Main contactor?

Electric Motors and Controllers
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HrKlev   10 mW

10 mW
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Joined: Dec 06 2019 10:24am

Main contactor?

Post by HrKlev » Jan 23 2020 6:58am

Hi, all!

Im about to start an ATV conversion (QS138 motor, Votol 72150SP controller, 20 LEV40 cells, "bluetooth bms" from aliexpress are in the garage, ready for install) and I think I have got the most figured out. I've been lurking around this forum and learnt a ton the last few weeks, but I still dont quite get the main contactor. Why do I need it? Or do I? The votol has a input that turns it on and off, and I was thinking I can wire it to a key switch. In addition, the BMS can also be switched off, so I was thinking about wiring it up to a another "hidden" switch in case the ATV will be parked for longer periods of time. Do I need a main contactor in addition to this? Thankful for any insight!

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amberwolf   100 GW

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Re: Main contactor?

Post by amberwolf » Jan 23 2020 3:25pm

contactors are used as a high-current-capable connection device (also high-voltage) to prevent damage from arcing during connecting battery to controller (because capacitors in controller take a large current momentarily as they charge up during power-on at connection).

they are also used to remotely disconnect the batteyr in case of system failure / emergency shutoff, and to completely disconnect a battery from controller/etc to prevent low-power-drain over time from killing the battery.


if your bms has a power switch then this should eliminate the above needs, except in one case, which while not common, does occur.

if there is a failure of the system in a way that causes the bms output fets to fail in their most common failure mode (shorted on, but at higher than normal resistance), the bms power switch no longer works, and there is then no way beyond physical disconnect of the battery from system to cut power.

often enough something tht damages the fets in this way is dramatic enough to require such disconnection to stop the drama part. ;)

so, instead of a main contactor, you could instead use a battery cutoff switch that's rated for teh voltage and current you'll be using, along with whatever main fuse you're also using to protect against shorts causing a fire.

the switch can be an actual switch, or it can be something like the anderson sb-series (or any other large sturdy connector that requires sufficient force to unplug that it can't be done by accident) that's got one block bolted to the atv, and one that's plugged into that block with a t-handle to pull in case of emergency disconnect needs. the plugin block is simply wired as a jumper across both terminals. the bolt-on block is wired to batery positive output on one side, and to controller battery positive input on the other.


or you can go without, and just physically unplug the battery as a final line of defense.

HrKlev   10 mW

10 mW
Posts: 29
Joined: Dec 06 2019 10:24am

Re: Main contactor?

Post by HrKlev » Jan 24 2020 1:39am

Thank you for a well written answer! I think I will stick to the plan then, and turn the ATV on and off with the controller for daily use, and wire up a switch for the BMS for emergency or storage for longer periods. I will also have a Anderson 175 between the battery and main fuse for a last line of defence as you say. It is a good tip to rig them up with handlebar and bolt the other end to the ATV, in case I will be in a hurry, I didnt think of that. I'll see what I can rig up!

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amberwolf   100 GW

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Re: Main contactor?

Post by amberwolf » Jan 24 2020 2:15am


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