Electric vehicles basically have two electrical systems, the first and most dangerous is the main drive motor(s) electrical supply, this high voltage can range from 250 to 350 volts depending on design and capacity. The voltage is not really the dangerous part, what creates the hazard is the amperage at which the voltage can be delivered at, which is about 300 amps. This can cause immediate electrocution and death if touched by a person, service to the main battery pack should always be handled by a trained professional. Now for the second electrical system, other than the main drive motors the remainder of a Tesla or any electric car's is the additional accessories such as the users control computers such as navigation, HVAC, window operation, seat control, LED lighting systems, cooling fans, coolant pump and so on, are all handled by a regular 12 volts system which is supplied by a traditional 12 volt car battery.
A lead acid 12 volt car battery has a duty cycle or expectancy life range of about 4 years. This can be cut short if the battery has an internal problem due to defects in manufacturing which will cause the output voltage of the battery to be lower than the accessory systems are designed to run on.
When a Tesla is plugged in for charging, there is an auxiliary output on the charge inverter that supplies voltage to the traditional 12 volt battery along with the main 300 volt battery pack. Additionally, when the car is in braking mode it will return charge to the main battery pack and smaller 12 volts battery.
Dim exterior and interior lights are a sign that the smaller 12 volt battery
is having a problem, in other words the output voltage of the 12 volt battery is
too low, hence the dim lights. Open the front compartment, you will see two plastic
covers, the uppermost cover simply pops off using clips. We have removed the lower
cover by undoing the four 10mm bolts, then it pops off and out as well (simple).
Here you can see under the covers, there is a
radiator which is used to cool the
battery packs, an electric air conditioner compressor, water pump and front drive
motor. You may have also noticed the 12 volt regular car battery, this is what needs
to be replaced.
To confirm the failure, let's use
a voltmeter to check the surface voltage. Here you can see the voltage is well
below the 12.4 volts which is normal. Here you have confirmed the dead
battery that needs to be replaced.
Our certified technicians are ready to answer Tesla questions for free. We hope you saved money and learned from this guide. We are creating a full set of car repair guides. Please subscribe to our 2CarPros YouTube channel and check back often for new videos which are uploaded regularly.
This guide created by Ken Lavacot: 35 Year Master Mechanic and Author