Batteries and the BMS · Hardware Academy

From one cell to a 400 V pack

Every EV battery is a large number of small electrochemical cells. During discharge, lithium ions leave the graphite anode, cross a liquid electrolyte through a porous separator and slot into the metal-oxide cathode, while electrons take the long way round through the inverter and motor. A nickel-rich lithium-ion cell swings from about 3.0 V empty to 4.2 V full, with 3.6-3.7 V as its nominal value. Those limits are hard walls: above 4.2 V the cathode and electrolyte degrade quickly, and deep discharge below the minimum costs capacity for good. A pack's usable range ends when its weakest cell group touches either wall.

Chemistry and format

Cathode	Nominal	Cell energy	Strengths	Trade-offs
NCA (nickel cobalt aluminium)	3.6 V	about 250 Wh/kg	High energy; Tesla's Panasonic cylindrical cells	Less thermally stable; uses cobalt
NMC (nickel manganese cobalt)	3.6-3.7 V	about 250 Wh/kg	High energy; made by LG, CATL, Samsung SDI, SK On	Same safety and cobalt concerns as NCA
LFP (lithium iron phosphate)	3.2 V	roughly 160-200 Wh/kg	Long cycle life, very stable, no nickel or cobalt, cheaper	Heavier for the range; flat voltage curve makes SoC hard to estimate

Cylindrical cells are named by size. The 18650 (18 mm × 65 mm) powered the 2008 Roadster and the Model S; the 2170 (21 × 70 mm) arrived with the Model 3; the 4680 (46 × 80 mm), announced at Tesla's Battery Day in September 2020, has about 5.5 times the volume of a 2170 and a tabless electrode that shortens the path for current and heat. Bigger cells mean fewer cans, welds and sense wires per kWh but a longer path for heat to leave the core. Prismatic and pouch cells, common for LFP and NMC, pack without the gaps between cylinders but rely on the module or pack to hold them under pressure.

Series, parallel, modules, pack

Cells wired in parallel form a group that shares one voltage and adds capacity; groups wired in series add voltage. The notation 96s46p means 96 groups of 46 cells, with V = s × V_cell, Ah = p × Ah_cell and energy V × Ah. The original Model S 85 pack holds 7,104 18650 cells in 16 modules; each module is 6 groups of 74 cells, so the pack is 96s74p, about 346 V nominal and 403 V full. Every group carries the same current, so the pack is empty when its lowest group reaches 3.0 V and full when its highest reaches 4.2 V. One weak group costs the whole pack range.

Cell, module, pack: the practice pack groups 46 cells in parallel, six groups per module and 16 modules in series (96s46p).

What the BMS does

Measures every group's voltage to a few millivolts, temperatures at many points, and pack current through a shunt or Hall sensor.
Estimates state of charge by coulomb counting (integrating current) and corrects the drift with open-circuit-voltage readings after the pack rests; production BMSs usually blend the two in a Kalman filter.
Tracks state of health: capacity fade and resistance growth over the pack's life.
Balances the groups. Passive balancing switches a resistor across the highest groups and burns their excess as heat, typically at tens to a couple of hundred milliamps.
Drives the contactors: a precharge relay and resistor charge the inverter's DC-link capacitors first, then the main positive and negative contactors close without an inrush that could weld them.
Monitors isolation between the high-voltage bus and the chassis. FMVSS 305 and UN ECE R100 ask for at least 100 Ω per volt on a DC bus, about 40 kΩ for a 400 V pack.
Sets limits: allowed charge and discharge power versus temperature, state of charge and health, sent continuously to the inverter and charger, and it opens the contactors on any fault.

Heat, cooling and thermal runaway

Cells are happiest around 15-35 °C. Tesla's cylindrical packs pump a water-glycol mix through flat, wavy cooling ribbons that snake between rows of cells and touch each can; prismatic and pouch packs usually sit on cold plates. The same loop heats the pack in winter. Abuse, an internal short or a crash can start thermal runaway: the protective film on the anode breaks down from roughly 80-120 °C, the separator shrinks and melts at around 130-170 °C and shorts the electrodes, and nickel-rich cathodes release oxygen at higher temperatures still, feeding a fire that spreads to neighbouring cells. Packs fight propagation with per-cell fuses (Tesla bonds each cell with a thin wire that opens like a fuse), spacing and mica barriers, vent paths that steer hot gas away from the cabin, and BMS alarms. LFP releases far less oxygen and heat, one reason it is chosen for standard-range and stationary packs.

Charging: C-rate, CC-CV and taper

C-rate is current relative to capacity: 1C would fill or empty the pack in about an hour, so for a 76 kWh pack 1C is about 76 kW and a 250 kW charge is about 3.3C. Lithium-ion cells charge CC-CV: constant current until a cell reaches its ceiling of about 4.2 V, then constant voltage while the current decays to a cutoff such as C/20. Fast-charging curves taper long before the constant-voltage phase because the BMS lowers current as the graphite anode fills. Its potential approaches that of lithium metal, and pushing harder would plate lithium on its surface instead of inserting it, a permanent capacity loss that can grow dendrites. Cold makes plating far more likely, so a cold pack may accept only a fraction of its warm power. That is why cars precondition, heating the pack on the way to a fast charger, and why 10-80 % is the sweet spot of a road trip.

Structural packs

Modules add mass and cost, so newer packs drop them. Cell-to-pack designs such as CATL's CTP and BYD's Blade fill the enclosure with cells directly. Structural packs go further: Tesla's 4680 pack for the Model Y bonds the cells into a pack whose lid is the cabin floor, so the battery becomes part of the body. The gains are mass, cost and stiffness; the price is repairability, because one bad cell can no longer be swapped with its module.

Interactive: build a pack, then charge it

Start from the practice pack, then load the Model S layout and the 800 V rearrangement. Zoom to the module to see how parallel cells and alternating bus bars form series groups.

Battery pack explorer

Zoom from one cell to a whole pack. Series groups (s) add voltage; cells in parallel (p) add amp-hours. Energy is their product.

Cells: 4,416
Pack voltage: 345.6 V
Window: 288-403 V
Capacity: 220.8 Ah
Energy: 76.3 kWh
Cell mass: 300 kg

Zoom

Cell

Series groups (s)96sCells in parallel (p)46p

400 V class. A 250 kW charge on this pack is 3.3C. Cell values are representative nickel-rich cells; a production pack's badge uses its own cell rating and usable-energy rules.

Now charge the practice pack. At 25 °C the BMS map limits the current from the start; precondition to 40 °C and the charger becomes the limit until about 20 %. Drop the temperature to 0 °C and watch the 10-80 % time triple.

DC fast-charging curve

A 96s46p pack of 2170 cells (76.3 kWh) charging from 10 %. The colour shows which limit sets the current at each moment. Cool the pack, then precondition it, and compare the 10-80 % time.

Charger power limit
BMS current map
Constant voltage (4.2 V per cell)

State of charge: 10 %
Power: 188 kW
Current: 508 A
Pack voltage: 370 V
C-rate: 2.30C
Elapsed: 0 min 00 s

Now limited by the BMS current map (temperature and state of charge). About 0.0 kWh added so far.

Pack temperature25 °C

Charger

Playback

Teaching model: open-circuit voltage plus resistance per cell, an illustrative BMS map and a constant pack temperature. Real packs warm up while they charge, and every manufacturer's map is different.

In practice

Practice

Size the practice pack

A pack uses 2170 cells rated 4.8 Ah and 3.6 V nominal (3.0-4.2 V), wired 96s46p. Each cell weighs 68 g. Find the cell count, the nominal, full and empty pack voltage, the capacity and energy, the mass of the cells alone, and the C-rate of a 250 kW charge.

text

cells     = 96 × 46               = 4,416
nominal   = 96 × 3.6 V            = 345.6 V
full      = 96 × 4.2 V            = 403.2 V
empty     = 96 × 3.0 V            = 288.0 V
capacity  = 46 × 4.8 Ah           = 220.8 Ah
energy    = 345.6 V × 220.8 Ah    = 76,308 Wh ≈ 76.3 kWh
mass      = 4,416 × 68 g          ≈ 300 kg of cells (before modules, cooling, enclosure)
C-rate    = 250 kW ÷ 76.3 kWh     ≈ 3.3C

sensitivity: one more cell per group adds 96 × 17.28 Wh = 1.66 kWh, so 96s48p is 79.6 kWh

Answer: 4,416 cells; 345.6 V nominal (288-403 V); 220.8 Ah; about 76.3 kWh; about 300 kg of cells; a 250 kW charge is about 3.3C. If a spec sheet quotes a different energy for the same layout, check its cell capacity and nominal voltage before you doubt your arithmetic.

Practice

How long must the balancer run?

One group in a 96s pack has an internal micro-short that leaks 25 mA. A pack-wide balancer bleeds every other group down to it at 100 mA, with cells around 3.7 V. How long must each healthy group bleed per day, and how much energy does the pack waste?

text

leak per day   = 0.025 A × 24 h          = 0.6 Ah
bleed time     = 0.6 Ah ÷ 0.1 A          = 6 h per day, every healthy group
energy wasted  = 95 × 0.6 Ah × 3.7 V     ≈ 211 Wh per day
per year       = 211 Wh × 365            ≈ 77 kWh, about one full pack

Answer: about 6 hours of bleeding a day and about 211 Wh a day, roughly a whole pack's energy every year burned as heat to hide one leaking cell. The scenario below is this failure seen from the fleet dashboard.

Real-world scenario

Module 7 balances every night

A last-mile delivery fleet runs 40 identical electric vans. Each 96s pack is built from 16 modules, and each module has its own monitor board that balances its six series groups toward the lowest one. The fleet dashboard flags van 23: module 7's balancing resistors run six to eight hours every night, while every other module in the fleet balances for a few minutes a week. The van's usable range has dropped about 6 % in three months.

Symptoms

Module 7's board logs hours of bleeding on groups 1-3 and 5-6 every night; group 4 is never bled.
With balancing disabled for a 48-hour park, group 4 falls about 4 mV below its neighbours; every other group in the pack stays within 1 mV.
Discharge ends when module 7 reaches the low-voltage limit, while the other modules still hold about 5 % charge.
Module 7's board runs about 6 °C warmer than the others overnight. Cell temperatures and charging currents look normal.

Interview questions

screenHow do you get pack voltage, capacity and energy from a series-parallel layout?

Series groups add voltage and parallel cells add capacity, so pack voltage is s times the cell voltage, capacity is p times the cell amp-hours, and energy is their product. For 96s46p with 4.8 Ah, 3.6 V cells that is 345.6 V, 220.8 Ah and about 76.3 kWh. Full charge is 96 × 4.2 = 403 V and empty is about 96 × 3.0 = 288 V, so the inverter and charger must work across that whole window. The same 4,416 cells wired 192s23p give the same energy at twice the voltage and half the current, which is the 800 V trade.

screenWhat does a battery management system monitor and control?

It measures every series group's voltage to a few millivolts, temperatures across the pack and the pack current. From those it estimates state of charge and state of health and continuously publishes charge and discharge power limits to the inverter and charger. It balances the groups, sequences the precharge and main contactors, monitors insulation resistance between the high-voltage bus and the chassis, and opens the contactors on overvoltage, undervoltage, overcurrent, overtemperature, an isolation fault or a crash signal. It also asks the thermal system to heat or cool the pack.

deepWhy does DC fast charging taper, and what does preconditioning change?

Charging is constant current until a cell reaches its voltage ceiling, about 4.2 V for nickel-rich cells, then constant voltage while the current decays, so the last 10-20 % is always slow. Well before that, the BMS lowers current as state of charge rises because the graphite anode is filling and its potential approaches that of lithium metal; pushed harder, lithium plates on the surface instead of inserting, which permanently costs capacity and can grow dendrites. Cold makes this much worse because resistance and diffusion times rise steeply, so a cold pack may be held to a fraction of its warm power. Preconditioning heats the pack on the way to the charger so the BMS allows high current from the moment you plug in. That is why the useful fast-charging window is roughly 10-80 %.

deepWhy is state of charge harder to estimate on an LFP pack than on an NMC pack, and what do you do about it?

Coulomb counting integrates current but drifts with sensor offset and gain error, so it needs periodic correction from the open-circuit voltage after the pack rests. NMC's open-circuit voltage changes steadily with state of charge, but LFP sits on a plateau near 3.3 V across most of its range, so a few millivolts of error can mean many percent of state of charge. LFP systems therefore lean on the steep ends of the curve: a regular charge to 100 % recalibrates the estimate, which is why some makers recommend frequent full charges for LFP. Model-based estimators such as Kalman filters, with temperature and hysteresis models, carry the estimate in between.

screenWhat is the difference between passive and active balancing, and which do EV packs use?

Passive balancing switches a resistor across the highest groups and burns their excess charge as heat until they match the lowest, typically at tens to a couple of hundred milliamps. It is cheap and simple but only removes charge, so it cannot recover capacity from a weak group. Active balancing moves charge from high groups to low ones with inductors, capacitors or small DC-DC converters, which saves energy but costs parts, board area and complexity. Most EV packs use passive balancing because well-matched cells only need small, occasional corrections.

deepHow do you keep one cell's thermal runaway from destroying the whole pack?

Start at the cell: a vent and current-interrupt device in the cap, and per-cell fusing such as the thin wire bonds Tesla uses, which open if one cell shorts. Then slow heat transfer between cells with spacing, mica or aerogel barriers and coolant, and give hot gas a defined vent path out of the pack and away from the cabin. The BMS watches for early signs such as a sudden voltage drop or temperature rise, opens the contactors and warns the occupants. Chemistry is the last lever: LFP releases far less oxygen and heat than NCA or NMC, which is one reason it is chosen for standard-range and stationary packs.

Quiz

References and further reading

Where the ideas in this lesson come from, and what to read next. Books are the standard texts; papers are the original sources; standards are what employers work to.

Book
Gregory L. Plett (2015). Battery Management Systems, Volume I: Battery Modeling. Artech House.
Read: The cell modelling chapters; Volume II for state-of-charge estimation.
Book
Thomas B. Reddy (editor) (2011). Linden's Handbook of Batteries. McGraw-Hill, 4th edition.
Read: The lithium-ion chapters.
Paper
John B. Goodenough and Kyu-Sung Park (2013). The Li-Ion Rechargeable Battery: A Perspective. Journal of the American Chemical Society 135(4), 1167-1176.doi:10.1021/ja3091438
Read: The perspective paper for the chemistry in plain terms.
Paper
K. Mizushima, P. C. Jones, P. J. Wiseman and J. B. Goodenough (1980). LixCoO2 (0 < x ≤ 1): A New Cathode Material for Batteries of High Energy Density. Materials Research Bulletin 15(6), 783-789.doi:10.1016/0025-5408(80)90012-4
Read: The LiCoO2 paper.
Paper
A. K. Padhi, K. S. Nanjundaswamy and J. B. Goodenough (1997). Phospho-olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries. Journal of The Electrochemical Society 144(4), 1188-1194.doi:10.1149/1.1837571
Read: The LFP paper.