Battery Diagnostics, Health Measurement, and Capacity Testing

Section Overview

The foundations taught what a battery is; this bench measures what one has become, because verdicts leaning on symptoms and labels are guesses wearing aprons (battery-systems-overview-and-lithium-safety-foundations). Voltage retires first. Resting voltage reads state of charge at the curve's edges and nothing through the long flat middle where lithium lives — the healthy number an hour before the dinner-time shutdown (smartphone-battery-replacement). The capacity test measures the tank. A controlled discharge from full to the floor, milliamp-hours counted against the label, fade expressed as the percentage every verdict leans on (controller-connectivity-and-battery-repair). The internal resistance reads the aging. Sag under a known load step — the drop over the step, trended against twins and history — catching the pack that fills its tank but can no longer pour. The fuel gauge testifies as a bookkeeper. Coulomb-counting toward an estimate that drifts on shallow cycles and lies confidently after — checked against measured truth, recalibrated honestly, never allowed to outvote the meter (usb-power-delivery-diagnostics-and-charging-fault-isolation). And the verdicts fall out of numbers. Healthy serves, tired gets the evidence-backed offer, retired goes out taped — with the safety gates running before any test touches a pack. The curve, the tank, the sag, the bookkeeper, the verdicts — the diagnostics bench entire.

Why This Matters

Battery complaints are the queue's vaguest intakes — dies too fast, shuts down early, feels off — and this bench is what turns them into tickets with numbers (smartphone-battery-replacement). This matters because voltage is the industry's favorite wrong answer: a resting number in the flat middle looks like information and is not, and benches that quote off it replace healthy packs and return dying ones (battery-systems-overview-and-lithium-safety-foundations). This matters because capacity and aging are different diseases: a pack can hold its label and still sag into shutdowns, or fade quietly while sagging not at all — the tank and the pour are two measurements, and verdicts need both (controller-connectivity-and-battery-repair). It matters because the gauge's lies are confident: a drifted bookkeeper reports thirty percent at the moment of shutdown, the customer blames the battery, and half the time the honest fix is the gauge's education, not a new pack. It matters because the numbers sell honestly: a replacement offered with measured fade and grown sag attached closes without argument, and a healthy verdict with numbers saves a customer from buying a pack they did not need (usb-power-delivery-diagnostics-and-charging-fault-isolation). And it matters because the safety gates keep the bench a bench: a domed or suspect pack is a containment case before it is a data point, and the foundations' rules run first, every test, every time. Measure the tank, measure the pour, audit the bookkeeper, gate everything — and the vaguest intakes become the cleanest tickets.

Required Prerequisites

• Battery Systems Overview and Lithium Safety Foundations — the chemistry, the law, and the safety gates every test in this section runs behind.
• Smartphone Battery Replacement — the replacement bench these verdicts feed, capacity honesty and all.

Recommended Consumables

• A printed health-log card per ticket — capacity, sag, gauge claim, verdict — to file numbers where callbacks can find them (smartphone-battery-replacement)
• Kapton tape at the bench — to blind the terminals of every pack a verdict retires (battery-systems-overview-and-lithium-safety-foundations)
• Labeled twin-pack references for the families the shop serves most — to give the sag method its comparison (controller-connectivity-and-battery-repair)
• Storage bags at documented charge for packs awaiting verdicts — to keep the queue itself under the foundations' rules

Recommended Practice Hardware

• A USB load tester or smart charger with a discharge mode — to run real capacity tests on packs nobody mourns (usb-power-delivery-diagnostics-and-charging-fault-isolation)
• Three donor packs of mixed age, same family where possible — to see healthy, tired, and retired as numbers instead of adjectives (battery-systems-overview-and-lithium-safety-foundations)
• A multimeter and a known load — to drill the sag method until the arithmetic is a reflex (smartphone-battery-replacement)
• A device with an accessible battery-diagnostics menu — to read a fuel gauge's testimony beside the measured truth (controller-connectivity-and-battery-repair)
• The health-log card and a pen — to practice the record the close depends on

Real-World Applications

The diagnostics bench turns battery arguments into battery arithmetic. A counter facing a phone that dies at thirty percent every afternoon runs the split the numbers allow: capacity measures near the label but the sag under load has doubled against the family reference — a tired pack pouring badly, replaced with the two numbers printed on the quote (smartphone-battery-replacement). A rental fleet's power banks coming due for their quarterly grading cycle through the load tester in batches, each bank's measured capacity logged against its label and last quarter, the fade trend deciding which banks retire before customers find out first (controller-connectivity-and-battery-repair). A bench handed a laptop whose owner arrived asking for a new battery finds capacity at ninety-two percent and sag in family norms — the gauge had drifted on months of shallow desk cycles — and sells a recalibration cycle and an explanation instead of a pack the machine did not need (usb-power-delivery-diagnostics-and-charging-fault-isolation). And a technician intake-checking a tablet pack before any test finds the faint dome the owner had not noticed — the safety gate converts the ticket to containment and replacement before a single test lead touches it (battery-systems-overview-and-lithium-safety-foundations). The failures this prevents: a healthy pack replaced on a drifted gauge's word, a sagging pack returned because its capacity measured fine, a fleet graded by labels instead of trends, and a domed pack wired to a load tester because nobody looked first.

Common Challenges

• The flat middle feels like data. A resting voltage is a number, and numbers feel like measurements — the difficulty is internalizing that this one only speaks at the curve's edges, and building the habit of measuring instead (battery-systems-overview-and-lithium-safety-foundations).
• The two diseases mimic each other at intake. Dies-too-fast describes fade and sag equally — the difficulty is running both measurements every time, because the complaint cannot tell you which one you have (smartphone-battery-replacement).
• The gauge argues back. The device's own screen says thirty percent, and the customer trusts the screen — the difficulty is walking a customer from the bookkeeper's claim to the meter's truth without calling their phone a liar (usb-power-delivery-diagnostics-and-charging-fault-isolation).

Safety Notes

Risk Level: Medium. Deliberate discharge of stored energy under supervision — routine exactly as long as the foundations' gates run first.

Professional Tips Before Starting

• Build the family reference file. Known-good capacity and sag numbers per pack family the shop serves — and one measurement method per file, because an AC-IR meter and the DC load step read honestly different numbers on the same pack, and a file that mixes them manufactures fake trends — verdicts compare, and the file is what they compare against (controller-connectivity-and-battery-repair).
• Log all three numbers even when one decides it. Capacity, sag, gauge claim — every ticket — the trend file is tomorrow's diagnostic (smartphone-battery-replacement).
• Schedule capacity tests like appointments. A discharge run has a start, a duration, and an attended end — the test that outlives the workday is the test that got skipped or got risky (battery-systems-overview-and-lithium-safety-foundations).

The Diagnostics Bench — The Curve, The Tank, The Sag, The Bookkeeper

Recap and Frame

The foundations built the chemistry and the law; this bench builds the instruments on top of them (battery-systems-overview-and-lithium-safety-foundations). The volume's earlier verdicts get their upgrade. The phone bench's capacity honesty and the accessory bench's graded replacements were measurements by proxy — supplier history, callback logs — and this section replaces proxy with meter (smartphone-battery-replacement). The accessory arithmetic carries forward. Small packs get the same three measurements; the verdicts just meet lower ceilings on the way out (controller-connectivity-and-battery-repair). The metering craft transfers whole. The USB-C chapter's discipline — numbers logged, baselines built, anomalies read against normals — is this bench's method with the instruments swapped (usb-power-delivery-diagnostics-and-charging-fault-isolation). What is new is the trio. The capacity test, the sag method, and the gauge audit — three measurements that turn dies-too-fast into a diagnosis. And the frame for what follows: these verdicts feed the replacement bench next, and the charging-circuit diagnostics after that — measured packs first, because everything downstream trusts these numbers. Hold the frame — law beneath, instruments on top, verdicts flowing downstream — and the vaguest complaints in the queue become the best-documented.

The Curve — Why Voltage Lies

The first instrument to master is the one being retired, because knowing why voltage fails is what makes the real measurements make sense (battery-systems-overview-and-lithium-safety-foundations). The discharge curve has a shape. Full sits high and falls fast; empty sits low and falls faster; and between them stretches the long flat middle — most of the pack's usable energy delivered across a few tenths of a volt. The flat middle is the trap. Two packs resting at the same mid-range voltage can hold wildly different remaining energy — one minutes from the cliff, one hours — and the resting number cannot tell them apart, which is why the healthy-looking read precedes the dinner-time shutdown (smartphone-battery-replacement). The edges still speak. A pack resting near its charge limit is genuinely near full; one sagging toward the floor is genuinely near empty — the curve's steep ends carry real information, and the bench uses them for exactly that much (controller-connectivity-and-battery-repair). Temperature bends everything. Cold raises the pack's effective resistance and drops its delivered voltage under load — the winter-morning shutdown at forty percent is the curve and the cold conspiring, not the gauge inventing (usb-power-delivery-diagnostics-and-charging-fault-isolation). Shape, trap, edges, temperature — the curve entire. Resting voltage is a state-of-charge hint at the edges and a coin flip in the middle — health lives in the measurements that follow.

The Tank — The Capacity Test

The capacity question has exactly one honest answer: discharge it and count (smartphone-battery-replacement). The test is a controlled emptying. From verified full, through a load the label sanctions, down to the floor voltage and not a millivolt past it — with the tester counting milliamp-hours the whole way down (battery-systems-overview-and-lithium-safety-foundations). The number reads against the label. Measured over labeled is the fade percentage — the single most load-bearing number in every battery verdict, the one the customer sees on the quote, and the one the trend file tracks quarter over quarter (controller-connectivity-and-battery-repair). The method owns its conditions. The load current, the temperature, the floor used — logged with the result, because a capacity number without its conditions cannot be compared to anything, including itself (usb-power-delivery-diagnostics-and-charging-fault-isolation). The run is scheduled, not abandoned. A discharge has a start, a predictable duration, and an attended end — the safety callout's rule restated as workflow. And the recharge after is part of the test. The pack returns to storage or service charge deliberately, the full cycle noted — because the test itself was a cycle, and the record should say so. Verified full, sanctioned load, counted down, conditions logged, ended attended — the capacity test entire. The tank is measured, never inferred — and the fade percentage is the verdict's spine.

The Sag and the Bookkeeper — Aging and Its Auditor

Capacity misses a whole disease, and the sag method catches it (controller-connectivity-and-battery-repair). Internal resistance is aging made electrical. Every cycle thickens interfaces and dries pathways inside the cell; the pack pours less willingly; and the symptom is voltage sag under load — the pack that holds its tank but drops its pressure (battery-systems-overview-and-lithium-safety-foundations). The sag method measures it with two readings. Terminal voltage at rest, terminal voltage under a known load step, and the arithmetic: the drop divided by the current step, the reading taken at the same moment after the step every time because the sag keeps growing under load and a trend is only honest when its timing is — an internal-resistance estimate good enough to trend, compared against a known-good twin and the family file (smartphone-battery-replacement). The growing number is the story. Absolute values vary by family and meter; the trend does not lie — a pack whose sag doubled since intake is aging out loud, whatever its capacity says. The fuel gauge audits from inside — and needs auditing. The device's bookkeeper counts charge in and out, estimates capacity from what it sees, and drifts when it sees only shallow cycles — reporting thirty percent at the shutdown, confidently wrong (usb-power-delivery-diagnostics-and-charging-fault-isolation). The audit is a comparison. The gauge's capacity claim beside the measured test; the gauge's percentage beside the sag behavior — agreement means a trustworthy bookkeeper, divergence means the gauge is part of the fault picture. And recalibration is bookkeeping repair, honestly framed. A guided full cycle teaches the gauge its pack's present truth — it fixes the ledger, never the chemistry, and it is sold as exactly that. Aging measured, trend read, bookkeeper audited, ledger taught — the sag and the gauge entire. The tank says how much; the sag says how well; the gauge says what the device believes — the bench needs all three testimonies.

The Verdicts — Numbers In, Decisions Out

Three measurements converge on one of three words, and the words carry money and safety both (smartphone-battery-replacement). Healthy serves, with its numbers logged. Capacity near label, sag in family norms, gauge in agreement — the pack returns with its baseline filed, and the customer keeps the money a lesser bench would have taken (usb-power-delivery-diagnostics-and-charging-fault-isolation). Tired gets the offer, with evidence attached. Fade past the family's honest threshold or sag grown past its twin — the replacement quoted with the measured numbers printed, because evidence closes what adjectives argue (controller-connectivity-and-battery-repair). Retired leaves by the law. Below the honest floor, swollen, damaged, dead-flat, or suspect history — taped terminals, the recycling stream, and the foundations' reasons on the ticket (battery-systems-overview-and-lithium-safety-foundations). The gates outrank the verdicts. No pack reaches any test without the inspection — the dome found at intake converts the ticket before the first lead lands, every time. And the close shows the numbers. Capacity, sag, gauge claim, verdict — on the ticket, in the trend file, and in the customer's hands at pickup. Healthy filed, tired evidenced, retired taped, gates first, numbers shown — the verdicts entire. A battery verdict without numbers is an opinion; this bench does not sell opinions.

Common Mistakes

• Quoting off resting voltage. A mid-curve number read as health — the flat middle is a coin flip; the tank and the sag are the measurements (battery-systems-overview-and-lithium-safety-foundations).
• Testing only capacity. The tank measures fine and the pack still shuts down under load — fade and sag are different diseases; run both (smartphone-battery-replacement).
• Letting the gauge outvote the meter. The screen says thirty percent, so the battery must be fine — the bookkeeper drifts; the discharge test does not (usb-power-delivery-diagnostics-and-charging-fault-isolation).
• Selling recalibration as repair. The guided cycle fixes the ledger and the customer hears the chemistry was healed — bookkeeping repair is sold as bookkeeping repair (controller-connectivity-and-battery-repair).
• Skipping the gate for a quick test. The pack looked fine from the fast angle — the inspection runs before every test lead, because the dome does not announce itself twice.

Troubleshooting Guidance

The diagnostics bench troubleshoots complaints into measurements: tank, pour, ledger. If a device dies too fast but predictably: the capacity test — steady fade is the tank shrinking, and the fade percentage prices the replacement (smartphone-battery-replacement). If it dies suddenly from healthy-looking percentages: the sag method — the cliff is the pour failing under load, and cold makes it worse; measure the step, compare the twin (battery-systems-overview-and-lithium-safety-foundations). If shutdowns cluster in winter or outdoors: temperature's tax on internal resistance — the sag measured warm and cold tells the story the customer lived (controller-connectivity-and-battery-repair). If the percentage jumps, sticks, or dies at thirty: the gauge audit — claim against measurement — and the guided recalibration where the ledger alone has drifted (usb-power-delivery-diagnostics-and-charging-fault-isolation). If the measurements disagree with each other: trust the discharge test over the gauge, the trend over the single reading, and the family file over the absolute number. If anything at intake fails the inspection: no test — the foundations own the ticket now. The throughline: every vague complaint is one of three failures — tank, pour, or ledger — and one afternoon of measurement names which.

Verification & Testing Methods

Confirm the diagnostics bench end to end before the queue tests it:

• [ ] I can explain the discharge curve's flat middle and use resting voltage only for what the edges actually say.
• [ ] I run the capacity test as a controlled, attended, condition-logged discharge and express fade against the label.
• [ ] I measure internal resistance by the sag method — the drop over a known step — and trend it against twins and the family file.
• [ ] I audit the fuel gauge against measured truth and sell recalibration as bookkeeping repair, never chemistry repair.
• [ ] I issue verdicts with numbers attached — healthy filed, tired evidenced, retired taped — and the foundations' gates run before any test touches a pack.

Then try the practice exercises below — donor-pack instrument drills and desk work; scenarios differ from the quiz.

Practice Exercises

1. Read the curve against the truth (5 minutes, three donor packs, multimeter, device gauge where available). Take resting-voltage readings on three packs of mixed age, write down what each number seems to promise, then check each against the device's gauge claim and any prior test data — logging where the flat middle fooled you and where the curve's edges told the truth (battery-systems-overview-and-lithium-safety-foundations).
2. Set up and start a measured discharge run (5 minutes, load tester, one donor pack). Verify the donor full, configure the label-sanctioned load and floor voltage, log the conditions — current, temperature, floor — start the run with its end time scheduled and attended, noting where the counted result will land on the log so the fade computes the moment the run ends, and while it counts, compute what the label promises in milliamp-hours and what fade percentage would tip this family's verdict from healthy to tired (smartphone-battery-replacement).
3. Drill the sag method (5 minutes, two same-family donors, multimeter, known load). On each donor: resting terminal voltage, voltage under the known load step, the arithmetic — drop over step — then compare the two packs' numbers head to head and against the family file, and write one line on which pack is aging out loud and how the trend would read next quarter (controller-connectivity-and-battery-repair).
4. Write the verdict card (3 minutes, desk). Draft the bench's card: the three measurements with their thresholds per family, the gauge-audit comparison and the honest recalibration line, the safety gates printed at the top where they run — inspection before leads, container in reach, dead-flat retires untested — and the close line that puts capacity, sag, gauge claim, and verdict in the customer's hands — then fill the card once for real: pull the diagnostics-menu device's gauge capacity claim, set it beside that pack's measured or family-file capacity and Exercise 3's sag numbers, and write its verdict line with the numbers attached (usb-power-delivery-diagnostics-and-charging-fault-isolation).

These core steps — the curve read, the scheduled capacity test, the sag drill, and the verdict card — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

• Resting voltage speaks only at the curve's edges — the long flat middle hides everything, which is why the healthy-looking number precedes the dinner-time shutdown (battery-systems-overview-and-lithium-safety-foundations).
• The capacity test is the tank's only honest answer: a controlled, attended, condition-logged discharge counted against the label, with fade as the verdict's spine (smartphone-battery-replacement).
• The sag method reads internal resistance as the aging signal capacity misses — the drop over a known step, trended against twins and the family file, catching the pack that holds its tank but cannot pour (controller-connectivity-and-battery-repair).
• The fuel gauge is a bookkeeper that drifts on shallow cycles and lies confidently after — audited against measured truth, recalibrated as ledger repair, and never allowed to outvote the meter (usb-power-delivery-diagnostics-and-charging-fault-isolation).
• Verdicts are numbers wearing words — healthy filed, tired evidenced, retired taped — and the foundations' gates run before any test touches a pack.

Skills Learned

• You can now explain why voltage alone misjudges health and where on the curve it says anything at all.
• You can now run a measured capacity test and express the fade against the label.
• You can now measure internal resistance by the sag method and trend it against twins and history.
• You can now read fuel-gauge claims, check them against measured truth, and recalibrate honestly.
• You can now issue battery verdicts with numbers attached — after the safety gates run first.

Glossary Additions

• capacity test — the controlled discharge that measures a battery's true remaining capacity instead of trusting its label or its gauge: the pack verified full, emptied through a load the label sanctions down to the floor voltage and no further, with the milliamp-hours counted the whole way and the result expressed against the label as a fade percentage. The test owns its conditions — load current, temperature, and the floor used are logged with the result, because a capacity number without its conditions compares to nothing, including itself. Runs are scheduled and attended, never left to finish in an empty shop, and the recharge afterward returns the pack deliberately to storage or service charge. The fade percentage the test produces is the spine of every battery verdict the bench issues.
• internal resistance — the opposition a battery presents to its own current, and the electrical signature of aging: cycles thicken the cell's interfaces and dry its pathways, the pack pours less willingly, and voltage sags under load even while capacity holds. The bench estimates it by the sag method — terminal voltage at rest, terminal voltage under a known load step, and the drop divided by the current step — a number whose absolute value varies by family, method, and timing — read at the same delay after the step every time, and never mixed with AC-meter values in one file — but whose trend, honestly kept, does not lie. Grown internal resistance is the disease capacity testing misses: the sudden shutdown from a healthy-looking percentage, the winter-morning death at forty percent, the pack that fills its tank and cannot pour it.
• fuel gauge — the device-side bookkeeper that estimates a battery's state and capacity, typically by counting charge in and out and adjusting its model as the pack ages. The gauge is testimony, not truth: shallow cycles starve it of the full-swing data it learns from, its estimate drifts, and a drifted gauge lies confidently — reporting comfortable percentages at the moment of shutdown. The bench audits it by comparison — the gauge's capacity claim beside a measured capacity test, its percentage behavior beside the sag evidence — and where only the ledger has drifted, a guided full cycle recalibrates it: bookkeeping repair, honestly sold as fixing the record rather than the chemistry, and never a substitute for the measurements it gets audited against.

Suggested Next Sections

Must read next:

• Battery Replacement Across Device Families — Section 8.3 takes the verdicts to the bench: adhesive-trapped phone slabs, laptop packs with connectors and screws, accessory cells at every ceiling — the pull disciplines per family, the sourcing honesty that keeps counterfeit cells out of the drawer, and the closes that prove the swap.

Recommended:

• USB Power Delivery Diagnostics and Charging Fault Isolation — the metering craft this bench inherits: numbers, baselines, and anomalies read against normals.
• Controller Connectivity and Battery Repair — the accessory bench whose capacity honesty these instruments upgrade from proxy to meter.