What is Precharge, and How Do You Use It?

You’ll often hear people shouting about how you have to precharge your controller or inverter, or that you’ve got to precharge your DC-DC converter. They’ll say it’s a requirement, but they won’t necessarily say why. Well, let’s take a look at why it matters, and how you can go about implementing precharge in your conversion.

First of all, what the heck is precharge?

When people refer to precharge, they’re talking about the process of gently charging up the capacitors in a component, usually the controller/inverter or the DC-DC converter. These devices have large capacitors which are used to help filter the input (and sometimes the output) of the component. Since controllers, inverters, and DC-DC converters all rely on some form of switching, the input and output from these devices can be very jagged, as rapidly turning the solid state switches on and off results in a constant stream of on-off pulses of power. Depending on the application, this isn’t exactly great. This “ripple” can be pretty stressful on a lot of components, especially batteries. One way to help reduce this ripple is through the use of a large capacitor, which helps to even out the pulses. Instead of an on-off square wave, the capacitor will help smooth the output into more of a reverse saw-tooth, easing the life of everything involved. Here are a few pictures to help you visualize this.

Example of a square wave

This is what the input or output of your device might look like without some kind of filtering. As the switches turn on and off, the power correspondingly turns on and off to whatever you have connected to the output.

Example of a reverse sawtooth wave

This is what the input or output of your device might look like with filtering. The large capacitor smooths out the pulses. It’s not perfectly smooth, but it’s a lot better, and it’s much easier on whatever you have connected to the output.

OK, so there’s a capacitor, and it’s smooths things out. Cool. So what?

Well, a capacitor with no charge appears to be a short in the circuit. You may have shorted out a car battery at some point, and if you did you know that you get a shower of sparks as a massive amount of current flows through the shorted connection. With a car battery, it can be enough to blow out small chunks of a steel wrench and lead terminals or liquefy copper wire. Imagine that on the scale of an EV battery pack… not pretty. All of that current, best case, will blow your fuse. Worst case, it could blow out your components or may even cause other, serious damage. Gently charging up these capacitors is a big deal.

This monstrosity is the capacitor that’s in the controller in my MG. The connections are wired together after manufacturing so that it does not pick up a charge during shipping/storage and possibly hurt you when you touch the contacts! These things are serious!

How do I actually precharge my components, then?

It’s actually very simple. Before you apply full power to the devices, such as by closing the main contactors, you first run the circuit through a large resistor. This resistor will trickle power into the capacitor, and it will be ready to go once you close the main contactors. These resistors are easy to get online, and I’m using one out of the precharge unit from a Nissan Leaf battery pack. An additional relay closes to put the resistor in the circuit. Once the precharge is complete, the main contactor will close, and the precharge relay will open. At that point, you’re charged up and ready to go!

The wire bar in the center of this precharge unit is the precharge resistor. The small black box with 4 wires immediately to the right of it is the precharge contactor. The larger boxes to the left and right are the positive and negative contactors. For those that need an entire assembly, these are actually pretty darn nice! I used all of the components from one to make my own precharge box or the Mercedes. Note the fuse on the resistor in the lower right corner. I don’t think it’s strictly necessary, but it’s a nice safety upgrade!

Precharge Circuit Diagram

I designed this circuit in TinyCAD, which worked pretty well. This circuit represents the positive and negative contactors, as well as the precharge contactor. It’s very simple, and it needs to be controller by something else, which is labeled as the precharge controller in this diagram. I’ll get to that in a moment.

My precharge controller only handles a single contactor, like many do. I like to have a contactor on both the positive and negative leads of the battery pack so that it is completely disconnected from the vehicle when it’s powered off. To facilitate that, I’ve connected the negative contactor to a Key-on 12v source of power which will enable the contactor whenever the car is running. The positive contactor and the precharge contactor are both controlled by the precharge controller. If the precharge controlling you’re using is bundled with something else, or if it will control multiple contactors, you can adjust accordingly.

Of special note is that the resistor in this diagram, R1, is not always the same. Follow the manufacturer recommendations when choosing a precharge resistor for your controller/inverter.

ZEVA Precharge Controller Example

I’ll use ZEVA’s precharge unit to give you an idea of how to hook these up. You’ll need to follow whatever documentation you’re using for your own device, but they’re all pretty straightforward.

ZEVA’s precharger incorporates the precharge contactor and resistor into the unit. Functionally, the diagram would be exactly the same as above. In practice, however, it’s a little simpler to wire up. Here’s their diagram.

In this case, the ZEVA unit bridges the terminals on the contactor with the precharge relay and resistor inside the unit. Once precharging is complete, it will apply power to the contactor to close it. The controller needs Key-on 12v so that it knows when to start precharging, it needs to be connected to the terminals on the contactor so that it can actually do the precharge, and it needs to be connected to the coil of the contactor so that it can close it when the precharge is finished.

I use a similar unit in my MG, except that it is integrated into ZEVA’s EVMS3.

This is the EVMS3 that is in the MG. A little more functionality than just precharging…

I heard I could just use a timer, and that sounds easier. Why not just do that?

You already know how important precharging is, so you know that it needs to be done correctly every time. What would happen if precharging were to fail, however? What if the precharge contactor has failed for some reason? What if the 12v wire to it were disconnected? What if you just forget to hook it up (come on, we all make mistakes…)? Well, in the case of a simple timer circuit, you get no precharge. There’s no way for a timer to tell that the precharge failed, and there goes your inverter! A unit like ZEVA’s precharge controller or EVMS3 or Thunderstruck Motor’s VCU actually checks for a successful precharge before closing the main contactor. In the event of a failed precharge, it won’t close the contactor, saving all of your expensive EV goodies from being turned to slag.

Always precharge.

Precharging is straightforward and easy. Always do it. If you have any questions, feel free to leave a comment and I’d be happy to reply.

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How about that, eh? What do you think?