Over rated batteries?
 

I wonder how many battery distributors are overly optimistic when rating the capacity of their batteries. I have formed and tested several batteries lately and found that the best performance seems to be about 70% or rated capacity for NIMH batteries. I checked six batteries from two different distributors and found they all fell far short of the 2500 MAH rating. All of these batteries are the AA size NIMH and I suspect that the best one can expect from good quality AA size NIMH is a Maximum of 2000 MAH capacity. I am using good equipment to form and test the batteries and have borrowed other equipment from friends to compare with the same results. However, I did find that one 500 MAH battery that I have had for several years was actually under rated and tested at almost 600 MAH capacity. Are over rated claims typical for RC battery suppliers? Are there industry and/or government standards for NIMH battery ratings?

Thanks for your input.

Best wishes, good luck and good flying.

Heli-Nu-Bee (Roger)

RE: Over rated batteries?
 

What are your specific test parameters for discharge termination voltage and discharge current/amperage? You'll likely find that these parameters are not the same as the OEM's, thus your purported shortcomings.

RE: Over rated batteries?
 

Batteries unmarked with the manufacturers name and capacity are suspected to be quality control rejects that have slipped into the gray market. I have made a number of packs and checked capacity and they always turn to be less than rated. If the battery was unmarked you were probably cheated. The slight of hand also exists with computer chips.

Bill

RE: Over rated batteries?
 

Thanks for your input. I am using 1000 MAH discharge rate and a cutoff voltage of one volt per cell which is the equipment mfg's recommendation. From discussions with other modelers, the one volt per cell seems to be the value used by most RC hobby folks. Again, I ask, are there industry or government standards for rating and testing NIMH batteries? If so, what are they? In my opinion, battery ratings are meaningless if there are no standards.

Best wishes, good luck and good flying.

Heli-NuBee (Roger)

RE: Over rated batteries?
 

No true standard but always listed in datasheets.
BTW most spec. will state that you are not testing fair....less than C/5 to 0.9 V and form charge 16 hrs @ C/10... 5 or more cycles.
Rich

RE: Over rated batteries?
 

You are discharging at too high a rate (C/2) to compare to the manufacturer's ratings. To check the ultimate capacity, you should probably discharge at a c/5 or c/10 rate to around 0.9V. If you are trying to determine the practical capacity, you need to discharge at the same rate you will be using the cell and to the lower voltage limit you will use the cell. Especially for lower quality AA's, the discharge rate can have a significant effect on the measured capacity.

Ross

RE: Over rated batteries?
 

RE: Over rated batteries?
 

The difference in capacity between C/2 and C/5 is quite small, while the difference between 1.15 (where many dis chargers cut off) is somewhat larger. The C/2 vs. C/5 difference is smaller than the variation in capacity one would see from lot to lot production of the same design.

Those that expect to get rated capacity at 20C also expect to get the published mileage on their SUVs when pulling a boat at 75 mph.

RE: Over rated batteries?
 

OK, we are getting some interesting perspectives here. Personally, I am on a learning curve here and appreciate all of your opinions. One person indicated there is an ANSI standard for NIMH batteries and I wonder if they could lead me to the ANSI standard number? In the mean time, I think I will try to discharge these batteries to .9 volt at 500 MAH discharge rate this next round and post the results. I have at least three forming (slow) 24 hour charges and four fast charges on each of these batteries. I have read somewhere (I think it was on RC Universe) that a NIMH battery that tests at 80% or less of its rated capacity should be discarded. I plan to install these batteries in a nice scale gas powered aircraft and would like to be confident they are good quality.

Thanks again for all of your input and opinions.

Best wishes, good luck and good flying.

Heli-NuBee (Roger)

RE: Over rated batteries?
 

OK, here are the results of the tests:

Five cell NIMH AA size battery rated at 2560 MAH comparison:


First test: using 5 volt cutoff and 1000 MAH discharge rate, results = 1932 MAH which translates to 75.4% of the rated value.

Second test: using 4.5 volt cutoff and 500 MAH discharge rate, results = 2006 MAH which translates to 78.4% of the rated value.

I used the same charger and charging method for both tests. I talked to the charger mfg. who gave me additional information on how to charge batteries with their charger to get the maximum capacity. I also used the same precision instrument for discharging the batteries. The test equipment mfg guarantees his equipment is accurate to 1% or less.

As you can see, lowering the cutoff voltage and discharge rates only slightly improved the test results.

I now have used seven different chargers and three different test instruments on six different NIMH batteries from two different distributors. I have been testing batteries for over a month and all of these tests fell far short of the distributors rated value. I can only conclude that at least some of these battery mfg/distributors are over rating their products. Test yours and see.

Best wishes, good luck and good flying.

Heli-NuBee (Roger)

RE: Over rated batteries?
 

I have always struggled getting rated capacity out of Nimhs also, but I think I found a cure. I've only used it on 2 batteries, but both cycled above the rated capacity after a 24 hour forming charge. I am curious how they hold up.

http://www.siriuselectronics.com/ind...b8510fede6a846

RE: Over rated batteries?
 

Roger,

First checking batteries will not be an exact science. Second as you may have discovered most battery packs will increase in capacity during the first few cycles. Strangely enough I recently saw a capacity increase after six months use.

Reliability can easily be cured with redundancy.

Beating your self up doesn’t accomplish much. Batteries are strange.

Bill

RE: Over rated batteries?
 

I have had similar problems with NiCd batteries. I did find that decreasing the rate from C/2 to about C/4 made a substantial improvement in my case. I think part of the problem was voltage drop in the various connecions and wiring of my test setup. I have not tested NiMH....

RE: Over rated batteries?
 

Nu-Bee

Please share the charge/discharge regimen and equipment used to charge and discharge your cells maybe we can find an answer...

I have had very good success with claimed capacity ratings over the years when cycling batteries.

Are your batteries name brand, or maybe generic stuff with an existing shelf life? Some of the earlier NiMH stuff sucked.....

RE: Over rated batteries?
 

Most of my good quality MiMh packs get very close to or exceed their stated capacity. I have inadvertantly purchased unmarked cells from reputable distributors that have not met spe. and not lasted for more than a few months. I stick with quality battery suppliers and get quality packs.
I dump NiMh when they have deteriated to 80% of their actual starting capacity, not rated capacity!

RE: Over rated batteries?
 

And I saw a 25% or more reduction in capacity in cold winter weather, returning by a top up before flying and keeping them warm before flying. Yes, 'strange' is a good technical term when it comes to these wonderful little power houses.

I was happier with all my packs before I got a charger that told me what was really going on inside them!