Posted on October 13, 2010
Shopping for batteries for some of my projects I came across bubble packed NiMH AA cells at a local store (WM) for a good price for rechargeable cells ($13 and change for an 8 pack). The package promised a lot of things including that they were charged and ready to go right out of the pack. They stated 1.2 V per cell and hundreds of charge cycles but lacked any specs. for milliamp hour energy capacity (mAH).
Having not much to lose I impulse bought a pack and after exhaustive searching on the net found that they were 1400 mAH cells and others wondered why they didn’t publish this spec. on the website or package. I think it is because there are similar cells which have a 2200 mAH capacity at a higher cost so why call attention to the difference?
I performed a discharge test for two of these in series right out of the package. The are Ray-O-Vac LD-715 cells. I used the .2C discharge rate (a 10 ohm resistor load) with a terminus of 1.1 V per cell to determine the mAH performance. It was 1440 mAH. Nice so far. The voltage remained impressively unchanged at 1.25 V/cell until the end of the discharge when it dropped to 1.1 volts during the last one half hour.
I then charged them using a laboratory current limited supply at C/10 while monitoring the voltage and case temperature. After 10.5 hours the case temperature rapidly rose from 74 to 84 degrees F signaling the cells were fully charged. This is impressive since charge efficiency is usually around 66% and this should take 14 to 15 hours. Still nice.
So far I am impressed. They will replace the NiCad AA cells in my 2 meter hand held and will give me more energy storage at the compromise of a drop in output power from 2.25 to 1.75 Watts due to the lower voltage but the extended operating time makes it worth this tradeoff.
I didn’t buy a charger since I can charge them in my lab but chargers can be purchased for $15 to $30.
Others cautioned that their experiences with this particular brand was variable and may depend on the lot number or sub contracted manufacturer.
My intent is to experiment with 1 hour fast charging out of curiosity since this isn’t a priority for me and I will report here when I have some data.
I hope this is useful for those who are using or contemplating battery operation.