A measuring method for vehicles equipped with an internal combustion engine, the engine including a starter battery and a driven staffer motor used when an attempt is made to start the engine, is disclosed. According to the method, a voltage across a measuring shunt inserted in a current circuit between the battery and the starter motor is repeatedly measured, at a sampling rate, during each revolution of a crankshaft of the engine during activation of the starter motor. Corresponding values of a current delivered from the starter battery to the starter motor are derived from voltage measuring results. Values of the current are registered in a memory of a measuring computer. The registered values are used to establish a status of the combustion engine and its starting system. A value of a derivative of a rotational speed of the combustion engine crankshaft is obtained from the registered values. The sampling rate is sufficiently high to permit obtaining the value of the derivative of the rotational speed of the combustion engine crankshaft.

A watercraft battery control system monitors a battery charge. The system informs the watercraft operator when the battery charge is below a predetermined value. Alternatively, the system automatically controls engine operation by starting the engine or by increasing the speed of the engine to allow a generator to replenish the battery charge when the battery charge has fallen below a predetermined value. By informing the user of an inadequate battery charge, the user can start the engine an recharge the battery. Alternatively, the system automatically maintains the battery at a predetermined charge level to ensure safe and enjoyable watercraft operation.