How to measure capacity of a 3S NiMh battery?

[zapta]

I have a 3S NiMh battery that with nominal 800mah capacity. What is the procedure to measure the actual capacity?  I do have a CC/CV power supply and a Maynuo electronic load.

1. How do I fully charge it? Is it ok to just apply CC, e.g. 80ma and let it run until the voltage stabilizes?  (topping off NiMh is ok, right?)

2. How do I discharge to measure the capacity, for example CC mode, let's say 80ma until it reaches voltage X (what would that X be)?

My goal is to follow the standard procedure, whatever it is, for measuring NiMh battery capacity.

[billfernandez]

I do not know the answers to your specific questions, but when I have similar questions I often go to http://batteryuniversity.com and find out that life with batteries is more complicated than I had hoped.

[crank2giri]

Have never worked with a NiMh battery, but will give my Li Ion Example, hope that helps.
am building a battery pack with discarded laptop batteries and hence many of them are there with varying capacities & have to measure the capacity of each one of them and sort them in order (450nos). What I do is. Fully charge the cells using a off the shelf cell charger rest it for a week and then discharge it over a 2ohm 10w power resistor, while I record the voltage on a Arduino Analog in pin. have an additional circuit to cut off automatically at a preset voltage & prevent cell damage. (you can get the current, voltage & capacity of the cell with the known parameters). now Use power and time to find the total capacity of the cell.

Hope that's useful.

[Andreas]

My goal is to follow the standard procedure, whatever it is, for measuring NiMh battery capacity.

Then refer to the datasheet.

usually it is charging with constant current (C/10) = 80mA for a 800mAh cell
for 14-16 hours. (depending on manufacturer).

discharging is usually done with constant current down to 0.9-1V at NiMh.
eg C/5 to 1.0V for eneloop cells.
Note that this test can only be done with single cells not with a battery.

With best regards

Andreas

[IanB]

For a capacity test you can apply a standard charge of 0.1C (80 mA for 800 mAh) for 16 hours. This will also condition the battery somewhat and will ensure all three cells reach a maximum state of charge. There is no problem with overcharging NiMH chemistry as long as the charge current is low enough.

For the discharge use a constant current down to a terminating voltage. As Andreas said, NiMH cells are best tested individually, in which case an end point voltage of 0.9 V is fine. Low discharge voltage doesn't hurt single cells, but 0.9 V is beyond the knee and the voltage will drop vertically after that with no point continuing. A reasonable discharge current is 0.2C.

You can test a series pack in a similar way, but the terminating voltage should be increased to 1.1 V per cell. With series connected cells, discharging a battery too far can cause cell voltage reversal which is harmful. This is a particular problem with power tool packs. If you run the tool until the motor groans you most likely will damage the battery.

[zapta]

Thanks everybody for the info. I also found this on Eveready's site http://www.eveready.com/faqs/Rechargeable/Pages/faq.aspx

How is the capacity of a rechargeable battery determined?
Capacity is how much charge a battery can hold often measured in units of mAh. An industry standard test procedure for determining rechargeable battery capacity is used by Energizer®. The American National Standards Institute (ANSI) procedure is as follows: Step 1 discharge new cells at 0.2 C to 1 volt. Step 2 Charge cells at 0.1 C for 16 hours. Step 3 rest cells for 1 hour. Step 4 discharge cells at 0.2 C to 1 volt. Battery capacity is determined by the hours of service to 1 volt times the discharge rate (mA x hours = mAh)

.

We have old Uniden cordless phones with S3 800mah model BT-446 batteries. The batteries don't hold charge so we order this from Amazon http://www.amazon.com/gp/product/B00DFVPAKG . The new batteries, are about 20% shorter so I wonder if they are of lower capacity or just a newer and energy denser NiMh product.

I will try to measure the capacity, mainly for the learning experience.