2.7 Precharge

An inverter or motor drive has a large input capacitance. Closing a contactor straight onto it gives a very high inrush current that can weld the contacts and trip the short-circuit protection. This page explains how the BMS charges that capacitance first, which precharge strategy to choose and how to size the precharge circuit.

Applies to

Boards: BMS1820, BMS3610, BCU2002, BMS1601, BMS1810.

2.7.1 How It Works

A precharge circuit, a small contactor with a resistor across the main or discharge contactor (Fig.2.6.1 in 2.6 Power Path), charges the capacitance through the resistor. The main contactor then closes with almost no voltage across it.

  • When precharge runs: every time the discharge path is switched ON after being OFF, for example at start-up or after a protection releases. Switching the discharge path OFF resets the precharge.
  • During precharge the Pre Charge Contactor Output is ON and the discharge path is held OFF, so Discharge Contactor Output and Main Contactor Output stay OFF.
  • After precharge the discharge path turns ON. The Pre Charge Contactor Output stays ON for a short time more, so the precharge circuit is still closed while the main contactor closes.
  • Only the discharge path is precharged. The charge path is not held during precharge.

Fig.2.7.1: Precharge Logic

1 Request discharge path requested ON 2 Precharge? strategy not Disabled and Pre-Charge Time > 0? 3 Precharge Pre Charge output ON, discharge path held OFF 4 Early end? Time and Current Limit: settling time over, current below limit? 5 Time up? Pre-Charge Time has run out? 6 Current check Time and Current Limit: current still at or above the limit? yes no yes No precharge discharge path ON at once no no: keep checking Precharge ends discharge path ON; Pre Charge output OFF after a short overlap yes no Pre-Charge Fault both paths OFF until the BMS restarts yes PCHG Time Only: steps 4 and 6 are skipped; the precharge always runs for the whole Pre-Charge Time.

2.7.2 Precharge Strategies

Pre-Charge Strategy decides how the BMS ends the precharge. The figures show the current through the power path and the two outputs over time.

PCHG Function Disabled (default): no precharge. The discharge path closes at once, so the load capacitance is charged through the contactor with a high inrush current. Use it only for loads without input capacitance, for example a resistive heater.

Fig.2.7.2: PCHG Function Disabled

Pre-Charge Strategy PCHG Function Disabled 0 0.5 A Current below 0.5 A the BMS reads 0 A high inrush into the empty load capacitance Pre Charge Contactor Output OFF Main / DCHG Contactor Output ON 0 Time after the discharge path is requested ON Without precharge the discharge path closes at once onto the empty load capacitance.

PCHG Time Only: the precharge always runs for the whole Pre-Charge Time, then the discharge path turns ON. The current is not checked, so the BMS does not notice a shorted load. Use it when the precharge current does not pass through the current sensor.

Fig.2.7.3: PCHG Time Only

Pre-Charge Strategy PCHG Time Only 0 0.5 A Current below 0.5 A the BMS reads 0 A Pre Charge Contactor Output ON OFF Main / DCHG Contactor Output OFF ON 0 Time after the discharge path is requested ON Pre-Charge Time The precharge always runs for the whole Pre-Charge Time; the current is not checked.

PCHG Time and Current Limit: the BMS watches the current. As soon as it is below Pre-Charge Current Limit, the precharge ends early and the discharge path turns ON. This is the recommended strategy when the precharge current passes through the current sensor.

Fig.2.7.4: PCHG Time and Current Limit, Normal Start

Pre-Charge Strategy PCHG Time and Current Limit 0 0.5 A Current below 0.5 A the BMS reads 0 A Pre-Charge Current Limit current below the limit: the precharge ends early Pre Charge Contactor Output ON OFF Main / DCHG Contactor Output OFF ON 0 Time after the discharge path is requested ON Pre-Charge Time The precharge ends a short time after the current is below the limit, before the Pre-Charge Time runs out.

If the current is still at or above the limit when Pre-Charge Time runs out, for example because the load is shorted or already ON, the BMS sets Pre-Charge Fault (error bit 31, CRITICAL). Both paths switch OFF and stay OFF until the BMS restarts.

Fig.2.7.5: PCHG Time and Current Limit, Shorted Load

Pre-Charge Strategy PCHG Time and Current Limit 0 0.5 A Current below 0.5 A the BMS reads 0 A Pre-Charge Current Limit Pre-Charge Fault: both paths OFF until restart Pre Charge Contactor Output ON OFF Main / DCHG Contactor Output OFF 0 Time after the discharge path is requested ON Pre-Charge Time Shorted load: the current stays above the limit until the Pre-Charge Time runs out.

The current check sees only 0.5 A and more

The BMS reads currents below 0.5 A as no current (2.4 Current Measurement), so a limit below 500 mA acts like 500 mA. The check therefore catches a shorted load or a load that is already ON, not the exact voltage left across the main contactor.

2.7.3 Pre-Charge Fault

With PCHG Time and Current Limit the BMS expects the precharge current to fall below Pre-Charge Current Limit before Pre-Charge Time runs out. If it does not, the load is not charging the way it should, and closing the main contactor would be unsafe. The BMS then stops the start (Fig.2.7.5).

  • What the BMS does: it sets Pre-Charge Fault (error bit 31, CRITICAL), switches the Pre Charge output and both paths OFF, and sets User Attention Required. The fault stays until the BMS restarts; Clear All Errors does not clear it.
  • What you see: Pre-Charge Fault in Events › Event Status, both paths OFF in Live View, and bit 31 in the error word on CAN and Modbus.
  • Typical causes: a shorted load; a load that is already ON and draws current, for example an enabled inverter or DC/DC converter; a load capacitance larger than planned or a Pre-Charge Time that is too short; or a current that flowed when the BMS started, so the current reading is offset (2.4 Current Measurement).

Fig.2.7.6: Pre-Charge Fault, Causes and Recovery

Typical causes load shorted load already ON, for example an inverter that is enabled capacitance larger than planned or Pre-Charge Time too short current flowing at BMS start-up (zero error of the sensor) Pre-Charge Fault error bit 31, CRITICAL both paths OFF Pre Charge output OFF User Attention Required stays until the BMS restarts 1 Read the events Events › Event Status 2 Disconnect the load check it for a short or a load that is already ON 3 Check the sizing Pre-Charge Time against the load capacitance (2.7.5) 4 Restart the BMS Restart in LiMon™ or power-up, with no current flowing Precharge again the next start runs a normal precharge Pre-Charge Fault comes only with PCHG Time and Current Limit. Clear All Errors does not clear it; only a restart does.

2.7.4 Settings

Table.2.7.1: Precharge Settings (LiMon™ Current tab)

Setting What it does How to choose Unit Default
Pre-Charge Strategy PCHG Function Disabled, PCHG Time Only, or PCHG Time and Current Limit (2.7.2). Any load with input capacitance needs a strategy other than Disabled. list PCHG Function Disabled
Pre-Charge Time Precharge time (Time Only), or the longest precharge time (Time and Current Limit). About 5 τ of the precharge circuit (2.7.5). ms 2000
Pre-Charge Current Limit Current that must be undercut to end the precharge (Time and Current Limit only). At least 500 mA and below the current your precharge circuit gives into a shorted load, V ÷ R. mA 2000

The LiMon™ reference describes every field of the screen: 7.4 Current Settings. The output for the precharge contactor is set on the Peripheral tab (2.6 Power Path).

2.7.5 Sizing the Precharge Circuit

With pack voltage V, precharge resistance R and load capacitance C:

Table.2.7.2: Precharge Sizing Formulas

Quantity Formula
Time constant τ = R × C
Current at the start (peak) Ipeak = V ÷ R
Current after time t I(t) = V ÷ R × e−t/τ
Voltage left across the main contactor after time t ΔV(t) = V × e−t/τ
Energy in the resistor per precharge E = ½ × C × V²
Peak power in the resistor Ppeak = V² ÷ R
  • After 3 τ about 5 % of the voltage is left, after 5 τ about 0.7 %.
  • Choose a resistor that takes the energy E on every start, not only its continuous power rating.
  • Set Pre-Charge Time to about 5 τ, and longer if the load capacitance is not known exactly.

Calculation example: 16S pack, 51.2 V, C = 2000 µF, R = 50 Ω

These are chosen values to show the calculation; use the values of your own system.

  • τ = 50 Ω × 2000 µF = 100 ms; Ipeak = 51.2 V ÷ 50 Ω ≈ 1.02 A.
  • E = ½ × 2000 µF × (51.2 V)² ≈ 2.6 J per precharge; Ppeak = (51.2 V)² ÷ 50 Ω ≈ 52 W for a moment.
  • PCHG Time Only, Pre-Charge Time 500 ms (5 τ): the main contactor closes with 51.2 V × e−5 ≈ 0.34 V across it.
  • PCHG Time and Current Limit, Pre-Charge Time 500 ms, limit 600 mA: the current falls below 0.5 A after about 70 ms, so the precharge ends a short time later, with a few volts left across the main contactor. If the load is shorted, the current stays at about 1.02 A, above the limit, and the BMS sets Pre-Charge Fault at 500 ms.

Fig.2.7.7: Precharge Timeline of the Calculation Example (PCHG Time and Current Limit)

Load voltage (V) Precharge current (A) 0 51.2 0 0.5 A 1 A Pre-Charge Current Limit = 600 mA below 0.5 A the BMS reads 0 A 1.02 A peak load almost charged when the precharge ends Pre-Charge Time = 500 ms Pre Charge Contactor Output ON OFF Main / DCHG Contactor Output OFF ON stays ON a short time (overlap) 0 500 Time after the discharge path is requested ON (ms) Calculation example: 51.2 V pack, 2000 µF load, 50 Ω resistor, Pre-Charge Strategy PCHG Time and Current Limit.

2.7.6 Checking the Result

  • At start-up the Pre Charge output switches ON first and the main or discharge contactor closes after it.
  • The CAN frame 0x04 Pre-Charge Path Status flag is 1 during precharge (CAN data frames).
  • No Pre-Charge Fault in the events.

2.7.7 Common Mistakes

  • Strategy left at PCHG Function Disabled with an inverter on the bus: the main contactor closes onto the empty capacitance.
  • Pre-Charge Time too short for the load capacitance: the main contactor closes with a large voltage across it. Time and Current Limit catches this only if the current is still at or above the limit when the time runs out.
  • Precharge current not through the current sensor: the BMS cannot see it, so Time and Current Limit ends the precharge at once or never detects a fault. Use PCHG Time Only.
  • Pre-Charge Current Limit above V ÷ R: the current never reaches the limit, so a shorted load is not detected.
  • Charger and load on the same bus with separate charge and discharge contactors: the charge path is not precharged, so the charge contactor can close onto the empty load capacitance.