Quote:
Originally Posted by TurnNBurn
I'm up in the air about this. I have a bunch of small 12v lead acid batteries (nice small ones). So I was contemplating using the larger batteries just for the drive system and have everything else run on a separate 12v battery.
$0 for batteries I already have, or $80+ for a 72v to 12v DC converter? Hmm....
So what I'm gathering is the contactor is the same function as the key switch except it's built for more volts and more amps.
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If you have the space for the battery and you are OK with the extra weight, the separate battery will work fine.
For the contactor - Yes. I think you understand what it does. I'll try to explain why it's important, at least why I think it's important.
The key switch enables the controller. So if the controller is working properly, it pulses the DC from the battery according to the signal from your accelerator.
If the controller is not working properly (everything fails eventually), the failure modes are OFF and ON. OFF gives you a coast to stop. ON is full power, like your bypass contactor got stuck or you are at maximum throttle.
If the controller fails ON, you can pull in the clutch and destroy your motor, but you LIVE instead of accelerating into a building beside the road ... depending on whether your motor starts shedding pieces as it spins over it's maximum rpm.
If you have a contactor, rated for the voltage of your pack or larger, and rated to break the maximum current that your batteries will put out, or the maximum your motor will take before it melts, then the key switch should also drop out the contactor and in this ONE scenario, your motor is not sacrificed.
Going a bit further out there into once in a while conditions ... in case of an accident (I use an inertia switch to detect that) the contactor can open and make it a bit safer for people to untangle you from your bike. 72V can give you a nasty shock ... and blood is a pretty good conductor
In a more boring and more frequent (at least I expect it to be more frequent for me) condition - you turn off your key switch but forget to turn off your manual battery switch (or someone else who may ride the bike forgets), and you do not connect your charger for some reason - your controller is still powered up and it MAY may run down your battery pack and EVENTUALLY experience some damage if left for a period of time ... like maybe a week or a month.
In my personal experience with packs (not with a car as yet) I have had the contactor save the pack from discharging a few times, while the pack is sitting on my bench and I got interrupted doing a test of some sort.
But EVERY pack that I put in a vehicle WILL HAVE both an inertia switch and a contactor, in addition to fuses and at least one manual maintenance switch.