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Precharge

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 limits that inrush current, without limiting the operating current.

A precharge circuit between a battery and its load is required if any of the following are issues:

  • The load has input capacitors will be damaged by the inrush current
  • The main fuse will blow if asked to carry the inrush current
  • The contactors, if present, will be damaged by the inrush current
  • The battery cells are not rated for the inrush current
back to topFunction

The precharge circuit consists at the minimum of:

  • A precharge resistor, to limit the inrush current (R1)
  • A contactor (high power relay) across the precharge resistor (K2) to bypass the resistor during normal operation

Additionally, the precharge circuit may have:

  • A precharge relay (K1), to keep the load from being powered through the precharge resistor when the system is off
  • A contactor in line with the other end of the battery (K3) to isolate the load when the system is off
Precharge circuit
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.
While you are free to use any designators you wish, the ones in this schematic (R1, K2, K2 ande K3) appear to be industry standard, so you are encouraged to use them as well.

back to topOperation

In the most basic form, the precharge circuit is operated as follows:

  • Off: When the system is off, all relays / contactors are off
  • Precharge: When the system is first turned on, K1 and K3 are turned on, to precharge the load, until the inrush current has subsided
  • On: After precharge, contactor K2 is turned on (relay K1 may be turned off to save coil power)
Precharge waveforms
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.
Such methods are specialized and are usually proprietary to the manufacturer.

The Elithion Lithiumate™ can be programmed to perform such tests.

back to topComponent selection

Here are some tips on component selection.

Contactors anchor

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.
For example, a contactor with a 50 A breaking current rating and a 100 A carrying current rating will work with a load that draws 100 A average.

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:

Precharge relay anchor

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.

Resistor anchor

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:
T = R * C
The current is reduced to a manageable value after approximately a time of 5 * T.
So, if the desired precharge time is 500 ms, and the load capacity is 10,000 µF, then:
R = T / C / 5 = 500 ms / 10,000 µF / 5 = 10 Ohm

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:
E = (C * V^2) / 2 = (10,000 µF * 100 V^2) / 2 = 50 Joules.
The power dissipated by the precharge resistor during precharge is that energy over the precharge time. For example, with a precharge time of 500 ms:
P = E / T = 50 J / 500 ms = 100 W

At the very beginning of the precharge, the instantaneous power will be quite high:
P = V^2 / R = 100 ^2 / 10 = 1000 W!

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.".
In that case, a 50 W resistor will be handle 500 W (well above the 100 W of the example above).
Ultimately, you should ask the resistor manufacturer if a particular resistor will work in your application, and try the resistor in the application.

Wire-wound resistors are recommended, typically encased in ceramic, cement or extruded aluminum. Inductive resistors are OK.
For example:

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.)

back to topReliability

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.

© 2008~2014 Davide Andrea. All rights reserved, except where noted by CC mark. Page published on Feb 22 2011. Graphic design by morninglori
 

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