Battery World Professional

In addition to the actual battery change, we also recommend maintaining the voltage during extensive troubleshooting. When the engine is switched off, power is still drawn from the battery during vehicle diagnostics. In the case of a longer diagnosis, the battery installed in the vehicle can be discharged by the control units and other electrical loads to such an extent that the on-board system voltage falls below the permissible under-voltage limit of the control units and a corresponding entry is made in the fault memory and data is lost in the volatile memory section (e.g. steering angle sensor or end positions of the electric window lifters).

A constant voltage of the vehicle electrical system is therefore a prerequisite for professional troubleshooting or battery replacement and to avoid additional difficulties from the outset due to any undervoltage

What is voltage stabilization?

Voltage stabilization describes the process of connecting an external power supply in order to maintain the voltage in the vehicle at a constant level when the battery is disconnected. This prevents the control units in the vehicle from registering a critical undervoltage and recording this as a fault in the event record.

Mercedes sometimes reacts sensitively to this if you simply disconnect the battery and connect a new one: We already had a head up display that no longer works and problems with driver assistance systems. You have to use a diagnostic tool for that anyway. It could also affect the start/stop behaviour for a longer period of time.
R. Nickel – CLARIOS Vehicle Application Engineer

Recommendation for voltage stabilization

The external power supply shall be connected before starting battery replacement or extended vehicle diagnostics.

NOTE: Never use a car battery charger to maintain voltage. Modern chargers use a special charging algorithm with different phases of charging. Sudden disconnection of the battery can lead to malfunctions in the charger, which can damage the vehicle electronics or charger.

When changing the battery, do not disconnect the installed battery until the external power supply has been connected and switched on.

The external voltage source must always be connected at points that are capable of safely carrying the required currents. For this reason, we always recommend connecting the external voltage source to the battery connection terminals for diagnostic purposes.

In the event of a battery change, the connection to the battery terminals is naturally somewhat more difficult to handle. Working on the cables can also cause the clamps of the external voltage source to drop or cause short circuits. If available, we recommend using the jump start connection points in the vehicle in this case. Information on where these are in the vehicle can be found in the VARTA Partner Portal.

NOTE: We advise against connecting the voltage maintenance via the OBD interface. According to the ISO 15031-3 specification, the maximum current carrying capacity of the connections is limited to 10A - a value that can quickly be exceeded if several control units wake up. This can lead either to serious and expensive damage to the OBD interface or to the fuse being blown, unnecessarily delaying diagnostic or repair work.

The easiest way certainly is to use a second battery as a secondary power source. With this approach, it is important to make sure that the system is protected against unintentional short circuits by a suitable fuse.

In everyday workshop use, stationary constant power supplies with sufficient power from 350W have proven their worth. In this power class, even the unintentional wake-up of one or more control devices does not immediately lead to a breakdown of the supply voltage.


Whenever a battery change or an extensive troubleshooting needs to be performed an additional voltage stabilization should be ensured. Always try to connect the external power source to the battery connection terminals. If this is difficult to handle we recommend using the jump start connection points. Beware of short circuits. Never use a voltage maintenance via OBD. Better go with a secondary battery or a constant voltage power supply of at least 350W instead.

You want to learn more about the different electrical systems in modern vehicles? Then our Advanced Automotive Training is definitely the right thing for you!