Rechargeable AA batteries

[ianess]

Why are my AA rechargeables only rated at 1.2 volts? A check with a voltmeter shows an Energizer AA battery at 1.5v but why should there be a difference?

I thought all AA`s were 1.5 volts............also.. what`s the difference between Ni-Cad and Ni-Mh?

[KARL]

All batteries/accumulators work on the principle of chemical reactions whereby (during discharge) electrons are given up during the reaction and these provide the voltage (potential) which in turn generates current. Different chemical combinations provide different voltages and in the case of rechargeables it happens to be 1.2V whereas the "non-rechargeables" (although these can be and are recharged using special chargers no longer commonly available)result in 1,5V. The two are compatible in normal applications because the lower voltage ones give better current characteristics and therefore "drive" as well as the slightly higher voltage ones. I am not a chemist, but the trend toward MH (metal hydride) has to a considerable extent come from a desire to reduce heavy metal (Cadmium is on) pollution - in the UK nearly all normal batteries end up in landfill but are recycled more carefully elsewhere in Europe.

[Hippy]

The output voltage of a single cell depends on its chemical action. Zinc/carbon (Leclanche) cells like your Energiser etc will produce 1.5v, lead acid accumulators like in the car are 2v, and the rechargeable NiCd or NiMh are 1.2v. It is to do with how the chemicals produce the electricity.

 

NiCd stands for Nickel-Cadmium, and it is the interface of these metals in an electrolyte and the oxidisation and reduction that stores the energy and then releases it again. Nickel can exist in a number of oxidised states and it can lose a different number of electrons per atom, depending on how hard it is coerced. Fully charged, the cathode is composed of Nickelic Hydroxide, the anode is free cadmium with no oxidisation  and the electrolyte is potassium hydroxide. When you discharge the battery electrons flow from the anode causing it to oxidise, and into the cathode causing it to reduce to nickelous hydroxide. Charging reverses this process.

 

Because cadmium is toxic alternatives have been developed and the NiMh cell is nickel-metal hydride. The anode material is a non-toxic metal capable of storing hydrogen. The same transfer of electrons takes place, and NiMh cells can pack more of a punch than NiCd, however, they are trickier to charge, have a worse self discharge rate, and don't work well at low temperatures.

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[Kev_UK]

I've tried normal batteries such as Ever-Ready, Duracel batteries, Ni-Cd and Ni-Mh batteries in my digital camera. Ni-Mh are definitely the best.. they last much longer than the Ni-Cd between charges.