Inside the Battery: the BMS and the Cell

How a software-defined protection board and a 100Ah LiFePO4 prismatic cell work together to keep a lithium pack safe, efficient and long-lived.

Battery Fundamentals · Part 1 — Anatomy and Instrumentation · Chapter 1 · 24 min read

All chapters

BMS protectionLiFePO4 cell specCell balancingSafety testing

Every lithium battery pack strapped under an e-rickshaw or stacked in an energy storage cabinet is, underneath the housing, a conversation between two very different pieces of engineering: a chemical energy store, and a digital nervous system that watches over it. This chapter takes both apart and then maps one onto the other.

8–24S

BMS cell count

50–100A

BMS continuous current

100Ah

Cell rated capacity

≥1500

Cell cycles to 80%

1.1 — Why a battery pack is really two products in one

The two halves are the cell — the chemical energy store — and the battery management system (BMS), the board that monitors and protects it.

Neither is safe to use alone. A cell without a BMS has no way to stop itself being overcharged, over-discharged or short-circuited. A BMS without a well-specified cell has nothing reliable to protect.

In plain English

Think of the cell as a fuel tank, and the BMS as the fuel gauge, the safety valve and the ignition cut-off all rolled into one small circuit board. The tank stores the energy; the board decides when it is safe to fill it up or draw it down.

Part A

The brain: the protection board

A software-defined board built around a front-end acquisition chip and an MCU, designed for 8 to 24 strings of LiFePO4 cells at 50–100A continuous current.

1.2 — What a BMS actually does

A BMS stands between the raw cell stack and the outside world. Every time current tries to flow into or out of the pack, it flows through the BMS first. That gives the board veto power: it can open the charge or discharge MOSFETs the instant a cell voltage, current or temperature drifts outside a safe window.

Technical framing

A front-end acquisition IC measures every cell’s voltage while an MCU runs the protection logic, the capacity algorithm and the communication stack. Two MOSFETs switch charge and discharge current, and the board reports state over UART, optional RS485, optional CAN, or Bluetooth.

Table 1.1 — Core electrical parameters

Parameter	Value
Cell specification	8–24 strings of LiFePO4
Interface type	Charge and discharge share the same port
Charging voltage	3.60V × number of strings
Cell voltage range	2.20V – 3.75V
Continuous charge / discharge current	50–100A
Running power consumption	≤20mA
Sleep power consumption	≤500µA (GPS off)
Circuit resistance	≤10mΩ
Operating temperature	−20°C to 75°C
Board dimensions (L×W×H)	134±2mm × 100±1mm × 12±2mm

In plain English

The board is a base model with add-on options, like a car trim level. The core job — protecting up to 24 cells, balancing them, logging history — is always there. Bluetooth, GPS, CAN bus and an LCD screen are optional extras.

1.3 — The four pillars of protection

Every BMS protection scheme is built on four pillars: voltage, current, temperature and balance. All four are applied at the individual-cell level as well as the whole-pack level — which matters, because a pack can sit within total-voltage limits while one weak cell is already out of range.

1.3.1 — Voltage protection

Table 1.2 — Per-cell voltage thresholds

Parameter	Min	Typical	Max	Unit
Over-voltage (overcharge) trip	3.700	3.750	3.800	V
Over-voltage delay	1000	2000	3000	ms
Over-voltage release	3.550	3.600	3.650	V
Under-voltage (over-discharge) trip	2.100	2.200	2.300	V
Under-voltage delay	1000	2000	3000	ms
Under-voltage release	2.500	2.600	2.700	V

Figure 1 — Typical per-cell thresholds, from the over-discharge cut-off at 2.20V through the safe operating band to the over-charge cut-off at 3.75V. Note that the cell datasheet caps charging at 3.65V, below the BMS trip point.

In plain English

The BMS watches every single cell, not just the total pack voltage. If any one cell creeps above about 3.75V or drops below about 2.20V and stays there for roughly one to three seconds, the board cuts off charging or discharging to protect that cell — even if the rest of the pack looks fine.

1.3.2 — Current protection — two levels of discharge defence

Rather than a single trip point, the board runs a two-stage discharge over-current scheme: a slower first stage tuned to the pack’s rated current, and a much faster second stage that engages only during severe overload. A brief surge, such as a motor starting on an e-rickshaw, does not nuisance-trip the pack, while a genuine fault is still cleared in milliseconds.

Table 1.3 — Over-current trip points by current tier

Continuous current	Charge OC trip	1st-stage discharge OC	2nd-stage discharge OC	Short-circuit trip
50A	60±5A	60±5A	200±40A	800±160A
60A	70±5A	70±5A	200±40A	800±160A
80A	90±5A	90±5A	300±60A	1200±240A
100A	120±5A	120±5A	400±80A	1600±320A

Figure 2 — Trip current scales with the pack’s rated current across all four protection stages (log scale). The wide separation between working current and the short-circuit trip is what prevents nuisance tripping when a motor starts.

Technical framing

Charge over-current uses a 5–15s delay (typically 10s) with automatic recovery after 32s. First-stage discharge over-current uses the same 5–15s delay; the second stage reacts in 32ms flat, independent of the configured current tier. Short-circuit protection responds in 62µs–1000µs and self-releases roughly five seconds after the load is removed.

1.3.3 — Temperature protection

Cell-surface temperature is read through one built-in and up to four external NTC thermistors, with different limits for charging and discharging, plus a separate higher-temperature cut-off for the MOSFETs themselves.

Table 1.4 — Temperature trip and release points

Parameter	Min	Typical	Max
Charging high-temperature trip	62°C	65°C	68°C
Charging high-temperature release	52°C	55°C	58°C
Charging low-temperature trip	−13°C	−10°C	−7°C
Charging low-temperature release	−8°C	−5°C	−2°C
Discharging high-temperature trip	72°C	75°C	78°C
Discharging high-temperature release	62°C	65°C	68°C
Discharging low-temperature trip	−23°C	−20°C	−17°C
Discharging low-temperature release	−13°C	−10°C	−7°C
FET high-temperature trip	85°C	90°C	95°C
FET high-temperature release	65°C	70°C	75°C

Figure 3 — The charging window is narrower than the discharging window, reflecting the risk of lithium plating when charging a cold LiFePO4 cell. The FET cut-off at 90°C protects the board itself, independent of cell temperature.

In plain English

Charging a cold battery is riskier than discharging one, so the board is stricter about temperature when current flows into the pack than when it flows out. And whatever the cells are doing, if the circuit board itself exceeds 90°C it shuts everything down to protect its own components.

1.3.4 — Passive cell balancing

During charging, once the highest cell reaches the balance turn-on voltage and the spread between highest and lowest exceeds the opening differential, the board bleeds charge off the highest cells through resistors, using a pulsed pattern. Two physically adjacent balance channels never fire at the same time, which limits local heating.

Table 1.5 — Passive balancing parameters

Parameter	Min	Typical	Max
Balance turn-on voltage	3.37V	3.40V	3.43V
Opening differential voltage	15mV	—	—
Balance current	20mA	—	100mA

This is why letting a charge complete occasionally matters: balancing only runs at the top of the charge. Chapter 2 covers the faster bench alternative — active balancing — which moves charge between cells rather than burning it off, and our explainer on cell balancing covers what happens when balancing never gets the chance to run.

1.4 — Capacity, sleep mode and communication

The BMS calculates state of charge by integrating current over time — coulomb counting. A cycle counter increments each time cumulative discharge reaches the configured cycle capacity. Newly commissioned packs need a one-time capacity learning pass — a full charge to over-voltage protection, a full discharge to under-voltage protection, then one more charge — or the state-of-charge reading drifts.

Technical framing

With no communication, no current flow, no active balancing and no over-voltage condition, the board waits five minutes and enters sleep, dropping to ≤500µA. Any communication, the dial switch, or charge/discharge current wakes it again.

•UART (non-isolated, standard) — 9600 baud, no parity, 8 data bits, 1 stop bit; used by the host-computer software for reading and writing parameters.
•RS485 (optional, isolated) — for longer wiring runs in electrically noisy environments.
•CAN bus (optional, isolated) — default 500K baud, for talking to a vehicle controller.

Important

Before changing any protection parameter, the host software must first read the existing parameters from the board. Writing blind can overwrite settings you did not intend to change.

1.5 — Connector and pinout map

J1 (15-pin) / J2 (10-pin)

Cell voltage-sense taps covering cells 1 through 24, wired sequentially from the lowest cell up.

J3 / J4 (4-pin)

NTC temperature probe inputs.

J5 (2-pin)

Alarm driver output (13–15V enable).

J6 (4-pin)

UART, non-isolated — GND / RXD / TXD / 5V.

J7 (2-pin)

Charging current-limitation control (CM-EN, I/O).

J8 (4-pin)

UART / GPS, isolated — includes a switched B+ power pin.

J9 (2-pin)

Charger identification (CH-ID, C−).

J10 (4-pin)

Shared connector: CAN bus (CANL/CANH) or RS485 (A/B), isolated.

J11 (2-pin)

External switch input (K−/K+).

J12 (4-pin)

UART / Bluetooth, non-isolated — includes a 13–15V BVCC pin.

In plain English

J1 and J2 are the measuring tape that lets the board see every individual cell’s voltage. J3 and J4 are its thermometers. J5 through J12 are its mouths and ears — different ways to talk to a charger, a phone app, a vehicle controller or a fleet-tracking platform.

Part B

The muscle: the LiFePO4 cell

A 100Ah, 3.2V nominal prismatic LiFePO4 cell — the energy-storage half of the pack that the BMS exists to protect.

1.6 — The cell on paper

Table 1.6 — Cell headline specification

Item	Specification
Charge voltage	3.65V
Nominal voltage	3.2V
Rated capacity (0.33C to 2.5V @ 25°C)	100Ah
Standard charge current	0.33C
Standard charge method	0.33C constant current to 3.65V, then constant voltage until current declines to ≤0.05C
Maximum charge current	1C (100A)
Maximum discharge current	3C for 30s at 100% SOC, 25°C
Discharge cut-off voltage	2.5V
Internal impedance	≤2.0mΩ
Cell weight	2.2kg
Charging temperature	0°C–45°C
Discharging temperature	−20°C–60°C
Cell dimensions	40.2mm × 130.2mm × 205.0mm (approx., with wrap)

In plain English

This is a large, flat brick of a cell about the size of a hardback book, weighing just over 2kg, storing 100 amp-hours — typically wired in series with 15 or 16 others to build a full pack.

1.7 — Electrical performance and test criteria

Table 1.7 — Electrical test conditions and pass criteria

Test	Condition	Pass criteria
Standard charge	0.33C CC to 3.65V, then CV until ≤0.05C	Charge voltage 3.65V, rate 0.33C
Rapid charge	1C CC to 3.65V, then CV until ≤0.05C	Charge voltage 3.65V, rate 1C
Initial impedance	AC 1kHz within 1h of standard charge	≤2.0mΩ
Cell voltage (shipment)	As shipped	≥3.0V
Rated capacity	0.33C discharge to 2.5V after standard charge	100Ah
High-rate discharge	1C discharge to 2.5V after 0.33C charge	≥95% rated capacity
Cycle life	0.5C / 0.5C charge-discharge, 25°C	≥1500 cycles to 80% of nominal capacity
Low-temperature performance	20h soak at −20°C, then 0.2C discharge to 2.0V	≥60% rated capacity
High-temperature performance	3h soak at 55°C, then 0.33C discharge to 2.5V	≥95% rated capacity
Charge retention / recovery	30-day stand at 25°C after full charge	≥93% retention, ≥95% recovery

Figure 4 — The characteristically flat LiFePO4 plateau, followed by a sharp voltage knee near full discharge. This shape is exactly why the BMS estimates state of charge by counting coulombs rather than reading voltage.

Technical framing

The flat discharge plateau is exactly why coulomb counting is the BMS’s primary state-of-charge method rather than voltage look-up — voltage barely moves across 80% of the discharge curve, so it is a poor proxy for remaining charge until very near empty. Chapter 3 picks this up in detail when it looks at how state of charge is estimated.

Figure 5 — Capacity fade under a 0.5C / 0.5C cycling regimen at 25°C, reaching the 80% end-of-life threshold at the specified minimum of 1,500 cycles. Note that the cell manufacturer’s own retirement guidance is stricter still, at 70% capacity or double the initial internal resistance.

1.7.1 — Charge and discharge rate against temperature

How hard the cell may be worked depends on both temperature and state of charge — a table any charge controller or BMS firmware needs to respect.

Table 1.8 — Permitted charge C-rate by temperature and SOC

Charge C-rate — SOC range	≤0°C	0–10°C	10–20°C	20–30°C	30–45°C	>45°C
0%–70% SOC	0	0.2C	0.5C	0.5C	0.5C	0
70%–100% SOC	0	0.1C	0.33C	0.5C	0.33C	0

Table 1.9 — Permitted discharge C-rate by temperature and SOC

Discharge C-rate — SOC range	<−20°C	−20–0°C	0–20°C	20–40°C	40–50°C	50–60°C	>60°C
0%–30% SOC	0	0.1C	0.2C	0.5C	0.33C	0.2C	0
30%–100% SOC	0	0.2C	0.33C	0.5C	0.5C	0.33C	0

In plain English

Cold batteries do not like fast charging. Below 0°C the cell should not be charged at all, and even in cool conditions the allowed rate is throttled back. This is exactly the kind of rule a well-designed BMS enforces in firmware, rather than leaving it to the charger alone.

1.8 — Safety testing

Five abuse tests every cell design must pass, all requiring no fire, no explosion as the outcome:

Table 1.10 — Abuse test methods and pass criteria

Test	Method	Pass criteria
Overcharge	1C constant current charge to 5V after standard charge + 10min rest	No fire, no explosion
Over-discharge	0.33C constant current discharge to 0V after standard charge + 10min rest	No fire, no explosion
External short circuit	External circuit resistance ≤10mΩ for 10 minutes	No fire, no explosion
Free-fall	1.5m drop onto wooden floor, twice per face	No fire, no explosion
Crush	Crushed perpendicular to plates until 0V or shell rupture	No fire, no explosion

Important

These are laboratory abuse tests performed by trained personnel with proper protective equipment. Improper handling during such tests can cause serious injury, and they should never be attempted outside a professional test lab.

1.9 — Handling, storage and end of life

1.9.1 — Charging and discharging rules

•Never exceed the maximum charge current — doing so degrades performance, mechanical integrity and safety margin.
•3.65V is the absolute maximum charge voltage per cell; charging beyond it is prohibited by design.
•Charging is permitted only between 0°C and 45°C. Below 10°C, charge current must be reduced to no more than 0.2C; below 0°C charging must stop entirely.
•Reverse charging is prohibited — confirm polarity before every connection.
•Discharge is permitted from −20°C to 60°C, with 25°C–35°C as the optimum range.
•Idle cells should be recharged periodically to hold voltage between 3.2V and 3.4V.

1.9.2 — Storage

Table 1.11 — Storage conditions

Scenario	Condition
General storage	−20°C to 45°C, 65%±20%RH
Long-term storage (>3 months)	15°C–35°C, 65%±20%RH, voltage held at 3.2V–3.4V
Warranty-period top-up	Recharge with 10–50A every 3 months if cell voltage falls below 3.2V, until it reaches 3.4V

1.9.3 — Pack design notices

•The enclosure must have sufficient mechanical strength in flame-retardant material, and cells must not be able to move inside the pack.
•Wiring must be sufficiently insulated from the cells to prevent internal short circuit, smoke or fire.
•Different part-number cells must never be mixed within the same battery system.
•Electric-box design must account for heat dissipation and meet the relevant waterproof and dustproof standards.

Technical framing

Impedance and capacity should be tracked across the cell’s service life. A cell should be retired once internal resistance exceeds 200% of its initial value, or capacity fades to 70% of typical capacity — a more conservative cut-off than the 80% / 1500-cycle figure used to define standard cycle life. Chapter 3 formalises both of those numbers as state of health.

Part C

How the two talk to each other

Mapping the BMS thresholds onto the cell ratings, to see how the protection board’s settings sit relative to the cell manufacturer’s own limits.

1.10 — Board thresholds against cell limits

Table 1.12 — Where the BMS trip points sit relative to the cell datasheet

Parameter	Cell datasheet limit	BMS typical threshold	What it means
Max charge voltage	3.65V (absolute max)	3.75V over-voltage trip / 3.60V charge target	The BMS trip sits above the cell’s stated maximum — the pack-level charger should still be configured to stop at 3.65V per cell.
Discharge cut-off	2.5V	2.20V under-voltage trip	The BMS allows a deeper discharge than the cell datasheet recommends.
Max charge current	1C (100A on a 100Ah cell)	Up to 100A continuous / 120±5A trip	The board’s continuous rating matches the cell’s 1C limit; the trip point gives modest headroom.
Max discharge current	3C for 30s (300A) at 100% SOC	400±80A second-stage trip / 1600±320A short-circuit trip	Both discharge trip stages sit above the cell’s rated 3C pulse limit.
Charging temperature window	0°C–45°C	−10°C to 65°C	The BMS window is wider than the cell’s recommended charge range — pack-level control must respect the tighter limit.

Important

This comparison illustrates a general principle, not a certification. A BMS’s factory-default thresholds are a safety backstop, not a substitute for configuring the pack to the cell manufacturer’s actual recommended operating window. The cell manufacturer stops charging below 0°C entirely, while the board’s hardware window extends to −10°C — closing that gap is the pack integrator’s job, not the BMS default’s.

In plain English

The BMS’s built-in thresholds are like the circuit breaker in your house: a last line of defence against a serious fault. They are deliberately looser than the cell manufacturer’s “please treat me this way” recommendations, which is why good pack design also needs charge-controller logic that follows the tighter, temperature- and SOC-aware curves the cell datasheet specifies.

1.11 — Quick-reference glossary

Term	Meaning
BMS	Battery Management System — monitors and protects a battery pack.
LiFePO4	Lithium iron phosphate — a lithium-ion chemistry valued for thermal stability and long cycle life, with a nominal cell voltage around 3.2V.
C-rate	Charge or discharge current expressed as a multiple of rated capacity; 1C on a 100Ah cell is 100A.
SOC	State of charge — the percentage of remaining usable capacity.
NTC	Negative temperature coefficient thermistor — a sensor whose resistance falls as temperature rises.
Passive balancing	Equalising cell voltages by bleeding excess charge from the highest cells through resistors.
Coulomb counting	Estimating state of charge by integrating measured current over time.
MOSFET	The solid-state switch the BMS uses to open or close the charge/discharge path.
Cycle life	The number of full charge/discharge cycles a cell delivers before capacity fades to a defined threshold — 80% here.

Chapter summary

✓A pack is two products in one — a chemical energy store and the protection board that governs it. Neither is safe to deploy without the other.
✓BMS protection rests on four pillars applied per cell, not just per pack: voltage, current, temperature and balance.
✓Discharge over-current protection is deliberately two-stage, so motor inrush does not nuisance-trip a pack that a genuine short still clears in microseconds.
✓The charging temperature window is narrower than the discharging one, because charging a cold lithium cell risks permanent lithium plating.
✓Passive balancing only runs near the top of a charge — which is why occasionally completing a full charge counts as maintenance.
✓A BMS’s factory thresholds are looser than the cell datasheet’s recommended operating window. Closing that gap in charge-controller logic is the pack integrator’s job.

The figures in this chapter are charts constructed from values published in manufacturers’ product specifications, not reproductions of manufacturer-provided plots. This chapter paraphrases and reorganises that technical data for educational purposes.

Frequently asked questions

What does a battery management system actually do?+

A BMS sits between the cell stack and the outside world, so every amp flowing into or out of the pack passes through it. It measures each cell individually and opens the charge or discharge MOSFETs whenever a cell voltage, current or temperature leaves a safe window. It also balances cells, counts coulombs to estimate state of charge, logs history, and reports state over UART, RS485, CAN or Bluetooth.

What voltage should a LiFePO4 cell be charged to?+

A typical LiFePO4 cell has a nominal voltage of 3.2V and an absolute maximum charge voltage of 3.65V. A protection board will usually trip at around 3.75V per cell, but that trip is a safety backstop rather than a target — the charger should still be configured to stop at the cell manufacturer’s 3.65V limit.

Why does a BMS have different temperature limits for charging and discharging?+

Charging a cold lithium cell risks lithium plating, which permanently damages it, so the charging window is deliberately narrower. A typical board allows charging from about -10°C to 65°C but discharging from about -20°C to 75°C. Cell manufacturers are stricter still, often prohibiting charging below 0°C entirely — closing that gap is the pack integrator’s responsibility.

What is passive cell balancing?+

Passive balancing equalises cells by bleeding charge off the highest ones through resistors as heat, typically starting once the highest cell reaches around 3.40V and the spread exceeds about 15mV. It only runs near the top of a charge, which is why occasionally letting a charge complete fully is genuine maintenance rather than just topping up.

Why does a BMS count coulombs instead of reading voltage for state of charge?+

LiFePO4 has a very flat discharge curve — voltage barely moves across roughly 80% of the discharge. That makes voltage a poor proxy for remaining charge until the cell is nearly empty, so the BMS integrates current over time instead. Newly commissioned packs need a full charge, full discharge and recharge so the algorithm learns the true capacity.

How many cycles does a LiFePO4 cell last?+

A quality prismatic LiFePO4 cell is typically specified at 1,500 or more cycles to 80% of nominal capacity under 0.5C charge and discharge at 25°C. Cell manufacturers often set a stricter retirement point for service use — commonly 70% capacity, or internal resistance reaching double its initial value.

Reviewed by

Sahil Goyal

Co-founder, Wingzman

Sourabh Goyal

Co-founder, Wingzman

Battery Fundamentals is an original educational series on lithium battery technology. Threshold tables, cycle-life curves and worked examples use representative values drawn from published product specifications and widely-observed industry patterns, not measured data from a specific product. Always consult the current manufacturer datasheet before making design, purchasing, warranty or certification decisions.