Originally Posted by ItsJustMe
An additional comment on the "run the batteries dead" comment.
You don't want to run them DEAD dead. NiCad and NiMH should NEVER EVER EVER be run deader than 0.95 volts per cell - doing so will do them GREAT HARM, far worse than not discharging them regularly.
If you have a digitally controlled system, odds are it'll shut the light off when the battery gets this low. But if you just have a regular ole' light on a battery, you could run the batteries stone dead, and that will kill them fast.
In fact the whole "memory effect" thing barely has any truth to it anymore. It came about years ago, and the original observations of it were on a satellite that went into earth shadow like clockwork every x minutes and then into sunshine (charging) every x minutes. If you load NiCad like that, discharging them PRECISELY the same amount every time, you can get a noticable memory effect, but it's not that bad anymore; modern NiCads are much better than they were 10+ years ago.
NiMH barely has a memory effect at all. You would probably need a digital voltmeter and a lot of time to detect it. The big nemesis of NiMH is running them too dead, and overtemperature when charging. If you charge a whole pack at a time, be sure to get a charger that has a temperature probe to keep from boiling your batteries.
Actually, the memory effect is quite real, it's just not the cyclic effect that you describe. From
Batteries for a Portable World:
The problem with the nickel-based battery is not the cyclic memory but the effects of crystalline formation. There are other factors involved that cause degeneration of a battery. For clarity and simplicity, we use the word ‘memory’ to address capacity loss on nickel-based batteries that are reversible.
The memory phenomenon can be explained in layman’s terms as expressed by Duracell: “The voltage drop occurs because only a portion of the active materials in the cells is discharged and recharged during shallow or partial discharging. The active materials that have not been cycled change in physical characteristics and increase in resistance. Subsequent full discharge/charge cycling will restore the active materials to their original state.”
When NiMH was first introduced there was much publicity about its memory-free status. Today, it is known that this chemistry also suffers from memory but to a lesser extent than the NiCd. The positive nickel plate, a metal that is shared by both chemistries, is responsible for the crystalline formation.
In addition to the crystal-forming activity on the positive plate, the NiCd also develops crystals on the negative cadmium plate. Because both plates are affected by crystalline formation, the NiCd requires more frequent discharge cycles than the NiMH. This is a non-scientific explanation of why the NiCd is more prone to memory than the NiMH.
The effects of crystalline formation are most pronounced if a nickel-based battery is left in the charger for days, or if repeatedly recharged without a periodic full discharge. Since most applications do not use up all energy before recharge, a periodic discharge to 1V/cell (known as exercise) is essential to prevent the buildup of crystalline formation on the cell plates. This maintenance is most critical for the NiCd battery.
All NiCd batteries in regular use and on standby mode (sitting in a charger for operational readiness) should be exercised once per month. Between these monthly exercise cycles, no further service is needed. The battery can be used with any desired user pattern without the concern of memory.
The NiMH battery is affected by memory also, but to a lesser degree. No scientific research is available that compares NiMH with NiCd in terms of memory degradation. Neither is information on hand that suggests the optimal amount of maintenance required to obtain maximum battery life. Applying a full discharge once every three months appears right. Because of the NiMH battery’s shorter cycle life, over-exercising is not recommended.
A hand towel must be cleaned periodically. However, if it were washed after each use, its fabric would wear out very quickly. In the same way, it is neither necessary nor advisable to discharge a rechargeable battery before each charge — excessive cycling puts extra strain on the battery.
No battery should be drained until it is dead. This just kills the cells and renders the battery inoperable. The discharge should be tightly controlled which means the use of a smart battery charger to monitor the cell voltage. Most of the ones that come with light systems are pretty dumb, in more ways then one
.