Custom Electric Motorcycle Packs 6 KW

If you only need to travel at around 25 mph around town or to get a short distance to work, an electric bicycle might just be the best thing you can ride. It’s cheap, quick, and fun, and sometimes a great way to get some exercise too. If you want to dial up the amount of excitement, though, you’re going to want something with a little more power and speed. Something like an old dirt bike converted to a 6 kW electric motorcycle.

This is the latest build from [Boom Electric Cycles] and uses the frame from an early-90s Suzuki dirt bike as the foundation. From there it’s all new, though, as the engine was removed and replaced with 3 kW hub motors in each of the wheels. A 72-volt custom battery with 240 18650 cells pushed the amps through the motors, making this bike able to keep up anywhere except the fastest highways (if it’s street legal at all…).

Having about eight times more power than is found in a typical electric bicycle is sure to be a blast, but this build isn’t quite finished yet. Some of the trim panels need to be finished and the suspension needs to be adjusted, but it looks like it’ll be out and about any day now. Until then you’ll have to be satisfied with other projects that managed to cram in 3 kW per wheel.

16 thoughts on “Custom Electric Motorcycle Packs 6 KW

      1. Sure, so other than a hobby experimental scenario one would select appropriate cell types for a commercial product in accordance with classic engineering risk assessment strategies across product availability, then employ tactics to ameliorate/warn of causal risk factors eg change intemperature, discharge state, change in cell resistivity during charge etc
        There are a few quite new single chip level products that differentially measure cell voltages in a serial stack (20+ iirc) and report numerical or even statistical data too. Many low end single chip solutions can also be connected ah lah 4-20mA current loop fashion to effectively do the same thing with a couple of levels of calibration and alerts thus probabilistically catching change in risk and free of single supplier reliance…

        1. The “appropriate cell types” cost considerably more, and weigh considerably more per kWh, both of which are a serious issue for the EV market.

          That’s why the “risk assesment” that companies like Tesla make are more like the case of Ford Pinto – they know their batteries are unsafe, but they can’t afford to use safer batteries because it would eat up all their profits, so they’re rather crossing their fingers that battery tech improves soon and keeping an army of lawyers in the mean while.

          Also, the video illustrates one important point: as you increase the energy density of ANY chemical battery which contains both reactants in the same casing, it becomes unstable to the point that it behaves like gunpowder simply due to the amount of energy packed in.

  1. How would you manage/balance a 240 cell battery pack?
    The ICs I’ve been able to find would be horrendously expensive with a pack this size and the circuits I’ve seen would be horribly inefficient at this scale. I’m trying to make a small lithium pack and it seems much of the advice online boils down to “get 2-4 brand name cells from the same batch and don’t bother managing them”.

    1. you buy cheap, pre-made BMS modules from ebay/aliexpress like everybody else does… https://www.aliexpress.com/w/wholesale-bms-72v.html

      BTW it’s not 240 cells in series, the pack is probably 20s12p, so you’d have 20 packs in series, each made up of 12 cells in paralel. The ballancing circuit only deals with balancing the series modules, individual cells within each module “sort it amongst themselves” (they balance naturally).

      1. You could also add a charging wire tap halfway on the 72V pack so you can SEQUENTIALLY charge 2x 36v packs with a 10s balance charger, just make sure you charge BOTH or you end up killing one side….

  2. I’m building a couple of 13S3P packs for my ebike
    Conversion – suitable Module’s readily available in eBay/ AliExpress etc.

    Despite the availability of the parts it’s still no walk in the park to put it all tougher (depending on how far down the tree you want to start) I’m using salvaged batteries from medical equipment- they all need to be individually tested and matched then I decided to build the spot welder to go with it …. yep it would have been a LOT easier to buy a prebuilt pack :lol:

    1. the again since you built the pack and sorted the cells yourself, you KNOW what you can and can’t safely do to the pack AND you know how to troubleshoot it should there be a problem…

  3. its annoying that when looking up hub motors the companies will not list the actual specs of the motor. for example, instead of listing a max rpm, they list a max speed for a hub motor with no wheel, considering that there are many different kinds of wheel hoops that you can get, the maximum speed is related to the diameter of the hoop which is not included with the motor. I get that the majority of these motors are geared towards people with a more casual knowledge of electronics but is it really too much to ask for a torque and power curve graph and max RPM.

      1. Understood, but variables can be controlled and stated to produce the specifications.

        So how can they spec a maximum road speed given that it relies on the maximum rpm and the wheel diameter?

        1. It also depends on your driver configuration. For example, whether you use FOC or block commutation, or some other commutation scheme.

          The maximum RPM is rather unimportant, because you run out of power before you reach it. The limitation for speed comes from your driving voltage and the current/power handling capacity of the BLDC driver.

          It works out to be relatively standard, because the power you need to go faster goes up in v^3 anyways and you’re essentially fighting against an aerodynamic wall. If your bike has better aerodynamics, you hit that wall at slightly higher speeds but not by much. Hence, if your motor is nominally let’s say 350 Watts, doesn’t matter what size of wheel you put in, you’ll get to about the same top speed.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.