Options for high voltage precharge

The MEB battery, but also CCS charge ports require an external high (precharge) voltage to be applied before contactors are closed.

One of the options is the HIA4V1 board original or modified.

HIA4V1

The HIA4V1 has been used successfully to precharge the MEB battery's inverter port. It can be found on aliexpress. Without modification it requires manually tuning the resistor to match the external voltage to the internal battery voltage. Given the thin traces on the board, it is probably a good idea to put a circuit breaker between the battery and this board.

Modified HIA4V1

The HIA4V1 contains a 555 based oscillator, with the output connected to a MOSFET. This MOSFET is 3v3 compatible, which means we can directly control it from an ESP32 based board like the lilygo.

To do this:

• first remove the 620 ohm smd resistor (marked 621) from the board. Easiest way to do this is put some solder on top of the resistor such that it covers both ends, melting the solder of both ends and thus releasing the resistor.
• In place of the above resistor, on the pad closest to the MOSFET, solder a small wire.
• Connect this wire via a 330 ohm resistor to a pin on the lilygo (eg. io25).
• Solder a gnd wire to gnd pin of the connector on the underside of the board (if the low voltage gnd of the HIA4V1 is already shared with the lilygo, this step is not necessary). And connect this to the gnd of the lilygo.

By using the ledcWriteTone(PRECHARGE_PIN, freq); function you can now tune the voltage.

Results while powering the board with 12V:

• 23kHz : 370V
• 28kHz : 390V

ledcWrite(PRECHARGE_PIN, 0); to turn the output off.

Note that these values depend on the current the HIA4V1 has to provide. I also biased my board with 4x 140k resistors in series across the HV output (4x to increase voltage handling capability), to prevent very high output voltage in no-load situations (which may damage connected equipment).