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A question: why are small rechargeable batteries a lower voltage than (ahem) 'disposable' ones? Is it possible to get true rechargeable equivalent-voltage batteries?

Apologies if this is a really basic question, to which every other PowerSwitcher knows the answer.

Ah, basic chemistry.

The voltage of a given electric cell is a function of the change in electochemical potential between the oxidised and reduced elements.

Rechargable batteries use different, and reversible chemical reactions. The specific chemical change generates a smaller voltage per unit cell.

Nickel Cadmium, zinc chloride, silver chloride, metal hydride, alkaline, lithium, lead acid, etc. all have different characteristic voltages between 1.0 and about 1.7 V.

If you want higher voltages, you put cells in series, as in the old fashioned
9V batteries used in smoke detectors.

Thanks for your reply, Ralph. So, is there such a thing as a 1.5 volt - say AA - rechargeable battery? Is it possible, necessary or desirable?

http://en.wikipedia.org/wiki/Rechargeab ... chnologies

Not sure that all these are easily rechargeable. Normal alkaline batteries can be recharged with modest success in my experience.

All standard small rechargeable cells have a nominal voltage of about 1.2 volts. Other voltages such as 2 volts for lead acid or 3.7 volts for lithium ion, exist but are not normally replacements for standard alkaline cells.

In many cases though, 1.2 volt rechargeables are a suitabe replacement for 1.5 volt alkalines.

When current is drawn from a cell, the voltage drops due to the internal resistance of the cell. In the case of alkaline cells this drop is appreciable at all but very low currents, and may result in an actual on load voltage of about 1.2/1.3 volts.
With rechargeable cells, the drop in voltage is very much less at any normal current, and the on load voltage is almost the same as the off load voltage.

Also the voltage of alkaline cells declines with use, only brand new cells will be the full 1.5 volts. Used, but still useful cells, will be about 1,4 volts off load, and will will drop to about 1.2 volts on load.

Therefore the voltage of alkaline cells varies with degree of use, load, and temperature, but in many applications the average voltage in use will be about 1.2 volts, the same as rechargeables.

It should be noted that the voltage ratings of most torch bulbs reflects this fact.
A bulb intended for 3 alkaline cells will normally be rated at about 3.6/3.8 volts, and not for the 4.5 volts that one might expect.
A "6 volt" bulb would normally be for 5 cells, not 4.
Some manufacturers however rate torch bulbs differently which adds to confusion !
Maglite dont state a voltage rating, but simply mark bulbs with the number of cells that they are intended to be worked from.

PowerSwitch is an EverReady source of info.

Thanks guys!

The difference between 1.2v and 1.5 volts is not at all relevant in nearly all applications. Though for alkaline cells the nominal cell voltage is 1.5V this falls rapidly upon the first discharge, in contrast NiMH batteries have a much longer plateau in the discharge profile.



For NiMH cells the discharge profile is pretty constant, provided they're charge properly. Consequently the appliance shouldn't experience any different in power availability compared to an alkaline cell.

However, the difference does manifest itself in extreme high discharge situations. Alkaline can produce a much higher current compared to NiMH so for model planes/car there might be problems as the NiMH cells will not produce as much power (they'll work, but they might not go as fast). If you're effectively shorting the battery high discharge can also damage/shorten the life of NiMH cells. The cost of this high current capacity is of course THE COST -- discharging single-use batteries very quickly burns money, and it would be better if you were not doing that activity in the first place!