Why are rechargeable batteries 1.2v....

[daveham]

....it really annoys me not knowing why....someone must know why AAs and AAAs that are normally 1.5v are 1.2v when rechargeable.......why????????!?!?!?!?

.....if this I all have to worry about, my life must be simple!

[Funkstar]

i think it is a nominal voltage on rechargables because they tend to drop the voltage right at the start of their discharge. Standard batteries stay at their given voltage until they are around 75% through dischange (or something like that).

I have seen graphs for this somewhere before...

[Funkstar]

have a look here for one set of graphs

http://shdesigns.org/batts/battcyc.html

[KW1816]

If I remember back to my physics lessons (and it was a few years ago ) ..

Each chemical cell (a battery is technically a combination of a number of cells) like a normal AA battery produces 1.5V. This is a one of them physics laws that only make sense to the teacher. A rechargable battery doesn't create electrical charge it just stores it and so the voltage is always less.
So in summary, class, a cell that creates electricity will be 1.5V (and a battery multiples of e.g. 3V, 6V, 9V etc) whereas a rechargeable, due to the inefficiency of the transdextral hypermatrix linear algorithmic model, will never be able to match this.

I'm sure that's the jist of it, of course I could have just made this up but I'm sure someone who knows what they're talking about will be along soon

[Ruggerbugger]

The voltage (or potential difference) in the cell is produced by a combination of two metals and an electrolyte. The further apart the metals are in the reactivity series, the larger the voltage set up. Voltage can be likened to a "push" given to the electrons in the wires making up the circuit. The larger the voltage, the larger the current that will flow.

The exact chemistry escapes me at the moment, but in a standard battery, one or more electrodes (the metals we mentioned earlier) will corrode away as the battery discharges, eventually rendering it useless. In a rechargeable battery, the electrodes don't corrode (or at least do it more slowly), so they can be reused by passing a current through them (charging). The logical conclusion is that the properties that enable this to take place mean that the electrodes set up a smaller potential difference (voltage) than a standard cell. This probably also has something to do with the way rechargeable cells gradually discharge over time.

It's incorrect to talk about creating electricity. All cells, batteries, power supplies etc set up a voltage which causes a current of electrons to flow around the circuit. This current carries energy (measured in joules) to each component of the circuit. Think of it like a line of buckets carrying water, some of which is transferred to each component along the way. At the end (back at the cell), the buckets (electrons) are still there, but the energy (water) has been transferred. The chemical reactions in the cell provide "fresh" energy for the whole thing to start again.

Anyone who can add more details is welcome to.

[Rave]

Simply, it's because different battery chemistries give different terminal voltages. Alkaline and Zinc Carbon are both around 1.5V (maybe 1.6V when fresh), but that's basically a coincidence, it's nothing to do with the fact that they're both use once type batteries. Lithium disposable cells give 3V, whereas lithium-ion rechargeable cells are 3.6V, so in that case the rechargeable type gives a higher voltage. A lemon battery gives just under 1V. Lead acid cells give 2V, and so on and so on. IIRC from chemistry classes, the maximum potential difference possible in a single cell is about 4V, but I have no idea which two metals you need to use.

The terminal voltage of the battery is nothing to do with it's capacity in joule terms, or other properties like its internal resistance- rechargeable AAs don't have the same theoretical joule capacity is alkalines (although they're getting close) but they're way better for high drain applications.