Life is easier in DSLR world where batteries are concerned if you stay still.
Move ahead, and you find yourself a little overwhelmed.
Time was, I had 4 cameras with two batteries between them.
20D, D60 IR conversion on the tried and true BP511, and 1D mkII and older 1D with it's behemoth NiCad. In fact, at this time, these were the only two Canon batteries that mattered.
Life was good with only two sets of batteries, but I still dreamed of a better world where we ditched the NiCad for a BP511 lithium for the new 1D MkIII and had only one battery to contend with.
Of course they did go Lithium in the 1D MkIII, and they went better with the LP-E4 which offered such amazing shot life.
....but fast forward a few years and the landscape is really sad.
I'm still in the BP511 business, I still have my IR converted D60, and the Wife now has a 30D. I still have to deal with the behemoth NiCad, since selling the MkII is a total waste. I still like my 1D-3 as a main shooter, so I'm still relying on the LP-E4.
Now, add in the 5D MkIII LP-E6 battery and you'd think it couldn't get any worse, BUT to top it all off, I have my tiny camera, a Nikon 1 V1, and it's battery is about the same size as the LP-E6 and BP511 but of course is incompatible.
Its truly silly in the latter cases, as all three batteries (LPE6, BP511 and the Nikon EN-EL15) hold within them the exact same set of little cells making the exact same 7.2V final output.
So five sets of batteries, five different chargers.
This is progress?
I can't keep up!
But why would someone have 4 cameras?!
I mean, I could imagine having maxumim 2!
Now for those 2 cameras you can use the same batteries, like:
BP511A for the 30D, 40D, 5D, etc...
LP-E6 for the 7D 6D, 5D2, 5D3, etc..