Using the Wangdd22 1500W 30A DC Boost Converter on an ebike.
Why bother? Short answer - economy, versatility and fun to try.
Longer answer: This is my first e-bike and I'm trying to keep costs down. I bought a used donor mountain bike ($180) and a $225 rear wheel kit (48V. 1000W) on ebay (CNEBIKES.CO) . I live in a hilly suburb of Phoenix, AZ (Fountain Hills) and want to commute to my work about 15 miles away in nearby Scottsdale. The idea behind the e-bike is to "level" the hills that I'd encounter making a long commute practical. Given my expected commute distance, I figured 13 amps would be a minimum battery capacity.
I really liked the look of these 36v 10s 4.4ah battery packs currently on ebay. Five for $150!!. That's 22 amps and almost 800 watts, but the voltage is lower than my system's 48V. Maybe these would work with a boost converter? If not, I guess I could rewire them to 48V or 52V. Though I'd really rather not.
The Boost Converter:
I paid $40, but you can get it for nearly half that if you buy on ebay and are willing to wait a bit. I was impatient.
The 30A rating only applies to input voltages between 10V and 30V, so my real input current limit is 25A which works out to 1050 watts at 42V - not the nominal 1500 watts. Further, it has a 20A max output which works out to 1080 watts at 54v. So realistically, this 1500 watt boost converter will only deliver around 1000 watts in my application - which isn't going to be great for hard accelleration. But will it be enough to get up a 10% grade and help me climb out of Fountain Hills to Scottsdale on a commute?
So I finally got everything installed and the bike put together last night (though some of it is still temporary). So I took it for some shakedown cruising this morning. The short answer to my question is that yes, it will work for my intended purposes. The main caveat is that you may have to feather the throttle in demanding situations, and it really does help going up hills if you give a moderate amount of pedal assist.
The general booster behaviour is that it tries to always output whatever output voltage you set regardless of the input voltage. I set mine to 54V and the system's LCD reported 53.9V.
On accelleration, if you place too high of a demand via the throttle, the converter simply stops delivering significant amounts of current. It still powers the system LCD panel but the battery icon goes "empty" and the motor stops working. When the icon restores, you've got power to the wheel again.
The LCD doesn't show watts or amps, but it shows volts. Higher throttle demands cause the voltage to drop. It seems that when voltage drops below 50V or so, the risk of losing power to the wheel goes up. So monitoring the voltage is a good way to gauge the demands on the system and prevent any power cut-off.
So I rode the bike a total of 25 miles around Fountain Hills this morning. I recorded the last half of my testing ride using Sport Tracker Pro. Here's a link to that second ride. You can see that it does pretty much what I wanted. It "flattens" the hills a bit allowing me to maintain around 20mph going up many hills. I think the system will work better when I get a better 20mph gear. Right now, I'm over-spinning at 20mph in my tallest gear. The current gear feels comfortable at around 18mph. This jibes with my Sports Track Pro average moving speed of about 17mph. Spinning beyond 20mph is almost pointless. I need taller gearing.
http://www.sportstracklive.com/track/de ... st/2346622
I don't have a separate watt meter, so I'm not sure if I have the boost converter configured to deliver its full power. But I think it is, or is at least close to it. Its an analog converter and adjustments are made with pots. You need measure with a meter while adjusting to know exactly what's happening. I've just spun the pot a lot in the clockwise direction. But I can still estimate total power usage based on the change in battery voltage.
I started with four 4.4ah packs wired in parallel showing 41.5V (about 94% capacity). I rode 25 miles with a net elevation gain of zero. My ending voltage was 35.9V (about 5% capacity). So I used about 89% of the 17.6 ah potentially available at an average voltage of 38.7V. That works out to 606 watts or about 24.25 watts/mile - not too far off from my benchmark expectation of 30 watts/mile for a commute.
It was around 90 degrees F when I was doing these rides and it was sunny. I measured the hub temp at the top of a long (1.5 miles 4.4% average grade sometimes 10% grade) climb up Palisades Blvd. It was 119.5 degrees F. Given that the hub is painted flat black, I think some of that temperature is just from being out in the sun. The booster itself never got hot enough so that it would turn on its cooling fan (somewhat disappointing - but maybe my placement for good airflow helps keep it cool). Spot measurements showed some booster components at around 115 degrees F, but most closer to the mid 100s. No wires or connections seemed hot or noticeably warm.
This isn't the optimal performance setup. It isn't for drag racing. But it works nicely for commuting. In fact, there is a nice unintended benefit from the booster cutting out under heavy load. It makes it hard for the rider to over-tax the batteries or cook the motor.
One small disadvantage to the booster is that system's LCD panel always thinks it has a full battery since it sees always sees 53.9V - regardless of the battery condition. You can set the booster to a low voltage supply limit. When reached, the booster stops boosting and simply passes direct, un-boosted battery current to the output. In that way the system can slap you upside the head when you've nearly fully depleted your battery source - at least keeping you from over-discharging your batteries. As it is, I think I need one or two (before and after the booster) watt meters so I can better monitor battery condition and figure out the efficiency of the booster. (They claim 92-97% efficient.)
FWIW, the motor and controller worked when directly connect to the 36V battery pack, my concern / question is whether the controller will cut out due to low voltage well before the batteries are near empty. That's something I'll test next. The 25 mph or so top speed that the 36V limits me to really isn't a big deal for my application.