Battery Charging

Motor Inverter » Battery Charging

  • Boost Mode Connection
    Boost Mode Connection
  • Boost mode variant
    Boost mode variant
  • Buck Mode Connection
    Buck Mode Connection

A unique feature of the inverter is it's ability to charge the battery. It can take a DC input voltage and either step it up or down to the battery voltage with no additional components. The DC voltage may be supplied by a simple 1 or 3-phase bridge rectifier, a PFC correction circuit or any other DC source like a solar panel.

At least for lower power charging (say below 5kW) this allows you to get rid of a separate charger built into your car. In most practical situation slow charging is ok. You always have the freedom to add an external fast charger (e.g. in your garage) or add DC fast charging to your conversion.

Charge mode is enabled when both FORWARD and REVERSE pins are high at inverter startup. The schematics assume that the FORWARD signal is already connected to 12V. If this is not ensured you'll have to do so with another diode.

Step Up (Boost)

The first diagram depicts how to connect the inverter for boost mode charging, that is stepping up the input voltage. This mode is used if the DC input voltage is LOWER than the lowest battery pack voltage. For example rectified 230V AC results in 320V DC.

Precharge and main relay should already be present in your car anyway. If your main contactor has a free contact it can be used for breaking the input voltage. Otherwise a separate relay is needed.

The AC input voltage first passes though a ground fault detector. This is an important feature as the charger is non-isolated. The AC input terminates at the input relay K1 to be completely disconnected when unused. A free contact of K1 supplies 12V to the inverter. In the drawing it is assumed that the car is in forward drive position. The 12V are then also supplied to the reverse pin. This combination tells the inverter to run in charge mode.

The normally open relay K3 makes sure no AC is supplied to the inverter while it is still in drive mode.

In a variant the precharge signal is used to lock out drive mode. Charging will only start when the inverter is initially off and goes through the precharge sequence.

When you plug in your AC mains, the inverter will power up in charge mode. It will precharge the bus caps to the specified voltage and then close the main relay. After a delay of 500ms it will start charging. It keeps charging until it is powered down, so you need a BMS to find out when to stop charging. The BMS relay output can be put in series to the green control line.

The parameter "chargemode" must be set to Boost (3) to use this mode. The parameter chargecur sets the charger INPUT current in this mode. chargekp and chargeflt can be optimized to minimize oscilation of the control loop.

Step Down (Buck)

Step Down mode is used if the DC input voltage is higher than the highest wanted battery pack voltage.

Precharge and main relay are not used in this mode. Two input relays are needed. The first, K1 is closed immediatly as you plug in your AC mains. It charges the bus caps through a resistor, locks out the precharge and main contactor and powers up the inverter in charge mode. The second relay, K4 is closed by the usual precharge logic in place of the main contactor.

Like in the second boost variant drive mode is locked out via the precharge signal but with a separate relay (K5).

The battery is connected to one of the phase outputs via (delayed) K4.

The parameter "chargemode" must be set to Buck (4) to use this mode. The parameter chargecur sets the charger OUTPUT current in this mode. chargekp and chargeflt can be optimized to minimize oscilation of the control loop.

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