How to Spec a Custom Battery Pack: Voltage, Capacity, BMS

Step 1: voltage from the device, not from habit

A custom battery pack starts from your device, not from a voltage you used on a previous project. The pack has to sit inside a product that already exists or is being designed, and the device defines the electrical window the pack must fit. Start by writing down the supply range your electronics actually accept, then check what the charger and the load can tolerate at both ends of that range.

Series count sets the pack voltage. The cell chemistry you choose gives a nominal voltage and a working range per cell, and the number of cells in series multiplies that range. If the device runs from a 48V class bus, the series count follows from that requirement, not from what a supplier happens to stock. Parallel count does a different job: it sets capacity and shares the current, and it does not change the voltage.

Confirm the voltage decision against real operating states. A device that draws a short burst of high current will pull the pack voltage down, and a device that charges at low temperature will push it up. Both extremes must stay inside what the electronics accept. This check is where a spec that looks fine on paper fails in the field. If you are still choosing between chemistries and configurations, the custom battery pack design process walks through how voltage, capacity and current are checked against each other before a specification is written.

Write the voltage section of your spec as a range, not a single number, and state the conditions where each end applies. A supplier can then select cells and a series count that stay inside the range across the device's working states rather than only at room temperature.

Step 2: capacity from load current and runtime

Capacity follows from two inputs you already have: the current the device draws and the time it must run. Multiply average load current by required runtime and you get the charge the pack must deliver. That figure is the starting point, not the answer. Convert it into a cell configuration only after you have accounted for how the device actually behaves in service.

Work through the load profile rather than a single current figure. Many devices draw a low current most of the time and a much higher current in bursts, and the average over a duty cycle is what sets capacity. If the device sleeps, transmits, then sleeps again, build the average from the share of time spent in each state. A pack sized from the burst current alone will be larger and heavier than the product needs.

Capacity in ampere-hours is not the whole story, because the usable part of it depends on the discharge rate and on the cut-off voltage your device can accept. Faster discharge delivers less of the rated capacity; a higher cut-off leaves more charge unused. Therefore state your runtime target together with the load current and the cut-off, and let the configuration be calculated from those. A pack that meets the runtime target at the wrong current is not a match.

Temperature and aging belong in this step as well. A pack sized with no margin will meet its runtime when new and fall short later in life. Agree with your supplier how much margin the design should carry and write it into the spec. The number that matters to the end user is the runtime at the end of the product's service life, not the runtime on the first charge.

Step 3: continuous and peak current set the BMS

The BMS is chosen from current, not from capacity. Two packs with the same voltage and capacity can need very different protection boards if one drives a small motor and the other powers a radio module. Continuous current, peak current and the duration of that peak are the three inputs that decide whether the protection circuitry is adequate.

State the continuous current as the level the device can hold indefinitely, and the peak current as the level it reaches briefly, with the time attached. A peak with no duration is not usable information. The protection FETs, the sense resistor and the board layout all have to carry these currents without overheating, and the BMS must trip only when the device has genuinely faulted, not during normal start-up surges.

Overcurrent, overvoltage, undervoltage and short-circuit thresholds should be agreed as part of the spec, because they interact with the charger and the load. A threshold set too tight causes nuisance trips in the field; one set too loose leaves the cells unprotected. Temperature sensing is part of the same decision, since charge and discharge limits change with cell temperature.

Communication is a separate requirement that many specs miss. If the device needs to read state of charge, cell voltages or fault flags, say so early, because it changes the BMS selection and the connector pinout. If the device only needs power, a simpler board is enough. Describe what your host system needs to know, and the protection and communication options can be evaluated together with the cell configuration. The custom battery solutions overview shows how these options are grouped into a single pack specification rather than chosen one at a time.

Step 4: enclosure, leads and connectors on the drawing

The mechanical side of the pack has to be on a drawing before sampling, because space is usually the constraint that decides the configuration. Give the supplier the available envelope in the device, the mounting points, and the direction the pack slides or fastens in. A pack that meets the electrical spec but does not fit the housing is a redesign, not a sample.

The drawing should show the pack outline, the position of the leads, the connector type and orientation, and the clearance around the pack for swelling and for the wiring. Lead length and gauge follow from the current and from where the connector sits in the device. Specify both, and specify the connector mating part so there is no ambiguity about what the pack plugs into.

Enclosure material and construction follow from the environment and from any mechanical loads the pack sees, but they also have to be decided early because they change the internal layout and the thermal path. Add the tolerances you can accept to the drawing, and mark the surfaces that must stay flat or clear. Manufacturing a pack to a drawing with no tolerances produces parts that fit some units and not others.

A simple 3S2P pack for a handheld instrument and a 48V class pack for a larger machine follow the same rule: the configuration is only settled once the enclosure, leads and connector are on the same drawing as the cells and the BMS. Send that drawing with the electrical requirements so both are reviewed together.

Step 5: environment and target-market documents

The environment the pack operates in decides several specification items at once. State the operating temperature range for charge and for discharge separately, since they are usually not the same. Add humidity, vibration, shock and any exposure to dust or water that the device is rated for, and describe how the pack is mounted, because vibration through a rigid mount is not the same as through a cushioned one.

Transport and market documentation are a separate workstream, and it should start with the spec rather than after the design. Tell the supplier which market the product ships to and what the device itself must hold, so the pack design does not create a documentation gap. UN38.3, IEC 62133 and MSDS documents can be confirmed and provided for specific models; tell us the model you are buying and we will confirm which documents cover it. Cell selection for a target market can affect the configuration, so this belongs in the first conversation.

The company's own quality system supports this step. We are ISO 9001 and ISO 14001 certified and hold National High-Tech Enterprise status, with multiple utility model patents in pack design. Those credentials matter to procurement, but they do not replace the model-level document confirmation your shipment needs.

Describe the environment and the target market in the same document as the electrical requirements. When the operating conditions and the destination are known, the cell selection, protection thresholds, structure and documentation can all be evaluated against the same inputs.

The one-page spec sheet to send with your inquiry

A single page with the right inputs is enough to start. Write it in the order the design is actually checked, so nothing is decided in isolation.

Cover the device and its application in a few lines: what it powers, how it is used, and the duty cycle. Then the available space and the drawing, including the connector position. Add the target voltage range with the conditions at each end, the capacity derived from load current and runtime, and the continuous and peak current with the peak duration. Round it out with the operating environment and the target market and documents required.

Project flow follows from that page. The device, application, space, voltage, capacity, current, environment and target-market requirements are submitted first. Cell selection, series and parallel configuration, protection and management circuitry, structure and communication options are then evaluated. Samples are developed after the specification and drawings are confirmed, verified against the agreed test items, and the documentation for the target market is checked. Production, packaging, quantities, delivery and acceptance conditions are confirmed last.

Use arithmetic on your own inputs to sanity-check the sheet before sending it. If the load current and runtime give a capacity, compare that with the space available and confirm both can be satisfied at your voltage. If they cannot, the spec needs adjusting before sampling, not after.

You can send your spec as a drawing plus a requirements page, and it will be reviewed as one package rather than as separate electrical and mechanical questions.