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1. What is NiCd and NiMh?
The nickel–cadmium battery (NiCd battery or NiCad battery) is a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes. The abbreviation NiCd is derived from the chemical symbols of nickel (Ni) and cadmium (Cd)

A nickel–metal hydride battery, abbreviated NiMH or Ni–MH, is a type of rechargeable battery. The chemical reaction at the positive electrode is similar to that of the nickel–cadmium cell (NiCd), with both using nickel oxide hydroxide (NiOOH). However, the negative electrodes use a hydrogen-absorbing alloy instead of cadmium. A NiMH battery can have two to three times the capacity of an equivalent size NiCd, and its energy density can approach that of a lithium-ion battery.

2.  Whats the difference between NiCd and NiMh?
Nickel metal hydride (NiMH) advantages:                         Nickel cadmium (NiCd) advantages:
** lighter than NiCad                                                         ** Longer Life Cycles
** 2-3X capacity to equal size NiCd                                  ** Performs in cold tempertures (good performance down to 20 F)
                                                                                          ** Lower self-discharge level than NiMh
                                                                                          ** No voltage drop at/near discharge level
​                                                                                          ** Less expensive

Nickel metal hydride (NiMH) disadvantages:                   Nickle Cadmium (NiCd) disadvantages:
** fewer life cycles compared to NiCad                          ** Heavy
** shorter run time                                                         ** May suffer from "Memory Effect" if                                                                                              constantly discharged  half-way and then recharged
** performs the worst in cold temperatures
** higher self-discharge level than NiCd
** voltage drop at near-discharged levels
** More expensive

The major differences between the two types of batteries are is capacity, memory effect and environmental friendliness. NiMH batteries have high capacity, no memory effects and environmentally friendly. If you are under the illusion that rechargeable batteries are not very good then the chances are that you have been using older Alkaline rechargeable batteries (which are rubbish) or NiCd batteries suffering from memory effect and you have not been aware of the effects of Memory Effect.

3. What is the mAh rating mean?
This is a rating of energy storage capacity mAh = “milli-ampere hours”. So if you are comparing batteries to a AA with a 2000 mAh rating, it will have twice the capacity of a 1000 mAh rating.

4. What is the Memory Effect of a NiCd battery? How can I avoid it?
If a NiCd battery has not been entirely emptied before the next charge, the charge will not be complete and will set a new lower energy capacity. Over time it will appear that the battery is no longer holding a charge. This is called the "memory effect". To avoid this, it is best to occasionally discharge NiCd batteries entirely as this will ensure optimum performance and help lengthen the life of the battery. You should discharge NiCd cells before every fifth to tenth process of charging in order to avoid the feared memory effect. The memory effect has the consequence that the not used parts of the cell due to partially discharging are no longer in a position to receive and produce energy. Consequently their operating times become shorter and shorter and the cells have to be recharged more frequently. This often misunderstood feature of NiCd batteries has possibly given some users a bad impression of rechargeable batteries in the past. NiMH batteries do not have memory effect.