{ "node": { "id": "term:weakest-cell", "type": "Term", "label": "Weakest cell", "properties": { "id": "weakest-cell", "name": "Weakest cell", "aka": [ "limiting cell", "weakest link" ], "section": "topic-state", "definition": "The cell in a series string that reaches a voltage limit first — through lower capacity, higher resistance or a different SOC — and so sets the usable capacity and power of the whole pack. Balancing can correct an SOC offset but not a capacity deficit, so one weak cell among hundreds retires the pack early unless replaced.", "related": [ "cell-to-cell-variation", "cell-balancing", "series-parallel", "state-of-health", "remanufacturing" ], "url": "https://lemonergy.com/learn/glossary/weakest-cell/", "mentions": [ "usable-capacity", "state-of-charge", "string" ] } }, "edges": [ { "id": "edge:6561", "source": "term:weakest-cell", "target": "topic:topic-state", "type": "belongsTo", "properties": { "sourceFile": "terms.json" } }, { "id": "edge:6562", "source": "term:weakest-cell", "target": "term:cell-to-cell-variation", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:6563", "source": "term:weakest-cell", "target": "term:cell-balancing", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:6564", "source": "term:weakest-cell", "target": "term:series-parallel", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:6565", "source": "term:weakest-cell", "target": "term:state-of-health", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:6566", "source": "term:weakest-cell", "target": "term:remanufacturing", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:mentions/weakest-cell/usable-capacity", "source": "term:weakest-cell", "target": "term:usable-capacity", "type": "mentions", "properties": { "sourceFile": "terms.json", "method": "build_glossary.mention_index + mentions", "field": "definition", "meaning": "Text mention; not a causal claim" } }, { "id": "edge:mentions/weakest-cell/state-of-charge", "source": "term:weakest-cell", "target": "term:state-of-charge", "type": "mentions", "properties": { "sourceFile": "terms.json", "method": "build_glossary.mention_index + mentions", "field": "definition", "meaning": "Text mention; not a causal claim" } }, { "id": "edge:mentions/weakest-cell/string", "source": "term:weakest-cell", "target": "term:string", "type": "mentions", "properties": { "sourceFile": "terms.json", "method": "build_glossary.mention_index + mentions", "field": "definition", "meaning": "Text mention; not a causal claim" } }, { "id": "edge:6570", "source": "term:weakest-cell", "target": "visual:balancing", "type": "illustratedBy", "properties": { "sourceFile": "visual-pages.json" } } ], "neighbors": [ { "id": "topic:topic-state", "type": "Topic", "label": "State estimation", "properties": { "id": "topic-state", "name": "State estimation", "lede": "The quantities a battery management system estimates but cannot measure directly.", "context": "Distinguish internal states from noisy measurement estimates.", "example": "Current-sensor bias accumulates in coulomb counting. Exact model SOC does not establish real estimator accuracy.", "lesson_url": "https://lemonergy.com/learn/fundamentals/capacity-and-c-rate/", "lesson_label": "Open charge balance" } }, { "id": "term:cell-to-cell-variation", "type": "Term", "label": "Cell-to-cell variation", "properties": { "id": "cell-to-cell-variation", "name": "Cell-to-cell variation", "section": "topic-state", "definition": "The spread in capacity, resistance and self-discharge between nominally identical cells, from manufacturing and from unequal temperatures in service. It is why packs need balancing and why grading matches cells before assembly.", "related": [ "cell-balancing", "grading", "end-of-line-testing", "thermal-management" ], "url": "https://lemonergy.com/learn/glossary/cell-to-cell-variation/", "mentions": [ "self-discharge" ] } }, { "id": "term:cell-balancing", "type": "Term", "label": "Cell balancing", "properties": { "id": "cell-balancing", "name": "Cell balancing", "section": "topic-state", "definition": "Equalising the state of charge of cells in series, since the pack is limited by its emptiest cell on discharge and its fullest on charge. Passive balancing bleeds charge from high cells through resistors; active balancing moves it between cells.", "related": [ "bms", "series-parallel", "state-of-charge", "cell-to-cell-variation" ], "example": { "title": "Pack average versus cell limits", "text": "Average pack SOC can look acceptable while one series cell reaches its voltage limit. One aggregate SOC state does not demonstrate cell balancing." }, "url": "https://lemonergy.com/learn/glossary/cell-balancing/", "mentions": [ "active-balancing" ] } }, { "id": "term:series-parallel", "type": "Term", "label": "Series and parallel connection", "properties": { "id": "series-parallel", "name": "Series and parallel connection", "aka": [ "xSyP" ], "section": "topic-pack", "definition": "Cells in series add their voltages; cells in parallel add their capacities and share current. A pack is described as, for example, 96s4p: 96 groups in series, each of 4 cells in parallel. Series strings need balancing; parallel groups self-balance through their shared terminals.", "formula": "V = n_s·V_cell, Q = n_p·Q_cell", "related": [ "module", "cell-balancing", "pack-voltage", "volt", "fuse" ], "lesson": "fundamentals/battery-basics/#cell-module-pack", "example": { "title": "Assemble an ideal 4s2p pack", "text": "Eight identical 3.6 V, 5 Ah cells arranged as four series groups with two cells in parallel give 14.4 V nominal, 10 Ah and 144 Wh. Interconnect losses, cell variation and usable voltage limits are additional constraints." }, "url": "https://lemonergy.com/learn/glossary/series-parallel/", "mentions": [] } }, { "id": "term:state-of-health", "type": "Term", "label": "State of health", "properties": { "id": "state-of-health", "name": "State of health", "abbr": "SOH", "section": "topic-state", "definition": "A summary of how far a cell has aged, often expressed through remaining capacity or increased resistance compared with when it was new. By convention a cell has reached end of life at 80 % of its rated capacity.", "unit": "%", "related": [ "capacity-fade", "impedance-rise", "end-of-life", "remaining-useful-life", "state-of-charge", "state-of-certified-energy" ], "lesson": "fundamentals/capacity-and-c-rate/#real-cells", "url": "https://lemonergy.com/learn/glossary/state-of-health/", "mentions": [ "nominal-capacity" ] } }, { "id": "term:remanufacturing", "type": "Term", "label": "Remanufacturing and repair", "properties": { "id": "remanufacturing", "name": "Remanufacturing and repair", "aka": [ "refurbishment", "module replacement" ], "section": "topic-recycling", "definition": "Restoring a pack to its original duty by replacing failed or weak modules, cells, electronics or seals, then retesting it. It keeps the most value of any end-of-life route, and depends on packs that can be opened and on cells that can be matched.", "related": [ "battery-triage", "second-life", "design-for-recycling", "grading" ], "url": "https://lemonergy.com/learn/glossary/remanufacturing/", "mentions": [] } }, { "id": "term:usable-capacity", "type": "Term", "label": "Usable capacity", "properties": { "id": "usable-capacity", "name": "Usable capacity", "aka": [ "available capacity" ], "section": "topic-ratings", "definition": "The part of a cell’s capacity that a system actually lets it use, after the BMS reserves margins at the top and bottom of the state-of-charge window. An electric-car pack may use 90–95 % of its gross capacity.", "related": [ "nominal-capacity", "depth-of-discharge", "state-of-charge", "bms" ], "url": "https://lemonergy.com/learn/glossary/usable-capacity/", "mentions": [] } }, { "id": "term:state-of-charge", "type": "Term", "label": "State of charge", "properties": { "id": "state-of-charge", "name": "State of charge", "abbr": "SOC", "section": "topic-state", "definition": "How full a cell is, as a fraction of its available capacity. State of charge, like temperature, affects how fast a stored cell ages.", "unit": "%", "related": [ "depth-of-discharge", "ocv-soc-curve", "coulomb-counting", "state-of-health", "bms" ], "lesson": "fundamentals/battery-aging/#storage-soc", "example": { "title": "Interpret a charge inventory", "text": "If available capacity is 5 Ah and 3 Ah remains by the chosen estimator, SOC is 60%. A different estimate of available capacity changes this percentage, so compare SOC values only with the capacity convention recorded." }, "url": "https://lemonergy.com/learn/glossary/state-of-charge/", "mentions": [ "usable-capacity" ] } }, { "id": "term:string", "type": "Term", "label": "String", "properties": { "id": "string", "name": "String", "aka": [ "battery string", "series string" ], "section": "topic-pack", "definition": "A chain of cells or modules connected in series to reach the system voltage, the basic electrical unit of a large battery. In stationary storage each rack is usually one string of about 1000–1500 V with its own contactors and BMS, and many strings are paralleled on a common DC bus.", "related": [ "series-parallel", "pack-voltage", "stationary-storage", "parallel-string-current-sharing", "bms-architecture", "contactor" ], "url": "https://lemonergy.com/learn/glossary/string/", "mentions": [ "bms" ] } }, { "id": "visual:balancing", "type": "Visual", "label": "The limiting cell matters", "properties": { "title": "The limiting cell matters", "summary": "A pack average can hide a cell that reaches its voltage limit first.", "readout": "Average: 3.93 V. Highest cell: 4.02 V. The highest cell can reach the charge limit first.", "formula": "V_max = max(V₁, V₂, …); V_average = ΣV_i / N", "note": "The 4.2 V ceiling is an example, not universal. Voltage alone does not uniquely determine SOC.", "software": { "status": "reference", "label": "Reference concept", "body": "Cell supervision uses individual voltages. The native Studio battery is an aggregate model; this chart does not claim it simulates cell balancing.", "href": "https://design.lemonergy.com/cosim.html?example=charging-home" }, "html": "
Four cells before balancing
Four cells before balancingCell number versus Cell voltage (V). Individual voltages; Average; Example ceiling. Interpretation follows the graphs.3.713.851.7542.54.153.254.34Cell numberCell voltage (V)
Individual voltagesAverage (dashed)Example ceiling
" } } ] }