Introduction: Kids Polaris Sportsman Quad ATV EV Electric Motor Swap Conversion
I had some questions about my kids quad build so I decided to post info here to share. The idea in this build was to convert a quad maintaining the same 23mph top speed as the original 110cc motor in my 2012 Polaris Sportsman 90 quad … yes, they still call it 90 even though it has a 110cc motor.
I have figured as a general rule that if you want to replace an ICE with a brushless motor than you use half of the horse power converted to kW. This seems to hold true as demonstrated in this build. I hope it will on my next build of a side-by-side UTV, more on that in the future. Anywho, the original motor is about 8hp so I halved it to 4hp which is 3,000 Watts. I then went cheap and purchased this motor https://www.aliexpress.com/item/4000028463136.html You might want to consider a QS 138 70H as it is better quality. I built it first without the gearbox and found that it had speed but was heating up. I realized that I robbed all the torque for top speed. I went back to the drawing board and found this gear box https://www.aliexpress.com/item/32835885761.html Next I back calculated the gearing I would need to achieve a top speed of 25mph, this would give me more torque. After some fabrication and 3D printed mounts I was happy to find that it work awesome! Not only could the quad reach top speed with my 100lb son but it could reach top speed with me weighing in at 170lb. I also am happy to report it has more acceleration that the ICE and I am sure my son feels even more!
What about the batteries? I used recycled drill 18650 batteries and spot welded them together forming a Li-Ion 18s7p 68 V (75.6 V max) pack that is about 900Wh (~ 13 Ah). I used a 60 Amp BMS found here https://www.aliexpress.com/item/4000086899408.html I then used a cheap cell phone that I mounted to the handlebars that connects to the BMS via Bluetooth to display the speed (using GPS) and battery capacity. Though it displays in kph it does a good job estimating how much power is left. I can also see how much current it is using. To charge the machine I used a constant current DC power supply that was adjustable and set it to 75.6 V and 6 Amps. The BMS handles the charging safety measures. This way it will charge in just over 2 hours which is quick enough.
For lights and other 12v accessories I used a 72V DC to 12v DC converter like found here https://www.aliexpress.com/item/4000449378433.html
* only use brushless motors. DC motors are a waste of time as they are too inefficient
* I wish it had regenerative braking, my next build will. I think the range would increase significantly if it did
* you must use a gear box, if you don’t you are not using all the potential torque. Do the math and gear it down, web pages like this will help https://www.blocklayer.com/chain-sprocket.aspx
* I needed to add a heat sink as the motor was getting warm. I bought a Delavan Heat Sink, Fits all 4" Diameter Delavan Motors (7870/7970/7871/7971 FB Series) found here https://www.pwmall.com/p-210643-50_126-delavan_heat_sink
* consider LifePo4 batteries, they are more robust and can take the heat but they are a little heavier. I plan on using LifePo4 in my side-by-side UTV build
* for an adult quad I would consider at least a 4000 W motor, a 11000 W motor would be even better
* 3000 W motor will replace a 110cc (using gear box)
* I get about 45 min of run time on a 900 Wh pack ( will update after I get more run time in )
* adults can ride it
UPDATE: I had a question about not using a gear box and only using a drive & wheel sprockets. Sprockets are not enough. I have seen many use two sets of sprockets which is the easiest way to build a gear box or better yet use the new qs motor with the built in gear box https://www.aliexpress.com/item/1005001996869603.html To help make my point; to achieve what I did with only two sprockets you would need a 14T to 160T combo! Here is the math on how to calculate using a gear box and sprockets:
RPM / Gear Ratio x Motor sprocket / wheel sprocket x Tire Dia in inches x 0.002973
... for my setup I used: 5500 / 4.54 x 14 / 35 x 17 x 0.002973 = 24.5 mph