When the engine begins to RUN the generator takes over as the source of electrical energy needed to operate all vehicle circuits. The battery goes off-line and becomes a load as it receives electron current from the generator to recharge. Generator voltage is often referred to by the term “charging voltage” which is a higher voltage than battery voltage. Generator electron current which replaces battery electron current is often referred to by most technicians by the term “charging amps” implying the electron current provided by the generator.
|Want more ? Enjoy a free subscription to Motor Age magazine to get the latest news in service repair. Click here to start you subscription today.|
If we are to test and evaluate the charging system and vehicle electrical circuit performance it would require that we measure both generator voltage and generator electron current underload to verify the charging system is performing properly. Figure 1 below shows the generator operating during engine run and the changes in electron current that occur. But first our focus is on generator electron current as it flows through the vehicle.
Electrons always travel from voltage source negative terminal to voltage source positive terminal. Since the charging voltage created during engine run is about 2 V higher than battery voltage, we expect the current clamp readings to be higher during engine RUN. All electron current comes from -GEN. Normal charging voltage will be in the range of 13.6 - 13.8 V during hot weather to as high as 15.1 V in extremely cold weather. In extremely hot weather the charging voltage may drop slightly below 13.8 which is perfectly normal for very hot weather. For now, let’s stay focused on generator electron current because this plays a crucial role in testing a battery that is often overlooked. Generator charging voltage issues will be discussed in future parts of this series.
"Smart Charging Systems"
Some vehicles employ what is known as a “smart charging system” where an onboard computer controls the generator output. The computer monitors battery recharge current and when it detects battery recharge current has dropped to a predetermined level it indicates the battery is fully charged. At this point the onboard computer reduces the charging voltage to 13.0x V to prevent excessive battery gassing which is important when the battery is mounted in a confined compartment in the vehicle other than the engine compartment. Lowering the charging voltage also reduces the generator’s load on the engine which can result in a slight boost in miles per gallon.
(The topics of charging voltage and charging amps, as well as “smart charging” is covered at length in our book “Electrical SHORTCUTS” in Section 6, which discusses the operation of generators and testing charging system performance based on voltage measurements.)
For the remainder of this discussion we will consider this vehicle does NOT employ “smart charging.” The generator is not controlled by an onboard computer but with an internal voltage regulator. With the engine running, the generator is online and the battery is offline. Motor vehicle operation during engine RUN is important to understand because this is when the majority of electrical problems show up.
Current Clamp #1
This reading is taken on the primary, and only ground strap, so we are assured that the reading indicates total generator electron current output to all vehicle circuits. It does NOT show electron current that is recharging the battery.