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The battery percentage on a device is an estimate, not a direct reading of how much “fuel” remains. A coulomb-counting IC measures current flowing into or out of a battery and adds it up over time. That gives a useful running account of charge—but reliable state-of-charge estimates also need voltage, temperature, battery behavior and corrections for measurement error.
What a coulomb counter measures
A coulomb is a unit of electric charge. It is not power or energy. Current, measured in amperes, is the rate at which charge flows; one ampere-hour (Ah) is 3,600 coulombs. Battery capacity is commonly stated in ampere-hours or milliampere-hours: a nominal 3,000-mAh cell is rated to deliver a specified amount of charge under stated conditions. Its usable energy, measured in watt-hours, also depends on voltage and operating conditions.
The basic relationship is Q = ∫ I(t) dt: charge is current integrated over time. A gauge tracks the sign as well as the size of the current—charge entering the cell increases the estimate, and charge leaving it decreases the estimate. A coulomb counter can measure that flow; it cannot directly observe the battery’s exact remaining usable capacity.
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A common design puts a low-value shunt resistor in series with the battery current path. The IC measures the small voltage across it and derives current from Vsense = I × Rsense. For example, 1 A through a 10-mΩ shunt produces 10 mV. The IC converts its repeated measurements into current readings and accumulates them over time.
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- Brand NEW and Sealed from Switch OEM Factory that has never been used. Part numbers: MAX17050, MAX17050x
- Responsible for showing battery levels on your Nintendo Switch console (Model No: HAC-001).
- CAUTION: ***YOU CANNOT FIT THESE WITH A SOLDERING IRON ONLY HOT AIR IS REQUIRED! Requires technical soldering to install.
- If you are getting proper voltage but no sound or display, this IC may need to be replaced.
- Each Pack come with: 1x MAX17050 Fuel Gauge IC Chip (Maxim). (This is the same brand and specification IC used by in the Switch Console.)
Battery ── sense resistor ── system load / charger
│
differential ADC
│
coulomb-counting IC ── I²C / SMBus ── host
This is a conceptual arrangement, not a universal wiring diagram: sensor placement and current paths depend on the design. The essential rule is that every battery current the gauge is expected to count must pass through its measurement boundary. If a charger or part of the load bypasses the shunt, the gauge cannot count that current.
The shunt is a compromise between a measurable signal and wasted energy. Raising its resistance increases the sense voltage, which can help measurement, but also increases voltage drop and heating. A smaller shunt reduces those losses but makes offset, noise and layout errors more consequential. Separate Kelvin sense traces should connect directly to the shunt’s sense terminals rather than sharing the high-current path. The MAX17055, one single-cell example, supports sense-resistor values from 1 mΩ to 1,000 mΩ; that range is a device capability, not a recommendation to use any value in any design. See the MAX17055 specifications.
Why not estimate charge from voltage alone?
Battery voltage can help estimate state of charge, but it changes with load, temperature, chemistry, internal resistance, recent charging or discharging, cell age and the time allowed for the battery to relax. Under a heavy load, voltage may sag temporarily; just after charging, it may not yet represent the cell’s settled condition. A voltage-only indicator can therefore move in ways that do not match the charge actually used.
Current integration is useful because it follows charge flow through changing loads without treating every temporary voltage change as a capacity change. It is especially helpful while a device is operating. But it is not infallible: integrating an imperfect measurement does not make the accumulated result exact.
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Why a pure coulomb count drifts
Small current-measurement errors accumulate. ADC offset or gain error, shunt tolerance and temperature coefficient, sampling limits, sleep current, self-discharge and unmeasured paths can all contribute. So can an incorrect starting state of charge, changes in battery capacity with age, and differences between charge and discharge efficiency. A 1-mA offset sustained for 1,000 hours adds up to 1 Ah of apparent charge error.
That makes the initial reference important: if the gauge starts with the wrong state-of-charge estimate, later integration inherits that error. Designers use methods such as automatic offset cancellation, known full or empty events, battery models and periodic correction to keep estimates anchored. TI’s BQ26500 is an example of a device specified for coulometric charge and discharge integration with automatic offset cancellation; that feature addresses measurement offset, not every source of system-level error.
From coulomb counter to fuel gauge
A fuel gauge typically combines several inputs rather than relying on an accumulator alone:
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware match- Current integration provides continuous short-term tracking of charge in and out.
- Voltage behavior helps correct long-term drift when the reading is meaningful for the cell’s condition.
- Temperature helps account for changes in capacity and electrical behavior.
- Battery models can account for chemistry, discharge rate, impedance and aging.
- Full and empty events may provide synchronization points, depending on the gauge and design.
Vendors use different gauging approaches, including voltage correlation, coulomb counting, compensated end-of-discharge voltage (CEDV) and impedance-based methods. TI’s fuel-gauge overview compares these approaches. Analog Devices describes its ModelGauge method as combining coulomb-counter linearity with voltage-based long-term stability. Its MAX17055 is a compact example for one-cell lithium-ion applications: the manufacturer specifies 7-µA operating current, a two-wire I²C interface and package options as small as a 1.4 mm × 1.5 mm, 9-pin WLP. It reports state of charge and remaining capacity and provides battery-aging indicators. The part’s size and features do not imply a universal percentage-accuracy guarantee.
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- Communication: Supports I2C interface for communication with the host system (e.g., Switch Lite's SoC).
- Input Voltage Range: 2.5V¨C4.5V (typical for single-cell Li-ion).
- Accurate Battery Monitoring Designed to manage and report precise battery charge levels Compatible with Nintendo Switch Lite consoles.
- Ideal for Repair Professionals Suitable for technicians repairing battery-related issues such as incorrect charge display.
- Supports Power Management Helps stabilize charging behavior and optimize battery life for handheld gaming use.
At the pack end of the spectrum, TI’s BQ27Z855 combines an 18-bit, low-offset delta-sigma ADC for coulomb counting with a separate 16-bit ADC for cell voltage and temperature sensors. It also includes an embedded processor, protection and current-limiting functions, authentication support and an I²C-compatible interface. TI’s Dynamic Z-Track approach models impedance to improve estimates under changing loads. These are capabilities of that product, not inherent features of every coulomb-counting IC.
What “precision” means in practice
Datasheet resolution, current-measurement accuracy and the accuracy of a displayed battery percentage are different things:
- Resolution is the smallest change a measurement system can represent.
- Accuracy is how close a measurement is to the actual value.
- Repeatability describes how consistently a measurement is reproduced.
- Drift is how error changes over time.
- State-of-charge accuracy concerns the estimate of remaining charge; time-to-empty accuracy concerns a runtime prediction under a particular load.
An 18-bit ADC is a converter specification, not a promise that the battery icon will be accurate to a particular fraction of a percent. The shunt, PCB layout, calibration, temperature behavior, current profile, cell variation and quality of the estimation model all matter. Likewise, “remaining capacity” depends on what the cell can usefully deliver under current conditions—not only its original nameplate rating.
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With low-side sensing, the shunt sits between battery negative and system ground. This can simplify the measurement’s common-mode requirements, but it raises system ground relative to battery negative and can create trouble if ground-referenced connections or loads bypass the shunt.
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With high-side sensing, the shunt sits in the positive battery path. That can preserve system-ground integrity and make it easier to account for total battery current, but the measurement IC must handle the higher common-mode voltage and the design may need additional protection. Neither placement is automatically best: choose based on the gauge’s input limits, the pack and system topology, and which currents must be measured. The BQ27Z855 supports high- or low-side sensing, illustrating that this is an application choice.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose the right level of battery management
| Design need | Likely fit | What to check |
|---|---|---|
| Measure current or accumulated charge; host firmware will do the estimating | Current monitor or basic coulomb counter | Current range, offset, shunt, interface and how the initial charge estimate will be established |
| Provide SOC, remaining capacity or runtime estimates | Fuel gauge | Cell count, chemistry, model requirements, temperature range, quiescent current and load profile |
| Also control pack safety switching or limit current | Gauge with protection, or a broader battery-management design | Protection thresholds, FET control, balancing needs, authentication and system safety requirements |
A gauge does not automatically charge, protect, balance or manage the power path. Some products integrate several of those functions, as the BQ27Z855 does for certain protection and current-limiting functions, but the design must confirm what is included and what remains external.
Before choosing a part, define the battery chemistry, capacity, series and parallel configuration, voltage limits, maximum current, operating temperature and expected life. Then check whether the exact device supports that configuration and whether it needs battery characterization or specific parameters. For example, MAX17055’s ModelGauge m5 EZ is marketed for typical applications that do not require battery characterization; that is not a blanket claim that every cell and operating profile will perform optimally without validation. Other gauges may require discharge data, coefficients or impedance information.
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Also confirm current direction and measurement range, sleep current, interface voltage, package assembly constraints and any host duties such as reading registers, handling alerts, loading parameters or recovering after reset. Some gauges are intended to operate with little host involvement; others demand more configuration and characterization. Vendor evaluation hardware and software can materially affect engineering time.
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Validate the complete design
Test the gauge with the actual cell and system, not only a steady laboratory load. Include light and heavy loads, pulsed loads, charge and discharge transitions, hot and cold conditions, partial cycles, long idle periods, aged cells, charger insertion and removal, low-battery operation, resets and battery replacement. Check time-to-empty separately from SOC: a reasonable percentage reading does not guarantee a reliable runtime estimate under a changing load.
Investigate failure modes that hide current or corrupt the reference: a charger or load bypassing the shunt, poor Kelvin routing, shunt heating, a wrong battery model, uncounted standby current, or a cell replaced with a different vendor or capacity. Multi-cell packs add another concern: individual cells can become imbalanced even if a pack-level reading appears plausible. Confirm the gauge’s supported configuration and the protection architecture for the exact pack.
Finally, distinguish SOC, state of health and state of power. SOC estimates remaining charge relative to usable capacity. SOH estimates condition or degradation compared with a new cell. SOP estimates how much power the battery can safely deliver or accept under current conditions. Time-to-empty and time-to-full are predictions that depend on load and charging behavior. They are useful outputs, but they remain model-based estimates.
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