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Wiring Fundamentals: Battery and Alternator

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Battery and Alternator


00:00 - The battery is an often overlooked but critical part of our EFI system.
00:03 It performs a couple of very important functions.
00:06 It is the reservoir of electrical energy which we can use to power the EFI system while the engine is not running.
00:12 It's important we don't run this reservoir too low however as the voltage it can supply the system with will start to decrease.
00:19 As discussed in the electrical fundamentals part of the course, our EFI system components will only correctly work when supplied with a pretty narrow voltage range.
00:28 So we need to keep the battery within these limits.
00:30 When the engine is running it is the alternator attached to the engine which provides power to the EFI system.
00:36 The alternator also recharges the battery with any electrical energy that was used while the engine was not running, or by the starter motor during cranking.
00:44 The battery is still an important part of the system while the engine is running though as it helps to smooth the output from the alternator.
00:51 For this reason the battery must never be disconnected when the engine is running as this can cause the alternator to output a powerful voltage spike known as a load dump that can seriously damage components of the EFI system.
01:03 This is a good point to mention a common mistake when fitting a battery master kill switch to a vehicle.
01:09 The master kill switch must be installed such that it breaks the connection from both the battery and the alternator to the rest of the power supply system of the vehicle.
01:17 If it only breaks the connection from the battery, the alternator will continue to supply power to the EFI system, and the engine will still run.
01:24 Obviously this is not what we want, and even worse than this though is that you've just disconnected the battery from the system with the engine running.
01:32 So the alternator will produce a load dump spike which can seriously damage your automotive electronics.
01:38 The voltage supplied by the battery and alternator can vary substantially depending on whether the vehicle is running, the state of charge of the battery, and the overall electrical load.
01:47 If the system is operating correctly, it will fall anywhere between 11.5 volts and 14.5 volts.
01:53 However it is typical to refer to it as 12 volts.