The Nickel Metal hydride(NiMH) accumulator is a type of rechargeable battery. The NiMH was developed in the mid 1970's, but wasn't commonly available before the early 90's. At that time a concern for the negative environmental impact of the cadmium used in the NiCd accumulator started growing. This and the need for more compact power sources spurned the production of the NiMH cell. A HiMH cell consists of a positive electrode made from a fused nickel granulate, while the negative is made from an alloy of manganese, nickel, magnesium, cobalt, aluminium an some rare earth metals. Between the electrodes an alkaline electrolyte and a polymer separator is added(The manufacturers is of course reluctant to reveal the exact alloys and electrolytes used)

The advantages of the NiMH over the NiCd, in addition to being more environmentally friendly, is that it has a higher energy density and a negligible memory effect. The higher energy density means that a NiMH battery of the same size as a NiCd, will have a longer usage time. The low memory effect means you don't have to discharge the accumulator completely before recharging. But the NiMH cell needs a more accurate charging monitor system than the NiCd accumulator.
NiMH batteries have become the mainstay in mobile consumer electronics. The main reasons for this are:

  • They have a large capacity -as much as 2000mAh in a single AA cell.
  • They are light-weight. Again, an AA cell weighs about 40grams.
  • They are easy and safe to handle. Although the different kinds of lithium based technologies offer a higher energy density,
    they have an annoying tendency to explode when mishandled(that's why Li-Ion batteries only come in intelligent packs that can take care of themselves).
  • It's Really Worth It(tm) to use NiMH batteries instead of alkaline -although you can get something like four or five alkalines for the price of one NiMH battery,
    you have something in the vicinity of 800 charge cycles in one NiMH.
  • Over the last five years or so, NiMH capacities have been steadily on the rise. 2000mAh for an AA cell is the current maximum -expect to see larger capacities in the future. To put this in the right context, alkaline batteries typically store 1500-3000mAh, but it's important to remember that rechargeables only drive 1.2 volts per cell where alkaline batteries are 1.5 and that means slightly less hours in a NiMH battery per charge.

    Now, before you go out on a NiMH shopping spree, there is an important issue with NiMH batteries. This particular gremlin is called internal resistance.
    All batteries have this. It's very low in NiCD and lead-acid batteries and notoriously high in NiMH and (in particular) lithium-technology cells.

    The effect of a high internal resistance is that as you draw more current from a battery, the voltage at the terminals drops quite a bit. This means that NiMH batteries should mainly be used in electronic equipment that draws relatively small currents over longer periods. It's not that a handful of NiMH cells can't drive something that depletes them in about ten minutes reasonably.
    The problem is that if you need a large current here and now (most likely for an electrical engine - and probably R/C uses), you just won't get it. This is of no concern to users who don't need large currents, but at least I've warned you now.

    Charging NiMH batteries

    To properly charge a Nickel Metal Hydride (or NiMH) battery, the charger must be able to measure the battery's temperature, and it must also be able to determine the amount of current flowing in or out of the battery. When it approaches full charge, it will begin to heat up very rapidly.

    Never let the temperature exceed 60 degrees C, because the battery could leak or explode!

    To reduce wear on the batteries, it helps to replace only the amount of charge which has been spent. If this value is unknown, try to replace battery's rated capacity instead, but monitor the temperature to know when the battery is nearing full charge.

    Charging might proceed in four phases:

    1. For the first phase, charge the battery at 1 Amp for as long as it takes to put in what was taken out. For example, if 2.1 Amp-Hours were spent, then phase one should last for 2.1 hours. Keep track of how long this phase has gone on. If the battery temperature exceeds 45 degrees C, proceed to phase 2.

    2. In phase two, charge at 0.5A for as long as it took to complete phase 1. Start checking temperature 20 minutes into this phase. If the temperature exceeds 45, move on to the next phase.

    3. In phase three, charge at 0.2A for as long as it took to complete phase 1. As in phase 2, start checking temperature 20 minutes into this phase. If the temperature exceeds 45, move on to the last phase.

    4. In this last phase, the battery is considered to be fully charged. NiMH batteries can be damaged by trickle charging (charging at low currents) for longer than 10 hours. Instead, keep the charging current just high enough to stop charge from leaking back out of the battery.

    NiMH (Nickel-Metal Hydride) rechargeable batteries are newer than NiCd (Nickel-Cadmium) batteries. NiMH has up to 30% to 50% more run time per volume than NiCd; i.e. it has a higher energy density and delivers about twice the capacity. NiMH are also more environmentally friendly because they do not have heavy metals. NiMH batteries also can be charged without suffering from the "memory effect".

    Battery performance is improved by doing a charge pulse and then a discharge pulse or with a Delta V terminating charger. This is known as "burp" or "reverse load" charging. A NiMH battery may be damaged by extensive over-charging (on a charger for more than 24 hours). NiMH batteries are more stable while being charged and have less voltage drop when full capacity has been reached but NiMH batteries don't respond well to being stored in a completely discharged state. NiMH batteries can last up to 1000 recharges.

    sources: various sites from around the web. I wanted to learn about NiMH batteries after purchasing a set for my digital camera.

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