Camytrade International
72V / 84V NCM Prismatic Battery Build Guide
20S Configuration
Introduction
Building a 72V NCM Prismatic Lithium Pack
This guide covers everything you need to build a professional 72V (84V full charge) lithium NCM prismatic battery pack for electric motorcycles, e-bikes, or custom EV applications. A 20S configuration gives you 72V nominal — the industry standard for high-performance builds.
What is a 20S Pack?
20
Cells in Series
Series Count
84.0V
Full Charge
20 × 4.20V
72V
Nominal
20 × 3.60V
60V
Cutoff
20 × 3.00V
21
Balance Pins
S + 1
Pack Voltage Reference
| State | Per Cell | 20S Pack |
|---|---|---|
| Full Charge | 4.20V | 84.0V |
| Nominal (rated) | 3.60V | 72.0V |
| 50% SOC (resting) | 3.75V | 75.0V |
| Low Warning | 3.40V | 68.0V |
| BMS Cutoff | 3.00V | 60.0V |
| Absolute Min (dead) | 2.50V | 50.0V |
NCM Chemistry: Lithium Nickel Manganese Cobalt (NCM/NMC) cells have high energy density but require precise voltage management. Never overcharge above 4.20V per cell or discharge below 2.50V. Always use a quality BMS.
Prismatic vs Cylindrical: Prismatic cells (pouch or hard-case) are easier to assemble into large packs, handle higher continuous current, and are better for this voltage range than 18650/21700 cylindrical cells. This guide focuses on prismatic format.
Step 1
Selecting Your Prismatic Cells
For a 20S prismatic pack you need exactly 20 cells in series. Choose your capacity (Ah per cell) based on the range you want. All cells must be from the same batch — never mix old and new cells.
Popular NCM Prismatic Cell Specs
| Cell Format | Nominal V | Capacity | Max Discharge | Internal R | Pack Wh (20S) | Notes |
|---|---|---|---|---|---|---|
| NCM 50Ah prismatic | 3.6V | 50Ah | 3C = 150A | ≤0.5mΩ | 3,600Wh | E-moto standard |
| NCM 100Ah prismatic | 3.6V | 100Ah | 2C = 200A | ≤0.35mΩ | 7,200Wh | High range build |
| NCM 150Ah prismatic | 3.6V | 150Ah | 1.5C = 225A | ≤0.28mΩ | 10,800Wh | Long range/cargo |
| NCM 200Ah prismatic | 3.6V | 200Ah | 1C = 200A | ≤0.22mΩ | 14,400Wh | Maximum capacity |
Cell Grading — What to Check
1
Voltage Match
All 20 cells must read within 0.02V of each other before assembly. Measure with a calibrated multimeter.
2
Capacity Test
Charge each cell to 4.20V, discharge at 0.2C, measure actual Ah. All cells must be within 2% of rated capacity.
3
Internal Resistance
Use an impedance tester (YR1035+). All cells must be within 0.1mΩ of each other. High IR = heat and imbalance.
4
Physical Inspection
No swelling, dents, corrosion, or electrolyte leakage. Terminals must be clean and undamaged.
Cell Orientation — Series Connection
In a 20S series pack, the POSITIVE terminal of Cell 1 connects to the NEGATIVE terminal of Cell 2, and so on. The first negative (Cell 1 −) is B− and the last positive (Cell 20 +) is B+.
NEVER connect two positives or two negatives together directly — this causes an immediate dead short and can destroy cells instantly.
B− → [Cell 1 −/+] → [Cell 2 −/+] → ... → [Cell 20 −/+] → B+
Step 2
Pack Configuration Calculator
Configure Your Pack
Pack Results
84.0V
Full Charge
S × Vmax
72.0V
Nominal
S × Vnom
60.0V
Cutoff
S × Vmin
100Ah
Total Capacity
P × Cell Ah
7,200Wh
Energy
V × Ah
20
Total Cells
S × P
21
Balance Pins
S + 1
84.0V
Charger Voltage
Required
Current & Wire Sizing
Step 3
Choosing the Right BMS
The BMS (Battery Management System) protects your pack from overcharge, over-discharge, overcurrent, short circuit, and temperature extremes. For a 20S pack you need a BMS rated for exactly 20S at 72V nominal. The BMS continuous current rating must exceed your controller's peak draw.
BMS current rating: Must exceed your controller's peak current draw plus 20% safety margin. A 5000W motor at 72V draws ~70A — use a minimum 100A BMS. For 10000W builds, use 150A+.
JBD Smart BMS
Most popular — Bluetooth app monitoring
20S60-200ABluetoothApp: Xiaoxiang
Real-time cell monitoring via phone app. Shows individual group voltages, temperature, SOC%. Excellent for diagnostics. Available in 60A, 100A, 150A, 200A versions.
DALY Smart BMS
Reliable — Good for high current
20S80-500ABluetoothApp: DALY
Excellent for high current applications. Available up to 500A continuous. Separate charge and discharge ports. Good short circuit protection response time.
ANT BMS
Premium — Active balancing option
20S100-300ABluetoothActive Balance
Premium option with active cell balancing — transfers energy between cells rather than just burning off the excess. Best for packs that see heavy use and deep cycling.
JBD BMS — Wiring & Settings
| Terminal | Connection | Notes |
|---|---|---|
| B+ | Cell stack positive | Direct from Cell 20 + |
| B− | Cell stack negative | Direct from Cell 1 − |
| P+ | Load positive output | To fuse → controller |
| P− | Load negative output | To controller negative |
| C+ | Charge positive input | From 84V charger + |
| C− | Charge negative input | From 84V charger − |
| BAL | Balance connector | 21-pin JST-XH from cells |
| NTC | Temperature sensor | 10kΩ @ 25°C, on cells |
JBD App Settings for 20S NCM:
OVP: 4.20V per cell
UVCP: 3.00V per cell
OCP: Match BMS rating
Balance start: 3.90V
Balance diff: 0.02V
OVP: 4.20V per cell
UVCP: 3.00V per cell
OCP: Match BMS rating
Balance start: 3.90V
Balance diff: 0.02V
NEVER load from B+/B− — always use P+/P−. The B terminals bypass all protection.
Step 4
Complete Pack Wiring Diagram
B+ / B− (cell side — never load directly)
P+ / P− (protected discharge output)
C+ / C− (charge input)
Balance connector (JST-XH 21-pin)
NTC temperature sensor
Always fuse the B+ positive wire as close to the cell stack as possible — before any switch or BMS connection. Use an ANL fuse rated 1.5× your maximum expected current at 84V+.
Step 5
Balance Connector — 21-Pin JST-XH
A 20S pack needs exactly 21 balance wires — one at every cell junction plus both ends. The diagram below shows where every wire connects. Click any cell or pin to highlight it.
Pin Assignment
Click any pin to see its measurement point. Each adjacent pair (Pin N to Pin N+1) should read 3.60V nominal for a healthy cell.
How to Measure Each Cell Group
1
Set multimeter to DC voltage, 20V range
Use the balance connector — DO NOT use B+ and B− for cell group measurements.
2
Black probe on Pin N, Red on Pin N+1
Example: Black on Pin 1, Red on Pin 2 = Cell Group 1 voltage. Should read 3.50–4.20V.
3
Record all 20 readings
All groups should be within 0.05V of each other at rest. Any group 0.10V+ below others = weak cell.
4
After full charge verification
All groups should read 4.15–4.20V at full charge. Groups not reaching 4.15V have reduced capacity.
Balance Connector Wiring — Cell by Cell
| Pin # | Connection Point | Expected Voltage vs Pin 1 | Status Check |
|---|---|---|---|
| Pin 1 | B− (Cell 1 negative) | 0.00V (reference) | Ground reference |
| Pin 2 | Cell 1+/Cell 2− junction | ~3.60V | Cell 1 voltage = Pin2 − Pin1 |
| Pin 3 | Cell 2+/Cell 3− junction | ~7.20V | Cell 2 voltage = Pin3 − Pin2 |
| ... (cont) | Each cell junction | +3.60V per pin | Each pin = cumulative voltage |
| Pin 21 | B+ (Cell 20 positive) | ~72.0V | Full pack voltage vs Pin 1 |
Step-by-Step Build
Complete Build Procedure
READ BEFORE STARTING: Lithium cells at full voltage contain enormous energy. A short circuit can cause fire, explosion, or severe burns. Work on an insulated mat. Remove jewellery and metal watches. Keep a Class D or CO₂ fire extinguisher nearby. Never work alone on a live pack.
1
Grade and Sort All 20 Cells
Test every cell: voltage, capacity, and internal resistance. Group cells by matched IR. All cells must be within 0.02V of each other before assembly begins. Label each cell 1–20.
⚠️ Never skip grading — mismatched cells will create imbalance that degrades the pack within months.
2
Top-Balance All Cells to 4.20V
Using a bench power supply, charge each individual cell to exactly 4.20V at 0.1C (e.g. 10A for a 100Ah cell). This is the most critical step — all cells must start at the same voltage for proper BMS balancing throughout the pack's life.
⚠️ Top-balancing must be done on individual cells BEFORE series assembly, not on the assembled pack.
3
Prepare Cell Housing / Enclosure
Build or source a suitable enclosure — aluminum extrusion, steel, or ABS. Cells must be clamped with compression plates and threaded rods (5–10 PSI). Do NOT overtighten — prismatic cells need controlled compression, not crushing. Line interior with foam/fish paper insulation.
4
Insulate Between Cells
Place 0.2mm fish paper (Nomex) between each cell face. This prevents cell-to-cell shorts if a case is compromised. Cut fish paper to exactly cell face dimensions. Also insulate both end plates from the terminal cells.
5
Connect Cells in Series — Busbars
Use copper or nickel-plated copper busbars. For 100A+ packs use 2–3mm thick copper busbars. Torque terminal bolts to manufacturer spec (typically 4–6Nm for M6 terminals). Use conductive paste on terminals to prevent oxidation. Connect + of Cell 1 to − of Cell 2, etc.
⚠️ At this point the pack is LIVE with 84V potential. Work with insulated tools. Never bridge B+ and B− — even a momentary short will weld the busbar and potentially ignite the cells.
6
Verify Series Voltage at B+/B−
Measure the total pack voltage at the B+ and B− terminals. Should read approximately 20 × 4.20V = 84.0V ± 0.2V. If the reading is wrong, recheck your series connections — one reversed cell will show a lower voltage.
7
Install Balance Wires (JST-XH 21-pin)
Solder a wire to each cell junction point (the busbar connecting adjacent cells). Pin 1 = B− (Cell 1 negative). Each subsequent pin connects to the positive terminal of each cell going up. Pin 21 = B+ (Cell 20 positive). Use 22–24 AWG wire for balance wires — they carry only milliamps.
⚠️ Double-check polarity on every balance pin before connecting to BMS. Reversed pins will destroy the BMS balance IC instantly.
8
Install NTC Temperature Sensor
Mount the NTC thermistor (10kΩ at 25°C) directly against the center cells in the pack — the hottest location. Secure with thermal paste and kapton tape. Route the sensor wire to the BMS NTC connector. This enables over-temperature protection.
9
Connect BMS
Connect B+ and B− from cell stack to BMS. Connect the 21-pin balance connector. Connect NTC. Do NOT connect the load (P+/P−) yet. Power up the BMS — check the app shows all 20 cell voltages correctly and all reading 4.15–4.20V.
⚠️ If any cell reads 0V or incorrect voltage in the app, disconnect immediately and recheck balance wiring before proceeding.
10
Install Main Fuse on B+
Install an ANL or MIDI fuse on the B+ wire as close to Cell 20 positive terminal as possible — before any connection to the BMS or load. Rate the fuse at 1.5× maximum current. For a 100A BMS build, use a 150A fuse. Must be rated for 100V+.
11
Connect Load (Controller) to P+/P−
Connect controller to P+ and P− output of BMS — NOT to B+ and B−. Verify polarity before connecting. Use appropriately rated cable (10 AWG for 100A, 8 AWG for 150A+, 6 AWG for 200A+). Add a pre-charge resistor (50Ω, 25W) to limit inrush current on startup.
12
First Charge Test
Connect the 84.0V charger to C+ and C−. Verify charger output is exactly 84.0V before connecting. Monitor the BMS app during charge — all cells should rise together. After full charge, verify all 20 groups read 4.15–4.20V and spread is under 0.05V.
13
Seal and Weatherproof
Seal all enclosure penetrations with self-amalgamating tape or marine sealant. Route cables through grommets. Apply corrosion inhibitor to terminals. Label B+, B−, P+, P−, C+ and C− externally. Add a voltage display (optional but recommended).
What You Need
Tools & Materials Checklist
Essential Tools
Bench Power Supply
0–100V, 10A+ for top-balancing individual cells
REQUIRED
Calibrated Multimeter
4.5+ digit accuracy — e.g. UNI-T UT61E
REQUIRED
Internal Resistance Tester
YR1035+ for cell matching
REQUIRED
Torque Wrench
For terminal bolts — 4–6Nm range
REQUIRED
Soldering Iron
60W+ temperature controlled — for balance wires
REQUIRED
Wire Crimper / Stripper
For AWG 6–24 cable work
REQUIRED
Smartphone + BMS App
Xiaoxiang (JBD) / DALY / ANT app
REQUIRED
Insulated Tools Set
1000V rated — never use uninsulated tools on live pack
REQUIRED
Thermal Camera (optional)
Detect hot spots on first charge cycles
RECOMMENDED
Materials List — 20S NCM Prismatic Pack
| Item | Spec | Quantity | Notes |
|---|---|---|---|
| NCM Prismatic Cells | 3.6V, your chosen Ah | 20 cells | Same batch, matched IR |
| BMS (20S) | 72V, 100A+ | 1 unit | JBD / DALY / ANT |
| Balance Connector | JST-XH 21-pin | 1 harness | With 21 wires pre-crimped |
| Copper Busbars | 2–3mm thick, cell width | 20 pairs | Nickel plated preferred |
| Main Cable (B+/B−) | 8 AWG silicone, 84V+ | 0.5m each | Short as possible to BMS |
| Output Cable (P+/P−) | 8–6 AWG silicone | As needed | To controller/fuse |
| ANL Fuse + Holder | 150A, 100V rated | 1 unit | On B+ close to cells |
| Balance Wire | 22 AWG, silicone | ~5m | For 21 balance wires |
| Fish Paper (Nomex) | 0.2mm, cell-sized sheets | 22 sheets | Between every cell + end plates |
| NTC Thermistor | 10kΩ @ 25°C | 1–2 sensors | Mount on center cells |
| Pre-charge Resistor | 50Ω, 25W | 1 unit | For controller inrush |
| Compression Hardware | M8 threaded rod + end plates | 4 rods | 5–10 PSI on cell stack |
| Charger | 84.0V CC/CV (20S) | 1 unit | Verify output before use |
| Kapton Tape | 25mm wide | 1 roll | Insulation and securing sensors |
| Thermal Paste | Non-conductive type | 1 tube | For NTC sensor mount |
| Anderson / XT90 Connectors | 100A+ rated | 1 pair | For P+ output connection |
Critical Information
Safety Rules & Emergency Procedures
THERMAL RUNAWAY: If a lithium cell begins venting (hissing sound, unusual heat, acrid smell, or smoke) — DO NOT attempt to extinguish with water. Move the pack outdoors immediately. Use a CO₂ extinguisher or submerge in a bucket of sand. Call fire services. Thermal runaway is self-sustaining and cannot be stopped once started.
Before You Start
!
Remove ALL jewellery
Rings, bracelets, watches — even a thin gold ring bridging a terminal can weld instantly and cause severe burns.
!
Insulated mat mandatory
Work on a rubber or silicone insulated surface. Never on metal workbenches.
!
Fire extinguisher nearby
CO₂ or Class D extinguisher within arm's reach at all times when working with assembled pack.
!
Never work alone
Have another person present when working on live packs above 48V. Medical emergency can occur from arc flash or electrocution.
Operating Limits — NCM 20S
| Parameter | Limit | Consequence if exceeded |
|---|---|---|
| Max charge voltage | 84.0V (4.20V/cell) | Electrolyte decomposition, fire risk |
| Min discharge voltage | 60V (3.00V/cell) | Permanent capacity loss |
| Absolute min voltage | 50V (2.50V/cell) | Cell death, copper dissolution |
| Max cell temperature | 60°C discharge | Accelerated degradation |
| Min charge temperature | 0°C | Lithium plating — permanent damage |
| Max charge current | 0.5–1C recommended | Heat, reduced cycle life |
| Storage voltage (ideal) | 75V (~50% SOC) | Best for long-term storage |
Common Build Mistakes
Loading from B+/B− instead of P+/P− — bypasses all BMS protection
Using a charger with wrong voltage — 72V charger on 20S pack will undercharge; 96V charger will destroy pack
Reversed balance connector pin — destroys BMS balance IC immediately
Skipping top-balance — cells will diverge rapidly and reduce pack life
Always verify charger output with multimeter before first connection
Check BMS app shows all 20 cell voltages before connecting load
Label every terminal — B+, B−, P+, P−, C+, C−
Store at 50% SOC (~75V) if not using for more than 2 weeks
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