Let's take a quick look at the ever popular PP3, 9 volt battery. 6LR61. What does the IEC number tell us? Well it's a 9 volt battery (6LR) made from AAAA size cells (LR61).
Now I just happened to have one of these on the bench as I was writing this so out came the camera and here it is before and after opening. A quick check with a steel rule and referring to the AAAA page for the size, I can confirm that the 6 cells inside the battery were AAAA size. Some PP3 types are "6F22" and they have 6 flat cells stacked up inside.
I do not recommend doing this yourself as there is electricity, chemicals and sharp metal edges to be careful of.

The IEC CR series (Coin cells and Camera batteries).
These are Lithium Manganese Dioxide chemistry in a round (usually, some camera batteries have shaped plastic casings) package.
Since LiMnO2 cells produce 3 volts there are usually no alternate chemistries for a CR coin cell. Conversely one LiMnO2 cell can replace two alternate chemistry cells, in a 3, 6, 9, or 12 volt battery.
CR cell numbers correlate with the cell dimensions, being the diameter in millimetres (except for the extra half millimetre in some cases) followed by the height in tenths of a millimetre, this also holds true for most of the camera types as well.
Some notes about Button Cells.
LR series are Alkaline chemistry and provide 1.5 volts, dropping to about 0.8 volt at the end of their life.
MR series were Mercury chemistry and provided 1.35 volts and were stable until end of life. These are now banned.
MRB series are made by a company called WeinCELL and are a Zinc Air voltage compatable replacement for the Mercury cells.
SR series are Silver Oxide (Ag2O) chemistry and provide a steady 1.55 volts to end of life.
Since there are no 'common' names beyond the AG designation, many places use the LR and SR terms interchangeably, and they will both fit and work. The only difference is that the SR series typically have 50% greater capacity than the LR series. In low-drain devices like watches (without lights) this isn't very important, but in high-drain devices like novelty key chain torches, or laser pointers the SR type is preferred. Typically SR and LR will be the same price so there is no reason not to get the SR version. Often the free 'demo' batteries that come with a device are the LR version.
The MR series can usually be replaced by either LR or SR but some equipment is voltage dependent so an adaptor or the MRB series are really the only choices, the options are given on the battery details pages. The adaptors I have linked to are supplied by a British company called The Small Battery Company. I am not connected with this company in any way, I have, as yet, had no dealings with them either. I have used their website as one of the cross referances for the equivalents and placed the adaptor links as examples of what is available. Other suppliers are available.
Important notes about the MRB series:
- They are expensive. Typically over £5.00 each here in the UK.
- You have to activate them by removing a sticky label that covers some holes, thus letting the air in, and wait for about 30 minutes until they are fully activated and ready to use.
- Once they have been activated they will "lose" power whether they are being used or not, you will tend to get 8 to 10 months or so use before they "die". (Some people claim that a sticker replaced over the holes after use until next needed prolongs life).
- Due to the manufacturing tolerances for watches the Zinc Air cell is not recommended as insufficent air reaching the cell reduces performance and can cause what appears to be early failure.