JK BMS Precharge: Stop Sparks and SCP Trips on Inverter Start
Precharge is the BMS charging the inverter's input capacitors slowly, through a resistor, before it closes the main MOSFETs. Without it, an inverter's empty capacitor bank draws a huge spike the moment the battery connects. That spike can trip JK BMS short circuit protection, weld or burn contacts, and give you the familiar spark at the terminal. On JK boards that have it, precharge runs only when the BMS turns on from the off state, and its length is set by the precharge time.
Why inverters trip short circuit protection
An inverter has a large bank of capacitors on its DC input. When they are empty they behave like a short circuit for a few milliseconds. A battery can deliver thousands of amps into that, and the BMS sees exactly what it is built to stop: a current far above its short circuit threshold. It opens the MOSFETs within microseconds, the inverter never starts, and the BMS logs a short circuit.
The typical symptoms:
- a loud spark when you close the breaker or plug in the cable;
- short circuit protection right when the inverter is switched on;
- everything works if you switch the inverter on with the battery breaker already closed, but not the other way round.
What the precharge time does
On boards with a precharge circuit, the BMS first connects the output through a resistor for the precharge time, letting the capacitors fill gently, and then closes the main MOSFETs. Two details matter:
- It works only at turn-on. Precharge runs when the BMS starts from the off state, for example after you press the power button. It does not run when the BMS is already on and you close a breaker between battery and inverter.
- The unit is not documented. JK does not state the unit of the precharge time. Compare the value with what the JK app shows before you change it, and increase it in small steps.
If the inverter still trips short circuit protection at start, a longer precharge time is the right fix. Raising the short circuit delay instead makes the BMS slower against a real short. The OCP and SCP guide explains why that is risky.
Which JK models have precharge
Precharge is a feature of the JK inverter BMS line (the PB series) and of some newer boards. Many standard boards do not have it, even though the settings frame always carries a precharge time field. On a JK-B2A8S20P with hardware 19H, the BMS refused precharge time as a setting it does not have.
You can check your board: connect your BMS over Bluetooth. Right after connecting, the editor tests which optional settings the BMS really accepts and marks the missing ones as not on this BMS. If precharge time is missing, you need an external solution.
Precharging with a resistor
For boards without precharge, for very large capacitor banks, or when you connect an inverter to a battery whose BMS is already on, precharge by hand:
- Leave the main breaker open.
- Connect a resistor of a few tens of ohms, rated for a few watts, across the open breaker or between the battery and inverter terminal. Some breakers and battery switches have a precharge button with a built in resistor.
- Wait a few seconds. Measure the voltage across the open breaker: it should fall to almost zero as the capacitors charge.
- Close the breaker and remove the resistor.
The resistor only carries current for a moment, so it barely gets warm. Never leave it in place: with the inverter running it would limit the current and overheat.
Startup order that avoids trips
- Inverter off, battery breaker open.
- Precharge, by the BMS at turn-on or by hand with a resistor.
- Close the battery breaker.
- Switch the inverter on.
- Connect loads.
With parallel batteries, precharge each pack before you connect it to the bus. The parallel batteries guide covers the current limiter that helps when packs at different voltages meet.
FAQ
What is precharge in a BMS?
Precharge charges the capacitors of the inverter or motor controller through a resistor before the BMS connects them fully. It turns a destructive inrush spike into a small, slow current, so the short circuit protection does not trip.
Do I need precharge on a JK BMS?
With an inverter, yes, in one form or another. Small DC loads without big capacitors do not need it. If your BMS has no precharge time setting, precharge with a resistor before closing the breaker.
Why does my JK BMS trip short circuit when the inverter starts?
The empty input capacitors of the inverter draw a spike that looks like a short circuit. Use the precharge time, or a precharge resistor, instead of raising the short circuit delay.
Connect your BMS to see whether your board has a precharge time and what it is set to.
Related guides
- JK BMS Overcurrent Protection Settings: OCP and SCP GuideJK BMS overcurrent protection settings explained: charge and discharge OCP limits, delays, recovery times and SCP, with ranges and sizing for inverters.
- JK Inverter BMS Communication: Deye, Victron and Growatt SetupConnect a JK inverter BMS to Deye, Victron or Growatt over CAN or RS485: protocol number, port mode, DIP address, cable pinout and the inverter-side settings.
- JK BMS Model Numbers Explained: B2A8S20P, PB2A16S20P and MoreDecode any JK BMS model name: series, balance current, cell count and current rating, plus the -H, -HC and -R suffixes and what they mean for heating, CAN and RS485.
- How to Set Up a New JK BMS: First Start Step by StepSet up a new JK BMS the right way: wiring order, first power-on, passwords, cell count, a chemistry preset, current limits and a verified write over Bluetooth.