Pre-Charge Circuit Description

The SK48v100 contains an Internal Pre-Charge Circuit. Before powering up, we highly recommend understanding the pre-charge circuit and how it plays a factor in powering up the system safely.

Common in Inverters, battery chargers, and solar controllers is a sizable bank of capacitors on their DC bus to smooth out any ripple that may cause interference with normal operation.

When you apply a voltage across a capacitor it initially appears to be a short-circuit, that is, the voltage across the capacitor is zero. LiFePO4 Batteries in general have a very low internal resistance. This means that it is easy for very large currents to be delivered instantaneously. If there is very little resistance in the circuit, such closing of a circuit breaker, then the current will be extremely high for a brief instant. In this instant extremely high currents will flow through the closing contacts.

The large Voltage difference and sudden high current (known as an inrush current) can cause damage to various components such as circuit breakers, fuses, BMS units, and the capacitors themselves. This may not show up as a failure immediately, but the damage can reappear as a different issue later. This inrush current certainly will exceed the rating of the BMS, fuses, capacitors and circuit breakers.

This can all be prevented using a pre-charge resistor circuit. The pre-charge resistor allows the capacitors in the controller to slowly charge BEFORE the BMS fully activates. This means that the powering-up of inverters and chargers is regulated and graceful, rather than abrupt and volatile.

Proper Power-Up with Pre-Charge Circuit

In order to properly utilize the pre-charge circuit, you must take special care to follow the power-up steps in the correct order, otherwise the benefits of having a pre-charge circuit are negated.

1. Confirm all DC circuit breakers and switches in the system are in the “OFF” (open) position – Typically they should already be off during the wiring process.
2. Ensure the BMS(s) in all batteries have been de-activated – this means none of the LED lights on the front panel are illuminated.
   • If the BMS is active, you can turn it off by pressing and holding the “RST” button with a small screwdriver or similar object for approximately 3 seconds, releasing the button when the SOC% lights begin to flash. A few seconds after releasing, all lights and the display display will turn off, which indicates the battery has shut down.
3. Turn on (close) all DC breakers and switches in the battery system. This includes turning on the DC breaker located on the front panel of all the SK48v100 unit(s).
4. Briefly press the “RST” button of any battery. This will immediately engage the pre-charge circuit of that battery, which will charge any capacitors in the system. After pre-charging is complete, the pre-charge circuit will be automatically disengaged and the main power circuit will be enabled.
5. If additional batteries in the system have not turned on automatically when the first battery was turned on, turn them on now by briefly pressing their “RST” button.
6. The system is now ready for use!

Warning: The pre-charge only works during the power-up procedure of the BMS. If you shut off any disconnects or trip any breakers during normal operation, the system must be fully shut down and this process must be repeated, otherwise damage can occur. Inrush damage is not covered under warranty.