When it comes to batteries, there is always the choice between rechargeable and non-rechargeable batteries, which are great for most gadgets but not so great for powering LEGO power functions creations effectively because of the voltage drop when the battery gets discharged. Even LEGO's own rechargeable 8878 battery drops in voltage as the battery gets discharged, plus the 8878 Li-Po battery can only output a maximum of 7.5 V.

There are a few hacks out there on the Internet that show you how to power your LEGO creations using anything from a 9V battery to a RC LiPO battery, which actually LEGO's 8878 battery already uses (2 cells 1150 mAh LiPO battery).

If you are tired of wasting money on double-A and triple-A batteries but don't fancy doing any of the hacks mentioned above, check out BuWizz! BuWizz is a powerful 10V DC rechargeable battery that will give you even more speed and more power than the standard battery pack, as well as remote functionality for your LEGO creations.
BuWizz is essentially a remote control battery brick the size of two LEGO IR receivers that will help you replace the LEGO Power Functions IR Receiver, battery and remote control with a compact (even smaller than LEGO AAA battery box) nifty solution that you can control via an app on your smartphone.

With BuWizz, you also get left-center-right controls just like with LEGO IR controls, as well as 15 other positions plus variable voltages to control speed from slow (4 V), normal (7.5 V) and fast (10 V). The constant voltage output throughout is really a huge plus because LEGO rechargeable battery packs and rechargeable batteries always lose power, even with 75% remaining battery! I have yet to come across a better solution than BuWizz, but be sure to drop me a line if you know of any others.