CHAPTER 18 ■ ADDING GEARHEAD MOTORS
Determining the Least Current (Best Case)
The no-load current of the selected motor (after a breaking-in period) at 9 V measures 31 mA. Simply
connect the motor to a battery to experience this test case. A 150 mAh 9 V rechargeable battery would last
almost five hours powering this motor when allowed to turn freely.
150 mAh battery capacity / 31 mA no-load motor = 4.8 h (hours)
Determining the Most Current (Worst Case)
By briefly applying pliers to stop the motor shaft from turning, the stall current at 9 V measures 380 mA.
A gentler approach is to measure the resistance of the motor coils and then calculate stall current.
With a stall current of 380 mA, a pair of these motors that are prevented from turning (like if the robot
was stuck against a wall) would drain the battery in less than 12 minutes.
150 mAh battery capacity / (380 mA stalled motor × 2 motors) = 0.2 h
0.2 hours × 60 minutes in an hour = 12 minutes
Of course the robot also has lights and chips that consume power, so the battery would actually drain
in less than 12 minutes. The 150 mAh battery-capacity rating is based on a fairly slow drain, so the battery
would exhaust very quickly when drained at over 800 mA.
■ Caution Battery manufacturers warn you to not draw so much current from a battery that it lasts
significantly less than one hour. For example, a 150 mAh battery shouldn’t be drained at more than 150 mA.
Greater currents may damage the battery. In practice, most radio-control models and robot designs push this
guideline, draining fresh batteries in as short a time as 15 minutes.
Fortunately, these motors have enough torque (twisting force) that they don’t ever stall on Sandwich.
Even if the robot is pushing against a wall, the wheels spin in place.
Figure 18-4. Testing motor current with a multimeter and pliers