Dual Battery, Range Extender Or None, How to Choose

• Why additional batteries and range extenders are a big plus
• Advantages of range extenders compared to double batteries
• Advantages of double batteries compared to range extenders
• Some tips to better enjoy double batteries and range extender

Even the ancient Greeks understood the need of stocking energy to freely master it, inventing the myth of Prometheus, who tried to steal fire from the gods. Nowadays, electric transport is spreading rather fast. Its many advantages increasingly persuade companies and individuals to adopt it, be it ebikes, electric cars or even e-boats. Yet, its diffusion is hampered by this century-old problem: storing electric energy is not very practical. Through lithium-ion technology, batteries have come a long way since the lead batteries of the first ebikes in the 90s. Nevertheless, they are still expensive, heavy, and a bit weak, allowing limited riding ranges and lasting a mediocre 10-12 years span before significantly losing capacity. The latter is roughly 13 times inferior to that of fossil fuels for each kilogram of weight, although fossil fuels are not exactly energy, for they need to be burnt to produce it. For example, a small city car can drive about 350 km with 20 kg of gasoil, whereas a similar electric city car needs a 300 kg battery to deliver the same range. To say nothing about the average 2 to 5% yearly loss of capacity affecting lithium-ion batteries. No wonder that many ebike manufacturers endeavor to satisfy the public demand for longer-range batteries, offering an ever larger choice of range extenders or double battery adaption kits.

Why additional batteries and range extenders are a big plus for urban and trekking ebikes

To put it bluntly, the average range of ebikes is just barely sufficient for many usages. If we are to abide to the widely accepted rule of thumb, that one shouldn’t discharge his ebike lithium-ion battery below 20% and avoid charging it beyond 90%, we can shave off 30% from the battery capacity declared by the manufacturer. Consequently, in a flat city, the range of a decently powerful 625 Wh battery would be about 50 km, largely sufficient for commuting to work. That notwithstanding, should one ride with the maximum level of assistance in a hilly city with cold temperatures, that range would drop to barely 30 km. Not much, albeit still plenty of range for commuting.

For touring, such a range would be sufficient only if you recharge at a bar or at a restaurant every 2.5 hours, provided that the bartender lets you do it. A big battery capacity is even more decisive for cargo ebikes.

All the more for e-mountain bikes

What’s the point of buying a mighty e-mountain bike if you don’t dare venturing into uncharted territory, where you are to challenge all sorts of energy consuming hindrances and trials? Indeed, compared to many e-mountain bikers, Indiana Jones would seem a turtle. Why risking to remain stuck in the middle of nowhere because of an exhausted battery, compelled to drag home a heavy bicycle, often times through steep and muddy paths? Indeed, additional battery capacity is most demanded by e-mountain bikers.

Analyzing the advantages of range extenders compared to double batteries

The most popular ebike and battery manufacturers offer at least a type of range extender: Bosch, Giant, Trek, Mahle, TQ, Brose, Fit, Orbea, Sram, XLC to quote just a few.

• They are not cumbersome, being rather small, for instance the Bosch PowerMore is 218 x 79 x 88 mm.
• They are light, weighing an average 1.5 kg
• You can mount and take them off quite easily, many ebike models being equipped with special fastenings for range extenders. They are handy and s ergonomic to use. Some manufacturers offer special fastenings for double batteries, such as  the Haibike MRS system or the Scott E-MTB Battery Range Booster adapter plate, or Bosch Dual Battery both for Smart System and other Bosch drives. Sadly, they aren’t nearly as easy to mount and handle as range extenders.
• Provided that they are compatible, you can easily switch them from one bike to the other.
• There are aesthetically discrete and pleasant, often looking like water bottles.
• They are ideally placed on the down tube, thus not altering the ebike balance.
• They can easily interact with the main ebike battery, for instance automatically kicking in when the former is exhausted, or displaying their charge status. For such fonctions, double batteries can have troubles.
• Especially for e-mountain bikes, their light weight is safer, should your e-mountain bike hit you during a fall.

The advantages of double batteries compared to range extenders

• They have at least twice the capacity of a range extender, the latter seldom exceeding 250 Wh, in some cases being limited to merely 185Wh (for instance, the Mahle X20 range extender).
• They can be locked.
• They sport a much better price to power ratio, averaging roughly 1.2 euros per watt of power, whereas range extenders cost on average 1.9 euros per watt of power. A Bosch Smart System Powerpack with 800 Wh costs 895.90 euros, 1.12 euros per watt, whereas a Bosch Smart System PowerMore range extender costs 1.98 euros per watt, giving 250 watt for 495.90 euros.
• Even their weight to power ratio is better than that of range extenders. A Bosch Smart System Powerpack with 800 Wh weighs 4 kg, thus 5 g per watt of power. A Bosch Smart System PowerMore range extender weighs 1.6 kg, resulting in a 6.4 g per Watt ratio.

Some tips to better enjoy double batteries and range extenders

• Reckon that they lose some 2-3% of their capacity per year, even when staying idle. That’s why you should carefully plan if and when to get one, possibly when you plan to use it frequently.
• It’s better to procrastinate than to anticipate your purchase, because practically every year there are better range extender and double batteries novelties.
• Ideally store them at 10°C, or 50°F.
• If you have difficulties attaching your double battery to the frame, you can always carry it on a luggage rack bag, which also conceals it, preventing thefts. In case you have no other choice, before deciding to get a double battery to carry it in your backpack, just make a test with a couple of mineral water bottles having the same weight of the battery. Can you bear such weight for a long ride?
• If it’s feasible, after an hour of heavy-duty usage of your battery, especially if it’s hot outside, switch to the other idle battery. Once the former will have cooled down, you will notice that it will display a slightly wider range. Moreover, it’s better to prevent batteries from wearing or even damaging overheating. This was easily noticeable with the old double battery Giant ebikes, Twist and Aspiro. Switching from a rather warm battery to the cold one would add, once you had switched back to the former after 10 minutes, some 10% more range. Be it riding or parking, a scorching sun does affect your battery performance, because any temperature above 25°C, or 77°F, diminishes its range and power output.