:40:42 UTC 2007

On Jul 29, 2007, at 8:33 PM, f-myth-users at media.mit.edu wrote:
> [This also means that you're wasting your money on that spiffy AGM
> battery; if you only need to ride out a tiny outage and are willing to
> have things die in a few minutes, then almost any cheap half-dead
> battery will do.  In fact, several cheapies in parallel may work
> better if you can keep them balanced, because that way one dead one
> with high resistance won't hose you during a 10-second outage.  Come
> to think of it, maybe you can use the one from your car the next time
> you have to replace it?

Remember, if you use a car battery in an indoor application you'll  
have to vent it.

AGMs have the advantage of being totally sealed, which makes them a  
bit friendlier to use indoors.  Probably still overkill for a UPS  
application, though -- ordinary gel-cells are cheaper.  They have  
some pretty strict charging current restrictions, though.  Charging  
too fast will overheat them and pop the vent caps, and then they  
quickly become worthless.

On the whole I think building this kind of thing is only worthwhile  
in three circumstances:
- You have some oddball application where a commercial product won't  
- You really like to tinker.
- Your time isn't worth anything.

Otherwise you're better off going out and buying a battery charger  
soldered together by some kid in a factory in China.

If you want to tinker and your total battery capacity is low, you  
might check garage sales and surplus outlets for an old Panasonic VHS  
camcorder, the kind with the square battery that slides into a slot  
in the camera.  They used gel-cell batteries and had pretty well- 
designed external chargers.  They're intended for batteries of around  
4 AH, though, so if your battery bank is large they'll take quite a  
while to charge it.

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