Backup battery

[Eric Barbour]

Jens Johansson wrote:
> 
> A general question: can I replace NiCads that are used for static RAM backup
> with something better, such as Lithium cells, without modifying the charging
> circuit?

Of course...but you must use RECHARGEABLE lithiums. IF you use regular
Li cells, the charging circuit MUST be removed, or the batteries will
overheat and burst.

> (The instruments in question are polysixen BTW -- notorious for having their
> backup NiCads blow up and belch all sorts of corrosive spew over the PCBs. :)

See above.......same thing happens to Li cells. 
Why don't you just move the batteries off the PC board to a corner of
the
case, where they won't harm anything???? Regular NiCds are cheap.
 
> The voltages are a bit off as well -- the original design has a three-pack
> of NiCads, making 3.6 volts, and the Lithiums are what, three volts each?

The voltages are not critical. I believe that as little as 2.2 volts
will
save the contents of most SRAMS.
 
> As an aside, does anyone know of a good, cheap source of a selection of NiMH
> cells? (Why aren't these readily available in electronics stores? A
> conspiracy of some sort?)

Search me....they are VERY expensive, for one thing. And they have the
same
problems as NiCds. The only advantage is greater capacity for a given
weight and size, AFAIK.

-- 
Eric Barbour
Svetlana Electron Devices
Portola Valley CA USA