I have a 3 mile daily commute that I bike in Toronto. I enjoy it very much, but there are times I'd like a bit of an e-boost; for example when a car cuts me off while I'm in the middle of climbing a steep hill. I'd like my e-bike to weigh as little as possible, and to be inexpensive. I would be happy with a battery range of about 1 mile, but the smallest ones I can find are at least 10-15 miles. Is there a reason why this range segment of the market does not exist?
I think the ultimate issue boils down to a lack of demand for such a short-ranged product. Looking broadly at the entire electric vehicle market, range anxiety is fairly common. Most people are worried about not having enough range (even when it's an unreasonable or unwarranted concern)--very few people are actively seeking a short-ranged product when longer-ranged versions are available for little additional hassle. The bike still needs all the wiring, the motor, the controller, etc. None of those get simpler or go away due to a smaller battery. Hence, if you're going to go through the effort of motorizing a bicycle, you might as well give it a nice big battery. Also, pateksan points out in a comment that the manufacturer's pricing model would be disrupted by such an addition. To quote them:
A bike with 1 mile range would probably only be very slightly cheaper to make than one with 10-15 miles. It could therefore be hard to price competitively and without undermining the overall pricing model. If the 10-15 mile bikes were substantially pricier, nobody would buy them. And if the 10-15 was only a little extra, most people would prefer them, so few would buy the 1 mile.
There might be technological issues involved as well. Note that the electrical power needed to provide a given acceleration is independent of what battery capacity you use. Also, under a given load, a smaller battery is being used "harder" than a larger one. For example, to provide 1A of current, a 1000mAh battery is discharging at 1x its capacity, whereas a 100mAh battery would be draining at 10x its capacity. 1 mile is a very short range, and so a very low-capacity battery would be involved. I would not be surprised if the required current draw is infeasible for such small batteries.
Of course, with lithium-polymer technology, such current draws are possible, but lipo batteries tend to be delicate and have an increased fire risk. I believe existing ebike batteries all use regular lithium-ion technology with arrays of cylindrical cells for safety (and cost). Supercapacitors would be a good solution if this product was actually created.
From a convenience point of view, a 1-mile e-bike would need to be charged every ride, for the use mentioned in the question (that would be a red flag for me, I have already enough batteries to manage in my life, but that's personal). For people living in cities, charging is very often a constraining aspect of e-bike ownership, as the bike is often parked outside the owner's premises – whether outside or in bike rooms. For such users, the minimum considered range required is not 1 ride, but the commuting distance of several days, or one week (+ a margin to consider battery wear, and cold-related discharge, if applicable). On top of that, charging takes time, but that point is not an issue with a pure commuter.
Another reason is that it would make such a bike only usable for that particular use. Some people prefer to buy specialized tools, but I would also assume that the vast majority still prefers general-purpose tools, especially if they get more expensive. From observations around me, an e-bike increases the bike use, and just for that reason I would not consider a bike that I can "only use" for my commute, and that I would have to charge after each ride. For instance, that would defeat the purpose with grocery shopping, if above that range: one of the legs of the journey would need to be done without assistance.
Now in terms of weight, not sure it would make a significant difference. A Bosch Powerpack 500 weighs 2.6kg. Typical ebikes using them weigh around 21-25kg. A battery with 10 times less capacity would probably weigh 2kg less (you still need housing, connectors and weather protection if the battery is removable), that would still make the bike too heavy for most people.
A small battery does not provide enough power, as there is a limit to speed of discharge
The largest safe continuous discharge rate for lithium ion batteries is about 10C, i.e. fully discharged in 6 minutes. So for a 1 mile range, the top speed of the bike would be 10 miles per hour (16 km/h).
And even that rate is only safe in the sense that the battery doesn't overheat. Discharging at 10 C and always to 0% charge would wear out the battery much sooner than a larger battery discharging at e.g. 1 C and to 50%.
Different battery models have varying maximum discharge rates, but the effect on the cycle life (number of charges before battery capacity falls to 80% of original) is dramatic. At the 10 C discharge and a daily charge, a typical battery would be degraded within one year of use:
There are two limitations on a battery, that are quite strongly linked, in any given technology and price point at least
You need the battery to be able to provide enough power to assist the bike, say 100 W absolute minimum, preferably twice that.
You cannot get this power handling in a small, economical, battery. Once you can handle the power, the capacity will be up into double figures of miles.
Another way to look at this is how long it will take the battery to discharge. How long does it take you to go one mile? A battery that fully discharges in 4 minutes is working way down on its discharge rate / capacity curve, is discharging very inefficiently.
For a 36V motor, you need 10x18650 batteries. With all the equipment to make an e-bike, the additional weight would be over ~3kg's, at best.
Such a bike would look weird on a catalogue, and price would still be too high for a 1 mile range e-bike.
With so much cost, effort and additional weight involved, no one would bother. But there's a DIY option as always. Or, you can buy an e-bike with no battery, and build a smaller battery pack, which would definitely give you more than 1 mile range, as it would be 1.5Ah at worst.
I would buy a bike with a reasonable range - if it only has a 1 mile range, then you will be recharging every day. Because you are fully cycling the battery pack every day, it will only last maybe a couple of years max, and show range reduction within months.
If you have a bike with a range of 10 miles, you will be recharging 10x less frequently and so the battery will last many years.
The motor and speed controller will still be the same weight as it still has to deliver the same power. All you save is the weight of the battery pack, and as others have stated, each cell has a maximum allowed current drain.
Which means for a given power output, the lower limit on the battery pack capacity is defined by the peak motor power, not the range.
The reason you cannot find an e-bike with such a small range is probably due to the lack of demand for that short of a range along with an inability for costs for a reduced capacity model to be lowered much if at all from the 10-15 mile range costs. The 10-15 mile range IS the bottom of the current market based on current battery capacity, motor efficiency, and general customer demand. A one-mile range e-bike is simply not practical (or profitable) to produce commercially.
It's unusual to find cells that small in the ebike segment. Depending on the voltage, you have to join the batteries in a series to get i.e. 24 volts. If you join 6 18650 cells together, you get 2.5 amps at 24V, which is 2 miles. The problem is that arrangement would get very hot. You can use a 15 mile range ebike to do 1 mile aday for 15 days in a row before you recharge, and the price between 6 and 12 batteries is like 20 dollars, so why not pay 20 dollars for the extra batteries and top it up less frequently?
My "40 mile range" electric skateboard goes a real-world 15 miles when on full-throttle and going up and down hills in San Francisco. That's 15/40 = 37.5% of the "sticker rating".
After 5 years, I expect the range to reduce by half, down to 7.5 miles. That's 7.5/40 = 18.75% of the original "rating".
So, a "1 mile sticker range" electric bicycle (75 lb kid, flat smooth streets, 7 mph speed, 1 mile range) might have a real-world range (180 lb person + gear = 205 lbs, 15 mph speed, uphill) of 0.38 miles. After 5 years you'd get 0.19 miles. That's 0.19 miles x 5280 ft/mile = 1003 ft, or about 3 times the length of a football field.
Pretty useless, right? So is the complexity of an electric bike that has a near-zero range. The time it takes to get out your bike and gear is about how long it takes to just walk the 1000 feet.
The amortized component weight. You need a motor, speed controller, wiring, and battery. Getting the battery down to be tiny still leaves a decent amount of other components, weight, and complexity. You might as well make the battery big enough to actually provide the power you need.
Smaller-capacity batteries also have smaller currents, as the max deliverable current is directly proportional to the capacity of the battery as a "C-rating multiplier." Ex: a 20-C discharge battery rated for 4 Ah can discharge at a maximum of 20 x 4 = 80A. So, if you make the battery too small, the max continuous and burst output current gets too small to be useful, and you might as well choose a larger-capacity battery to be able to supply more current. You can offset this by getting a smaller capacity higher-C-rating battery, but then you increase cost to get higher C-rating. You might as well lower the C-rating and increase the capacity for the same cost.
Ex: two equivalent-cost and equivalent-max-discharge batteries might be these:
- $100 4Ah 37V 10S Li-Ion 15C discharge (max continuous current = 15*4 = 60A)
- $100 1.5Ah 37V 10S Li-Ion 40C discharge (max continuous current = 40*1.5 = 60A)
As you can see, they can each deliver the same max continuous current, and they cost the same, but one has 4Ah/1.5Ah = 2.7x the capacity. You might as well get a bigger capacity and have 2.7x the range for the same price and max-current capability.
I can imagine that a short range e-bike with with minimum features would really be suitable for a huge number of casual commuter cyclists.
Why there isn't a market for it? Probably because of "range anxiety" as other have mentioned. But also I think it's a bit of a chicken and egg scenario: nobody has offered the product, because the perceived demand isn't there, and nobody wants it because it isn't something to be had.
I can imagine strapping a cheap cordless circular saw to one of the front sprockets with a chain, could achieve basically what you're asking for... save for all the details that make that less viable in practice.
The e-bike market is just a mess of businesses trying to make huge profits off something that should be very cheap and simple. To get what you want, you need a DIY/kit solution where you supply your own battery and it's easily removable. This also solves the objection some answers have raised that "it would be hard to charge the battery because you parked your bike somewhere outside not at your home": you just carry the battery inside with you wherever you're going and plug it in. (Aside: this also completely solves the risk-of-theft issue.)
Put a friction coupling or hub motor kit on a normal inexpensive road bike, connect a battery of the appropriate size, and you're done.