The AM Forum

Looking for a simple way to discharge (2) 1.2V NiMh batteries at a constant current of ~25ma.
I have several batteries that I need to compare.
I'm having a senior moment - can't come up with the easy way.

I really miss my old Radio Shack data graphing meter. It used a really nice piece of software that ran under DOS. Now, also need to look for an updated version. Would have made this project so easy!
Suggestions!

Forward operating voltage of a typical LED is 1.2V at about 12 ma. Put two in parallel across each battery.

Nickel Cadmium batteries used to be quite common.
They have a voltage of about 1.3 volts when fully charged, but drop down to about 1.2 volts for most of their discharge curve, assuming that the load is compatible withe the cell capacity and temperature is taken into account.
The article here covers it pretty well.

http://www.hangtimes.com/nicdbasics.html

These cells have very low internal resistance all the way down to the point where you must stop discharging them.
When the voltage is down to about 1.1 volts, they must be recharged or they can discharge to the point that they reverse polarity and then it is hard to bring them back.
This peril is most dangerous when cells are placed in packs and their charge position is different. Having several fully charged cells mixed with cells that,are not fully charged will result in the charged cells pushing the partly discharged cells into reverse polarity hell and then redemption is difficult or impossible…😬😉

The other snake in the NiCd garden is a reduction of useable capacity called “memory” that results from keepimg the NiCd packs on charge all the time and then only pulling out 10-15 percent of the rated capacity over and over. Then when you need that full capacity, it will not be there…
I used NiCd in portable receivers and solar powered wildlife transmitters, so I,suffered with them for years until Nickel Metal,Hydride packs were available. The suffered not from memory or reverse polarity, but their seff discharge rate was high, so a piece of equipment that sat for a minthmwas often dead.
I am guessing that a fixed resistor or a rheostat would serve to do what you want, govem that themrate will,be higher at 1.3 volts for,about 10% of discharge and then pretty constant. However, the size of the cells will affect capacity at a fixed discharge rate, so small cells will not give good results for 25mA and larger ones will.
10%’of capacity for discharge rate is about right.
73, Mike

Assuming the cells are in series, use an LM317 regulator and a 50 ohm resistor.  Look for LM317 app notes, couldn't be simpler.

By no means an expert here, but did you perchance mean LM117?

Wouldn't the LM317 require too-high of an input voltage to be used here?

But, I may be overlooking something or, even worse, wrong! (That happened once last year.) :)

"https://en.wikipedia.org/wiki/Nickel%E2%80%93metal_hydride_battery#Discharge"

A fully charged cell supplies an average 1.25 V/cell during discharge, declining to about 1.0–1.1 V/cell (further discharge may cause permanent damage in the case of multi-cell packs, due to polarity reversal). Under a light load (0.5 ampere), the starting voltage of a freshly charged AA NiMH cell in good condition is about 1.4 volts.

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If your NiMH battery is 1.25V charged and 1.1V discharged (1.0V if you dare), then normal current regulators don't like the low voltages. So maybe adding some volts will help. If two are in series, some current regulators still don't like the low volts. Lets add some to fool it just in case junkbox parts are to be had.

How about a series circuit composed of something like an LT3092 two terminal current regulator set to 25mA, and a power supply source using a 5V regulator, and the battery? The 5V supply will act like 3-4 other cells in series with the battery under test, and the current regulator will be the load.

Adding a -5V supply, and a comparator to detect when the battery is discharged to 1.1V (my circuit puts the battery + at GND, so the detection is for Negative 1.1V), a relay to break the discharge circuit, and LED/alarm could make it more automatic and not over-discharge/ruin the battery.

The LT3092 has a precision of 10ppm, but no doubt others can be considered if they are accurate enough. The voltage regulator(s) should also be high precision if this is a scientific experiment.

A schematic for a general suggestion is attached. It is just a brazen idea and not ready to rely on. Maybe it can be done more simply or not need a dual polarity supply by moving some elements around.

Sorry for PDF - the LTspice schematic does not output in simple PNG.

Here is a jpeg of it Patrick.

Ok, I’ve got a few ideas to try. Suppose to be 100 today, so it’s a good day to start. Thanks to,all