NiMH -- Nicad user lore

[Sub_Umbra]

Up until just recently my rechargeable battery usage would have to be classified as 'casual'. Just buying them, using them, recharging them and eventually replacing them. (I'm sure that compared to many of you, I'm still a casual user.)

I was using (4) old NiMH AAs (old enough to be rated at 1400 mAH!) in my Nuwai AT-100 and something didn't seem right. After a few charge/discharge cycles I finally got the idea to test the voltage of the individual cells before I popped them into the charger. One of the old cells was reversed at -.3V. The others were fine and all very close to each other.

It took me so long to even figure out that I should check this that I started thinking about starting a new thread. Here goes...

I'm not an electronic whiz. For testing I have a basic $25 RatShack digital multimeter. I use a stock Accumanager20 charger. When I buy cells I try to find a compromise between price and capacity. I just bought some 9500mAH Ds and some 2250mAH AA -- I'll only buy 'bleeding edge' well after it's stopped bleeding and costing so much.

I'm interested in practical users tips regarding NiMH and Nicads. I use both. Things you've learned from experience. Two classes of tips I'm interested in pop right into my head:

-- 1.) Brand new cells. Simple evaluation tests and tips on getting those new rechargeables through the first five charge/discharge cycles. (Both NiMH and Nicad) What to look out for. Sure signs of when to just toss one.

-- 2.) Cells in service. (Both NiMH and Nicad) Periodic tests? What to look out for. Sure signs of when to just toss one.

I'd like to know how some of you have routinely handled the above situations.

Thanks.

 

[JohnnyB]

Great topic /ubbthreads/images/graemlins/popcorn.gif

Does the normal self discharge count as a real discharge for the first 5 cycles?

 

[Steelwolf]

Interesting. I don't think I've ever really examined what I do, when I do it and why I'm compelled to do it. I check my cells routinely with a multi-meter, especially just after a charge, before I put it in to a particularly crucial application.

Most of the time though, I don't really bother, unless, like your experience, I notice that the run time seems a little short, or the power output is not quite right. Then it will be a whole series of tests (open circuit, short circuit and loaded at 10R, 1kR and 100kR) coupled with conditioning cycles on the charger before just tossing it and getting new cells. I don't think I even have a scientific criteria for readings to expect.

 

[SilverFox]

Hello Sub Umbra,

I classify my rechargeable’s into two categories: Routine use and Special use.

Special use are evaluation batteries and special tests that I have running.

Routine use batteries are used pretty much like throw away batteries, except after they run down, they go on the charger instead of in the trash.

I don't worry about cycling them artificially. I figure that in normal use the first few cycles will be a bit shorter, but it is not a big deal. With that said, I do a complete discharge/recharge cycle every two years.

NiCd's can handle overcharging better than NiMh. NiMh's have higher capacity. If you plan to leave the cells in the charger for extended periods of time, you will get better life out of NiCd's.

Self discharge is an issue that you need to pay attention to. I do not believe it contributes to "conditioning" the cells (to try to answer JohnnyB's question). The self discharge rate varies throughout the life of the cells. You will hear figures of 1% per day, but I think that is an average rate. I have noticed in my TigerLights that it is below 0.5% per day. The bottom line is that you will periodically have to charge up the cells even if you do not use them. I charge my standby cells up every 3 months. I should add that others may charge them more frequently wanting maximum run time, but I am a bit slack in that regard. I do try charge them up prior to putting them in use, but if I am unable to plan that far ahead, it's no big deal. I just get reduced run time. For example: at a 1% per day discharge rate, the cells will be at around 40% of full capacity after 3 months.

The time to toss the cells are when they no longer work. Your light will not be as bright and (as you figured out) you find a weak or reversed cell, it's trash time for it.

A good battery charger is the key to making rechargeable cells last a long time. I use the Vanson BC-1HU. You can do a search on battery chargers and find a lot of discussion on other chargers.

Some people steer away from rechargeable batteries because there is too much "fiddling" around with them. I find that if you set up a standard routine that is keyed towards something else in your life, it is not much hassle at all.

Hope this helps.

Tom

 

[Lurker]

I don't think it is necessary to give rechargeables much special treatment or testing. I just use mine and throw them in the charger. If they seem to die quickly I would put them in the battery tester or use a volt meter to verify that they are above 1.2V fully charged. If not or there is any performance problem with one, I would just toss it and replace it. They are pretty cheap and pay for themselves in the first few uses.

The only special considerations I give them are:
- I don't store them in a hot car or other hot location.
- I don't leave them on the charger longer than necessary.
- I top them up before use if they have not been charged in a month or two.

I have gotten away from Ni-Cd cells, but with those I would recommend some kind of conditioning on a monthly basis such as a full discharge.

I also try to keep my inventory as low as possible. I have just as many cells as I actually use, and maybe one extra set for charging. I think they perform best when they are being actively discharged and charged on a regular basis.

 

[jayflash]

20 years of using rechargeables has shown me extremes in cells. Some (few) will last up to 15 years while others fail in three.

I periodically test all the cells to determine how much charge they will accept. As they age, they lose capacity and a 1000mA cell may only hold 200 mA at the end of its life.

The quickest method for checking the capacity of a cell is to compare its "flash amps." When new, most NiCd & NiMH cells will sink ten or more amps into a short circuit, as they lose their amp hour capacity the flash amps go down accordingly. A cell that provides ten amps when new will eventually give 8 then 5 and down to only 2 amps. Of course some cells will go bad suddenly and short out internally before a large reduction of amps is measured.

This method of testing a cell's capacity works for NiCd's but I'm not 100% certain if it's as accurate an indicator for other types (NiMH & SLA's). My NiMH cells are four years old and still close to 100% of rated capacity and still provide 8 - 10 flash amps.

I've found that my 500mA NiCD's sink more current than my higher capacity NiMH cells. I'm guessing there's quite a variation between cell types with some being made, specifically, for high drain and others for high capacity.

 

[cheesehead]

Won't flash shorting cells reduce their capacity? I thought this caused some large crystal formation and then a reduction in voltage. Eh, on the other, half the stuff I read about NiCD/NiMH seems to be made up.

cheese

 

[jayflash]

If crystal formation occurs it must be a minor effect as I have not experienced any voltage or capacity reductions. At the most I test my cells twice a year.

RC racing places very heavy demands on the cells - just short of a short in some cases. I'd be curious to know the experience of that group of users.