High-Voltage converters: Electrification of vehicles

Reading time: approx. 5 minutes – High-voltage converters play a central role in the electrification of vehicles of all kinds. In this article, we look at some of the special features. These features should be taken into account when operating a DC/DC converter in combination with a low-voltage battery.

High-voltage wiring system

In the course of the electrification of the car and lorry sector, many existing components are becoming obsolete or need to be replaced by alternative solutions. This applies not only to the combustion engine, but also to the alternator. The alternator was necessary as a generator to supply the 12 VDC or 24 VDC vehicle electrical system. In electrically powered vehicles, a high-voltage converter performs this task. It takes energy from a high-voltage battery and transforms it to the respective low-voltage.

To prevent a dip in the on-board power supply when the converter is deactivated, a 12 VDC or 24 VDC lead-acid battery is installed, similar to vehicles with combustion engines. This means that an installed high-voltage converter is not only responsible for supplying the low-voltage on-board electrical system. It is also responsible for charging the batteries installed there. Charging is a complex process. It can lead to increased ageing or destruction of the battery if unsuitable parameters are selected.

Robust high-voltage converters for electric vehicles

When supplying a Pb battery with a fixed secondary voltage at the high-voltage converter, various unwanted reactions can occur within the battery:

– Sulphation

Sulphation is the formation of poorly soluble lead sulphate crystals, primarily on the negative plates of the battery. This leads to a reduction in the available capacity of the battery. One reason for sulphation may be that the supply voltage at the high-voltage converter is too low.

– Acid stratification

High charging currents can occur if the charging voltage is set too high without current limitation. As a result, a corresponding reaction gradient of the diluted sulphuric acid, which serves as the electrolyte in the battery, occurs. This is due to a resistance gradient across the electrode areas. Consequently, a density gradient of the acid within the battery results, which accelerates further unwanted reactions.

– Gassing

If the charging voltage is permanently too high, the battery is overcharged. This results in electrolytic decomposition of the water contained in the diluted sulphuric acid. As a consequence, accelerated ageing of the battery occurs due to cell imbalances. There is also a risk of corrosion of the positive electrodes.

Due to these facts, a high-voltage converter should be able to adapt the output voltage to the battery conditions. It should thus minimise the ageing effects of a battery. At the same time, the supply to the vehicle electrical system must not be impaired by the charging process. This makes it necessary to adapt the charging parameters to the respective vehicle requirements.