----- Original Message -----
From: "David Martindale" <dave.martind...@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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