NiCd batteries in test equipment (memory retention)

[Joerg]

Hello All,

Once in a while an older piece of equipment comes up with a
'non-volatile memory loss' or some similar message. Usually the little
NiCd in there is flat and when it is equipment that's out of support
it'll be either unobtanium or very expensive. Even when price wouldn't
matter, most likely the replacement had been on the shelf forever and
may not last. Unfortunately these are mostly 2 cell bundles, so that is
2.4V.

So, here is the question: The usual batteries in stores are 3.6V (for
cordless phones etc.). Would there be any reason not to be able to use
these instead? Charging is typically from a 5V rail and they are meant
to retain some static RAM content. I don't know if there were static
CMOS RAM where Icc would go up if it Vcc were 3.6V instead of 2.4V on
standby.

Regards, Joerg

 

[mike]

Hello All,

Once in a while an older piece of equipment comes up with a
'non-volatile memory loss' or some similar message. Usually the little
NiCd in there is flat and when it is equipment that's out of support
it'll be either unobtanium or very expensive. Even when price wouldn't
matter, most likely the replacement had been on the shelf forever and
may not last. Unfortunately these are mostly 2 cell bundles, so that is
2.4V.

So, here is the question: The usual batteries in stores are 3.6V (for
cordless phones etc.). Would there be any reason not to be able to use
these instead? Charging is typically from a 5V rail and they are meant
to retain some static RAM content. I don't know if there were static
CMOS RAM where Icc would go up if it Vcc were 3.6V instead of 2.4V on
standby.

Cordless batteries are three cells, usually. Just move the wire to use
two of them and you're done.
There are some issues.
If the new cells are much bigger than the old cells, you may get into a
situation where the leakage current (self discharge) is higher than the
charge current when averaged over the on/off cycle. Probably not, but
you should measure it.

If you try to use 3.6V, the memory retention current can go up
substantially. Ran into this problem when I tried to replace two
nicads with a lithium...yes I disconnected the charge current.
Ditto when I tried to replace a rechargeable lithium backup with a
nicad. Not a problem if you use the system regularly. I needed
something that could stay dormant for a year.
mike

Regards, Joerg

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[Tam/WB2TT]

Hello All,

Once in a while an older piece of equipment comes up with a 'non-volatile
memory loss' or some similar message. Usually the little NiCd in there is
flat and when it is equipment that's out of support it'll be either
unobtanium or very expensive. Even when price wouldn't matter, most likely
the replacement had been on the shelf forever and may not last.
Unfortunately these are mostly 2 cell bundles, so that is 2.4V.

So, here is the question: The usual batteries in stores are 3.6V (for
cordless phones etc.). Would there be any reason not to be able to use
these instead? Charging is typically from a 5V rail and they are meant to
retain some static RAM content. I don't know if there were static CMOS RAM
where Icc would go up if it Vcc were 3.6V instead of 2.4V on standby.

Regards, Joerg

Joerg,

I suggest measuring the standby current at 3 Volts. If it is down in the
microamp region, you would be better off with lithium or alkaline batteries.
I replaced the standby battery in a radio with a stack of three AA alkaline
batteries, and they are still going strong after 3 years. The big advantage
over nicads is that you don't have to charge them every few weeks, as the
nicads run down even with no load.

Tam

 

[Joerg]

Hello Mike,

If you try to use 3.6V, the memory retention current can go up
substantially. Ran into this problem when I tried to replace two
nicads with a lithium...yes I disconnected the charge current.
Ditto when I tried to replace a rechargeable lithium backup with a
nicad. Not a problem if you use the system regularly. I needed
something that could stay dormant for a year.

That's what I was concerned about. Thanks for the info. I'll also follow
Tam's advice of measuring what the current consumption is in retention
mode, to see if NiCd is really called for here. The only downside is
that any lithiums or alkalines here in the stores cannot be soldered and
I don't trust battery holders much. So it may have to be NiCd or NiMH
just for that reason. We don't live in a metropolis, actually some folks
call it a truck stop (arghhh) and the next electronics store is an hour
away. Heck, even our little Radio Shack shut down. Sigh.

Regards, Joerg