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Rechargable batteries

  1. Jan 2, 2004 #1


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    Nickel-Cadmium (nicad) batteries have the strange tendency to stop working unless they are completely drained then completely recharged. If the batteries are fully charged, drained maybe 20% then charged again, and this is repeated several times, the batteries simply don't work any more. If the batteries are fully charged, then drained down completely before being recharged, they virtually last forever.

    Why is this?

    Do Lithium batteries act the same way? I've heard that lithium batteries act completely different and that they should be kept as close to fully charged as possible at all times (charge em as often as possible).
  2. jcsd
  3. Jan 5, 2004 #2
    In NiCads the anodic reaction is Cadmium metal turning into Cadmium Oxide (on discharge) and vice versa on recharge. Both are solids. In low discharge the Cadmium Hydroxide crystals are relatively big, and therefore they passivate the anode. Less and less active surface of Cadmium is accessible for further dissolution (discharge). Hence the capacity drops. Deep discharge and subsequent recharge will break the crystals and form a new layer of small crystal, and thus restoring capacity.

    Full discharge doesn't mean "drain down completely". The deep discharge means going below 1 volt. The nominal voltage is 1.2 volts. This corresponds to 60% discharge. The equipment is designed to stop operating below 1 volt, therefore the user may think that the battery is "fully discharged". If indeed you discharge it to death, for instance via a simple ohmic circuit, will simply die....
    NiCads do not "last forever". Their lifetime is about 1000 cycles.

    Lithium batteries have a totally different chemistry. In Lithium batteries, the solid lithium of the anode dissolves into a lithium ion, and there are no problems with crystals A safer battery (lithium metal is very reactive) is the lithium ion battery, where the anode is not metallic lithium, but rather lithium ion intercalated in graphite. Both types do not have a memory effect because the discharge product is not a solid coating the anode.

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