Tesla drivers very rarely run out of charge, but it may give peace of mind to know what to expect if the worst happens.
What actually happens?
Long before you actually run out, the car will start warning you. The navigation system will give a warning when you get too far from known charging places, and the green bar ‘fuel gauge’ under the speedometer will turn first yellow and then, at about 5% remaining, red. A dotted line will appear on the power display, indicating that maximum power is limited, although the car will still drive normally – it is just maximum acceleration that is not available.
If you ignore these warnings and carry on driving, when the gauge reaches zero it is replaced by a red notice saying ‘Charge Now!’. After that, the maximum power limit is gradually reduced and the car’s performance will become noticeably sluggish, although it can still maintain 60+ mph. Eventually, power is lost altogether and the car gives a warning chime and a notice to ‘pull over safely’. All of the car’s safety systems are powered by a separate small battery so that the brakes, power steering, lights, instruments etc. will still work normally after the main battery has run out and switched itself off.
Having pulled off the road, you should now call Tesla Roadside Assistance for help. Usually, their response will be to arrange for your car to be transported to the nearest sensible charging facility – a Supercharger if one is nearby, or another public charging facility. Alternatively, they can arrange for your car to be taken to your home or your original destination, although you may have to pay for the cost of this if it is significantly further. Check the Roadside Assistance document or the Teslapedia article for coverage limits.
If you are a member of one of the standard motoring support organizations (AA, RAC, AAA, CAA etc), most of these now include towing of EVs within their standard policies. Occasionally they may be able to offer a faster response than Tesla’s towing contractors, so it is worth checking your coverage.
How low can you go? Zero means zero.
Some people fear running out and don’t like to go anywhere near empty, while others have read tales on internet forums and believe there is a ‘secret reserve’ below zero. The truth is between these extremes, so you can be confident driving down to zero on the gauge (but no further).
The uncertainty arises because it is not possible to measure the remaining capacity in a battery accurately. However, Tesla’s systems are now pretty good at this, and in particular they are designed to be slightly conservative: They attempt to ensure that the car will never stop before the gauge reads zero. Any error in the measurement means that there will be a little left when the car says ‘Charge now!‘. The accuracy has improved over time, so early reports from pioneer Model S drivers that they managed to drive 10s of miles beyond zero are probably more of historical interest than useful guidance today.
In general, once you know that you haven’t enough charge to reach your next charging point, it is better to stop straight away rather than pressing on to the bitter end. You can then choose a safer or more comfortable place to stop, and it will be much easier to get your car onto a tow truck if it can still move rather than having to be dragged dead-weight. On the other hand, if your destination is almost in sight when you reach zero then you may feel that it’s worth the risk of carrying on carefully, as there will often be a mile or two of low speed driving still available.
Once you have called Tesla roadside assistance, they should handle everything for you and all you need to do is wait. However, there are a few things that may be useful to know, especially if you want to solve the problem yourself rather than calling Tesla. Also bear in mind that, although Tesla will be giving instructions, the tow truck driver may never have towed a Tesla before and you can speed up the process if you are able to help.
The main complication is the 12-volt auxiliary battery that powers the car’s controls and safety systems. This battery is quite small. Usually you don’t need to worry about it since it is kept topped up automatically from the main battery, but once you have lost the main battery the auxiliary battery can run flat quite quickly. If this happens then you will not be able to use the car’s controls and, importantly, you won’t be able to release the parking brake to load the car onto a tow truck, or to start a charging session to charge the main battery. You (or the tow truck driver) will then need to ‘jump start’ the 12V battery, which is just like jump starting a conventional car with a flat battery. The type of battery trolley normally carried by roadside assistance services for jump starting is ideal for the job. If doing this at home, you may well have a 12V battery charger that is suitable, or you can use jump leads from another car (don’t start the other car’s engine if doing this). The auxiliary battery itself is not readily accessible, so a special pair of terminals is provided in the nose of the car for attaching jump leads. You will need to remove the nose cone of the car to access the terminals (ideally using a plastic pry tool, but the corner of a credit card may suffice in an emergency). The nose cone will usually need to be removed anyhow to access the towing eye.
The actual procedure for towing/transporting is detailed in the Model S manual (both the full version and the ‘quick guide’). You may like to keep a copy stored on your phone, or the relevant pages printed out in the car.
For cars with air suspension, the suspension must be set to Jack mode – you can to do this straight away after stopping. To release the parking brake to allow the car to be pulled onto the transporter, you need to select Tow Mode from the main screen (under Controls->Settings->Service&Reset->TowMode). This has the same effect as putting the car in neutral, but in Tow Mode the car remains in neutral even if the driver is no longer in the driving seat. If the car is on level ground, you can consider finding some suitable items to chock the wheels and then selecting Tow Mode so that the car is ready to load even if the 12V battery goes flat before the tow truck arrives. However, beware that in Tow Mode the car is free to roll and you should only do this if you are confident there is no risk of the car moving.
The auxiliary battery will run down more quickly if you are in the car with lights, displays etc. turned on. If practical, you should shut down the car and wait elsewhere (which is standard advice if stopped on the shoulder of a fast road regardless of the type of car – to avoid the risk of being struck by an inattentive driver in another vehicle). To shut down the car while remaining inside, press the Power Off button (under Controls->E-brake&PowerOff on the main screen). Once you have arrived at your charging point, don’t let the tow truck driver depart until the car is successfully charging as you may need to jump-start the 12V battery again to unlock the chargeport and start the charging process. In very cold conditions, be prepared for slower-than-usual charging, since the battery will have cooled down while you were waiting for the tow.
If circumstances force you to abandon the car with no charge and return to recover it some days later, you will certainly need to charge the 12V battery and it is possible that the 12V battery may be damaged if you leave it long enough. The main battery however should be unaffected, provided that you recharge it within two months.