In case of low SOC of battery, strange behavior of the engine is possible: it can become unresponsive.

Symptoms of the problem:

  • in neutral gear the engine gains revolutions slower than usual;
  • when driving, very slow reaction to pressing the accelerator pedal is observed.


Reaction to pressing the accelerator pedal is reduced for several seconds, but slowly (during these several seconds) the engine maintains full torque (of course, if the DSC/DTC system don’t limit it).

Solution: charge the battery, using a battery charger or – driving for a long time (several hours). If SOH is low or battery was not registered for a longer time – replace it and perform correct registration, using ISTA D/+ (or ISTA P, if the type or capacity of the battery is modified).


Explanation of the problem.

In the situation, when SOC of the battery is very low (below 40 %), the ability of battery both to give and accumulate energy reduces. The internal resistance of the battery grows.

Due to these reasons, the power (current) of the alternator has to be limited – too high power (power reserve) will not be stored in the battery (as it happens in case of normal SOC), but, as it won’t have where to leak – the onboard voltage will increase rapidly and can damage electronic equipment of the car.

To limit possible short-term over-voltage, which can be caused by the alternator in this situation (low SOC of battery), the growt rate of RPM is limited.

The reason, why such short-term over-voltage can appear (“leftovers” of power) in case of rapidly increasing RPM, is quite simple – alternator excitation coil, which defines alternator’s “toque”, has a certain inertia (this coil has high inductance – the current in it can not be stopped during one moment). If RPM of the engine grows too rapidly, even complete disconnection of coil won’t help – for a short moment, the power of the alternator actually is increased – onboard voltage is inevitably above allowed.

Of course, this lenient mode works correctly only in case, if IBS system is in working order, battery – registered, its data – correct. If IBS system doesn’t work as planned, increased fluctuations of onboard voltage are inevitable. In the best case – the error messages regarding over-voltage will be recorded in the electronic modules, in the worst case – they will get damaged.

And in the end – the error messages regarding under and over-voltage (in case of low SOC) will be recorded also in case, if alternator is in working order, but all other system is performing incorrectly (for example, IBS sensor – damaged or disconnected, unregistered replacement of the battery, incorrect data of battery, etc.). Unqualified “specialists”, of course, will immediately replace or repair the alternator, but – that can not even be the cause of these problems.