Adding a lithium cell to a product

The prototype ran from a bench supply. Now the product has to run from a single lithium cell, charge over USB, and survive being left in a hot car, a cold truck and a customer’s drawer for a year. The forum answer is a charger module and a cell from a marketplace listing. That gets a demo working. It does not get a product through shipping paperwork, a safety review or a warranty period.

Designing the battery in properly is not hard, but it is four separate jobs that people tend to collapse into one. This note walks through them, what to ask the cell supplier for, and what to test before the first unit leaves.

Four jobs, not one chip

A lithium-powered product needs charging, protection, power-path management and state-of-charge reporting. Some parts combine two or three of these, but each one has to be accounted for somewhere.

Charging. A single-cell charger runs constant current, then constant voltage, and terminates when the current falls to a set fraction. It should also precondition a deeply discharged cell at reduced current before fast charging. Small linear chargers from Microchip and Texas Instruments do this in a five- or six-pin package with one resistor setting the current. What separates them is what else they do.

Protection. The charger regulates under normal conditions. Protection is what acts when something else goes wrong: a shorted load, a charger fault that pushes the cell over voltage, or a firmware bug that drains the cell to nothing. A protector such as the TI BQ2970 family watches for “overcharge detection (OVP), over-discharge detection (UVP), charge overcurrent detection (OCC), discharge overcurrent detection (OCD), load short-circuit detection (SCP)” and drives external charge and discharge FETs to open the cell. Most pouch cells sold for products already carry a small protection board of this kind under the tab tape. Treat it as a last line, not the only one. It disconnects at limits chosen to prevent fire, not to preserve capacity.

Power path. With a plain charger, the system load sits in parallel with the cell. The charger sees the load current as charge current, terminates late or never, and a dead cell means the product cannot boot even when plugged in. Power-path chargers separate the two. TI describes the BQ24075 as using “dynamic power path management (DPPM) that powers the system while simultaneously and independently charging the battery,” and notes that the regulated system output “enables instant system turn-on when plugged in even with a totally discharged battery” (BQ24075). If the product must work while charging, or must show signs of life with a flat cell, this is the feature to pay for in board area.

Gauging. A voltage reading is a poor fuel gauge for lithium chemistry because the discharge curve is flat through the middle and moves with load and temperature. A dedicated gauge such as the TI BQ27421 needs “connections only to PACK+ (P+) and PACK– (P–)” and reports “remaining battery capacity (mAh), state-of-charge (%), and battery voltage (mV)” over I2C, adjusting for aging and temperature (BQ27421-G1). If the product only needs a low-battery warning, a voltage threshold in firmware may be enough. If it shows a percentage to a user, a gauge saves a support queue.

Picking the charger

The decision usually comes down to three questions.

Question	If no	If yes
Must it run while charging, or boot from a flat cell?	Simple linear charger	Power-path charger
Will it be charged outside room temperature?	Fixed thermistor resistor	Charger with NTC input and a JEITA-style profile
Will the cell chemistry or capacity change during the product’s life?	Resistor-programmed	I2C-programmed charger

The temperature question is the one most often skipped. Lithium cells should not be fast-charged when cold or hot. The cell datasheet gives the allowed charge temperature window, and it is narrower than the discharge window. Chargers with a thermistor input suspend or reduce charging outside that window. The TI BQ25180, for example, lists “configurable NTC charging profile thresholds including JEITA support” alongside a ship mode measured in microamps for products that sit in inventory (BQ25180). The thermistor only helps if it is thermally coupled to the cell. A 10k NTC on the PCB reads the board, not the cell. Ask for a cell with a built-in thermistor lead, or mount one against the pouch in the mechanical design.

Ship mode is worth a sentence on its own. A product built, tested and boxed will sit for weeks or months before someone presses the button. A charger or protector that can disconnect the cell from the system until wake-up keeps the cell from self-discharging through the quiescent current of every rail on the board. Without it, a product can arrive with a cell below the protector’s under-voltage cutoff, which many chargers will refuse to recover.

The cell is a purchased part with paperwork

Engineers who would never design in an undocumented regulator will design in a cell from a listing with a capacity number and a photo. The cell needs the same treatment as any other component.

Get the datasheet. It should state nominal and maximum charge voltage, standard and maximum charge current, cutoff voltage, charge and discharge temperature ranges, and cycle life to a defined capacity. If the charger’s regulation voltage does not match the cell’s rated maximum, one of them is wrong. Some high-energy cells are rated above the common maximum; some long-life or high-temperature cells are rated below it. The charger must be set to the cell, not the other way around.

Ask for the protection circuit spec. If the cell ships with a protection board, ask which limits it uses. A cell whose protector opens at a discharge current below your product’s peak draw will drop out under load, and it will look like a firmware crash.

Ask for the UN 38.3 test summary. Lithium cells and batteries have to pass a defined series of transport tests before they can be shipped, and since 2020 the manufacturer or supplier has to make a test summary available down the supply chain. The US Department of Transportation’s hazardous materials agency describes the requirement and what the summary must contain (PHMSA, lithium battery test summaries). A supplier who cannot produce one is a supplier whose cells will get stuck at a carrier.

Ask about the safety standard. IEC 62133-2 “specifies requirements and tests for the safe operation of portable sealed secondary lithium cells and batteries containing non-acid electrolyte, under intended use and reasonably foreseeable misuse” (IEC 62133-2). Whether the finished product must be tested against it, or a related regional standard, depends on the market and the product category. Starting with a cell that already has a report removes the largest piece of that work.

Specify the mechanical interface. Tabs, pre-crimped leads with a specific connector, or a bare pouch each drive a different assembly step. A connector that can be plugged in reversed will be plugged in reversed. Pick a keyed one.

Firmware’s share

The board can be right and the product still fails in the field because firmware treated the battery as a fixed rail.

• Read the gauge, do not guess. If a gauge is fitted, configure it with the cell’s design capacity and chemistry profile, and read state of charge from it. If not, use the cell datasheet’s discharge curve at the product’s real load, not an open-circuit table.
• Shut down before the protector does. Firmware should save state and sleep at a voltage above the protector’s under-voltage trip. Letting the protector open the cell is an unclean power loss, and everything we’ve written about firmware update recovery applies at that moment.
• Handle the charger’s status pins. Chargers expose charging, done and fault states. A fault that is silently ignored becomes a customer saying the product “sometimes doesn’t charge.”
• Respect the temperature window in software too. If the charger lacks an NTC input, firmware with a temperature sensor should be able to disable charging through the charger’s enable pin.

What to test before it ships

These are the checks that catch problems a working demo hides.

1. Charge from empty. Discharge a cell to the protector cutoff, then plug in. Confirm the product boots, the charger preconditions, and the gauge recovers.
2. Charge at the temperature extremes. Put the unit in a chamber or a warm enclosure at the edges of the cell’s charge window and confirm the charger suspends and resumes. This is where a board-mounted thermistor shows its error.
3. Full load while charging. Draw the product’s peak current with the charger connected and confirm the system rail holds and the charger terminates correctly afterward.
4. Shelf test. Fully charge, put the unit in ship mode or powered off, and measure the cell voltage after a defined interval. Compare against the cell’s self-discharge spec and the board’s quiescent draw.
5. Short the output. With the cell fitted, short the system rail through a fuse-rated wire and confirm the protector trips and recovers. Do this behind a shield.
6. Log every charge cycle on the first units in the field, with temperature. Cycle life numbers on the cell datasheet were measured at one temperature and one rate, and yours differ.

Fitting a lithium cell into an existing design is a scoped piece of engineering: a schematic block, a mechanical decision about where the cell and its thermistor live, a firmware layer, and a supplier conversation. We take the cell datasheet, the product’s load profile and the intended markets as the starting point, and return a design with the four jobs above each accounted for and a test list to prove them.