Welcome to my forum!
I will give you an insight on how to tell what is wrong with either your vehicle or a forklift when it comes to voltage.
A dead battery will cause your vehicle not to crank, little to no dashboard lights, or a clicking sound when you try to start it.
A bad starter will fail only when trying to start your vehicle, you will usually hear a single click when turning the key to the start position. Usually you can hit the solenoid of the starter to get the gear to fly open to start the vehicle, sometimes that may not work.
A bad alternator can be checked using a couple of techniques. A healthy alternator or some call it a generator will produce or read about 14 volts using a volt meter across the battery while running of if the positive lead is directly on the positive probe of the alternator and the negative probe is touching the casing of the alternator or the negative side of the battery.
The other way is if you turn on the car and at either idle or actually running the car and the car dies, when you try to start it it has a dead battery.
Bad battery:
A healthy bad battery would look likes it is in great shape, no spillage of acid and no GFS (green fuzzy stuff) on top of the positive terminal.
Be aware that a bad battery might still read about 12.5 volts but its the cranking amperage that is what makes is bad. If you take a battery reading and it reads 12.5 volts but while you are trying to crank the vehicle and your meter drops down to 9 volts you have an unhealthy battery and needs to be evaluated by charging it manually or simply just replacing it. You will also experience slow crank when trying to start it.
Dim lights and maybe not always though the battery light would display on the dashboard.
In some exotic European cars a car could die while running due to the battery not charging properly.
If you have a Lamborghini or a Ferrari when you have a dead battery you would need to tow to it the dealer to reset the computer to install a new battery, crazy I know
Bad starter:
A bad starter would cause a no start...because you have a bad well starter!
The dashboard lights will light up as it should and your meter should read around 12.5 volts across the terminals.
When trying to start it your meter should read no lower than about 10 volts, if it does you know that the starter is receiving voltage but the solenoid itself is not actuating therefore not engaging the flywheel to turn and beginning the start up sequence.
Bad alternator:
A healthy alternator should produce 14.5 volts when the vehicle is running,
An alternator or some call it generator keeps the voltage when it should to keep an engine alive without relying on the soul battery to keep it running. The battery is mainly used for starting and accessory purposes only, the alternator is what charges the battery and produces all the necessary energy to keep it going. When you have a bad alternator it is either shorted inside the casing or some type of wire winding is broken or faulty.
You can bench test an alternator by spinning the pulley while your positive meter probe is on the terminal and the negative terminal is on the casing it self, it should produce some voltage while spinning.
