This is the link to the battery/charger combination I recommend.
At $99 dollars it seems to be the perfect solution
for battery issues. I have one here and the Z4/MidiJet was still playing after
10 hours. The playing time may be reduced with constant
playing but I think it is an excellent solution for anyone eating up disposable
This is a Lithium Polymer Battery at 500MA and should not be confused
with other rechargeable technologies that are much weaker.
Also the quad charger is a fast charger and trickle charger to keep the
batteries topped off safely. Another good feature of Lithium Polymer is that
the four Batteries maintain their charge for months where others can go dead
just sitting inactive.
Hope this helps!