On Sat, 05 Aug 2006 13:57:19 -0700, Frederic Wenzel wrote:

> CN schrieb:
> >> There is a always-on hack in the wiki (Basicly restarting the slug as
> > soon
> >> as the power comes back). But it may fail in your situation if the power
> >> comes back too quickly ...
> >>
> >
> > I think a capacitor is practical only if the pwr stays off for a
> > fraction of a second,
> > anything past that you would need a huge capacitor.
> >
> > If you put 4 AA NiMh rechargeable batteries (about $4 in the USA) in
> > series right
> > across the output of your power supply, it will give you as much as 1 hour
> > (definitely many minutes ) of operation without any problem. Since at 5
> > v the
> > battery each will be at 1.25 volts, there is no risk of overcharging.
>
> That idea is just great. I got the parts and soldered it all together
> and it works. Now, when I unplug the power, the battery keeps the slug
> running. I still have to run a longer test (to see if the batteries
> don't explode ;) or discharge altogether or whatever) but for now it
> works just great.
>
> Including wire (which was only available as 35 ft. or more), it cost me
> 25 bucks now at Radioshack.
>
> I will blog and wikify that soon (giving you credit for the initial idea :))
>
> Fred

I am reasonably sure the batteries will not overcharge. When they are
discharged they have under 1.2 v each (4.8v), and when they are fully
charged, they go to about 1.4 (5.6v). So with the slug power supply at 5.0 or
very close to it, the bateeries will stop charging at some point in between (the
voltage vs charge curve is not quite linear). The only downside I see is that
the batteries will always be only charged to about 30% or so in your normal
'floating' operation, but even that should provide 10s of minutes of backup.

As for the cost, 4AA nimh batteries are routinely available for about $5 or less
on ebay...

This will have another significant advantage if you are running two usb
harddrives solely powered off the slug. My observation is that the powersupply
just does not have enough transient capacity for two drives (when the motor
starts, or the head moves). But a battery float like this will make the two
drives operation quite reliable --- this is a conjecture at this point, I have
not
tried this yet, but is planning to do this. In normal idle times in between
reading or writing, there is enough extra current available form the slug
charger
to keep the battery get the lost charge during the transient heavy loads.

Keep us posted about your experiences...

Cordially, Chacko