Current Draw on a 9V Battery

I am currently making a light follower and have tested several power sources. So far 6 1.5V AA batteries work and if I connect my Arduino to my laptop, I can run the motors directly from the 5V pin. In both cases, I use transistors to control the logic. A single 9V battery doesn't work. It turns the Arduino on but it doesn't power the motors. I assumed this was due to the amperage and a cursory googling confirms this. But what confuses me is that most forums I go to tell me that I could technically draw however much current I want from the 9V battery, just for a really short time. So I was expecting the battery I tested on to be dead by now. But its not. A multimeter shows 8.5V and it can still turn my Arduino on. Is the problem really the amperage? Is there some internal current regulator on the battery?

I'm using Mega 2560 rev 3 if thats relevant.

All batteries have some effective internal resistance that limits their maximum current. With some, like high capacity Lipos, it is extremely low. With others, like typical rectangular 9V batteries, it is very high.

Steve

I can run the motors directly from the 5V pin

You will destroy the Arduino that way.

Use a separate power supply for motors and servos, and don't forget to connect the grounds.

ClovisN:
I am currently making a light follower and have tested several power sources. So far 6 1.5V AA batteries work and if I connect my Arduino to my laptop, I can run the motors directly from the 5V pin. In both cases, I use transistors to control the logic. A single 9V battery doesn't work. It turns the Arduino on but it doesn't power the motors. I assumed this was due to the amperage and a cursory googling confirms this. But what confuses me is that most forums I go to tell me that I could technically draw however much current I want from the 9V battery, just for a really short time. So I was expecting the battery I tested on to be dead by now. But its not. A multimeter shows 8.5V and it can still turn my Arduino on. Is the problem really the amperage? Is there some internal current regulator on the battery?

I'm using Mega 2560 rev 3 if thats relevant.

No battery can produce unlimited current. The internal resistance limits the current, and at high discharge the internal resistance usually increases markedly due to chemical byproducts blocking the electrodes.

Small 9V batteries are happy to produce say 50mA or so without much stress, but not much more. (Rechargable ones are much more potent though)

No pp3 battery will provide enough current to drive motor circuits. As JRemington says, 50-100mA max, and even that will soon kill them.

Best to power the arduino and motors separately. A 9V block, or better 4 "1.5V" cells to power the arduino, and depending on the motors,weight and max slope of the terrain, another supply for the motors.

as you say your motors will run from a 5V source I'd recommend a pack of 4 rechargeable (NiCd or NiMH )cells

Thanks for the replies.

The chasis I'm working with doesn't have the space for 2 power supplies so I'm currently just using 6AA batteries to power both the Arduino and the motors.