Problem child 12 V battery: How you can prevent breakdowns

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The latest ADAC breakdown statistics also show that the most frequent source of defects is the 12 V battery. This applies to combustion engines as well as electric cars and hybrids. The fact that there is a 12 V network with its own battery in addition to the high-voltage network and why is a separate topic. Now it's about how to ensure that the car jumps reliably, i.e. preventing a defect and maintaining the 12 V battery.

Nothing works without the 12 V battery

Since everything is started from 12 volts, the typical battery breakdown looks like this: Nothing works. To be able to drive, you need to re-establish a 12 V power supply that provides enough power for starters (combustion engines, high power required) or to start the battery management system (BMS) in electric cars or hybrids (low power required). Consequently, an electric car that has broken down due to a 12 V defect can also be refloated by bridging it with a power donor car.

Conversely, an electric car can also help a combustion engine. To do this, the car should be switched to ready to drive after the cables have been connected so that the high-voltage battery can supply the 12 V grid via the DC-DC converter. Due to many comfort systems, the output of this converter is high enough to be sufficient for petrol engine starters in cars. With large diesel engines, however, a small 12 V battery and the converter of an electric car may not provide enough power to allow the starter motor to work against the high compression ratio. In this case, it is better to bridge the gap with a similar diesel drive.

How did this happen?

After a battery breakdown, the cause must be identified and rectified. Often it is simply because the battery was discharged too deeply. This happens, for example, due to the classic "accidentally switching on the parking lights and leaving for the weekend". However, it can also be that the distances driven are too short and the consumption in the 12 V network is too high, meaning that the battery discharges more than it charges during use. This can happen if extremely short distances are driven in winter with the combustion engine and seat heating. The starter motor and other consumers then draw more current from the battery than the generator can recharge during the short journey. Electric cars are more resistant to short journeys. For the sake of completeness: If I leave a lot of 12-volt consumers running in the electric car and only drive short distances, this can also become a problem - mitigated, of course, by the fact that the small battery is usually charged when the large battery is charged.

In such cases, it is sufficient to recharge the battery first. Low charge levels are particularly problematic with lead-acid batteries. Low-charged lead batteries, for example, lose a lot of their frost resistance. You should therefore address this by changing user behavior, generator output or battery size. If the battery in the vehicle does not charge at all, it is not to blame, but the fault must be sought in the alternator or DC-DC converter. On motorcycles, the alternator and regulator are designed separately. Here, especially with Japanese makes, the regulator installed at too hot a point often burns out.

The second common cause is a faulty battery. You can sometimes recognize it by the fact that the voltage is not correct despite sufficient charging time. To measure this, the battery must be left to rest for a few hours after charging. The voltage of lead-acid batteries should be between 12.4 and 12.8 volts, depending on the type, and 13.4 volts for lithium iron phosphate (LFP) batteries. The defect usually only becomes apparent under load when, for example, the dipped beam quickly darkens noticeably after switching on (without the engine running/ready to drive) with incandescent lamps. The more economical LED lights often do not show this so precisely, but here a voltage measurement under load reveals a disproportionate drop: With the ignition switched on (standby off), lights on and consumers that can be switched off, the voltage should remain above 12 V with a charged battery.

In an electric car, do not switch on the standby mode, otherwise the DC-DC converter will continue to supply current from the high-voltage battery. Instead, it is sufficient to wait with the lights and radio in the smaller electric car battery to see whether the voltage drops when the vehicle electrical system is activated without the vehicle being ready to drive. The manufacturers now all know that 12 V also causes problems in electric cars and make the battery contacts easily accessible. However, the situation can be very different in older models.