Daren’s Cr125 E-conversion

img source: suwalls.com
I have seen a dozen electric conversions of dirt bikes, but this one really impressed me, so I definitely wanted to write about it to provide readers with an example of the type of parts that would work well for this kind of build. The beautiful scenery in the background is from Kentucky, in the USA.

Honda CR125

img source: ytimg.com

Some frames are easier to convert than others, so one of the things I most want to accomplish is compiled an index of builds that present successful examples that are the result of an enthusiastic builders’ countless hours of research.

As a side-note, I have often heard of a builder finding a running candidate of a certain model, and then…they were able to sell the running engine to someone who had the same model, but with a fried engine. Doing this often meant you ended up with a donor frame for almost free.


img source: ytimg.com

If an EV is a conversion (instead of being designed from the ground up to be an EV), the available space for the battery pack will not have an optimum shape or size. In the pics below, you can see the odd shape that Daren chose in order to cram as many volts and amps into this build as he could. Daren chose the highly-regarded Sony VTC6 cell. It is a 18650-format (18mm diameter, 65mm long) that is factory-rated at a capacity of approximately 3000-mAh, and 20A continuous with a temporary burst rating of 30A.

This pack uses 20 cells in series (the 20S), which has a nominal voltage rating (the average between empty and full) of 72V. We recommend that builders do not charge these cells to over 4.1V per cell, so 20S would be roughly 82V when it’s hot off the charger.

Since the cells are rated for a burst of 30A per cell, having 12 cells in parallel means this pack can put out a peak of 12P X 30A = 360A. Darren has wisely chosen to take the extra effort to add forced air-cooling for the battery pack to help it live as long as possible.

A dry fit of the pack into the frame, before spot-welding the electrical buses

In the pics below, Daren documented his method for forming dimples on the conductive metal buses that connect all the cells. The custom jig he designed and made allowed this task to be performed so that it provided a consistent pattern. These nickel sheets will electrically connect one 12-cell paralleled group to the adjacent 12-cell paralleled group.

The spot-welder he used to connect each bus-dimple to the cell-tips. The power for the welding is provided by a large car starter battery, which can typically provide 800A for a split-second. Attached to the negative post of the car battery is an Arduino-based precision timer that provides an amp-burst that is measured in milliseconds. The completed battery pack on the left, and the BMS balancing wires on the right.

The Battery Management System  (BMS) does several important things, but one of the most important is keeping the cells balanced. The bulk charger will fill the pack up to 82V. The problem is that due to minor variations with the internal resistances of each cell, one cell might be at 4.2V, while another might be at 4.0V (for an average of 4.1V for both). A minor variation in the voltages is not a problem, but…if one of the cells gets up to 4.3V, it can overheat and set off a chain reaction that could set the entire pack on fire. Using quality BMS is very important.

Since the balancing currents are very small (usually less than 0.5A), the wires that accomplish the balancing can be quite thin, as seen in the pic on the above right.

The twelve cells in each paralleled group (12P) act as one large cell (3000-mAh X 12 =) 36-Ah. If you run the motor easy, you could get a little more range than just 36-Ah, but…Darren did not design this for taking it easy.


img source: silixcon.com/

The controller Daren chose is a Silicon SL 84V unit, which is rated for a continuous 24-kW and a temporary peak of 36-kW (72V X 500A = 36,000W). The Silicon SL 24-kW nominal controller is surprisingly compact. The conductors on a system like this should be as fat and short as you can fit, and Daren’s motor phase cables (seen above) certainly qualify as properly-sized. Getting ready to bench-test the battery, controller, and motor. The controller is mounted out in the air-flow and anchored to a fat aluminum finned heat-sink. The large 88-tooth sprocket on the rear wheel was custom-ordered from Rebel gears in Tennessee.

The front 11-tooth drive-sprocket was made by Daren’s company, Amp Sprockets, which can be contacted on the Facebook page “Electric Motorcycle Builds Marketplace”, and the pic above shows the custom 3D-printed cover that Daren also made.

The rear sprocket Darren had made on his electric conversion was so large, he needed to move the chain-guide farther away from the swingarm. The 88T/11T sprocket combo provides an 8:1 reduction in RPMs between the motor and wheel.

Gasoline motors make their best power at the higher RPM’s so they need a clutch with a multi-speed transmission to help keep the RPM’s up, regardless of their road-speed. Electric motors are famous for having outstanding torque from only one RPM. Also, by being able to have a very broad and flat torque-curve, electric motors provide excellent performance with a simple and reliable single-speed drivetrain, without ever needing to shift to get the best power for the situation.


img source: ytimg.com

If you can absorb and shed the heat of the high amps you use when accelerating your motor, then you can use even more amps without damaging it. Many motors simply use passive air-cooling, and a few even add active fan-cooling to allow the motor to use more power. Daren chose liquid-cooling, which provides the absolute best possible performance from a given size of the motor. The 12V DC / 20W Marvel Star water pump, rated for 160-gallons per hour  (GPH), or 10 liters per minute Motenergy makes several models of motor that would have fit, and one of the reasons Daren chose the ME-1304 is because it has well-designed water-cooling passageways, straight from the factory.

The Final Result

The modest size of his battery keeps this build very light and nimble, but it also only provides about 50-minutes of trail riding or about 25-minutes of hard MX track riding. It has been geared for a top speed of about 46-MPH (74 km/h), and by having a modest top-speed, the gears he has chosen provide significant wheel-torque for outstanding performance.

Daren reports that he can charge up the battery to 90% in only 60-minutes, which is one of the benefits of using high-amp cells.

Daren’s Honda CR125 electric conversion

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