Assume a high resistance electrical load on the panel, the current drawn will be low
(Ohms law applies V = R x I ).
If the load resistance is lowered more current will flow through the load. If the resistance
is lowered to a value that would allow more current flow than the panel can provide at
that light level, the panel provides its maximum current and the voltage at the panel
output drops very rapidly.
Because the voltage is reduced, so too is the power reduced in the load (i.e. the car
motor).
In the extreme, if you place an ammeter directly across the panels output you will read
current but the voltage will have fallen to near zero.
This means the POWER output from the panel is near ZERO.
(Power = Volts multiplied by amps)
The onset of this voltage drop occurs suddenly with practically no warning, the voltage
dropping to near zero almost instantly.
In this discussion, we will call this situation "Panel Stall"
- MOTOR PERFORMANCE (DC brush type)
The motor when at rest has very low electrical resistance across its input terminals.
As an example consider the commonly used Faulhaber Minimotor type 2232 006S with
0.81 Ohms rotor resistance. For this motor the instantaneous current when connected to a
15 Volt supply would be 18.5 Amps according to Ohms law, providing the power source
is capable of supplying this current.
Assume the motor is connected to a Solarex MX 10 panel (10 Watts output) capable of
supplying only 0.6 Amps at 100% sun. The motor starts up with lower torque and runs
up to speed more slowly than it would if the power supply was capable of supplying the
higher initial current that the motor would like to draw
What is really happening here? Hopefully the following explanation will help
understanding.
When the motor is connected to an electrical supply, it initially appears as nearly a short
circuit across the supply, and will draw a large current (provided the electrical supply is
capable of providing the large current required).