Electric mountain bike range: what 500, 625 or 750 Wh is really worth

What 500, 625 or 750 Wh actually measures

218 km. That is the range Bosch’s online calculator shows when you open it, with a Performance Line PX motor and an 800 Wh battery [1]. A promise it can keep, provided you read the default settings: ECO mode, an 18 km/h average, 100 kg including bike and luggage, a few inclines, a light breeze, mostly good asphalt, and summer [1]. Not, on the whole, a description of a mountain bike ride.

First, the unit. The watt-hour (Wh) measures a quantity of energy: that of a 1 W device running for one hour. A battery’s capacity is its voltage multiplied by its charge in amp-hours: Gazelle, for instance, quotes 13.8 Ah for its 500 Wh Bosch battery [2], which corresponds to a voltage of about 36 V. For a sense of scale, a 4,000 mAh phone battery at 3.85 V stores about 15 Wh: a 625 Wh battery is worth some forty of them.

So the figure tells you how much energy the battery carries, not how many kilometres it will last. As with the run time of a head torch, a stated distance only holds under its own assumptions. And on a mountain bike, it is mostly the ascent that sets the price of each kilometre.

What a climb costs, in watt-hours

Let’s do the maths. Lifting a mass takes energy equal to the mass multiplied by gravity and by the height. For 100 kg, rider, bike and rucksack included, and 1,000 m of ascent: 100 × 9.81 × 1,000, or 981,000 joules, which is 272 Wh. That is a minimum, which counts neither rolling resistance, nor the air, nor the motor’s losses.

You share that energy with the motor, and the assistance percentage says in what proportion. At 300%, the motor supplies three times your effort, so three quarters of the total: about 200 Wh for 1,000 m. Mountain bike motors go further: Bosch claims up to 600% assistance below 15 km/h for its Performance Line CX, once updated [3]. The motor then supplies six sevenths of the effort, about 234 Wh for 1,000 m.

On paper, then, a 625 Wh battery would climb about 3,000 m at 300% assistance. In practice, far less: rolling resistance on dirt, the air and the motor’s losses all add to the bill, and the descent does not recharge the battery. Compare that with Bosch’s calculator: its 218 km on 800 Wh assume less than 4 Wh per kilometre, whereas a 1,000 m climb spread over 10 km already needs about 20 Wh per kilometre at 300%.

Enlarge diagram

Weight, mode, cadence, cold: what drains the battery

Weight counts in proportion on climbs: 10 kg more on 100 kg means 10% more energy for the same ascent. Bosch duly advises against any unnecessary weight, bike and luggage included [4].

The assist mode matters even more. For the same bike fitted with a 500 Wh Bosch battery, Gazelle’s French website quotes up to 180 km in Eco mode and 70 km in Turbo mode [2]: a range two and a half times shorter, from the same battery.

Those figures translate into energy per kilometre: 180 km on 500 Wh is less than 3 Wh per kilometre in Eco mode; 70 km, a little over 7 Wh per kilometre in Turbo mode. These are maximum figures, obtained in good conditions, and Gazelle lists alongside them the factors that bring them down [2].

How you pedal matters too. Above 50 pedal revolutions per minute, the motor works more efficiently, whereas a very slow cadence uses a lot of energy; frequent starting and braking costs more than a steady speed [4]. Lastly, Bosch recommends riding at the maximum permitted tyre pressure to reduce rolling resistance [4]. That advice is meant for the road: on a mountain bike, you choose your pressure for grip first, and the range suffers accordingly.

Then there is the cold. As the temperature falls, the battery’s electrical resistance rises and its performance drops: Bosch warns that in winter you should expect a reduced range [4]. Age works in the same direction: Bosch’s calculator states that its values apply to a new battery, whose performance declines over time, as with any lithium-ion battery [1].

Range over time: ageing and storage

A lithium-ion battery loses capacity over the years, whether it is ridden or not, and the range falls with it. Bosch lists what speeds up this wear: heavy use, storage above 30 °C, prolonged storage fully charged or fully discharged, and leaving the bike parked in full sun [4].

Conversely, moderate use and storage at 15 to 20 °C, at around 60% charge, are kind to it [4]. The same guide states that it does not discharge by itself over a winter break: no need to recharge it before your next ride if it was stored at 60% [4]. In other words, for the winter: not full, not empty, not in a freezing garage and not on a sunny windowsill.

Manufacturers’ range calculators, and what they assume

Manufacturers publish ranges, always under assumptions. In 2015, Bosch’s battery guide distinguished three cases: ideal conditions (flat terrain, 15 km/h, no headwind, a rider under 70 kg), mixed conditions (moderate hills, 20 km/h, 70 to 80 kg) and difficult conditions (mountainous terrain, 25 km/h, headwind, over 85 kg) [4]. An electric mountain bike in the hills spends its entire life in the third column.

Bosch’s online calculator goes further: you can set the mode, average speed, total weight, cadence, terrain, wind, surface, tyres and season [1]. It is a good tool, provided you set it up for your ride rather than relying on its default values. At the very least, set the terrain, your real weight including kit, and the season of the ride: those are the three settings the defaults flatter most. Gazelle lists the same factors: gear changing, tyre pressure, weight, speed, terrain, headwind, battery age and level of assistance [2].

For a mountain bike ride, I prefer the calculation to the spec sheet: count the ascent, multiply it by the energy of the climb in the mode you plan to use, then keep a margin of at least a third for rolling resistance, air, losses and the cold. It is the same reasoning as sizing a campervan’s leisure battery: an energy budget, not a promise of kilometres.

On an electric mountain bike, range is counted in metres of ascent, not in kilometres.

Worth watching

EMBN goes through what decides an e-MTB’s range, from battery size, rider weight and power mode to ground conditions, tyres, elevation and riding style, and how a range finder helps.

The video is hosted by YouTube (Google): nothing loads until you play it, and Google may then set cookies.

Julien de Jurquet

Campervans, motorhomes, cycling and calculators

Julien de Jurquet writes for Outfarer about campervans, motorhomes, cycling and calculators. He reads instruction manuals to the very end and never lets a figure through without its unit, so that your choices rest on sums rather than promises.

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