Figure 1 – The booster
Remember that the booster batteries shouldnit be connected for long time intervals, since they allow the motor to draw a voltage that is higher than the nominal.
For short periods of operation (a few seconds), this is acceptable, but it can cause the motor to overheat over longer periods.
The diodes are 1N4002 for motors up to 1 A and 1N5404 for motors up to 3 A.