Building an EV motorcycle. Batteries: Need opinions.

[P-hack]

well you might be able to find a 72v headlight for that matter and 72v bulbs or something, you don't need much auxillary power on a bike, which dc converter are you looking at, it might be overkill.

[P-hack]

Oh and the starter is still 12V

[TurnNBurn]

Originally Posted by P-hack

well you might be able to find a 72v headlight for that matter and 72v bulbs or something, you don't need much auxillary power on a bike, which dc converter are you looking at, it might be overkill.

I wasn't looking at any specific converter. Just getting a price figure by browsing. I saw the Kelly converter was $119 minimum going all the way up to $200.

[P-hack]

well, a 50watt headlight and 4x3 watt turn signals and a 6 watt brake light is ~ 70 watts.

70 watts at 12 volts is like 6 amps.

This thing for $19 converts 72volts to 12volts at 10 amps
DC 24V 36V 48V 60V 64V 72V to 12V 10A Converter for Electric Storage Battery Car | eBay

But you will need one large 72v coil for the main contactor, and this would be powered when the contactor is closed (they can leak current)

[thingstodo]

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.

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.

[Ryland]

Originally Posted by TurnNBurn

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.

The contactor connects to the key switch, it is your main disconnect, your emergency shut off, your anti-theft device as well as keeping your high voltage, high current switching away from your hands and allowing you to keep high current wiring much shorter.
The key switch can handle maybe amps, basically it can handle the power to turn the headlight on and off, a 200 amp contactor can often open up even if it has 1000 amps going threw it, at that point your key switch would be lit up like a light bulb.

Even people who install a big red "kill" button, often wire it to a contactor, because those kill buttons can't handle as much current as a good contactor can.

[TurnNBurn]

I'm sorting through some of the old motorcycle parts and I was wondering what parts I can recycle. I think I found the turn signal relay, and I found the distributor and something else the spark plugs connect to. Is there anything I can use from this junk? Solenoid? Starter? Something?

And another question about the contactor. My setup will be 72v. I've been toying with the idea of increasing it to 96v. If I got the 96v contactor, would that work for the 72v until I expand it? Or would it be a bad idea?

Also. Pre-charge resistor. Recommended? Or just skip it?

[mechman600]

Originally Posted by TurnNBurn

If I got the 96v contactor, would that work for the 72v until I expand it? Or would it be a bad idea?

Depends on the coil windings. Is the coil made for 72V? My contactor is a 12V coil that switches basically any voltage on the contact side. A coil designed for 72V cannot be powered by 96V - it will overheat and fail.

Also. Pre-charge resistor. Recommended? Or just skip it?

YIKES!!! Absolutely mandatory.
Without it you will have a HUMUNGOUS inrush of current filling the many caps in your controller when the contactor closes. The result (maybe over time) will be damaged contactor contacts, and controller failure.

[TurnNBurn]

Originally Posted by mechman600

Depends on the coil windings. Is the coil made for 72V? My contactor is a 12V coil that switches basically any voltage on the contact side. A coil designed for 72V cannot be powered by 96V - it will overheat and fail.


YIKES!!! Absolutely mandatory.
Without it you will have a HUMUNGOUS inrush of current filling the many caps in your controller when the contactor closes. The result (maybe over time) will be damaged contactor contacts, and controller failure.

I was wondering about the other way around. Can a 96v contactor work for a 72v system, which will at a later date be upgraded to 96v?

As for the pre-charge resistor, what ohm is recommended for a small setup like a 72v motorcycle? What goes into determining what ohm rating to choose? I know what ohms are and I now what pre-charging does, but how does the ohm resistance matter?

[P-hack]

in a simple case, the precharge slowly charges the caps when you initially hook up the battery by bypassing the contactor. It will use about 5 watts initially, so don't use something too small.