Maximum NiMH voltage vs. Maximum NiCd voltage...

[alank2]

I'm looking to make a battery pack for a vintage computer (Epson PX-8) and it looks like after checking out the schematic, they don't seem to have any sort of voltage regulator on the battery, but seem to count on 4 cells of NiCD to give a valid range.  Feeding 5.4V from my bench supply into the battery connector and I checked the voltage on a port on the back and it was coming up 4.95V, so I think they have something that is reducing it a bit.  If I drop the 5.4V to 5.3, 5.2, the VCC goes down in .1V steps too, so it is following the battery voltage.  So my concern is could the higher maximum or fully charged voltage of the NiMH be too much for it?  How much higher is it than NiCD?

Here i the technical manual for it:
http://www.vintagecomputer.net/fjkraan/comp/px8/doc/px8-tech-manual.ch2.pdf

Page 2-4 starts the power area, but 2-13 says when fully charged "5.4V" and low voltage 4.5V to 4.8V.

I've got some NiMH batteries - should I try to make a pack out of them?  Or should I stick to NiCD?

[MosherIV]

Hi. As far as I know NiCAD and NiMH have near identicle max and min voltages.
Main differences between them:
NiMH has higher energy density.
NiCAD has better high current output characteristics.

[alank2]

I always that that NiMH had a higher charged voltage.  I ended up getting some NiCd's at a local place that had the machine to spot weld them together and I got them to join them for me at no charge.  All I had to do was use some hot glue on the pack.

[magic]

NiMH can go up to 1.45V or so when freshly charged to their full capacity.

But this site says NiCd are similar:
https://batteryuniversity.com/learn/article/charging_nickel_based_batteries

Possibly the built-in charger limits voltage to 1.35V per cell so using NiMH would have been safe too.

[tggzzz]

Beware of the different maximum trickle charge rates, and of the capacity specs claimed on fleabay.

[alank2]

Thanks for the info everyone; I ended up going to interstate and they had both types I needed (4/5 sub c and 1/3 aa) in NiCd and didn't charge me to spot weld them together so I've got the original type working.