Field Diagnostics: Symptom to Cause
Eleven symptoms mapped to their likely causes and first checks — and why about half of returned packs have nothing wrong with them.
Battery Fundamentals · Part 6 — Safety, Standards and Field Work · Chapter 16 · 12 min read
About half of returned packs have no fault. They have a charger problem, a connector problem or a usage problem — and the only thing that separates those from a genuine failure is data recorded before anyone started taking things apart.
4
Readings before you touch anything
3
Tools that find most faults
7 days
Rest test for a suspect cell
~50 %
Returns with no fault found
16.1 — Diagnostic discipline comes first
Important
Before touching anything, record four things: individual cell voltages, pack voltage, temperature, and the BMS fault log. Opening a pack, cycling it, or even letting it rest changes the evidence. Half of these measurements cannot be recovered once the pack has been disturbed.
This is not bureaucracy. It is the difference between diagnosing a fault and guessing at one, and in a warranty dispute it is the difference between a record and an opinion.
In plain English
The single most useful habit a service network can build: photograph the BMS app screen showing cell voltages before disconnecting anything. It takes four seconds and it is the evidence you will wish you had.
16.2 — Symptom to cause
Work left to right. The first checks are the cheap ones.
| Symptom | Likely causes | First checks |
|---|---|---|
| Range dropped suddenly | Cell imbalance from never reaching full charge; one weak or shorted cell; BMS cutting on a single low cell | Read individual cell voltages at both top and bottom of charge. Look at the cell delta, not the pack voltage. |
| Range dropped gradually | Normal capacity fade; heat exposure; habitually deep cycling | Capacity test, compared against cycle count and SOH. Check where the pack lives and when it charges. |
| Cuts out under load, recovers at rest | High internal resistance; loose connection; weak cell; cold | Measure voltage sag under load — V_load = OCV − (I × R). Thermal-image the busbars while loaded. |
| Will not charge at all | BMS UVP latch; cold lockout; charger fault; blown fuse; MOSFET failure | Compare pack voltage at the terminals against at the BMS output. Check pack temperature and charger output voltage. |
| Charges but shows full immediately | One cell hitting OVP early — severe imbalance | Watch cell voltages during charge and look for one racing ahead of the rest. |
| Very slow charging | Under-rated charger; BMS in balancing or CV taper; high resistance in the charge path | Measure actual charge current at the pack, and charger output voltage under load. |
| Gets hot in normal use | Undersized cable; bad joint; failing cell; mounting or ambient problem | Thermal camera under load. Measure joint resistances — anything above ~50 µΩ on a busbar. |
| One cell always lowest | Defective cell with self-discharge; loose sense wire; bad weld on that group | Rest test: charge, disconnect, measure that cell’s OCV over 7 days. Reseat the sense wire first — it is free. |
| Swollen cell or pack | Overcharge, moisture ingress, heat, or end of life | Isolate immediately, outdoors. Do not charge. Do not puncture. |
| Intermittent faults over bumps | Vibration fatigue on welds, connectors or sense wires | Wiggle test with live cell-voltage monitoring. Inspect welds and strain relief. |
| Good in summer, poor in winter | Normal cold capacity loss — or cold-charge damage if it never recovers | Compare warm and cold capacity. Then check the charge-event temperature logs. |
Important
The last row is the one to take seriously. Reduced winter range that fully returns in spring is normal physics. Reduced winter range that does not return is lithium plating from cold charging, which is permanent, cumulative and a dendrite risk — see Chapter 6. Check the charge temperature logs before concluding it is ordinary ageing.
16.3 — The three tools that find most faults
- 1The BMS app or diagnostic port. Individual cell voltages and the fault log. Most diagnoses start and end here, and the cell delta is the single most informative number in the pack.
- 2A milliohm meter. Joint resistances and cell ACIR. Finds bad welds, loose terminals and aged cells — the causes behind most voltage sag complaints.
- 3A thermal camera. Under load, it shows you exactly which joint or cell is dissipating power. Nothing else finds a bad connection this fast, and it also shows the pack gradient that predicts future imbalance.
Technical framing
Between them a milliohm meter and a thermal camera diagnose most field failures, because most field failures are resistance problems rather than chemistry problems. They cost less than a handful of warranty replacements.
16.4 — The packs with nothing wrong with them
Roughly half of returns are not battery failures. The common ones, in order of frequency:
- •A charger problem. Wrong charger, failing charger, or a lead-acid charger quietly overcharging and floating an LFP pack — see Chapter 13. Logging the charger serial number against the pack makes this a two-minute check.
- •A connector problem. A loose main terminal or a displaced sense wire looks exactly like a dead cell to a BMS, and exactly like a dead pack to a rider.
- •A usage pattern problem. A fleet that never charges past 80 per cent has a pack that has never balanced. A vehicle carrying twice its rated load has a range complaint that is not about the battery.
- •An expectation problem. Range quoted optimistically at the point of sale returns as a fault report six weeks later. This one is fixed before delivery, not after.
In plain English
Keep a bench log of every returned pack: cell voltages, internal resistance, fault codes, charger serial, and what it turned out to be. Within a year you will have a failure-mode database specific to your product, your suppliers and your customers — which is worth considerably more than any general guide, this one included.
Quick check: test yourself
1.A rider reports the pack “dies on hills but is fine on the flat”. Where do you look?
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2.One cell reads 0.15 V below the others every time. What is the cheapest thing to check first?
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3.A customer’s range fell sharply over winter and has not recovered by April. Is this normal cold behaviour?
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4.Why record cell voltages before opening a returned pack?
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Chapter summary
- ✓Record cell voltages, pack voltage, temperature and the BMS fault log before touching anything. Disturbing the pack destroys evidence you cannot recover.
- ✓Cell delta is the most informative number in a pack. Pack voltage tells you almost nothing on its own.
- ✓A pack that sags under load and recovers at rest has a resistance problem — aged cells, a bad joint, or cold — not a capacity problem.
- ✓Reseat the sense wire before condemning a low cell. It is free and it is the most common cause.
- ✓Winter range loss that recovers is physics. Winter range loss that does not recover is lithium plating, and it is permanent and cumulative.
- ✓A BMS app, a milliohm meter and a thermal camera diagnose most field failures, because most field failures are resistance problems.
- ✓About half of returns have no battery fault: a charger, a connector, a usage pattern, or an over-promise at the point of sale.
- ✓Keep a bench log of every return. Within a year it is a failure-mode database specific to your product and suppliers.
Frequently asked questions
What should I record before diagnosing a returned pack?+
Individual cell voltages, pack voltage, temperature and the BMS fault log — before touching anything. Opening, cycling or even resting a pack changes the evidence, and an intermittent connection often reseats itself the moment you handle the harness. Photographing the BMS app screen showing cell voltages takes four seconds and is the evidence you will wish you had in a warranty dispute.
The pack works on the flat but dies on hills. Why?+
Resistance, not capacity. Higher current on a gradient exposes a resistance the flat never reveals, following V_load = OCV − (I × R_internal). Measure voltage sag under load rather than at rest, then thermal-image the busbars while loaded — a bad crimp or weld will light up. Aged cells and cold produce the same signature, so check pack temperature and cell ACIR as well.
One cell always reads lower than the others. What is the cheapest first check?+
Reseat the sense wire. It costs nothing, and a poorly seated or mis-sequenced sense wire makes a perfectly healthy cell read low — it is the most common assembly and service error there is. If reseating changes nothing, run a rest test: charge the pack, disconnect it, and measure that cell’s OCV over seven days. A cell that drifts down has an internal micro-short and needs replacing.
Range fell over winter and has not come back in spring. Is that normal?+
No. Cold reduces capability temporarily — at 0°C an LFP cell may deliver 70 to 80 per cent of rated capacity with two to three times the internal resistance — and all of it returns as the pack warms. Range that does not come back means lithium plating from charging below 0°C, which permanently removes lithium and grows dendrites toward the separator. Pull the charge-event temperature logs.
What tools does a battery service bench actually need?+
Three. A BMS app or diagnostic port for individual cell voltages and the fault log, where most diagnoses start and end. A milliohm meter for joint resistances and cell ACIR. And a thermal camera, which under load shows exactly which joint or cell is dissipating power. Between them they diagnose most field failures, because most field failures are resistance problems rather than chemistry problems.
What do I do with a swollen cell?+
Isolate it immediately, outdoors. Do not charge it and do not puncture it. Swelling means gas generation from overcharge, moisture ingress, heat or end of life, and the cell is a live hazard in whatever state it is in. Treat it as a disposal problem, not a repair problem.
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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.