Battery pack design starts with an energy budget, not with a cell catalog. Measure or estimate current draw in every operating mode, multiply by how long the product spends in each mode during a typical day, and only then select a chemistry, a cell format, and a protection scheme. In a portable product the battery is usually the largest, heaviest, and most safety-critical component, and it drives volume, weight, cost, certification, and shipping. Deferring the decision is the classic mistake — teams design a beautiful enclosure and then discover the cell they need does not fit inside it.
Start With an Energy Budget
List every operating mode — active, idle, sleep, transmitting, charging — with its current draw and the fraction of a day spent in it. Sum to a daily energy figure, then size capacity for the runtime you promise, plus margin for capacity fade over the product's life and for reduced performance in cold weather.
What usually emerges is that standby draw, not active draw, dominates — especially in products that spend most of their existence waiting. Cutting standby current in hardware and firmware is almost always cheaper than adding cells. This is one of the clearest places where electronics, firmware, and mechanical decisions collide, and it is why component selection and code architecture should be discussed together rather than sequentially. Microcontroller sleep modes are a first-order variable here; see how to choose a microcontroller.
Rechargeable or Primary?
This is not a foregone conclusion. A primary (non-rechargeable) cell simplifies the product enormously: no charger, no charge connector, no charge management circuit, and often no sealing problem around a port. It suits very low-consumption or infrequently used products, and it can be the right answer for something that runs for years on a coin cell.
A rechargeable pack suits daily use and higher consumption, but it adds cost, volume, a charging path, and a set of safety components that are not optional.
What a Rechargeable System Must Include
- Protection circuit. Prevents overcharge, overdischarge, overcurrent, and short circuit. An unprotected lithium cell is a genuine hazard and will not pass safety certification.
- A charger matched to the chemistry. Every chemistry has its own charge profile, and mismatched charging degrades cells and creates risk.
- Temperature monitoring. Charging a lithium cell below freezing or when hot damages it and is dangerous. A thermistor and firmware limits are the minimum.
- State-of-charge estimation. Required if the product shows the user how much runtime is left. Voltage alone is a poor proxy for most chemistries.
- Cell balancing. Necessary as soon as you put cells in series, because divergent cells drag the whole pack down and can be pushed outside their safe window.
Collectively that is the battery management system. In a multi-cell pack it is a real subsystem with its own firmware, not a couple of parts you sprinkle on the schematic — and it belongs in your PCB layout planning from the start, since sense lines, current shunts, and thermal paths all constrain the board.
Buy a Module or Build a Pack?
For startups and low-volume production, a commercially available battery pack that already includes protection circuitry and existing test reports is almost always the better choice. Developing a custom pack only pays off at high volume, or when the form factor genuinely demands it.
The reason is certification. Using an already-tested pack dramatically shortens your own compliance path, because a large part of the electrical safety and abuse-testing burden has already been discharged by the pack maker. If you build your own, that work becomes yours — see UN 38.3 lithium battery testing and the broader product safety testing requirements.
Shipping Restrictions Catch People Out
Lithium batteries are classified as dangerous goods. In practice that means special packaging and labeling, required test documentation, restrictions on air transport, quantity limits per package, and carriers that will occasionally simply refuse the shipment. A product manufactured overseas and shipped in volume needs this planned early — see sea vs air freight, since the rules are one of the main reasons battery-bearing products travel by sea. Shipping individual units to end customers is subject to the same regime, so it affects your fulfillment plan as well as your inbound logistics.
Effect on the Mechanical Design
A lithium cell swells slightly over its life, so the enclosure has to leave clearance. A pouch cell clamped tightly between rigid surfaces will deform or be damaged. Beyond that, the pack needs a thermal path, secure retention so it does not move during a drop, and accessible routing if it is meant to be replaceable.
Drop performance is worth designing for explicitly — a heavy mass on flexible leads is the usual failure mode, and the approach is covered in designing for drop test. Enclosure and sealing implications are covered in IP ratings explained.
How the Product Charges Changes the Whole Design
Charging method is not a convenience question. A standard charge connector is the cheapest and most familiar solution, but it is a hole in the enclosure and therefore the primary weak point for sealing — a serious issue for anything used outdoors. USB-C is the default choice here, and it brings its own protocol and mechanical requirements. Wireless charging eliminates the opening entirely and permits a fully sealed product, at the cost of lower efficiency, added heat, and higher component cost. Exposed contacts in a charging cradle are a middle path, sensitive to corrosion and contamination.
Because the choice ripples into enclosure design, component placement, thermal design, and test requirements, it belongs to the concept phase rather than the detail phase.
Test on Real Units
Runtime is measured on real hardware under real conditions, never by calculation alone: capacity drops in the cold and erodes over hundreds of charge cycles. Build battery runtime and cycle-life tests into the reliability plan and into the build phases described in EVT, DVT and PVT. Emissions testing also interacts with switching chargers, so budget for it — see EMC testing cost. More on the surrounding discipline is in our electronics development hub.
If you are sizing a battery for a new product and want the energy budget, cell selection, protection scheme, and enclosure implications worked out together, tell us about your product through our contact form.