Page 2: Electric cars could save themselves, but often don't

Contents

The third common case is deep discharge, either due to consumers accidentally running on battery power for too long (parking lights) or due to extremely long idle times and self-discharge. Batteries can be so deeply discharged that chargers indicate them as defective and even so deeply that even powerful starter blocks from garages or jump starts from a second car no longer start the vehicle. If it was the first deep discharge, I wouldn't throw the battery away just yet, but charge it with an old charger without a microprocessor until the regulated chargers work with it again. This can take a day or two. Although every deep discharge damages car batteries, a battery that has been deeply discharged once can usually continue to provide good service for years. In the case of more frequent deep discharges, batteries can of course still be used if they still have the capacity, but you can no longer rely on them. This raises the question of prevention.

Precaution is better

Knowing that the 12 V battery is a frequent source of breakdowns, you can take care of its well-being at an early stage. I am a fan of regular maintenance and prevention. That includes the battery. Vehicles that are parked for a very long time need to be supplied with power. This applies in particular to classic cars, motorhomes, sports cars, motorcycles and other entertainment companions. The best way to supply power is via a so-called "trickle charger", which only supplies a low charging current and a voltage that is sufficient to maintain the charge. For lead-acid batteries, this is usually 13.6 V. Trickle chargers cost from 10 Euro from Shanghai or from 25 Euro with all the technical chicanes.

The thin cables of these devices can usually be easily routed through seals, e.g. on the trunk, so that it can remain closed. Many such devices offer "battery jogging", but as far as I know, there are no clear study results showing that this is more effective than the pure float voltage. So it works just as well without. Many combi chargers offer a trickle function, so you don't have to buy anything new. To save fiddling around, I like to install a plug from the battery, to which I can connect the charger during downtime.

(Some) electric cars save themselves

In principle, electric vehicles that have been standing around for a long time could save the starter battery themselves, with electricity from the high-voltage battery because it usually discharges itself to a lesser extent and is always protected from stationary consumers in its safety isolation. Many manufacturers now offer this, depending on the vehicle generation. See the table for new vehicles:

Brand	Can 12 V be recharged from high voltage?
Audi	new premium platform and e-tron GT yes, older designs on MQB no
BMW	yes
Fiat	n.a.
Ford	n.a.
Jaguar Land Rover	n.a.
Kia	no
Mazda	no
Mercedes-Benz	yes
Polestar/Volvo	yes
Porsche	pure electric vehicles yes
PSA	no
Renault	yes
VW	MEB: yes, MQB: no

Note that older vehicles from the same manufacturer were often not yet able to do this. Some manufacturers still assume that their cars will not sit around for months. Mazda, for example, can recharge 12 V from high-voltage, but only if you occasionally open the door to wake up the 12 V system. The manufacturers marked "yes" in the table do this on their own. With these, the BMS wakes up periodically by itself and measures the voltage of the 12 V battery. If necessary, the BMS recharges the starter battery via the high-voltage system. The vehicle can then remain stationary until the traction battery is so empty that the BMS does not provide any further current for 12 V or, alternatively, can be supplied with power via a type 2 plug for as long as required.

Please note: Plugged into the wallbox does not mean that every car can stand for any length of time. Tesla's Model S, for example, could become inoperable if left standing for too long because the 12 V battery discharged itself. The vehicle also had a battery that was difficult to access without electricity. PSA's e-platform charges fully once, then switches everything off permanently, including the BMS, and can therefore be self-discharged in both batteries after a long period of standing (12 V is never charged from high-voltage).

Such peculiarities can be found in the manual of your vehicle. Nissan's first Leaf was able to keep the 12 V battery alive via the optional solar panel - this only worked outside, of course. If in doubt, you can occasionally switch to ready to drive for a few minutes in electric cars that have been stationary for a long time, then 12 V is supplied. There are a number of reasons why starting and running a combustion engine just to charge the battery is not recommended, but this should be sufficiently well known by now. The trickle charge described above is the correct method here.

Conclusion

12 V remains a source of breakdowns. It is therefore worth paying particular attention to this battery - regardless of the drive system. A voltmeter and a trickle charger are inexpensive tools that even non-professionals can use. A weakening battery is then delayed for a long time and noticed early. You should replace it soon after noticing it. Otherwise, all your precautions will end up with the same result as those who don't look: a 12 V breakdown at the stupidest possible time.

(cgl)