Mobile Robot Power Consumption

[ejohns85]

Hi!

I am building a mobile robot that consists of two motors, two motor controllers, a linear regulated power supply, and a 9V battery. The regulated power supply is working well, producing a constant 5V. The motor controllers respond to their input signals as would be expected. However, I am having problems drawing power from the battery.

When I switch on the power supply, and tell the motor controllers to drive the motors, the motors start turning quickly, but they slow down rapidly for no apparent reason. I did some tests to investigate this; the voltage across the battery is about 7V when I turn the switch on, but within about 30 seconds of the motors turning, this drops to about 3V. The current similarly drops from about 150mA to 30mA during this period.

So for some reason, the battery cannot provide its full power when the motors are turning, apart from right at the start. The motors are rated at 6V, so the 9V from the battery should be enough.

Any thoughts on this?

Thanks!

[audioguru]

On the other web forum you said you did not use a new name-brand alkaline 9V battery. It is dead.

Even a good new 9V alkaline battery doesn't have enough power for your application.
There are tiny and weak AAAA cells inside a little 9V battery.

Use a real battery. Six AAA alkaline cells have twice the capacity. Six AA alkaline cells have 4.6 times the capacity.

Look at this graph of the alkaline 9V battery from Energizer's website: