DEC RK05 Emergency Retract Batteries

Doug Ingraham dpi at
Mon Mar 14 09:41:25 CDT 2016

On Sun, Mar 13, 2016 at 9:41 AM, Rick Bensene <rickb at> wrote:

> Michael Thompson wrote:
> >
> > The NiCad batteries for emergency head retract are toast. These look like
> > standard 1.2V 2/3AA 400mAh cells. It looks like some cordless phones use
> the
> > same batteries so I can buy an assembled 4.8V battery pack.
> >
> > Any other suggestions for replacement batteries for the RK05?
> >
> I've used those 4.8V rechargeable battery packs for cordless phones as the
> emergency retract batteries in a few RK05 drives, and they seem to work
> just fine.     Just make sure you get a NiCd pack rather than NiMH as there
> are charging differences and I'm not sure if NiMH batteries would be happy
> in the charging circuit of an RK05 (but, who knows, they may work, I just
> haven't tried it).
> -Rick
> The Old Calculator Museum

The only difference in NiMH and NiCd charging schemes occurs when rapid
charging them.  When rapid charging both types would use a DV/DT technique
coupled with a temperature sensor.  The DV/DT is much smaller for NiMH than
NiCd.  So if they went to the effort to recharge fast after an event (why
since you could probably do several retracts on a charge) you would not
want to use NiMH replacement batteries.  It is unlikely that anything other
than a trickle charge was used with these batteries since it is cheap,
simple and reliable.  The trickle current would have been 30 to 50 ma.  I
would not use the low self discharge NiMH cells (Panasonic Eneloop or
Energizer Recharge) as they would convert more of the trickle current to
heat and suffer a short life in this application.  Non low self discharge
NiMH should work fine.

I think I have a box of unused Sanyo 500-AR cells.  If you need some I
could do a quick test and see if any are still good enough to use.  The
500-AR are about 1/2 A size.  I am sure you can find the dimensions online.

Doug Ingraham
PDP-8 SN 1175

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