How to Design Custom Battery Management System (BMS)

Why Custom BMS Design Can be Helpful for Your Product?

A team building a battery-powered product starts with an off-the-shelf BMS module because it's the fastest way to get a working prototype. Six months later, the product needs a cell configuration the module doesn't support, protection thresholds tuned for a battery chemistry the module wasn't designed around, and a communication protocol the module's fixed firmware simply doesn't expose.

How to design a custom battery management system becomes the real question once an off-the-shelf module's limits collide with what the actual product needs and getting the answer right the first time avoids a redesign after the product is already close to shipping. This post covers why custom BMS design makes sense in the first place, and the factors that actually determine whether a custom BMS performs safely and reliably in the field.

How to Design a Custom Battery Management System: Why Go Custom

Off-the-shelf BMS modules exist because they solve the common case well - a fixed cell count, standard protection thresholds, and a generic communication interface. The custom BMS vs off-the-shelf BMS decision almost always comes down to a specific mismatch between what a product needs and what a generic module was built to support.

Factor	Off-the-shelf BMS	Custom BMS
Cell configuration	Fixed series/parallel count, limited options	Matched exactly to the product's pack design
Protection thresholds	Generic, often set conservatively for a wide range of chemistries	Tuned precisely to the specific cell chemistry and use profile
Communication protocol	Usually fixed (often UART or a proprietary protocol)	Designed to match the host system - CAN, I2C, custom protocol
Form factor	Fixed board size and connector layout	Sized and shaped for the actual product enclosure
Certification path	Module may carry its own certification, but integration still needs validation	Full control over the certification and validation process
Unit cost at volume	Higher per-unit cost, no NRE	Lower per unit cost at real volume, but real NRE upfront

A custom BMS earns its development cost when the product's cell configuration, form factor, or protection requirements genuinely don't fit an available module not by default, and not because custom sounds more capable. For a low-volume product with a standard cell configuration, an off-the-shelf module is often still the right call.

Factors to Get Right When Designing a Custom BMS

Cell Balancing: Passive vs Active

BMS cell balancing methods exist to correct for the fact that individual cells in a pack drift apart in charge level over time due to manufacturing variance and unequal aging, and an unbalanced pack loses usable capacity and stresses the most-charged cell disproportionately.

Balancing Method	How It Works	Efficiency	Best Fit
Passive balancing	Bleeds excess charge from higher voltage cells through a resistor	Lower - energy is dissipated as heat	Lower cost products, smaller packs, less frequent balancing need
Active balancing	Transfers charge from higher-voltage cells to lower-voltage ones	Higher - energy is redistributed, not wasted	Larger packs, products where every percentage of capacity matters

Passive balancing is simpler, cheaper, and adequate for most consumer and light industrial products, since the energy lost to balancing is small relative to the pack's total capacity. Active balancing earns its added circuit complexity in large packs - EVs, grid storage, larger industrial equipment - where the capacity and lifespan gains from precise balancing are worth the extra bill of materials cost.

Protection Thresholds: Tuned to the Actual Chemistry

Every custom BMS needs overvoltage, undervoltage, overcurrent, and thermal protection, and the actual threshold values have to match the specific cell chemistry in use, not a generic default. A threshold set too conservatively wastes usable capacity; a threshold set too loosely risks cell damage or a safety incident.

Overvoltage and undervoltage cutoffs should come directly from the cell manufacturer's datasheet for the specific chemistry (LiFePO4, NMC, and others all have different safe voltage windows), not a generic lithium-ion assumption
Overcurrent protection needs both a fast-response threshold for short-circuit conditions and a separate, more conservative threshold for sustained high-current draw that would overheat cells gradually
Thermal cutoffs should account for where the temperature sensor actually sits relative to the hottest cell in the pack — a sensor on the pack's exterior reads lower than the hottest interior cell during heavy load

State of Charge Estimation: Choosing the Right Algorithm

An accurate state of charge (SoC) estimate is what makes a "battery percentage" on a product's display trustworthy, and the algorithm choice trades off complexity against accuracy.

Method	Accuracy	Complexity	Best fit
Voltage based lookup	Lower - voltage alone is a weak proxy for charge under load	Low - simple lookup table	Basic products where rough SoC is acceptable
Coulomb counting	Moderate - accurate short-term, drifts over time without correction	Medium - requires current sensing and periodic recalibration	Most consumer and industrial products
Kalman filter / model-based estimation	Highest - combines voltage, current, and a cell model	High - significant firmware and tuning effort	Products where accurate SoC materially affects the user experience or safety margin

Coulomb counting combined with periodic voltage-based recalibration at known reference points (like a full charge) covers the large majority of products well, without the tuning burden a full Kalman filter implementation requires.

Communication Protocol: Matching the Host System

The BMS has to report cell voltages, pack current, temperature, and fault state to whatever system is consuming that data, and this interface is one of the biggest reasons teams go custom in the first place - an off-the-shelf module's fixed protocol rarely matches an existing product's host communication stack cleanly. CAN bus is common in automotive and industrial equipment already using CAN elsewhere in the system; I2C fits tightly-coupled embedded designs where the BMS and host controller sit close together on the same board; a custom UART protocol offers full flexibility when neither standard fits the product's existing architecture.

Safety Certification: Plan the Path Early, Not at the End

Battery protection circuit design for a product headed to market typically needs to account for relevant safety standards - UL 2054 for battery pack safety, IEC 62133 for portable battery safety, and UN 38.3 for transport of lithium batteries are common requirements depending on the product's chemistry, market, and shipping needs [verify before publishing]. These standards influence protection circuit design decisions, documentation requirements, and testing procedures from early in the design, not just at final certification.

Bringing in certification guidance early avoids a common and expensive mistake: finalizing a protection circuit design, then discovering during certification testing that a threshold or a redundancy requirement the standard demands wasn't built in from the start.

Factors That Show Up in Firmware, Not Just Hardware

BMS firmware design considerations are just as important as the analog protection circuit, because the firmware is what actually interprets sensor data, makes protection decisions, and communicates state to the rest of the system.

Design the fault state machine explicitly - what happens when a fault clears versus when it persists, and whether a fault requires a manual reset or clears automatically, needs to be a deliberate decision, not default behavior
Log fault history, not just the current state, since post-incident diagnosis depends on knowing what happened leading up to a fault, not just that one occurred
Test firmware against real fault injection, not just normal operating conditions - simulating an actual overcurrent event or a sensor failure surfaces bugs that clean-path testing never will

Checklist: Before You Commit to a Custom BMS Design

Answering these covers most of how to design a custom battery management system without discovering a gap mid-project:

Does the product's cell configuration, form factor, or protection requirement genuinely not fit an available off-the-shelf module?
Has the balancing method (passive or active) been chosen based on pack size and capacity requirements, not just picked by default?
Are protection thresholds set from the specific cell chemistry's datasheet, not a generic lithium-ion assumption?
Does the SoC estimation approach match how accurate the product's battery percentage actually needs to be?
Has the communication protocol been matched to the host system's existing architecture?
Has a certification path been identified early enough to influence the protection circuit design, not discovered during testing?

Custom Pays Off When the Requirements Actually Demand It

How to design a custom battery management system comes down to matching cell balancing, protection thresholds, SoC estimation, and communication protocol to what the specific product actually needs - not defaulting to custom because it sounds more capable, and not forcing an off-the-shelf module to fit requirements it was never built for. Getting these factors right from the start, alongside an early certification path, avoids the redesign that shows up when a mismatch surfaces late in development. If you're scoping a custom BMS and want a second opinion on cell balancing strategy, protection design, or certification path before committing to an architecture, CoreFragment's hardware team can review your requirements and flag the trade-offs early.