Battery - the drivetrain's energy source
The battery pack is the vehicle's fuel tank and stores all the energy needed to power the motor.
Cell chemistry
Lithium-ion batteries are the standard in modern electric vehicles thanks to their high energy density, low weight, and good cycle life. A battery pack typically lasts 5 to 6 years under normal use. Variants such as NMC and LFP have different strengths in energy density and stability, but both work very well in two-wheeled vehicles.
Lead-acid batteries (lead) used to be common but are heavy and have low energy density. A modern lithium battery can weigh as little as one tenth of a lead battery with the same energy content.
Nickel-metal hydride batteries (NiMH) are uncommon in drivetrains today because they have better energy density than lead but worse than lithium, along with higher self-discharge. For high performance and hobby builds, lithium is the practical choice.
Lithium polymer (LiPo) often offers even higher energy density and lower internal resistance than many other lithium variants, enabling high currents and low voltage sag under load. The downside is higher fire risk and stricter handling requirements for charging, mechanical protection, and temperature.
Internal resistance (r) is the battery cell's internal opposition to current. It causes voltage drop under load according to Uterm = Uoc − I·r and losses as heat according to Pforlust = I²·r. Lower internal resistance means less heat and higher available power.
- LiPo: Very high energy density, low internal resistance (high current capability), but higher fire risk and stricter handling requirements.
- Lithium-ion: High energy density, low weight, and good cycle life.
- NiMH: Medium energy density, higher self-discharge, uncommon in drivetrains.
- Lead-acid: Low energy density, heavy, shorter lifespan, used mainly for starting internal combustion engines.
Battery structure and BMS
A battery pack is built from many smaller battery cells. Cylindrical cells of type 18650 or 21700 (typically 18 mm x 65 mm or 21 mm x 70 mm). These are connected in series (S) to increase battery voltage and parallel (P) to increase capacity and current capability. For example, a 20S4P configuration means 20 cell groups in series, where each group consists of 4 cells in parallel.
A BMS (Battery Management System) is essential in series-connected lithium battery systems. It monitors each group of cells and limits current during both charging and discharging. It also protects against dangerous voltage levels and balances the cells so they stay at the same charge level. Without balancing, the battery can degrade or lose capacity over time, and in the worst case may lead to fire.
Voltage, capacity, and current
The amount of energy in a battery is measured in watt-hours (Wh) which are calculated as voltage times ampere-hours (U × Ah). For example, 72 V × 8 Ah gives exactly the same energy as 36 V × 16 Ah, namely 576 Wh. Most e-bike batteries are usually between 250 and 600 Wh. The battery in Gbike 1 has an energy content close to 600 Wh which gives good range even for such a powerful vehicle.
Fun fact: 600 Wh compared with gasoline
In pure energy terms 600 Wh corresponds to only about 0.065 liters (6.5 cl) of gasoline!
But here is the magic: An internal combustion engine's efficiency is only ~20-25% (the rest becomes heat and noise), while an electric motor reaches ~85-90% efficiency. This means 600 Wh of electrical energy can do much more work in an electric vehicle than 0.065 liters of gasoline in a combustion engine, even though the energy amount is the same.
Thanks to the electric drivetrain's high efficiency, we can go 3-4 times farther per unit of energy, so 600 Wh is enough for many miles of riding despite the "small" energy amount.
When a battery is loaded, the voltage drops slightly because of internal resistance (r) in cells and cables. A simple model is Uterm = Uoc − I·r, where Uoc is the open-circuit voltage. Output power is approximately Put ≈ Uterm·I while losses as heat are Pforlust = I²·r. A higher voltage (U) means you can draw the same power with lower current (I), which reduces both voltage sag and heat losses.
Connecting more cells in parallel (more P) lowers total internal resistance, which helps reduce voltage drop. Connecting more cells in series (higher S) increases system voltage. You balance these two approaches to get the desired performance in terms of power and range.
Estimate charge level from voltage
You can get an approximate idea of the battery charge level (SOC - State of Charge) by measuring voltage. A typical lithium-ion cell has approximately these voltage levels:
- Fully charged: ~4.2 V per cell
- Nominal (about 50%): ~3.6-3.7 V per cell
- Empty (discharge limit): ~3.0-3.2 V per cell
To calculate the voltage of a full battery pack multiply cell voltage by the number of series groups (S). For example, for a 20S pack:
If you measure 78 V on a 20S pack, each cell is about 78 ÷ 20 = 3.9 V, which corresponds to roughly 70-80% charge. Note that the relationship between voltage and SOC is not fully linear, the curve is flatter in the middle and steeper at the ends.
Deep dive: Battery Capacity & Voltage
Want to learn more? We have a full article with interactive discharge graphs, an SOC calculator, and tables for 48 V, 52 V, and 72 V systems. Read more about battery capacity →
Calculate electrical power
Electrical power is calculated using P = U × I (power = voltage × current). Since voltage drops under load, a good rule of thumb for maximum power is to use nominal voltage × maximum current at full charge.
Note that this gives the electrical input power to the motor. The mechanical output power (the actual work done at the wheel) is lower due to losses in the motor and drivetrain. To calculate mechanical power, you need to account for motor efficiency at the current load, RPM, and temperature, which is significantly more complex.
Want to calculate more precisely?
For more detailed calculations of mechanical power, torque, efficiency, and how different parameters affect performance, try our motor simulator.
Note
A higher system voltage allows high power without unreasonable current. At the same time, 72 V is generally low enough that incidental contact is usually not dangerous, but you should always treat electricity with caution.
GBIKE 1: 20S 72 V lithium-ion battery
The battery in GBIKE 1 is configured as 20S which means 20 cell groups in series. This gives a nominal voltage of 72 V and a fully charged voltage of 84 V, which is significantly higher than typical e-bikes often running at 36 or 48 V. The higher voltage is crucial for delivering high power. The battery can deliver up to 80 A of current.
Theoretically, that means a power output of 72 V × 80 A ≈ 5760 W (~5.8 kW). In practice, voltage drops somewhat under load, but this gives a good picture of the system capacity. All components such as cables and connectors are sized to handle these currents safely, and the BMS ensures everything works as intended.