[time-nuts] Power Back-up

[Joseph Gray]

Over the years, I have gotten several APC Smart UPS for next to
nothing. In every case, all they needed were new batteries. Unless
they are physically damaged or a circuit board is toasted, they seem
to work fine. The attitude seems to be "The battery died, throw out
the UPS and buy a new one."


On Sun, Aug 9, 2009 at 5:40 PM, Philip Pemberton<lists at philpem.me.uk> wrote:
> J. Forster wrote:
>>
>> When considering a battery backup you really have two ways to go:
>>
>> Battery -->  Inverter -->  Load
>> Battery ---> Load
>>
>> The first option is generally easy to implement, but a lot of the battery
>> capacity goes up as heat in the inverter. As others have pointed out,
>> inverters are not very efficient or electrically quiet, and they sop up
>> quite a bit of power w/o any load at all. For this reason, the second
>> option is better, IMO.
>
> In a lot of cases, you'd be stepping up from the 12V, 24V or 48V the
> batteries provide, then the PSU in whatever you're running would be stepping
> that back down to 5V, 12V or a mix of voltages.
>
> I've built a (relatively) low-power PC server out of a Mini-ITX motherboard
> (Jetway NC92-230), two hard drives (a Seagate 'cuda 7200.10 and a 7200.11,
> both 500GB), 2GB of RAM... and a 120W PicoPSU. The PicoPSU plugs straight
> into the ATX socket, takes 12V in, then converts it to the +5V, +12V, -5V
> and -12V the motherboard needs.
>
> Efficiency is supposed to be around 96%, but I haven't done any
> measurements. As a bonus, it doesn't need any form of active cooling (read:
> cooling fans) to keep it cool, so the whole machine is pretty quiet. Well,
> aside from the Seagate Barracuda hard drives, which seem to be designed to
> make as much noise as possible.
>
> My plan is to get a couple of lead-acid batteries and rig up a
> battery-backed AC-to-DC supply to run the server, external hard drive, DSL
> modem and WiFi access point. That's still on the drawing board, though --
> the power to this menagerie has only been interrupted once since it was
> installed, and that was down to a lightbulb blowing and tripping the RCD...
>
> As for the desktop... I'll probably end up buying a Back-UPS or something
> along those lines. I figure I only need 5 minutes to hit "Shutdown ==>
> Suspend to disc", and the laptops will run for a couple of hours without AC
> power...
>
>> There are even purpose built battery backup supplies available for running
>> oscillators, receivers, and such. Sadly, the NiCds tend to be bad and are
>> quite expensive to replace.
>
> In truth, most of the NiCd and NiMH packs I've used have been borderline
> useless. The Varta Mempac memory backup cells are OK, but most of the AA or
> AAA "off the shelf" cell above about 1600mAh have been dire.
>
> The Sanyo Eneloops are pretty nice though. A bit more like the alkaline AAs
> of old (as in, they don't go flat if you leave them for a week or two), but
> rechargeable.
>
>>  You can use diodes
>> to make an OR circuitto switch automatically between DC supplies when the
>> line fails.
>
> But watch out for the voltage drop, and use suitably sized diodes.
>
> A silicon diode drops about 0.7V (though big rectifiers are usually closer
> to 1V). If you have a supply with 2A going through it, a diode dropping 0.7V
> will be dissipating:
>
>  P = I * V
>    = 2 * 0.7
> which works out at 1.4 Watts
>
> At this point, you'll probably want something a little bigger than a
> 1N4001...
>
> --
> Phil.
> lists at philpem.me.uk
> http://www.philpem.me.uk/
>
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