What is pre-charging and how does it work?

When power is first applied to a capacitive load (such as an inverter or charger), a large inrush current is induced. This current creates an arc between the relay contacts, causing severe damage. For that reason capacitive loads need to be pre-charged with a controlled current. TAO EMS can do that for you as a standard feature.




What are the risks of not pre-charging?

In order to smooth the current, power supplies and inverters are fitted with large capacitors on their DC side. When first connected to a battery, those capacitors behave like a short-circuit for a very brief instant (microseconds), similar to connecting the positive and negative posts of the battery. The short-lived high current going through the short-circuit is called “inrush current”.

Inverter input capacitor  

Short circuit for microseconds when the relay closes


Lithium-ion batteries have a very low internal resistance and are able to supply inrush current of well over 1,000 Amps. Typical inrush current would be in the range 500 – 1,000 Amps depending on the load capacitance. This current creates an arc between the relay contacts as they close.

At worst, that arc can fuse the contacts together, making it impossible to open the relay again. In all cases, however, it will build up bumps and dig craters on the contact surfaces, increasing the resistance of the relay contacts and leading to relay failure.


A relay with damaged contacts should not be used as a protective device


How is pre-charging performed?

To avoid inrush current, the input capacitor in chargers and inverters must be charged with a controlled current BEFORE the relay is closed.

To pre-charge the equipment’s input capacitor, you must:

  1. connect a small resistor in parallel with the relay contacts before the main relay is closed
  2. wait long enough for the small current (limited by the resistor) to charge the inverter input capacitor
  3. close the main relay
  4. disconnect the resistor before the main relay is opened again



Inrush current limiter


The resistor must be sized to set the maximum charge current and charge time you want

Without going into detail on how to calculate resistor value and power rating, here are a few examples for limiting the current to 2 Amps:

  • 12 V installation: resistor value = 7 Ω/power rating > 30 W
  • 24 V installation: resistor value = 14 Ω/power rating > 60 W

To determine minimum pre-charge time, you must take into account the parasitic loads in parallel with the inverter. These loads will reduce the pre-charge current flowing to the inverter input capacitor, and they may even prevent full pre-charge. A pre-charge time of 5 to 10 seconds is usually sufficient, but the actual value will depend on your installation.


No one wants to run the risk of forgetting to activate a manual switch

In a lithium-ion installation, the EMS activates the relay (charge or load). For automated pre-charging, you can either connect a specialized “inrush current limiting device” (for around $100) between the EMS and the relay, or…


You can let TAO EMS take care of the pre-charge for you

For a few dollars, simply add the resistor that is suited to you installation


How to use the TAO EMS integrated pre-charge feature

TAO EMS output #1 can be configured for pre-charging. That output is connected to an external resistor. Simply set the capacitor pre-charge time. Nothing could not be easier.


Pre-Charge Connection Diagram:

  • EMS output “6” is used to control the load relay
  • EMS output “1” is the pre-charge circuit


EMS Configuration:

  • set EMS outputs “1” and “6” to normally open (NO) when the EMS is powered down. When the EMS is powered up they need to be closed (relays activated by default)
  • the pre-charge time is set to 5 seconds



A look at the event logs when the EMS is powered up:

The load relay is closed 5 seconds after the pre-charge circuit has been activated


Trigger Configuration

  • trigger #4 has been set to open the load relay when cell voltage is less than 2.85 V (low voltage disconnect)
  • the trigger controls relay outputs “1” (pre-charge) and “6” (load relay) with a wait time of 60 seconds
  • the load relay will close again when all cell voltages are above 3.15 V


Low-Voltage Disconnect

The EMS’s “fault simulation” feature is used to set cell voltage at 2.84 V:


  • trigger #4 becomes active (< 2.85 V)
  • 60 seconds later the pre-charge circuit is deactivated (to avoid draining the battery)
  • the load relay is then opened


The sequence of events is confirmed by the EMS event log


Recover from low-voltage disconnect

With the simulation set to maintain the voltage of all cells above 3.15 V:

  • trigger #4 is deactivated
  • the pre-charge circuit is activated
  • 5 seconds later, the load relay is closed


The sequence of events is confirmed by the events log


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