A design guide to DC precharge circuits.
When initially connecting a battery to a load with capacitive input, there is an inrush of current as the load capacitance is charged up to the battery voltage.
With large batteries (with a low source resistance) and powerful loads (with large capacitors across the input), the inrush current can easily peak 1000 A.
A precharge circuit between a battery and its load is required if any of the following are issues:
The precharge circuit consists at the minimum of:
Additionally, the precharge circuit may have:
Typical precharge circuit.
In the typical precharge circuit, the precharge resistor is on the positive terminal of the battery, though it could just as easily be on the negative terminal.
In the most basic form, the precharge circuit is operated as follows:
Typical precharge waveforms.
Additionally, the precharge circuit may be used in combination with sensors to detect problems with ground isolation, short circuit across the load, and problems with components in the precharge circuit itself.
Here are some tips on component selection.
In a well designed system, under normal operation, the contactors are not required to interrupt the operating current, because the system will reduce the load current to 0 before the contactors are opened.
Therefore, only the carrying current rating needs to be sufficient for the average load current.
The contactors must be rated for the maximum battery voltage, as that voltage will be across the contacts when they are opened.
The contactors must be rated for DC operation. AC rated contactors rely on the fact that the current waveform goes through 0 A at every crossing from + to -, to interrupt arcing across the opening contacts. That is not the case with batteries. Instead, a DC contactor incorporates other ways of quenching the arc that forms at turn off. One method is a magnet that creates a field across the path of the arc, bending it and breaking it. In that case, the contacts of the contactors are polarized (one terminal is labeled '+'); connect the contactor so that normally (that is, while discharging) the current flows into the '+' terminal.
Contactor manufacturers include:
The precharge relay needs to be rated for the full battery voltage, because, when the system is off, the full battery voltage appears across its contacts.
An AC relay may be used because by the time it is turned off the current through it has gone to 0 A.
The relay needs to be able to handle the peak of the inrush current; but, since the average current is low, and the breaking current is nearly zero, the current rating of the relay is not critical.
The resistance of the precharge resistor is chosen based on the capacity of the load and the desired precharge time.
The precharge surge current reaches 1/e of its initial value after a time of:
The precharge resistor needs to dissipate as much energy as the energy stored in the load's input capacitors.
So, for example, with a 100 V battery voltage and a 10,000 µF capacitance, the energy in the charged capacitors (and therefore the energy dissipated by the precharge resistor during turn on) is:
At the very beginning of the precharge, the instantaneous power will be quite high:
Now, over the long term, the precharge resistor will not need to dissipate any significant power (it will not get hot). But, during the precharge, the precharge resistor will be stressed by that high, sudden power. That is why the precharge resistor needs to be very sturdy and high power, yet it doesn't need a heat sink.
Some manufacturers specify the peak power dissipation. For example: "Overload: 5 times rated wattage for 5 seconds.".
Wire-wound resistors are recommended, typically encased in ceramic, cement or extruded aluminum.
Inductive resistors are OK.
An alternative to a resistor is an incandescent light bulb, whose resistance increases as it gets hot, therefore making it able to drive a shorted load without a problem. (Thank you to Lee Hart for the tip.)
Note that, should the precharge circuit fail (the resistor is open, or the K1 relay is not closing), the immense inrush current will occur at the end of the precharge period, damaging the contactors, the load capacitors, and possibly blowing the main fuse. Therefore, the reliability of the precharge circuit is paramount, or its proper operation should be confirmed before the contactors are closed.
The Elithion Lithiumate™ can be programmed to perform such tests.