Questions we will try to answer
Do you know how many battery cells are in your bike's battery? Does it matter?
Did you know you could have up to 40-60 cells?
Why can a 40-cell e-bike battery limit the maximum power that a certain motor can deliver?
Why is the Avinox M2S motor, which can provide a maximum of 1500W, limited to 1300W on certain bike models? Is the battery to blame?
How many batteries does the Avinox FP700 battery have and what type?
What is the history of these batteries? Did they come from the automotive industry? Do they have any connection to Musk?
Why can the Avinox FP700 provide 1500W to a motor while others cannot?
What do they have extra?
What do they lack?
About battery cells in e-bike batteries
Most e-bike battery manufacturers use the convenient and well-tested, proven in practice, 21700 cells. What does this code, this name, tell us?
First of all, the dimensions:
- Diameter: 21 mm
- Length: 70 mm
Being a Lithium battery, its nominal voltage is 3.6V. It's a technical convention because in reality, when fully charged, it reaches 4.2V and will discharge down to a minimum of 2.5V or even earlier, depending on how the BMS (Battery Management System) controller is programmed, to leave some juice at least for the controller to show you the way home.
To power the electric motor with 36V, we need 10 such batteries in series, right?
3.6V x 10 batteries in series = 36V
And we have solved the voltage issue. Plus, we will have a slim battery of 2 cm in diameter and 70 cm in length, which doesn't quite fit in the downtube, but if we group them in sets of 6 and stack 7 such packs, it would fit.
Why are bike frames so bulky to accommodate the square e-bike batteries? Why is the Avinox FP700 so slim?
Picture of Avinox FP700:
Dimensions:
Length: 558.5 mm
Width: 59.1 mm
Height: 56.1 mm
What batteries does Avinox FP700 use?
Here are some comparative images of Avinox 50480 cells versus classic 21700:
A bit different, right?
With such cells, an almost cylindrical shape can be achieved for an e-bike battery.
Of course, to make room for the complex measuring, connecting, and management circuits of the cells, some space is needed between the batteries and around them.
The cells lined up in an FP700 battery within a transparent Amflow PX frame
These cells were seen in a Tesla presentation.
They have a special construction called Tabless, which drastically reduces their internal resistance by connecting the metal electrodes along their entire length at both ends, not just at the beginning and the end like the 21700 cells.
Dimensions and capacities that allowed Tesla to reduce the number of cells from 7000 to under 1000 in an electric car:
For enthusiasts, you can see how such a battery is built, the folks at Munroe disassemble them and show the principles of tabless operation and manufacturing: https://www.youtube.com/watch?v=VEBY7rEUZiI
But if we put 10 4680 batteries in series without any spacers, we get a battery at least 800 mm long and it wouldn't quite fit in the downtube.
From the specifications, we see that the Axinox FP700 is a total of 558 mm.
Ampace 50480
Continuing the search, we found that it is a custom 50480 battery made by Ampace specifically for Avinox, which offers an energy density of 220Wh/kg. See the press release: https://en.prnasia.com/releases/global/-c-o-r-r-e-c-t-i-o-n-ampace--529515.shtml
“One cell outperforms six standard 21700 cells” one 50480 cell does the work of 6 21700 cells connected in parallel.
The nominal capacity is 19.4 Ah compared to 4 Ah for a 21700 cell.
Being almost cylindrical, the Avinox FP700 battery is the only battery in the world that can fit very nicely into the downtube of the Atherton:
By the way, because they only use a cylindrical profile for the frame, Atherton can offer 12 sizes to customers based on the unique personal dimensions of each rider. With an Atherton, you can't complain about not having the right position on the bike.
How many batteries do we need in an e-bike battery?
Let's return to our string of 10 batteries in series; we just solved the 36V voltage problem.
Is it enough to put 10 21700 batteries in series? Let's calculate what current and power a battery with 10 21700 batteries in series can provide.
Being connected in series, see Ohm's law (you are a person with me, I am a person with you, right?), the maximum current that can be obtained is that of a single battery.
The capacity of these types of batteries ranges from 3000 mAh to 6000 mAh (depending on the manufacturer), the batch, and of course the price.
Let's take a 21700 battery as a model, we won't name names as there are many, whose nominal discharge current is 4000 mA.
However, according to the catalog data, it can deliver 35A continuously or even 45A but with the temperature limited to 80 degrees, as stated in the catalog.
The beauty of Lithium batteries is that they can provide a very high current when peak power is needed, tens of times greater than the nominal current, reaching incredible maximum powers, but we must take care of the battery temperature and additionally, the operating time will drastically decrease, not to mention that the voltage will drop dramatically as the current increases. If the voltage drops, our power drops too, right? Let's not forget that P (W) = U (Volts) x I (Amperes)
How much does it drop? About this much:
That's why we talk about maximum power, because it can only be delivered continuously for short durations.
But how long can I use a fully charged battery (the one in the example)?
If I consume only 1A, the battery will last me 4 hours.
If I consume 4A, it will last me only one hour.
What power can we achieve at 4A?
The power P=U x I=36 V x 4 A = 144 W so we still have "a bit" to go until 1500 W, right?
So far we have learned that a string of 10 batteries is not enough to reach 1500W but only 144W... What solutions do we have to increase the power?
How many batteries do we need in an e-bike battery?
Let's also connect 21700 batteries in parallel, not just in series!
That is, 10 in series to have 36V at the motor and 4 in parallel. This combination is called 10S4P, 10 in series of 4 in parallel.
We answered the first question, that we need 40 batteries in the e-bike battery to achieve 576W (4 x 144W).
Below are internal construction details of a battery from Phylion. As you can see, it is a whole engineering work for connecting the batteries, their physical safety in case of impact, deformation, separation for cooling and safety, but also advanced electronics full of temperature sensors, current sources, comparators, controllers, which ensure proper operation both during charging and discharging of each cell.
How do we solve the 1500W power issue?
It seems we are still far from those 1500 W, right?
But since we know we can draw not only 4A but also tens of amperes from our cells, if we draw not only 4 A but about 12A from the batteries, we find that we can reach peak 1500W without problems. Well, our battery would run out in about 17 minutes but the fun would be maximum.
However, bicycles do not allow us to do this because there is also the issue of battery heating, so the controller will allow us to deliver maximum power but only until we reach a safe operating temperature for the battery. And if we could abuse it in current, we wouldn't achieve much because you saw how the voltage drops when we increase the current, the power will self-limit from the equation P = U x I.
In electric cars, having more space available, the cells are assisted by a cooling system but also heating when temperatures are too low, we all know how a phone battery discharges when skiing if we don't keep it close to the body, warm.
How long can we continuously deliver 1500W?
Avinox provides us with a graph showing that the power of 1500W, available only with FP700 batteries, can only be delivered for 6 minutes after which it drops drastically.
If we draw only 1000W, we can maintain this power for 28 minutes.
Basically, we have seen that Lithium batteries can provide exceptional performance, spectacular current excursions when needed, but we must take care not to overheat them to exploit them safely for as long as possible. We talk about safety because we know how Lithium behaves in contact with air, it burns violently...
Why is the declared power on some Avinox M2S bike models 1300W and not 1500W?
Because the internal resistance of most battery models does not allow reaching this value. Because the battery cells were somewhat chosen at the limit, to serve models of motors that appeared until April 2026 when the whole industry was turned upside down by the emergence of Avinox M2S which can offer 1500W.
Avinox together with Ampace created a new battery model, using the technology first adopted by Tesla, Tabless, meaning not just two thin connectors at the far ends of the foils, welded at points, but permanent contact along the entire length of the foil, resulting in a very low internal electrical resistance (3 times lower than 21700), low voltage losses on the battery at high currents (see the graph below), higher charging currents but also a charge density 5 times greater than classic 21700 cells.
You might say that by connecting several 21700 cells with high internal resistance in parallel, the resulting resistance will be much lower. And yet, at this moment there are constructive, functional limitations of the 21700 cells that do not make this achievement possible yet. I am convinced that it is possible to reach 1500W power even in configurations with 40-60 21700 cells with superior performance, maybe in Tabless technology, who knows? We promise to inform you when these e-bike battery models appear.
Transitioning from Ah to Wh
Speaking of sizes, it's very simple to convert from Ah (the battery's capacity, 1 Ah=3600 Coulombs) Ampere-hours to Wh (Electrical energy in Joules, 1 Wh=1W×3600 seconds = 3600 J, like at your home electricity meter) Watt-hours simply by multiplying the Ah value by the nominal voltage of 3.6 Volts.
The Ampace 50480 cell has 19.4 Ah, multiplied by 3.6V results in 70 Wh.
We multiply by the number of cells in series, which is 10, resulting in 700Wh. Now we understand why the Avinox FP700 battery with this type of Ampace 50480 cells can only provide us with 700 Wh at 36V. Neither more nor less.
That's about it for this episode on batteries.
We will continue in the next one with motors: power, torque, support percentages, gear ratios, types of planetary systems and sprockets, power transistors with Si or Gallium? Gallium chargers, GaN, clutches, bearings, and other mechanical details that allow reaching a power of 1500W at 150 Nm. Additionally, easy pedaling, without strange noises downhill and without feeling any braking or resistance in the pedals from the motor. About sensors, types of sensors, where they are mounted, and synchronization with transmission systems.
