Many Tamiya kits available on sale today still come with the TEU-104BK ESC. This ESC is perfectly fine for running brushed motors, but does not contain a BEC and its low voltage cut-off is too low for LiPo batteries.
The missing BEC is easily dealt with: either use "high voltage" compatible servo and receiver, or a cheap external BEC.
Fortunately it is also easy to modify the ESC to have a LiPo compatible voltage cut-off -- assuming you are able to solder a tiny surface-mount resistor.
WARNING: This and other DIY projects are purely "at your own risk". If you are at all uncomfortable or inexperienced working with electronics, please reconsider doing the job yourself.
By default the voltage cut-off is set to 4.5V. We measured this by connecting the ESC to a lab-power-supply, slowly lowering the voltage until the ESC cut off and started to flash the LED.
For a 2S LiPo 4.5V is way below the voltage that damages the battery, we rather need a cut-off at 6V or slightly higher.
The ESC implements the voltage cut-off through its microcontroller. A resistor divider comprising of R1 and R2 on the PCB divide the battery voltage down to a level that can be measured by the microcontroller. R2 is 10 KOhm, and R1 is 27 KOhm. By lowering the resistor value of R2 we can increase the threshold for cut-off from 4.5V to 6.2V. We can lower the resistor by adding a 20 KOhm resistor in parallel to R2. This way we don't have to remove components from the ESC.
The components used in the ESC are tiny 0402 SMD parts. But due to the way the board is layout it is possible to use a larger 0805 SMD 20 KOhm resistor, which is easier to handle by humans.
We laid the 20 kOhm resistor onto R2 and soldered it to the bottom pad. The other side of the resistor we soldered on the bottom pad of R1, which is directly connecting to R2.
Depending on your soldering skills it does not take more than 10 minutes to modify the ESC. With the modification the ESC shuts off at just the right time when your LiPo batteries deplete.
Update Aug 2015: the LiPo cut-off mode is now also available for the new Tamiya TBLE-02S ESC