Read the technical specifications of a lithium battery cell: capacity, current, internal resistance
A scooter battery is simply an assembly of identical cells, often in the 21700 or 18650 format. Its performance is therefore directly determined by the specifications of these cells. Understanding these specifications allows you to understand why two batteries with the same voltage and capacity might perform very differently at full acceleration.
The essentials
- The number of cells in series determines the voltage; the number in parallel increases the capacity and the available current.
- A high-capacity cell doesn't necessarily mean a cell that delivers a lot of current; often, a choice must be made.
- The internal resistance determines the voltage drop and heat generation under high current loads.
- The lifespan figures are measured under specific conditions, which are often milder than real-world usage.
The formats:
The name of a battery format indicates its dimensions: an 18650 battery measures 18 mm in diameter and 65 mm in length, while a 21700 battery measures 21 mm by 70 mm. The 21700 battery contains more active material and dissipates heat more effectively, which is why it has become the standard for high-performance batteries.
The figures from a datasheet, one by one
The voltages
- Nominal Voltage: The average voltage during discharge, typically around 3.6V for most lithium-ion cells.
- End-of-charge voltage: Generally 4.2V. Exceeding this voltage can damage the cell and make it dangerous.
- End-of-discharge voltage: This is the minimum voltage that should not be exceeded, typically around 2.5V. In practice, it's best to stop charging at a higher voltage to prolong the battery's lifespan.
The capacity
In milliampere-hours (mAh): This indicates the amount of charge the cell can deliver, measured at a low current. At higher currents, the actually usable capacity is lower because the voltage reaches the low voltage threshold sooner. Reputable datasheets provide discharge curves at various currents; these are the ones you should pay attention to.
Continuous discharge current
The current that a cell can continuously supply without exceeding its temperature limit, under the manufacturer's specified conditions. This is a crucial figure for a powerful electric scooter. Peak values, which are only sustained for a few seconds, are listed separately.
Internal resistance
In milliohms: This represents the resistance that the battery cell offers to the current. It causes a voltage drop proportional to the current, and generates heat proportional to the square of the current. A low internal resistance results in a battery that maintains its voltage under full acceleration and generates less heat. It increases with cold temperatures and with age.
The lifespan
The number of cycles is determined until the capacity drops to a certain percentage of its initial value, often 80%. This figure is highly dependent on the test conditions: current, discharge depth, and temperature. It is used to compare cells that have been tested under the same conditions, and is not intended to predict a real-world lifespan.
Capacity or power: a trade-off
Within a battery cell, manufacturers must strike a balance between the amount of active material (which determines capacity) and the ease with which current can flow (which affects power). This leads to two main types:
| "Energy" cell | Power cell | |
|---|---|---|
| Capacity | Maximum | Slightly less powerful, or equivalent in performance on newer, high-end cells |
| Direct current | Moderate | Raised |
| Internal resistance | Higher | Lower |
| Ideal usage | Range, smooth riding | Strong acceleration, track use |
Two battery models are featured in our batteries, illustrating our selection process: the Samsung 50S, a 21700 cell with a capacity of 5,000 mAh, and the Molicel P45B, a 21700 cell with a capacity of 4,500 mAh, designed for exceptionally high discharge currents. The manufacturers' datasheets provide the exact discharge current values according to the measurement conditions.
From the cell to the battery
A battery is characterized by the number of cells connected in series (S) and in parallel (P):
- Nominal voltage = number of cells in series × nominal voltage per cell. 20S results in 20 × 3.6 = 72 V, and 84 V at full charge;
- `capacity = number in parallel × capacity of a single cell. 10P cells with a capacity of 5,000 mAh provide 50 Ah;`
- Energy = nominal voltage × capacity. 72 V × 50 Ah = 3600 Wh;
- Theoretical DC Current = Number in parallel × DC current of a single cell, before considering the limitations of the BMS, the wiring, and overall heating.
It is this last point that explains the sudden cut-offs during full acceleration, as detailed in our guide Battery and BMS that cause acceleration cut-offs: a large-capacity battery with "energy" cells can deliver less power than a smaller battery with "power" cells. This is what distinguishes the standard and Performance versions of our electric scooter batteries.
Frequently Asked Questions
Why don't two batteries, both with a voltage of 72V and a capacity of 40Ah, have the same power output?
Because their cells have different internal resistances and different continuous current ratings. Voltage and capacity indicate how much energy is stored, but not how quickly it can be discharged.
Can you mix different types of cells in a battery?
No. Cells with different capacities or internal resistances will become unbalanced: the weaker cells will work harder, age faster, and eventually limit the performance of the entire battery, or even become dangerous.
Does cold weather actually reduce power output?
Yes. Cold temperatures increase the internal resistance of the cells: the voltage drops more significantly under load, and the battery reaches its cutoff voltage more quickly.