Possible battery limits
 

What determines the upper limit of possible battery size in a truggy? Is it the ESC or the motor? And what on the ESC or motor will tell me the largest battery possible with a particular car? I have a used Team Associated 1/8 car that has an upgraded motor and ESC in it and I do not know how to determine the biggest battery possible in the car. It has a Castle Creations ESC and motor with a cooling fan on it. I am currently running both 3S and 4S batteries in it but am not sure how to determine if it will handle 5 or 6S batteries.

Both the Motor and ESC have limits relating to voltage .
Motor - will have a safe Max RPM . Putting more volts in at full throttle will push it past the limit . It will work , but , it will get hot and something will fail , like bearing , rotor or wires .
ESC - are built to handle a certain voltage . Putting more volts in , the ESC will shut down if it has an Over Voltage Protection feature or quickly fail due to too much voltage .
Have a look on the ESC sticker , usually they have something on there for operating voltage . And on the motor there might be a max rpm or at least a KV rating which will give you some idea of the safe voltage .
Do you have any pics you could post to identify the ESC or Motor ?

I can't see the ESC well enough to read anything on it. All I can see on the motor is 60,000 RPM. I am currently rebuilding the rear differential and have the complete rear suspension and rear diff. removed from the vehicle. It would not be much more effort to remove the ESC at this point and identifying it.

Probably the most common 1/8 system is the Mamba Monster 2 . Try a google search and see if yours looks like the pictures .
You might need to to pull the motor to read what KV it is ( Im guessing it will be 2200 ) .
6S =25..2V
25.2 x 2200 = 55440 Rpm .
Just a tip ... things tend to break and wear out quicker on 6S !