The battery test function on a DMM is really designed for primary cells like carbon/zinc or alkaline. The load and the voltage range is tuned for that kind of battery.
It is quite difficult to test a NiMH cell under load because such cells are quite load tolerant. You would have to put a much heavier load on them to get a meaningful result, but even then there can be a lot of variation between cells caused by the age, usage and specific type. That's why battery testers like the ZTS are not very reliable when used on NiMH.
When you discharge a NiMH cell under test conditions or in normal usage, you typically wait until the voltage under load drops to 1.0 V or 0.9 V and then you remove the load. Once you do that the voltage of a healthy cell will gradually climb back up to about 1.2 V. It is important to stop discharging at that point. You must not keep applying the load until the voltage drops to 0.9 V and stays there. That will over discharge the cell and weaken it.
For practical purposes, the best thing to do with NiMH cells is to become familiar with the characteristics of the particular cells you have, and then just measure their open circuit voltage. It doesn't take long to learn what voltage your favorite cells have when fresh off the charger, when stored for a week or two, and when partially used. Any time the cell reads 1.2 V open circuit, consider it fully discharged and do not drain it any further.
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