I have in front of me a NASA publication on the care and feeding of NiCd batteries, as learned from 30 years in space satellite operations. What follows here are some excerpts from this text organized in some potentially useful manner.
This is not an exhaustive dissertation and comes entirely from the experiences of NASA scientists and engineers and what they have decided are the important points to discuss in their publication. This doesn't even touch on all of what they did, its only the parts that I thought would be interesting to those of us in the RC hobbies. For more information see my reference to the publication at the end of this page.
I am not an expert in NiCd batteries and I am stating right now that I am not even trying to pretend that I am one. I am just regurgitating information that I have found in a NASA publication on the care and storage of NiCd batteries.
A common term you will see in this C something. Just substitute your packs' rating for the C, for instance: C/100 for a 1700 pack would be 1.7ma or C2 would be 3.4Amps (3400ma).
Guideline No. 2 Flight batteries should be maintained in a discharged and shorted condition and stored at cold temperatures when not required for "critical" spacecraft testing. Optimal temperature is around 0 degrees C. NASA does it this way:
To re-charge such a stored battery Guideline No. 7 A battery stored discharged and shorted for a period greater than 14 days should be activated with a "conditioning cycle" prior to placing it in use. The conditioning cycle (20 deg C) is defined as follows:
*** Steps 4,5,7 and 8 are not needed if the batteries haven't been stored for an extended period of time. See note in Guideline 9 below.
What does this mean? Well it means that to store our packs we should use the lightbulb style (or similar) method to discharge our cells down to .9V per cell. Then we should use a discharge tray to bring them down to 0V per cell and then short them and store them in the freezer in a baggie with a dessicant. Then bring 'em up before we race with 'em if we want our packs to be in peak condition. Quite a process huh?
We have all heard of experiences similar to this one:
As for memory in ni-cads, it was described to me as follows re-charging a cell that is only partially discharged say 50% often enough will cause the battery to only accept a 50% charge. If you then discharge more than 50% say down to 25% the best a re-charge would get you would be 75% (25 + 50). After reviewing how my early camcorder batts died an early death this theory holds a lot of weight with me.
Unfortunately this conclusion is not entirely true. The real reason for his NiCd early death was probably over-charging. Most chargers that come with equipment that use NiCd batteries aren't very sophisticated and over-charging is very easy to do. This is why the manuals tell you to fully discharge your NiCds packs before charging, their chargers aren't peak detecting and the only way to "know" that a pack is properly charged is to drain it first. The reason that our NiCds lose their capacity are manifold and complex but usually come down to voltage depression and crystal formation on the electrodes. The former is caused by age and over-charging and the latter usually by low discharge rates.
The most common problem is the formation of large crystals on the electrodes, this shows up to us as a "soft" pack, that gets better each time you run it on race day. According to one paper, this is caused by trickle charging, according to this NASA publication it is caused by low level discharging (leaving the battery sit open circuit or under light load). High discharge rates tend to bust up large crystals though, so that is a good thing for us.
To avoid this low level discharge we should reduce the charge of our batteries as much as is feasible before storage. The procedure above details this process in far more detail than any of us would probably care to take. But it does give us pointers to way to keep our cells in good condition within the level we each choose to afford.
Voltage depression usually occurs because of overcharging. Fortunately, this condition can usually be overcome by a few discharge/recharge cycles. I got this tidbit of information from an article the Navy published on reconditioning its massive supplies of NiCd packs and the money they saved by doing it. Unfortunately, I can't find that article any more so I can't quote much from it.
Here are some useful bits of info in the form of other guidelines from NASA:
The use of flight batteries after an open circuit stand of 4 hours or longer should be initiated with a short (3 to 5 minutes) discharge prior to initiating battery charge. Using a normal load.
During short periods when the battery is not needed to support spacecraft integration and test, it should be maintained on a low rate trickle charge. Low rate is defined as C/60 to C/100.
Batteries should not stand on open circuit for more than 7 days without being charged. Charging should be initiated only after implementing Guideline No. 3.
And finally, this is a useful guideline for us racers:
A battery should be "reconditioned" if it has been on open circuit, subjected to intermittent use, i.e, open circuit, trickle charge, occasional discharge, etc., for a period of 30 days. Reconditioning is effected by performing the following sequence at 20 deg C:
NOTE: The re-charge method following step 2 is not critical if the cells have not been discharged and shorted for extended periods. After a few hours (4-8) at the C/20 rate, charging at high rates is acceptable. If the battery is integrated into the spacecraft, final charging can be accomplished with the spacecraft battery charger.
Some things come up again and again, you may have noticed them. The first is heat is the enemy, do everything you can to keep your packs cool. The second is Overcharging is bad, the use of a good peak charger is probably the single best investment you can make to protect your batteries. The final one is our most common fault, don't let your packs just lie around. When not in use they should be either stored shorted, or, if you are about to race with one, on a C/60 or C/100 trickle charge in preparation for use. When I say shorted I mean that each cell is shorted, not the whole pack. You can't short your whole pack at once without risking damage to it! Also note the careful process used to short cells.
This is a lot of information, and I left quite a bit of detail out to get to the general guidelines. If you are interested in more details on the care and feeding of NiCds, you can get this publication through an interlibrary loan or through NASA, its called:
NASA Reference Publication 1326, February 1994 Handbook for Handling and Storage of Nickel-Cadmium Batteries: Lessons Learned by Floyd E. Ford Swales & Associates, Beltsville, Maryland) and Gopalakrishna M. Rao, Thomas Y. Yi (Goddard Space Flight Center, Greenbelt, Maryland) Published by NASA Scientific and Technical Information Branch.
I figured that folks would like some real data over just regurgitated "common knowledge" that we all tend to pass around without thinking about whether its correct or not. Feel free to distribute this information as long as you don't edit it. We should all benefit from being better informed.
I hope someday to find the definitive guide to charging our NiCd packs!
Much very good information can also be found at this link. Red knows quite a bit about the NiCd battery industry.
Copyright Dennis Clark 1998 and I don't care what you do with this information.