Battery Management System for Small Devices

Concept render

A battery management system protects a lithium battery and reports its charge. In a small device it is usually a protection IC with two MOSFETs, a charger with a thermistor input, and a fuel gauge. Packs with cells in series add cell balancing. LiFePO4 cells need different voltage limits and harder gauging than Li-ion.

Every rechargeable lithium device needs a battery management system (BMS). In a ring or a tracker it is spread across a few small chips and some firmware. This guide explains what a BMS does, how the parts map to a single-cell wearable and to a multi-cell or LiFePO4 pack, and how to test it before you trust it.

This is an engineering summary. Part figures come from vendor datasheets and product pages, as of November 2026, and all sources were read on November 12, 2026. The parts named are examples, not recommendations. Your cell maker's datasheet sets the limits for your cell.

What is a battery management system?

A BMS is the set of circuits and firmware around a rechargeable battery that does four jobs:

• Protection. It disconnects the cell on overvoltage, undervoltage, overcurrent, short circuit and, in many designs, temperature faults.
• Charge control. It charges the cell with the right current and voltage, and stops at the right point.
• Gauging. It estimates state of charge, remaining capacity and sometimes state of health.
• Balancing. In packs with cells in series, it keeps the cells at the same voltage.

TI's LiFePO4 design note lists among Li-ion limitations that it is fragile and requires a protection circuit for safe operation, that peak voltage is limited during charge, and that temperature needs to be monitored (TI SLUAAR1, as of November 2026). Those three lines are the reason the BMS exists.

What does a BMS look like in a single-cell wearable?

A wearable or tracker with one Li-ion or LiPo cell uses a small set of parts. Some products merge them into a power management IC (PMIC).

Figures as of November 2026. A PMIC can replace several rows: Nordic's nPM1300 combines a charger at 32 to 800 mA, charge termination settable from 3.5 to 4.45 V, two buck regulators, two LDO or load switch outputs, and an algorithm-based fuel gauge that reports state of charge, state of health and cycle count with no extra components. Its ship mode draws 370 nA (nPM1300 product page, as of November 2026).

For the regulators after the battery, see LDO vs buck converter. For the standards a ring or band battery must meet, see the smart ring battery safety standards checklist.

How does battery protection work?

A protection IC watches the cell voltage and the current, and drives two MOSFETs in series with the cell: one blocks charge current, one blocks discharge current. When a limit is crossed for longer than a set delay, it turns the matching FET off.

TI's BQ2970 family is a typical single-cell protector. TI lists overcharge detection, over-discharge detection, charge overcurrent, discharge overcurrent and load short-circuit detection. It senses current as the voltage across the external FETs, within ±5 mV typical. Variants offer overvoltage thresholds of 3.85, 4.275, 4.28, 4.35 and 4.425 V. TI lists 4 µA normal current, 100 nA shutdown current and a 1.5 mm × 1.5 mm package, with factory-programmed fault and recovery timers (BQ29700 product page, datasheet Rev. I, as of November 2026).

Why two layers of protection?

The charger stops at its regulation voltage, and firmware shuts down at a low-battery cutoff. The protection IC trips only when one of those has failed, or when something shorts. Set its thresholds outside the charger and firmware limits, and inside the cell maker's absolute limits. If the pouch cell already carries a protection board on its tab, compare its thresholds with yours so the two do not fight.

Leakage counts in a small device. A protector at 4 µA is about 35 mAh a year, which is a real share of a ring battery in storage. Use ship mode for the warehouse.

How does the charger keep a lithium cell safe?

TI describes the standard charge profile as precharge, constant current and constant voltage. For Li-ion, TI gives the regulation voltage as about 3.9 to 4.2 V, precharge at about 3.0 V and the short threshold at about 2.0 V (SLUAAR1, as of November 2026).

Charge voltage accuracy decides how much capacity you use. TI works an example: with ±2% accuracy, the voltage target must be set 2% low so the worst-case part never overcharges, so a part at the other end of the tolerance stops 4% below the maximum. In its table, that costs up to 13.2% of a Li-ion cell's capacity. At ±0.5% the loss is 3.0% (SLUAAR1, as of November 2026).

The BQ25120A wearable charger lists 0.5% accurate battery voltage regulation, settable from 3.6 V to 4.65 V in 10 mV steps, a charge safety timer programmable from 2 to 540 minutes, and a thermistor input with four thresholds for JEITA compatibility (datasheet SLUSD08A, as of November 2026). JEITA-style charging reduces current or voltage when the cell is cool or warm and stops outside the allowed range. TI lists the Li-ion charge temperature range as 0 °C to 45 °C (SLUAAR1, as of November 2026).

The charger input is often USB-C. See USB-C power delivery for device design for what the port needs.

How does a fuel gauge estimate state of charge?

There are three common methods, and many parts mix them.

The BQ25120A lists a simple voltage-based battery monitor (SLUSD08A). The BQ27441-G1 has a coulomb counter on a 10 mΩ sense resistor and uses TI's Impedance Track algorithm. TI says it adjusts for battery aging, self-discharge, temperature and rate changes, and estimates state of health. TI lists its typical current at 3.6 V as 93 µA in normal mode, 21 µA in sleep, 9 µA in hibernate and 0.6 µA in shutdown (datasheet SLUSBH1C). The nPM1300 gauge runs in the PMIC with no extra components (product page). All as of November 2026.

Gauge current matters in a small battery. At 9 µA, the BQ27441 hibernate figure, a gauge uses about 79 mAh a year. Compare that with your standby budget and use the gauge's low-power modes. Our guide to testing battery life claims honestly covers how to measure the whole budget.

How does a multi-cell or LiFePO4 pack change the BMS?

Cells in series need balancing

Cells in series drift apart in capacity and self-discharge, so one cell hits full or empty first and limits the pack. TI's BQ76952 monitors and protects 3-series to 16-series Li-ion, Li-polymer and LiFePO4 packs. TI says it supports passive balancing by bypassing the current of a selected cell during charging or at rest, through integrated bypass switches or external FETs, with a voltage-based algorithm that can run without a host. It lists 286 µA in normal mode (datasheet SLUSE13B, as of November 2026).

LiFePO4 uses lower voltages and is harder to gauge

TI's design note compares the two chemistries. Li-ion has a nominal 3.6 V per cell and charges at 3.9 to 4.2 V. LiFePO4 has a nominal 3.2 V and charges at 3.5 to 3.65 V, with precharge at about 2.0 V. TI lists LiFePO4 energy density as 90 to 120 Wh/kg against 150 to 180 Wh/kg for Li-ion, and notes that its lower voltage may require a boost converter where Li-ion would not (SLUAAR1, as of November 2026).

Gauging is the harder part. TI says Li-ion has a fairly linear state of charge versus open-circuit voltage profile. LiFePO4 is fairly linear from about 85% to 100% and has an abrupt change in slope from about 10% to 85% (SLUAAR1, as of November 2026). TI ties the shape of the curve to charge voltage accuracy and does not discuss fuel gauging. The gauging point is our engineering reasoning. Across the flat middle of the LiFePO4 curve, voltage changes very little as charge changes, so a voltage reading alone says little about how full the cell is. A coulomb-counting gauge that resets at full charge suits LiFePO4 better than a voltage-only monitor.

Charge accuracy matters more too. In TI's table for charging to 80% state of charge, LiFePO4 loses up to 41.4% of available capacity even with ±0.5% charge voltage accuracy (SLUAAR1, as of November 2026). TI notes that most designers charge LiFePO4 to 100%, since its energy density is already lower.

How should you test a battery management system?

Test the BMS on real boards with real cells at each prototype build. The board, the FETs and the firmware all move the datasheet thresholds.

• Trip each protection on the bench with a source-measure unit or electronic load: overvoltage, undervoltage, charge overcurrent, discharge overcurrent and a hard short. Record the threshold and the delay.
• Charge at the coldest and hottest ambient you allow, and confirm the charger pauses or slows at the thermistor thresholds.
• Measure the charge regulation voltage at the cell terminals with a calibrated meter.
• Run full discharge cycles at your real load profile and compare the gauge reading with measured capacity at 100%, 50%, 20% and 5%.
• Measure shelf current in ship mode and in the deepest sleep state, so a device in a warehouse does not drain to cutoff.
• For series packs, unbalance the cells on purpose and confirm the balancer brings them back within your limit.

For shipping samples, see shipping a LiPo wearable: UN 38.3 and watt-hours. For how the battery work fits into the build gates, see engineering.

If you want the battery protection, charging and gauging of your device reviewed against your cell and use case, tell us about your device.

Sources. All pages accessed November 12, 2026. Vendors revise datasheets, so confirm current values before you decide.

• Texas Instruments, BQ29700 product page (BQ2970 family, datasheet Rev. I) (link)
• Texas Instruments, bq25120A datasheet SLUSD08A (link)
• Texas Instruments, bq27441-G1 datasheet SLUSBH1C (revised December 2014) (link)
• Texas Instruments, BQ76952 datasheet SLUSE13B (revised November 2021) (link)
• Texas Instruments, LiFePO4 Design Considerations, SLUAAR1 (July 2023) (link)
• Nordic Semiconductor, nPM1300 product page (link)