
R3 is the precharge resistor. I'm using a Kelly Controller, and they recommend 2k 20W. 150VDC is connected when I turn the keyswitch to ACC. This is not shown on the circuit, but it is because my pack is broken by a contactor which is enabled by ACC/ON/START (and also by 240VAC).
When 150VDC is first applied the Drive Contactor is open. Current runs through R3 and into the controller, which charges up over time. The voltage at point A therefore starts out at 0V, then goes up towards 150V over time. The process is logarithmic - it starts out fast and gets slower as it approaches 150V. When I asked the question "how high does it need to get?", weber replied that one rule is that the voltage over the (open) contactor should be 28V or less. On my Kelly it takes about 30 seconds to charge up to this value (122V).
The green circuitry (12V) is there to turn the Drive Contactor on when ready. I don't want to enable this contactor until the voltage across the contactor is less than 28V. Here is the process:
1. Turn key to ACC. 150V is connected. The Not Ready light shines. This is a spare light in my dash (the oil light for example). The idea is that like in a normal car, the light goes out once the engine is "running".
2. Turn key to ON. The Latch Relay is not energised, so the Drive Contactor is not energised.
3. Turn key to START. If the Precharge Relay, Charging Lockout Relay and Inertia Switch are all closed, the Latch Relay is energised. This energises the Drive Contactor, turns off the Not Ready light and you are ready to drive.
4. Turn key back to ON. The Latch Relay stays on and keeps the Drive Contactor on.
The new bit is the Precharge Relay. This is RS Components part number 8135031. The relay can handle up to 160VDC, and according to my experiments it switches on at about 50VDC, lets go at about 30VDC. I have it in the circuit so that it is switched on by the voltage across the Drive Contactor contacts. So when the 150V is turned on the contactor is open, and the Precharge Relay switches on, breaking its contacts and disabling the Latch Relay. It stays energised until the voltage across the Drive Contactor falls to about 30V, at which point it lets go and earths the Latch Relay.
R2 is there to provide a bit of protection for the Precharge Relay, which is only rated to tolerate 160VDC. The voltage in my car could be up to 175V straight after charging.
So the process from the Driver's point of view is:
1. Turn key to ACC. The Precharging LED on the dash will light, reminding you that you cannot START yet.
2. Turn key to ON. The Not Ready light will light.
3. Wait until the Precharging LED goes out. Then turn key to START. The Not Ready light will extinguish and you will be ready to drive.
If you turn the key to START too early then the Latch Relay will not energise, so you will not be able to drive. The lights on the dash will remind you what you have done.