The devil’s in the details, as the saying goes… all the theory is fine, but when it gets down to putting the rig together, how do you chose which bits and pieces are going to do the best for you? Let’s look at how I, at least, approached building my bike. For every builder there’s a different approach, of course, but this is how I did it, and basically as a result of the advice I got.
The first decision I made was the motor. Let’s, for the sake of discussion, say we want something cool and fast, like the Agni 95R- a pretty standard motor in any of the faster bikes out there. If you go to the performance specifications, you’ll see it’s rated at 220 amp continuous. Based on that, and what other bikes and builders are running, it seems like a good idea to run a system capable of handling 400 amps or so. This will determine the rating of the controller, the fusing and the cabling. In a similar system, I run 4 ga. welding cable and a 300 amp fuse. My controller is rated at 400 amps. This is also based on running a specific voltage- in most cases, for a bike, you’re looking at 72V or 96V- at least for a PMDC motor. The Agni can handle up to 96, so let’s figure on that.
Let’s consider the controller. On the FAQ page, Agni talks about the various applications and suggestions for controllers- that’s a great place to start. Another resource is the EV Album page– just about everyone who’s built anything is here- and you can see what they’ve used. You can even do a search via Google specifying your motor limited to the EV Album site, and see every listing.
Another little trail of breadcrumbs you can follow is to look at suppliers’ pages. Here is the Electric Motorsport page, where they list a kit that includes the motor, controller, even the throttle and other stuff.
Let’s say I want to look at something that I haven’t found out there… I can go to the Alltrax AXE page, for example, and go through the specs- I don’t find anything that’s over 72V. A quick look at the Kelly site gives you what they have too- nothing that is 96V, and 400 amp capacity. At this point, I even start reconsidering my idea to run this at 96V- it seems like it’s a lot more common to run lower voltage, so the stuff will be cheaper and probably more reliable.
Here, on Motorcycle News, is a spec for the Mavizen bike- 96V, but described as a Kelly/Mavizen controller- sounds like a box built custom by Kelly for Mavizen. I suspect we’re looking at some big bucks if we want to stay with the 96V. But 96V means a faster bike. Speed costs money.
As far as the contactor goes, that’s just needing to handle the voltage and current. A 72V bike needs a 72V contactor, like that… again, some place like Cloud Electric will show you a good listing. A contactor is pretty simple- just a big relay- but is crucial to protecting the entire system in the case of a runaway meltdown.
Switches and such- it’s a really good idea to run some basic high-current main cutoff switch to the battery pack- both as a simple, practical way to assure the system is unpowered, but also as a safety interlock- keeping observers and would-be uninvited riders from hurting themselves. This thing, $15 from Cloud Electric, only $8 or so from PepBoys- does the trick.
The one thing you see lots of questions about is the kill switch, or the switch for turning on the contactor. Most wiring diagrams have a main cutoff, then an on-off switch for the contactor.
The temptation is to use the stock motorcycle key switch- a very bad idea. Those switches are not made to handle high voltage above the stock 12V, and will probably melt quite dramatically if you feed them 72V.
You can do a few things. If you want to use the 12V key switch, you can wire it to a standard 12V automotive relay that will handle 72V. Or, you can run a handlebar mount rocker-type kill switch that can take the 72V. I used this, from Trailtech.
This just in- the final word on switch selection. Read it here. The bottom line? Read the label, and use a switch that is rated for exactly what you want to use it for. Needless to say, I’m rethinking my switches…
The startup procedure goes like this- flip the main cutoff switch on. Turn on my 12V lights and horn switch. Then hit my handlebar kill switch, and the contactor snaps closed. That feeds my battery pack to the controller, so the controller can feed my motor. I’m ready to roll.
One other basic thing- a volt meter. The voltage of the battery packs, and especially the voltage drop as you load the motor, is the best way to see the state of charge of the pack. You honk on the pack, the volts drop dangerously low, then you know you’re running on empty. I use a vintage panel-gauge style volt meter that runs up to 150V- the 72V pack runs it with the needle pointing to noon- it works fine, and has a kind of steampunky look to it. That, and a few Ironman LEDs and what’s not to love? I have a standard digital LED panel meter, but it changes numbers so fast it’s a little hard to track.
I also added a little LED that winks on when the pack voltage drops below about 40V, as a warning, that a buddy of mine David O’Brien, designed for me. I like kind of a minimalist traditional motorcycle approach, but if you want to go all tech, it seems like the Cycle Analyst is the favorite panel of most builders.
Again, this is one perspective, and it’s probably the most simple approach. When you add a BMS, various instrumentation and things like a DC-DC convertor, it gets a little more complex. It does, however, work- and it’s a fine place to start.