Is there such thing as a 9V NiMH battery with 600 mAh?
I just bought a set of Lithium ion rechargable batteries.  Here is the straight dope.  Lithium Ion has a much greater energy density than NiMh.  In addition, a 9v battery is made up of many cells.  Since a NiMh chemestry is 1.2v, usually they have 7 little cells in there to make 8.4v.  Since these cells are tiny, they do not hold that much energy.  A Lithium Ion cell produces 3.7v and so they put 2 of these in a 9v battery package. It runs at 7.4v but that's ok for most things however it may always indicate that your battery is low.  But since the 2 cells are much bigger, they can hold more total energy.  The ones I have actually have a micro usb port in the bottom to charge so its sacrificing some power for convienence while still having more capacity than the NiMh.  Its probably a good thing however because charging a Lithium Ion battery in a NiMh or NiCd charger will probably cause damage or explode.