Custom Lithium Battery for Robotics: 18650 vs 21700

A Custom Lithium Battery for Robotics has to be able to provide enough energy to power the device for the required operating time and enough discharge capability for motor loads to quickly change. Using 21700 cells to get a higher nominal capacity causes issues with current-sharing and packaging that 18650 cells would not cause.

At GLOFLUX, we look at cell-level considerations when designing battery packs, considering usable energy, resistance, series-parallel configuration, mechanical integration, and manufacturing before finalizing the design.

1. 18650 vs 21700: Cell-Level Differences

The difference between the two cells is mainly their shape. Both 18650 and 21700 cells share the same electrical and chemical characteristics.

Technical Parameter	18650	21700
Nominal dimensions	18 × 65 mm	21 × 70 mm
Individual capacity	Model-dependent	Often higher per cell
Energy density	Depends on chemistry and design	Depends on chemistry and design
Discharge current	Energy/power variants	Energy/power variants
Pack integration	Flexible in narrow compartments	Potentially fewer cells
Thermal behavior	Resistance and cooling-dependent	Resistance and cooling-dependent

Cell Energy Density vs Pack Energy Density

Two parameters determine whether a cell format benefits robotic equipment:

• Gravimetric energy density (Wh/kg): Affects battery mass, robot payload, and moving inertia.

• Volumetric energy density (Wh/L): Determines how much usable energy fits inside the battery compartment.

A smaller cell (e.g. 21700) takes up less space as fewer cells are required. A larger cell, however, might yield more space in a compact enclosure.

Energy density of a pack is a factor of not only the cell (i.e. the primary storage element), but also the cell holder, insulation, busbars, BMS circuitry and walls of the enclosure.

2. Series-Parallel Configuration: Capacity Is Only Half the Calculation

For a Custom Lithium Battery for Robotics, series connections provide a higher voltage, while parallel connections give higher capacity and split the load current.

The basic relationships are:

• Pack voltage: Vpack = Ns × Vcell

• Pack capacity: Cpack = Np × Ccell

• Nominal energy: E = Vpack × Cpack

• Approximate runtime: t = Usable Wh / Average Load W

Engineering Comparison: 18650 vs 21700 Packs

Consider two hypothetical configurations using 3.6V nominal cells.

Specification	18650 Design	21700 Design
Cell capacity	3,000mAh	4,500mAh
Configuration	4S3P	4S2P
Cell count	12	8
Nominal voltage	14.4V	14.4V
Pack capacity	9Ah	9Ah
Nominal energy	129.6Wh	129.6Wh
Current per cell at 18A pack load	6A	9A

Illustrative engineering values, not tested GLOFLUX product specifications.

Although both packs provide identical nominal energy, the 21700 arrangement places 50% more current through each parallel cell at the same 18A pack load.

This impacts cell temperature, voltage sag, and discharge capability. Engineers need to verify each selected cell supports the required current over the cell's operating temperature and SOC (state of charge) range.

3. Discharge Performance: Managing Robotic Motor Peak Loads

While robotic motors can operate at a somewhat-constant current, factors such as uneven terrain, and acceleration can introduce transient loads.

Internal Resistance and Voltage Sag

V voltage drop can be approximated by

ΔV = I × R

The effective resistance can be made up by cells, interconnections, protection devices, and wiring.

Excessive voltage sag can cause undervoltage protection on the motor driver, even when the battery has a high SOC.

For a Custom Lithium Battery for Robotics, we suggest evaluating:

• Continuous discharge current: The current the battery can supply continuously over the specified temperature and SOC.

• Pulse-current capability: Max current the battery can provide for a given time.

• DC internal resistance: Measured at a defined SOC and temperature.

• Minimum operating voltage: Tested under worst-case conditions.

• BMS overcurrent threshold: Coordinated with acceleration and stall protection.

Cell datasheets' current ratings should always be considered in the context of the complete pack's thermal and electrical limits.

4. Thermal Management and Cycle-Life Constraints

Battery heating occurs according to the relationship:

Pheat = I^2R

Therefore, doubling the current through the same resistance produces approximately four times the resistive heat.

Thermal Design Considerations

For enclosed robotic systems, critical factors include:

• Thermal gradients between cells

• Positioning of the NTC sensors in relation to thermal hot spots

• BMS charge/discharge thermal protection

• Heating and resistance losses in busbars

• Charging and SOC operating window

• Cycle aging and life

Thermal performance of a cell depends on current load, cell spacing, and design. A bigger cell doesn't always result in better thermal performance.

Battery lifetime should be considered at the same temperature and with the same charging and depth-of-discharge as compared cells.

5. Matching Battery Formats to Robotics Applications

Cell selection should be based on mechanical constraints and duty cycles rather than robot category alone.

Application	Engineering Priority	Selection Consideration
Compact service robots	Space and weight	18650 layout flexibility
AGV/AMR platforms	Runtime and peak power	21700 capacity and current capability
Inspection robots	Continuous operation	Heat dissipation and enclosure geometry
Humanoid robots	Mass distribution	Energy-to-weight ratio and peak load
Industrial robotic systems	Service life	Replaceability and maintenance cost

For mobile robots, the final comparison should include usable Wh, pack mass, installation volume, current capability, cooling requirements, and lifecycle cost.

6. Focus and Combination in Battery Pack Manufacturing at GLOFLUX

There are requirements for exact features in cells and mechanical assembly precision in custom Lithium battery manufacturing.

At GLOFLUX, we offer OEM/ODM services for selection of cylindrical cells, design of battery pack architecture and BMS, enclosure integration, and mechanical assembly. We also offer prototyping and production services.

Dimensional Accuracy and Assembly Control

Our published 14.4V smart battery specifications provide a documented example of dimensional control:

Finished Battery Dimension	Specification
Length	150.5 ± 0.3 mm
Width	77.4 ± 0.3 mm
Thickness	22.7 ± 0.3 mm
Weight	445 ± 10 g

These tolerances apply to that specific smart battery, not universally to all GLOFLUX robotics battery packs.

For robot applications, engineering drawings should separately define enclosure dimensions, mounting positions, connector alignment, insulation clearances, and allowable assembly variation.

Cell Matching and Quality Verification

We have over 60 inspection checkpoints in our quality system, beginning with incoming materials and concluding with finished goods inspection.

When considering the control of production, key factors include the following:

Maintaining consistency of Cell capacity, Ohm's law balance, and Internal resistance.

Weld integrity and interconnection resistance.

Insulation, polarity of connectors, and wiring.

Aging and protection functions of the BMS.

Where applicable to the design, we can configure BMS to incorporate protection and communication functions such as CAN, SMBus and I2C.

7. Verification and OEM Purchasing Criteria

Measurable acceptance criteria should be established by engineering teams prior to approving a Custom Lithium Battery for Robotics.

Validation Area	Required Verification
Electrical	Capacity, voltage sag, continuous/pulse discharge
Thermal	Peak temperature and cell temperature spread
Mechanical	Vibration, shock, mounting integrity
BMS	Current limits, balancing, fault response
Compliance	Applicable UN38.3 and product safety requirements
Manufacturing	Dimensional inspection and batch traceability

For industrial applications, IEC 62619 applicability should also be reviewed against the final battery system.

An OEM specification should include rated voltage, duty cycle, peak-current duration, compartment drawings, charging method, environmental conditions, communication interfaces, annual demand, and maintenance expectations.

Engineering Your Robotics Battery with GLOFLUX

When comparing 18650 and 21700 cells, it is important to validate the whole battery system and not just nominal cell capacity.

At GLOFLUX, we provide cell selection, BMS integration, and battery pack design and validation for robotics manufacturers. If you provide your electrical and installation drawings, our engineering team will evaluate battery configurations for your robot.

FAQs

Q1. Does GLOFLUX offer both 18650 and 21700 robot battery packs?

Yes. GLOFLUX offers customized 18650 and 21700 lithium-ion battery packs for AGVs, AMRs, service robots, and industrial robotic systems.

Q2. How does GLOFLUX select between 18650 and 21700 cells?

We evaluate required capacity, discharge current, energy density, installation space, thermal conditions, and pack configuration before selecting suitable cells.

Q3. Can GLOFLUX customize robot battery voltage and capacity?

Yes. We customize voltage, capacity, and series-parallel configurations according to the robot's electrical requirements and available installation space.

Q4. Can GLOFLUX design battery packs for high-current robotic motors?

Yes. We support high-discharge battery designs based on continuous current, peak-load duration, cell specifications, and BMS protection requirements.

Q5. What BMS protection functions does GLOFLUX provide?

Our customizable BMS solutions support overcharge, over-discharge, overcurrent, and short-circuit protection, with temperature monitoring where required.