NiMH health tester method?

[Infraviolet]

Can anyone recommend a circuit design I can produce easily with typical (imagine the sort of generic parts you usually have to hand) components to measure the "health" of NiMH single cells.

I use NiMH AA batteries instead of alkaline AAs in a lot of AA battery powered items, and after suficiently many charge-discharge cycles the batteries give worse performance in whatever they are running in. But it can be hard to notice this degradation when it happens slowly over time. For example, a DC motor powered from NiMH cells (single, or stacked in series) runs slower with less torque capability when an aging battery is used. Some of the cells are slightly abused, they run in scenarios where the device does NOT detect a critical voltage below which it turns itself off.

I'm not quite sure whether the problem with the batteries is that the voltage they reach when first charged is lower when they are old and unhealthy, or whether it is only their current delivering capacity affected, or whether they self-discharge much faster...

What measurement would I need to make to assess how unhealthy and old a cell has become, as versus against a brand new cell? Internal resistance?

I was thinking of this as a quick project to do, with the end goal of making a PCB which I can plug any NiMH in to and it will give me a single "health" reading I can compare to what I get from new cells.

Ideally it could give the same health assessment for a given cell whether that cell had just been charged, was taken out of a device mid-use, or whether that cell was discharged to the point I was about to recharge it again.

The health test circuit should not require putting batteries through a full dicharge and or charge cycle to get a reading, it should be something I can put the cell in to and get a reading in seconds.

Any thoughts on what specific parameter of an NiMH cell needs measuring to make this determination?
Thanks

[tszaboo]

You should measure the ESR of it, when fully charged. Load it to maybe 500mA and see how much the voltage drops. That's a good indication of it's lifetime.
These also have memory effect, so a few charge/discharge cycle can "repair" a badly behaving battery. There are some smart chargers that can do this.
Here are bunch of testers from one of the awesome forum member here:
https://lygte-info.dk/info/indexBatteriesAndChargers%20UK.html

[alligatorblues]

Battery testers work by charging the battery, recording the voltage, loading the battery, comparing the two voltage outputs,, and referring to a digital table. Battery conditioners add multiple charge cycles and monitor improvements.

[p.larner]

i thought it was only nicad that had memory not nimh ?

[bdunham7]

Any thoughts on what specific parameter of an NiMH cell needs measuring to make this determination?

The best NiMH charger I have, the AccuPower IQ 338XL, measures and displays the internal resistance of the battery as soon as you put it in the slot.  I believe--but don't know for sure--that it uses an applied AC method of measuring the impedance.  AC battery measurements are not unusual.  In any case, it seems to be pretty consistent about measuring whether charged or not.  As long as the battery isn't completely dead, reverse charged or damaged, the initial reading usually fairly well matches the reading if I test again after charging.  If you are specifically concerned with voltage drop under higher loads, this would be the measurement you should try first IMO.

[IanB]

What measurement would I need to make to assess how unhealthy and old a cell has become, as versus against a brand new cell? Internal resistance?

Internal resistance is the primary metric.

Capacity is the second metric.

A device like the MH-C9000PRO can often bring underperforming cells back to better performance by doing repeated and controlled charge/discharge cycles on them and reporting the measured capacity when finished.

When you first start charging a cell it will measure the internal resistance by putting a high current through it and displaying the voltage. The higher the voltage, the higher the resistance. If the voltage is too high it will reject the cell (alkaline cells will always fail this test, and worn out NiMH cells will tend to fail it too).

[kripton2035]

measure the internal resistance, basically an impedence meter @ 1KHz.
there are lot's of small devices that do this.
like the SM8124A around $60
https://www.aliexpress.com/item/1005006069950083.html

[radiolistener]

You can measure internal resistance with using EMF+one load or two load method. I think that will be most significant indication of battery state.