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

That fits with Doug's comment: 20 mA charge current (ignoring the internal resistance of the battery). Interesting that they used a battery; I thought the usual mechanism was a large capacitor, I hadn't seen batteries used for this. paul

The 7900 certainly does (there is one to go on my bench when I have some spare time). In that case it's some AA-size NiCds (not tagged ones) in those aluminium clip-in holders (Radio Shack used to sell them, I think Mouser still do). The DEC RK07 (and I assume RK06) used 8 1/2 AA cells in a pack (like 2 RK05 retract batteries in series). When I replaced those, I used 2 of the cordless telephone batteries (that have been recommended for the RK05) in series. I am told some later drives used the spindle motor as a generator when the power failed to provide the power for emergency retraction. -tony