High performance requires high amps, but LiPo batteries can be dangerous for someone who is inexperienced with them. LiPo is capable of putting out VERY high current (the C-rate), and if you are building a racing E-bike, they are simply the best choice.
A LiPo pack will be smaller, lighter, and will put out more peak and continuous amps than any other pack you can buy. The problem is that…for E-bikers who want something that is much safer, but has better performance than the average common battery pack, the options are not well known.
It is possible for LiPo to be handled safely, but Hobby King sells LiPo in their bare form, with no protective circuitry for charging or discharging, and each builder is the one responsible for adding a BMS and making a protective case, so…sometimes they become damaged and catch on fire!
First, I want to establish what I mean by “better performance”. For several years, the go to choice for a direct drive hubmotor has been the Nine Continents (9C). They were frequently sold with a 20A controller, and at 48V it would provide about 960W of performance. The hub would accelerate as fast as it could with only 960W available to it, but after reaching cruising speed, the amount of watts that it draws would slide back down to just a few hundred watts for the majority of the ride.
Several builders mentioned that when they raised the amp-limit to 25A (48V X 25A = 1200W), the acceleration was much more powerful and quick. Of course after reaching cruising speed, the amps settled back down to the normal amount. This low amp “cruise phase” is vital to allow the motor to cool back down to being just warm.
Then (of course!), many builders raised their amp-limit to 40A (48V X 40A = 1920W). They reported that the acceleration was wonderful now, but…several builders also reported that…if you put a heavy load on a 9C that can draw 40A, and then ride up a long and steep uphill (with no cooling-down phase), sometimes the motor would overheat. This means that 40A is a practical limit to aspire to for high performance using a common E-bike kit.
This is what this article is about. How do we reliably get a SAFE 40A peak from the smallest possible battery…without using LiPo? Just to remind our readers, if you only need 20A-25A for your E-bike, then there’s no reason to pay extra for high-current cells, but if a safe 30A-40A sounds good…read on!
The short story here is…buy (or build) a 40A battery pack made from from any one of three cell selections: the Headway 38120, the Samsung 20R cell, or the Sony VTC4 cell. I will do my best to describe these cells here, so our readers can decide if one of these might be right for them.
Before we start, in order to understand the numbered names of these cells, they are the dimensions of the cylinders. The 18650 format is a cylinder that is 18mm in diameter, and 65mm long (the size of an adult thumb), and the much larger 38120 Headway cells are 38mm diameter, and 120mm long (roughly the size of two “D” flashlight batteries).
All of the top three cells on this list are cylindrical in shape, and they are made by using the same material as the flat foil pouches, except that a ribbon of that material is rolled up, and inserted inside a protective metal shell.
First up is Headway from China. They were developed for EV’s, and they use the well-known LiFePO4 chemistry. This means that not only can you charge these up without worrying that your house will catch on fire while you are sleeping…it also means that a pack made from these will last years!
Another benefit of these Headway cells is that they come from the factory with threaded attachment points at both ends of the cylinder. This means that assembling a pack from these does not require a spot-welder, or a serious soldering operation. Our friend Alan is happy with the pack that he made from Headway cells, but their large size may prove to be difficult to fit in a common bike frame. Alan built up a longtail cargobike, so he had plenty of room to use these large cells. Their smallest cell is the 38120S, which has 10-Ah in just one cell! The slightly longer 38140 cell has 12-Ah per cell, and there are also larger sizes of Headways to choose from.
update June, 2014
EMPowered Cycles is now selling a hardcase triangle case that is designed to hold 16 Headways, so it provides 48V at 15-Ah, and includes a 35A BMS. At only 35-Amps max, this pack should run very cool.
We have the cordless tool industry to thank for high-current cells in the 18650 format. Laptop computers also use the 18650 size, but they do not draw the anywhere near the amount of current that industrial cordless tools do.
Sony (from Japan) makes the high quality IMR-18650-VTC4 cell. The capacity is 2100-mAh (be careful, the similar VTC3 cell is only 1600-mAh), and this incredible cell puts out 30A per cell in the paralleled group. For every cell that you add in parallel, you will also be adding another 30A of current capability to the pack. This means that a 12S / 5P pack that would provide 44V / 10.5-Ah…can also provide 150A of temporary peak current!
The high Current capability of this quality cell is why our friend Martin used them when he built up a Cromotor street commuter. If fact, this cell provides much more current than necessary, so you can save a few dollars by considering a cell that puts out only 15A per cell (in a common 5P configuration using 15A cells, each parallel string could provide 75 peak amps, so the 40A I am looking for would not stress the pack at all).
Of course a pack of VTC4 cells will run cooler if you use less current than 150A, and you will also get more range if you draw only the 40A that I recommend as the practical limit. The IMR prefix in it’s part number means that it uses the ultra-safe Lithium-Manganese-Oxide chemistry (LiMn).
One of the beneficial quirks of these cells (sometimes branded as Konion cells) is that the factory claims that they don’t need to have the parallel strings balance-charged. In theory, the entire pack can be bulk-charged, and each individual cell will stop taking a charge when it’s full, even though the charger will continue charging up the other cells…the ones that need just a little but more to be full.
A word of caution: the similar Sony VTC3 is only 1600-mAh, so make sure you are researching the VTC4, which is the more desirable 2100-mAh. (edit: In 2015, the VTC5 cell is now readily available, and it provides high current at a 2600-mAh capacity, but at a lower 20A per cell current)
This is my new favorite E-bike battery pack cell (Samsung is from South Korea). Since it is an 18650 format, it is small and configurable like the Sony VTC4, but it is less expensive per Watt-Hour (WH), like the Headway cells. The price/performance is good, but there is another reason I like them. The Sony VTC4 cells would require a serious soldering operation, or possibly a spot-welder…in order to assemble a pack. However, the 20R cell is available in a pre-made pack. (edit, now in 2015, the Samsung 25R cell that provides high current at 2500-mAh is readily available, and the 3000-mAh 30Q cell should be available in 2016)
As time goes on, more and more vendors will provide a variety of choices, but right now there is only one battery provider that will make you a pack with the 20R cell. And…as luck would have it, this vendor already has a well-known reputation for quality products and good customer service. It is Paul at em3ev.com, and his endless-sphere username is cell_man.
However, the good news doesn’t stop there…just like other battery vendors, he has several rectangular packs shapes to choose from, but he also is the only vendor I know of that makes and sells triangular packs. Having a pack that is shaped like a triangle allows you to mount it in the center of most bicycle frames. Not only that, he sells TWO different sizes of triangle packs (can you tell I’m a fan?).
The INR prefix on its part number means that the chemistry is Lithium Nickel Cobalt Aluminum Oxide (NCA). The smallest triangle pack available with 20R cells is capable of 50A of peak current, but Paul limits them to 40A with a Battery Management System (BMS) that is designed to make the pack last as long as possible.
Prismatic pouch cells
I recently asked respected ES member Luke /LFP about non-LiPo cells that provide high current and he mentioned there were some large prismatic cells that could fit on a bike frame. One of the companies he mentioned was Energy Innovation Group (EIG, from South Korea).
EIG makes foil packs in the large format, and they are designed to serve the EV market, such as electric cars, motorcycles, and scooters. They use the well-known LiFePO4 chemistry, and their F-series cells are designed to put out high current. Since each cell has quite a bit of amp-hours in it, you wouldn’t need to connect them in parallel to get a high-amp pack. Their F007 cell has 7-Ah per pouch, and provides an awesome 30C worth of current (200A?), so they could make a small and short range pack.
The EIG F014 cell is a useful 14-Ah per pouch size, and provides 20C of peak current, with 10C of continuous current, which would be 280A and 140A…much more than enough for us, so this pack would stay very cool when drawing only a 40A peak.
If you wanted a pack in the popular 16S configuration (16 cells in Series for approximately 50V), you only need to buy 16 pouches. However, I’m sure it’s obvious that these large format pouches are not very configurable. Pre-made packs are all going to be a large rectangular block.
Why don’t we see more pouch packs?
These flat foil pouch packs (made from several different chemistries) are popular with electric cars, motorcycles, and scooters. The main reason is that there is no wasted air-space between cells (as there would be with cylinders), so you can have a very power-dense pack.
That may sound good for an E-bike pack too, but there are downsides. First, they are more expensive than readily available packs made from common 18650 cells. Second, they are not very configurable, so you are limited to a large rectangle. The website em3ev.com also sells foil packs for scooters that are made from the excellent A123 company, but his 20-Ah pouches are even larger than the EIG cell shown in the pic above. Chinese battery manufacturer Farasis also makes high-current LiFePO4 foil packs that are the large 20-Ah size.
18650 cells are very configurable (like Lego’s), and since they are cost-effective compared to the other options, I believe that they will become very established this year as the premier cell type and size for E-bike battery packs. While studying for this article, I came across other high-current 18650 cell options.
Interestingly, the most readily available technical information about the various cells were from website forums that discussed flashlights, and also E-cigarettes/Vaporizers. The flashlight connection is self-explanatory, but a vaporizer is less well-known. With the recent legalization of recreational marijuana in Colorado and Washington (and the de-criminalization of medical marijuana in over a dozen states), a vaporizer is a device that heats the leafy material so the vapors can be inhaled without emitting smoke. Their heating element requires a small, high-current battery.
But regardless of who is using these cells, high-current 18650’s have been developed for the huge global market for cordless tools. The prices per cell should come down over time, but fortunately for us…we can have any characteristic you want right now. The “best value” cell from em3ev.com is the Samsung 22P, and his small triangle pack made from those easily delivers 30A (the larger packs made from the 22P would provide more amps), the extreme range cell is the Panasonic 29E (with 2900-mAh per cell). A small pack of 29E’s will still have a lot of Amp-hours (Ah) of range.
His high-current cell is the Samsung 20R (and soon the 25R in 2015). His smallest triangle pack of 20R’s is limited to 40A for long life (it is capable of more amps than 40A), but if you can fit the large triangle pack, you can also get 40A from the more affordable 22P cell. I actually tried to find a better selection of cells to see if there were any other options to add to this list.
Here is a (slowly increasing) list of high-current 18650 cells that I collected (all with a minimum capacity of at least 2000-mAh), and after considering their characteristics and cost, I realized that cell_man has really done his homework. The other high-current cell options either don’t have as much capacity as the 29E, they cost much more than the 22P, or they provide less current than the 20R. When the Samsung 25R becomes available this spring (a high-current 2500-mAh cell that costs less than the Sony VTC4), it will then become my newest favorite cell for a small high-amp pack.
Samsung 22P, 2200-mAh
Samsung 20Q, 2000-mAh
Samsung 20R, 2000-mAh
Sanyo 20EX, 2000-mAh
Samsung 25R, 2500-mAh
Sony US18650VTC4, 2100-mAh
Sony US18650VTC5, 2600-mAh
Panasonic 29PD, 2900-mAh
LG ICR18650HE2, 2500-mAh
LG DBE21865 (18650, NMC chemistry, 2500-mAh)
Sony US18650NC1, 2900-mAh
A quick note, there are many high-current 18650’s that have a capacity of 1000-1600 mAh, but that capacity is so small I did not consider them because I would have to add many extra cells just to get to a minimum 10-Ah.
edit: Thanks for the links that some of our readers have sent to expand this list.
Here are two high-current cells in the 26650 size (26mm diameter, 65mm long, high current, non-LiPo chemistry) that are worth considering:
A123 26650M1B 2500-mAh
Sony US26650VT 2600-mAh
Written by Ron/Spinningmagnets, February 2014