Any Easy way to test battery capacity (Mah)?

I'm wondering if there is there a fairly scientific way to test a battery's capacity, in Mah, without using any fancy equipment, like one of those chargers that will discharge at a set rate and give a result.
I have battery's such as 4 new NiMh D cells that claim to be 9000 Mah each, and I'd like to test them all and see how many Mah's they actually have.
I have a multimeter, but that's about all.
I could run them in a flashlight until they run out and time it,
but how would I work out the capacity...
What is the formula? And it would have to be a lamp without any electronic gizmos to maintain output.
I guess I could get a 1.2-1.5 volt bulb, with XX wattage, check the current when connected to the battery, time the runtime till it runs flat, and then use whatever the formula is? or is there an easier way?

Thanks!

Capacity (mAh) is a summation of mA over time. You could measure that by discharging the battery through a resistor and ammeter and then write down the current at fixed time intervals (Like each minute). When the battery is empty, add all the measurements together and divide by 60 (To convert from minutes to hours).
If you are using a spreadsheet you can measure either current or voltage and calculate the other (using ohms law and with the load resistor).

Oh wow, thanks HKJ, sounds like it will involve more time than I have available. I'll just have to assume that if they say 9000 mAh that they are ~9000...

With a linear regulator and a few parts and a heat sink you can make a precision current sink for about $5.00. After that, you just need some way of timing. At the end of the day, it is all going to come down to what your time is worth ... either building something or sitting there.

Semiman

A hobby charger will do what you are try to do. I know you said you did not want this but its very simple with a hobby charger. It wont be 100% acurate but it will be close enough for testing purposes. You can get them from hobby king reasonable priced. The Turnigy Accucel 6 seems to always get high reviews for its low cost to features. They are better ones that do cost more.

I think a hobby charger is what I'm heading towards, in the future some time. In the meantime, I just ordered a MH-C9000, which will take care of my AAA and AA batteries, as for the D Cells.... I'll just see how they last me in real life, and get reputable brands. (Like the 垃圾的垃圾電池公司 battery co.??? )

Do you have a digital camera with an intervalometer function?

If so, you could follow the method that HKJ described, but set your camera to take a shot of your multimeter every minute, rather than you sitting there and taking readings yourself. Then at then end of the run, you just need to shuffle through your pictures to get the readings.

That is a fantastic idea Stephen. Unfortunately I don't have a camera with that function any longer. Have gone to DSLR... All the manual stuff, none of the guff... I'm not going to worry. I've also ordered a MH-C808M, and I'll periodically condition the D-cells, and use known brand cells. But I still really like the idea. I could probably work something out with the laptop's webcam and some software. How complex would that be, resisters to drain the battery etc, DMM to measure current, batteries, timer, laptop, camera, capture images, enter data into spreadsheet.... calculate mAh based on current/voltage/resistance over time intervals... Just to figure out if my D cells are lasting 7 hours or 7.5 hours... or if they're really 9500 mAh or 9257 mAh. I like it.

Edit, PS, this is turning into a strange usage of the term "easy" only in use by those of us who frequent forums like this...

Edit, PPS, a much simpler way would be to just wire the D cells up to the C9000 and discharge.

SemiMan said:

With a linear regulator and a few parts and a heat sink you can make a precision current sink for about $5.00. After that, you just need some way of timing. At the end of the day, it is all going to come down to what your time is worth ... either building something or sitting there.

Semiman

Click to expand...

I was trying to come up with a way to do that myself with an LM350 or something, but I couldn't figure out how to supply voltage to the regulator to run it and discharge the battery both. How would you do it?

Yes there is now. Get a $11usd Charger Doctor from Drox. It will measure the mah if you put it between a battery pack and a load. I just noticed this on mine. The mah used won't reset unless you unplug the charger doctor for at least a minute. So, plug it in to the battery pack, add a load downstream from the charger doctor, and let it go unattended. When you come back, plug the CD into a charged power source, it will display the mah used for about 10 seconds then reset mah to zero.

Drox OLED charger doctor.