[time-nuts] [OT] Re: Power Back-up

phil fortime at bellsouth.net
Tue Aug 11 16:31:20 UTC 2009


----- Original Message ----- 
From: "David Martindale" <dave.martindale at gmail.com>

> There's another potential problem with connecting larger batteries, too:
> heat.  Small UPSes often have no cooling fan, and minimal heat sinking for
> the power transistors when operating in inverter mode.  The UPS designers
> know that, with the stock battery, the inverter run time is going to be
> about 4 minutes at full load and maybe 30 minutes at very light load.  So
> they just need to provide enough thermal mass to spread the heat around
> without temperatures getting too high for that short run duration, not
> enough cooling for the longer runtime provided by larger batteries.
>
> I have one UPS (a SL Waber) that has the inverter FETs and diodes mounted 
> on
> a big chunk of aluminum inside the case, without any cooling fins on the
> aluminum, no fan, and only small slots in the plastic outer case.  But the
> battery is only 12 V 4.5 Ah.  This pretty clearly wouldn't survive being
> connected to a 20 Ah battery, even if you provided an external charger to
> keep the battery charged.
>
> The APC UPSes at least have conventional heat sinks on the transistors, 
> but
> the smaller units have cases with no ventilation openings.
>
> If you "re-engineer" one of these for longer run time, you may need to
> change heat sinks, add ventilation slots, and/or add a fan.
>
>      Dave

The real pitfall with using larger than designed batteries in a UPS is the 
additional current required to charge the larger batteries. "Transistors" 
are not used in the output on most UPS's any more, most use power FET's and 
with it's lower resistance, doesn't dissipate as much heat.  Running in a 
lost power state is not the issue; it's when the power comes back on and 
starts charging the batteries.

     Phil 




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