I have two unmarked cells, circa 1.2V terminal voltage.
How can I tell which chemistry they are, and hence which charger to use?

 

I have two unmarked cells, circa 1.2V terminal voltage.
How can I tell which chemistry they are, and hence which charger to use?

My NiMH chargers also support NiCd.

 

My NiMH chargers also support NiCd.

Yes, generally any charger that works for NiMh batteries will work for NiCD, but not vice-versa (mainly due to the NiMh being more sensitive to end charge trickle current).

 

And you can fast charge NiMh but not NiCd.

 

If the cells are the same size, a Ni-MH has 2x or 3x the capacity of a Ni-Cad.
While charging but before being fully charged, a Ni-Cad gets a little cooler but a Ni-MH gets a little warmer.

 

"charge acceptance" ~about https://www.servocity.com/nicad-vs-nimh-batteries
+ https://en.wikipedia.org/wiki/Comparison_of_commercial_battery_types

 

The article wrongly says a Ni-MH battery self-discharges in 1 week.
Oh Yeah? Modern (since 2005) Ni-MH cells use the Sanyo/Panasonic chemistry to hold a charge for 1 year.

 

I have two unmarked cells, circa 1.2V terminal voltage.
How can I tell which chemistry they are, and hence which charger to use?

Consider weighing them.

Various sources show that for the same physical size, NiMH weigh more than NiCd and alkaline is in between. For AA size, alkaline = 24, NiCd = 21, and NiMH = 27 (all weights in grams): https://power.tenergy.com/battery-size-chart/

 

Ni-Cad batteries are banned in many countries because Cadmium is a deadly toxin.
I found one 300mAh AAA Ni-Cad cell in a solar garden light from China snuck in by Walmart. It weighs 5 grams and a Chinese 300mAh Ni-MH AAA cell weighs 6 grams.
Both cells were dead so I charged them at the same time at the same current (in series) for 5 minutes. They both charged to full charge voltage and operated a solar garden light like with a full charge (charged in only 5 minutes at 90mA). They both felt like the same temperature.