I have my old camera - canon eos 650d with many batteries lp-e8 (7,2V, 1120mAh). I would like to buy a new canon eos 100d, but I've read in specification that it uses lp-e12 battery. Can I use lp-e8 batteries with canon eos 100d? I see that voltage is the same in both.
No you can't use the lp-e8 in an lp-e12 as the make of both the batteries is different. So even with the same voltage, they can't be used interchangeably because they wouldn't fit. Hope that helps.
Short answer: No, the LP-E8 and LP-E12 batteries are different shapes and/or have the camera connection contacts in different places and are not interchangeable.
One of the nice things about Canon's upper tier series is that they often share a common battery over several successive models. The 5D, 6D, 7D, and x0D (60D, 70D, 80D) have all shared the same LP-E6 battery or its backwards/forwards compatible successor the LP-E6N since late 2008 with the introduction of the 5D Mark II that was the first model to use the LP-E6. The Canon 1-series has used the LP-E4 or its backwards/forwards compatible successors the LP-E4N and LP-E19 since early 2007 when the LP-E4 was introduced along with the 1D Mark III.
Not so with the Rebel/xx0D and xx00D series of EOS cameras. The BP-511/BP-512/BP-514, LP-E10, LP-E12, LP-E17, LP-E5, LP-E8, and NB-2LH have all been used with models in the Rebel line since the beginning of 2007. Other than the BP-511/BP-511A/BP-512/BP-514 series of batteries that are all the same form factor and voltage with varying total capacity and that are backwards/forwards compatible with each other to the best of my knowledge none of the other different batteries in the Rebel/xx0D and xx00D series of EOS cameras are interchangeable.