It’s what’s on the inside that really matters

All modern Class 1 e-mountain bikes are powered by some sort of lithium-ion battery pack. For the most part, there is little you need to know beyond how to recharge them. With range and battery-pack capacity becoming a popular topic for discussion, a lot of us wonder what’s happening on the inside of these battery packs. And, with these packs costing close to a thousand dollars, it’s in every owner’s best interest to know how to get the most out of what they have, as well as what to do with their batteries when they are past their service life. Although these batteries are considered safe in most regards, there are things that every eMTB rider needs to be aware of. Before we go any further, it’s worth noting that you should always read and thoroughly understand your eMTB’s owner’s manual.

The most popular battery pack in Tesla cars contains 7,104 cells roughly similar to this one. They are wired together in 16 groups, with each group containing 444 cells. That helps explain why replacement batteries for electric cars and eMTBs cost so much.



What’s inside an eMTB battery pack? Lithium-ion battery cells and battery management circuitry most likely. Most brands will not disclose the specifics, but Bosch says the common e-bike battery contains 40 to 60 lithium-ion cells. They say that about 130 e-bikes correspond to an average of about one electric car.

Rocky Mountain, however, is a bit more open about its battery construction. “Inside our battery we use 39 automotive-grade 21700 lithium cobalt manganese cells,” says Alex Cogger, Rocky Mountain’s Chief Product Officer. (The number “21700” refers to the physical size of the cells—21mm diameter by 70mm long.) He adds that automotive-grade 21700 cells typically aren’t available in the DIY (do-it-yourself) marketplace. How does this compare to what you might find in a Tesla? “We use Samsung 21700 cells under normal circumstances;” he says, “however, you may have heard the global supply chain is just slightly out of whack these days, so we also have contingency plans with LG in case of a shortage. We don’t work inside Tesla and haven’t dissected their batteries, but to the best of our understanding, Tesla has used these Samsung cells but has apparently switched to Panasonic. Both of our cells are functionally identical to the Panasonic Tesla cells.”

Also inside most battery packs is the complex circuitry that manages the battery cells. Cogger says that a BMS (battery management system) controls all the safety protocols and ensures the balancing of the cells. Rocky Mountain uses CAN BUS communication to provide information to the motor controller and the display screen. “Using this tech, we can display, in real time, a number of vital stats: cell voltage, temperature, charge cycles, and battery pack health status,” says Cogger.


Expert consensus is that the worst thing you can do to your e-bike’s battery is damage it in a crash or purposely tamper with it. Let’s assume that nobody is dumb enough to try to open a battery pack for any reason, since personal injury is as likely as the instantly voided warranty. Crashes happen, but the battery packs are designed to protect the cells inside. Bosch protects each cell inside its battery with a steel cup, and it’s held in place by a plastic or aluminum housing. Rocky Mountain uses an aluminum extrusion, as well as a completely rigid fill inside the casing to prevent any movement of components that could cause failure. Some designs further protect the battery by hiding it inside the downtube or adding extra-thick skid-plate type protectors on the exposed sections.

The second worst thing you can do to your battery is to not recharge it at three-month intervals if it’s not being used. Cogger says that over time the cell voltage will drop, and once it gets below its threshold, the chemistry will deteriorate and won’t allow charging anymore. Recharge it every three months to at least 75 percent. Rocky Mountain’s battery also has a protection mode: it enters a sleep state after one month, at which time you simply connect it to a charger to wake it up to party. A good pro tip is to set yourself recurring calendar reminders on your cell phone if you are not riding the bike often.

Shimano’s battery-storage guidelines state that if the bicycle will not be ridden for an extended period, store it with a battery level of approximately 70 percent. Also, charge the battery once every six months to ensure that it is not completely discharged. When you use the battery after a long storage period, make sure to charge it first.

Although they will not disclose exactly what battery cells they use, Shimano utilizes high-quality cells that enable their batteries to be recharged as many as 1000 times.



Heat and cold will influence the battery’s usable capacity and overall health. “Best practices are to keep it in a secure place that is not exposed to extreme temperatures (above 30 °C [86 °F], or below 0 °C [32 °F]), excessive sun exposure, humidity or condensation,” says Cogger. “Ideally, store the bike at temperatures between 10 °C (50 °F) and 25 °C (77 °F) and relative humidity below 65 percent.

Shimano also recommends that you store the battery or bicycle with the battery installed in a cool indoor location away from direct sunlight and rain (estimate: 10 to 20°C). If the storage temperature is low or high, the performance of the battery will be reduced, and its usable time will be shorter.

Rocky Mountain says that, in general, the hotter the battery, the lower the internal resistance will be, leading to more power and capacity. “Obviously more is not always better, and there comes a point when we need to control the temperature of the battery pack for safety,” says Cogger. “Furthermore, when you use a cell at high temperature constantly, you reduce the lifespan of the cell. The motor is the opposite; the internal resistance will be lower when it is cold and the motor will be more efficient when it is cold. Power output may drop at high temps.”

Extreme cold can affect a battery, too. “Since most batteries have no temperature conditioning like an electric car, the battery will function in cold weather, though at a diminished capacity,” Cogger says. “Expect 70 percent of full capacity at -10 °C (14 °F). It is recommended that you warm up an extremely cold bike prior to use.”

Heat can have the opposite effect and actually increase battery capacity. Cogger says that battery performance can actually improve when heated but at the cost of lifespan. “If the battery temperature is over 50 °C, you could see a 5 percent increase in the battery capacity. Due to the damage that continued hot running can cause to the battery, we program in thermal protection as stated above to maintain battery lifespan and performance.”

Bosch is one of the few manufacturers that have the Underwriters Labratories “UL” safety certification on their batteries.



Your battery’s life will vary greatly depending on use, conditions, storage, care and the pack itself. Shimano says its charge cycles are listed at 1000 full charge cycles to reduce the battery’s usable charge to 60 percent. That’s roughly 19 years charging the battery from dead to full once a week.

Nick Murdick, Shimano’s MTB Product Manager, says that for most users, their batteries will last several years. “As a general guideline, lithium-ion batteries can expect to get through 500 charge cycles before they drop to 60 percent of their original capacity, and that’s when people might want to replace them,” he says. “Shimano uses very high-quality cells, and we are able to push that to 1000 charge cycles under the same charging conditions. Of course, that’s just for a basic comparison; we can expect the actual number of charge cycles to be higher or lower depending on the conditions.”

“However, most people wouldn’t want to prioritize battery longevity over ride performance,” Murdick adds. “Since the batteries are fairly robust and durable, I wouldn’t feel bad at all about charging the battery fully before a ride in order to take advantage of the full range, but understanding how batteries work and following best practices when it doesn’t impact your ride is certainly a great idea.”

What do you do when your eMTB battery reaches the end of its lifespan? In North America, the most popular recycling company is Call2Recycle (https://www.call2recycle.org/). This ebike recycling program shows drop-off points and other recycling options for your spent batteries.


As long as eMTB owners follow the instructions in their owner’s manual and use common sense, batteries are largely considered safe. The big brands carry out extensive testing to ensure that the battery packs have the correct capacity and function within the designed specifications. Bosch even goes as far as having its battery packs certified by Underwriter Laboratories—a nonprofit organization that tests and certifies products for safety. Bosch is one of the few to do so. Still others pass rigorous global safety standards.

So, unlike your friend who bought that homemade conversion kit for his bike on Alibaba, you have little to worry about. “The Rocky Mountain battery packs are made from automotive-grade 21700 cobalt manganese lithium cells that are not available on the public market,” says Cogger. “The performance and battery life are much higher than what most consumers can purchase. It’s hard to say what the DIY market batteries (or lesser-known brands) go through in terms of testing and certification, but based on our personal experience, we wouldn’t give a substandard battery to our worst enemy. As an aside, there have been great strides in the last five years in terms of cell development, and lithium-battery chemistry and stability have improved dramatically.”

All owners’ manuals state that a bike’s battery should not be charged while unsupervised. Although we know many who simply end each ride by plugging the bike in and walking away until the next outing, care should be taken. Some of the more paranoid MBA wrecking crew members charge and even store their eMTBs outside. That’s probably over the top, but it’s a judgment call for each of us to make on our own.

Cobalt is a common ingredient in lithium battery construction. There is a big push for manufacturers to reduce it for environmental, human-rights and supply-chain-dependence reasons.



Shipping and traveling with a bike is easy, but eMTBs come with some rules that owners must be aware of. Until recently, anyone shipping an e-bike had to be hazmat certified to do so. That changed with a special battery-powered vehicle classification known as UN 3171. According to BikeFlights, under this classification, your e-bike is not considered to be hazmat. You don’t need to be a hazmat-certified shipper, and you don’t need to pay a hazmat surcharge.

BikeFlights is only shipping e-bikes via UPS within the contiguous U.S. (no Alaska or Hawaii) with ground shipping, and they cannot ship them internationally, such as to Europe. You cannot ship an e-bike with a battery via air. If you’re shipping an e-bike with BikeFlights, they say that the battery must be fully functioning and installed in the bike. Other important battery-related steps are as follows: Lower your battery charge to less than 30 percent to limit cell-to-cell combustion; power off your battery; remove any keys, and ensure that your battery cannot turn “on” during transit. Protect your e-bike and its battery by using lots of extra-dense foam padding.

One solution to flying with an eMTB is to remove the battery and ship or fly with it as you would a normal mountain bike. There are shops with battery rental programs that will allow you to vacation and travel with your e-bike. No matter how you ship or travel with your eMTB, always check with the carrier or shipping provider to confirm their policies.


As battery technology advances, we will likely see smaller and more powerful batteries. This will lead to even better eMTBs. What’s really on the horizon for battery technology? “We hope to see improved battery chemistry that will use less cobalt, bringing the price down,” says Cogger. “This is important, as the battery is the single-most expensive component on an e-bike. Lower battery costs will open the door to other exciting opportunities to reinvest these savings in other areas, integrating new features, or increasing performance. Also, power density of cells will be improved, creating smaller battery packs that will be easier to integrate into the frame.”        

Bosch’s General Manager of e-bike systems, Claudia Wasko, says that battery-cell technology will continue to progress and—depending on the cell format and cell chemistries—allow further improvements in the area of battery-pack design, size, safety, energy density and sustainability. “Talking about sustainability, since 2013 the cobalt content within the battery cells of Bosch e-bike batteries has been reduced by 65 percent,” she adds. 

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