Your electric car can be fully charged and still refuse to open its own doors
There is a second battery in your electric car, and it is the one that decides whether the first is allowed to do anything at all.
Strip an electric car back to its wiring and it looks less like a battery on wheels than a conventional car with a very large power station bolted underneath. The doors, the locks, the screens, the lighting, the brake and steering electronics and the computers that supervise all of it run on the same 12 volts cars have used since the 1950s, fed by an ordinary lead-acid or AGM battery about the size of the one in a diesel hatchback. The traction pack under the floor is isolated from every one of those systems by a pair of contactors, and contactors are electromagnets: they need 12 volts to pull in. That is the whole trap. If the small battery drops below the voltage needed to close them, the large one cannot be reached, cannot recharge the small one and cannot move the car, whatever the state of charge said last night. ADAC, which attends more breakdowns than any other organisation in Europe, traced slightly more than half of all its callouts to electric cars back to the 12-volt battery, a larger share than the 44.6 percent it records for petrol and diesel. The comfort in that statistic is that electric cars break down far less often to begin with, at 3.8 callouts per thousand cars aged two to four years against 9.4 for combustion. The discomfort is that when they do, it is almost always this.
Why the small battery goes flat while the big one stays full
An electric car has no alternator. Its 12-volt battery is topped up by a DC-DC converter that steps the pack's several hundred volts down to about fourteen, and that converter only runs while the car is awake. A parked EV is awake far more often than a parked petrol car: it answers the phone app every time you open it, it listens for the key, it runs whatever guard or sentry mode you have switched on, it keeps a modem alive for over-the-air updates, and on a schedule it will warm the cabin for eight the next morning or precondition the battery before a fast charge. Most manufacturers program a periodic top-up, waking the car every day or two purely to run the converter for a while, but that only happens if the traction pack still holds charge and the car has not been put into a transport or deep-sleep mode. Fourteen days in an airport car park with the app checked twice a day is a realistic way to come home to a car that will not open. Short daily trips do not fix it either, because the battery is being shallow-cycled by the parked wake-ups rather than by driving, and a lead-acid battery treated that way ages faster than the calendar suggests.
The symptoms do not look anything like a flat battery
On a petrol car a flat battery announces itself with a slow crank. An electric car has nothing to crank, so it fails in stranger ways. The screen boots and then reboots, or comes up with a dozen unrelated warnings at once, stability control and power steering and drive system and tyre pressure all together, reported by computers that are browning out rather than actually broken. The key fob stops working at a distance and then at all. The doors may lock you out entirely, which is why every electric car sold in Europe still hides a mechanical key blade inside the fob and a lock barrel behind a small cap on the driver's door, and why it is worth finding both once, in daylight, on your own drive. The detail that catches people out is the charge flap: on many cars its release is electric, so a flat 12-volt battery means you cannot plug the car in either, and on some the electric parking brake will not release far enough to winch the car onto a truck until someone has fed 12 volts back into it.
Jump-starting an electric car is about waking it, not cranking it
The job itself is trivially small. You are not turning over a cold diesel, you are supplying enough current to close two contactors and boot a few computers, which a modern lithium jump pack manages without effort. Every EV has designated jump points, usually a red-capped positive post and a bare earth stud in the front compartment, and the manual will name them; on some cars, the Tesla Model 3 and Model Y among them, the bonnet release is itself electric, and there are two leads behind the front tow-eye cover specifically so that 12 volts can be fed in to pop it open. Connect the positive first and the earth to the marked stud rather than to the battery's own negative terminal, then switch the car to Ready and leave it sitting there for at least twenty to thirty minutes so the DC-DC converter can put something back. Driving does the same job, but the 12-volt battery recovers on time rather than on distance, so a five-minute run to the shops achieves nothing at all. None of this involves the high-voltage system, which stays sealed behind its orange cabling and is not something to open a bonnet and poke at.
Never use your electric car to jump-start anything else
This is the instruction almost every EV manual contains and almost no owner has read. A petrol car being jumped pulls a few hundred amps for a second or two, and in a conventional donor that surge comes from a battery and an alternator built to survive it. In an electric car it would arrive through a DC-DC converter sized only for the vehicle's own lights and electronics, and through a 12-volt battery that is often physically smaller than the one in the car you are trying to rescue. The converter is not a cheap part and the damage is not the kind anyone treats as goodwill. Lend your jump pack instead, or your phone to call for help. It is a mundane rule about a mundane component, which is roughly the shape of electric car ownership once the novelty has gone: the pack and the headline charging speeds get all the attention, while the actual running costs land on the tyres you get through faster than you expected and on a lead-acid box under the bonnet that costs less than a tank of fuel.
What to actually do
Almost all of this is preparation rather than maintenance, and the most valuable ten minutes are the ones you spend on the driveway on a dry afternoon.
Look up three things in your manual now: where the 12-volt jump points are, where the mechanical key blade and door lock barrel hide, and how the manual boot or bonnet release works. All three are useless knowledge until the one morning they are not.
Carry a small lithium jump pack rather than cables. An electric car needs a fraction of the current a diesel does, so even a modest pack will wake it, and it needs no second car and no willing stranger.
Before leaving the car for more than a week, leave the traction pack with a healthy state of charge, and plugged in if your manufacturer recommends it. An empty pack cannot run the converter that keeps the 12-volt battery alive.
Switch off sentry, guard and cabin overheat modes when the car is parked somewhere safe for a long stretch, and resist opening the app to look at it. Every check wakes the car.
If the car mostly does short trips, switch it on and leave it in Ready for twenty minutes every couple of weeks. Time, not distance, is what recharges a 12-volt battery in an EV.
Replace the 12-volt battery on the calendar, around four to five years, rather than waiting for it to strand you. It is one of the cheapest parts on the car and by far the most likely to stop it.
Fit the same chemistry the car left the factory with. Lead-acid, AGM and the lithium iron phosphate 12-volt units now used by several makers are charged differently, and the car's software expects the type it was configured for.
After a jump, leave the car in Ready for twenty to thirty minutes before switching it off, and treat a second flat battery within a month as a battery that is finished rather than bad luck.
Never connect your electric car to another vehicle as the donor, and say so out loud if someone in a car park asks. The part it damages costs more than their whole repair would have.
Frequently asked questions
Can I charge the 12-volt battery from the big battery myself?
Indirectly, yes, and that is exactly what happens whenever you switch the car to Ready: the DC-DC converter starts working and the 12-volt battery is charged from the pack. The catch is the order of events. If the 12-volt battery is already too flat to close the contactors, the car cannot reach the pack to start that process, so an external boost has to come first. This is why a completely dead EV is a chicken-and-egg problem rather than a fuel problem.
Does a 12-volt battery last longer in an electric car?
Not necessarily, and often the opposite. It never has to crank an engine, which is the hardest thing a starter battery normally does, but it spends its life being shallow-cycled by a car that keeps waking up to talk to an app, a network and a timer. Four to five years is a realistic working life, and the lithium iron phosphate 12-volt batteries some manufacturers have moved to are partly an answer to exactly this pattern of use.
Can I use an ordinary battery charger on it?
On the 12-volt battery, yes, provided you select the correct mode for the chemistry and connect only to the terminals or jump points your manual identifies. A trickle charger is a sensible thing to leave connected on a car that sits for months. What you must not do is treat the orange high-voltage cabling as something you can work near, and you should not attempt to charge the traction pack with anything other than a proper charging cable.
My car has warned me about the 12-volt battery but still drives. How urgent is it?
Urgent, in days rather than months. That warning usually means the car's own monitoring has watched the voltage sag under load and concluded the battery is on the way out, and the failure mode is not a gradual weakening you can nurse along, it is a morning when nothing opens. Replacing it is a cheap job at a moment of your choosing and an expensive inconvenience at a moment of the car's.
