Saturday, March 14, 2009


After learning that we do in fact need capacitors, I bought a nice big 230 uF capacitor made especially for this purpose:

The problem of limiting inrush current still remains- the main contactor wouldn't last very long under the 1kA or so that cap would draw as it's connected across the battery. I ruled out having the cap upstream of the contactor, because that would mean the contactor would need to be in the fron of the car, and the main high voltage cables running underneath the car would have no means of disconnection, other than manually pulling the pack splitting connections in the back. So, I set off to design a precharge circuit. The principle of this is that when the key is turned on, it applies 12 volts to a small relay that charges the main cap through a 1k or so resistor. This takes about 1 second. Meanwhile, another RC timing circuit charges up and turns on the main contactor. I used the free circuit modeling tool LTSpice to model the timing, and I think it will work nicely. Here's a screen grab of the circuit tool:

Note that for modeling purposes, the 12v ground (V1) and the negative terminal of the battery (V2) are connected, while in the car, they are isolated. The main contactor is modeled by the winding resistance, R1, combined with a voltage-controlled switch, S1. As C1 charges through R2, Q1 turns on and draws current through R1 and turns the main contactor on. The main capacitor C2 has been charging through R4 and the small relay S2. The Schottky diode D1 is to make contactor turnoff instantaneous, and D2 is to subtract D1's bias voltage from the gate of Q1. R3 is to bleed off C1's voltage while the system is off.

Feel free to copy this design at your own risk!