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

[David Martindale]

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

On Mon, Aug 10, 2009 at 7:24 AM, Dave Baxter <dave at uk-ar.co.uk> wrote:

> Sadly, if you try to use higher capacity batteries, that is, "many
> times" higher (cabling in say 80AH batteries instead of the original
> 17AH types!) their internal chargers can't cope, and flag the batteries
> as bad all the time, even if they are the same "type" (AGM, Jelly etc)
> as the originals.
>
> (Some of the small desktop units, have pitiful chargers, literally just
> a trickle at best...)
>
> Also with second hand UPS's, as many of them are fan cooled when running
> Off Battery, and there is often little to no air filtering, remove the
> cover (after all the usual precautions) from time to time and blow out
> all the dust bunnies and dead bugs etc, & also check the fan is OK.
>