BMS Requirements for Custom Battery Packs: Protection, Balancing,Fuel Gauging, and Communication
A custom battery pack is not finished when the cells are welded together. For a lithium pack, the battery management system - the BMS - is what keeps the pack inside its safe operating area, extends service life, and tells the host product what the battery can still do.
That is why the phrase “include a BMS” is not a real specification. A BMS can be a simple protection board that disconnects the pack during a fault, or it can be a smart embedded system that measures every cell group, estimates state of charge and state of health, balances the pack, communicates over CAN or SMBus, logs faults, and coordinates charging.
The cost, firmware work, validation effort, and integration risk are completely different between those two cases. The goal is not to buy the most expensive BMS or the cheapest one. The goal is to define the BMS your product actually needs.
For OEM custom battery packs, the BMS requirements usually fall into four jobs:
Protection
Cell balancing
Fuel gauging: SoC, SoH, and sometimes power limits
Communication with the host product or charger
If you define those four jobs clearly, the rest of the pack design becomes much easier to quote, build, and validate.
First principle: the BMS measures, estimates, then acts
A BMS is an electronic control system. It does not magically know whether a pack is safe or full. It starts by measuring the pack, then estimates the states that cannot be measured directly, and finally acts on those values.
The main measurements are:
Cell-group voltage. In a multi-series pack, the BMS should monitor each series group, not only the total pack voltage. Overcharge or over-discharge happens cell group by cell group.
Pack current. Current is usually measured with a shunt resistor or Hall-effect sensor. This supports overcurrent protection, coulomb counting, and power-limit calculations.
Temperature. Thermistors are placed near expected hot spots: high-current cells, dense cell groups, BMS power components, or areas with poor airflow.
From those inputs, a smart BMS estimates state of charge, state of health, available power, and fault conditions. Then it acts by opening MOSFETs or contactors, limiting charge/discharge, balancing cells, or reporting data to the host.
One important design point: the BMS can only protect what it can measure. Poor sensor placement, inaccurate voltage measurement, or a current sensor outside its useful range can make a pack look safe on paper and fail in the field.
Job 1: Protection - the non-negotiable requirement
Protection is mandatory for lithium packs. Even a basic protection circuit must keep the cells within the limits defined by the cell datasheet and the pack design.
A typical lithium BMS protects against:
Cell over-voltage during charging. If any cell group reaches its maximum voltage, charge must stop.
Cell under-voltage during discharge. Deep discharge can permanently damage lithium cells and make later charging unsafe.
Over-current during charge or discharge. Current limits must protect the cells, interconnects, BMS switches, wiring, connectors, and fuse strategy.
Short circuit. A near-instant disconnect is needed when the output terminals are shorted.
Over-temperature and under-temperature. The BMS should block operation outside the cell’s safe temperature range. Charging lithium cells when too cold is especially risky because it can cause lithium plating.
The thresholds cannot be generic. A 4.2V NMC cell, a 3.65V LFP cell, and a high-power specialty cell do not share the same safe voltage and temperature limits. The BMS settings must match the selected cell, the charger, the load profile, and the desired cycle-life target.
Protection timing also matters. A motor-start pulse may be acceptable for two seconds but not for twenty. A good BMS spec defines both the current limit and the allowed duration, then validates that the BMS, cells, connectors, and interconnects survive the real duty cycle. For higher-risk packs, the design may also need secondary protection, independent over-voltage cutoff, fuses, thermal cutoffs, or redundant sensing; depending on the application, certification path, energy level, and consequence of failure. As a rule of thumb, the more energy stored and the worse the failure consequence, the more that second, independent layer earns its place.
For higher-risk packs, the design may also need secondary protection, such as independent over-voltage cutoff, fuses, thermal cutoffs, or redundant sensing. Whether that is necessary depends on the application, certification path, energy level, and consequence of failure.
Job 2: Cell balancing - keeping the series string usable
Cell balancing matters when cells are connected in series. In a series string, the same current flows through every cell group, but the groups are never perfectly identical. Small differences in capacity, self-discharge, temperature, and aging cause them to drift apart over time.
When that happens, the strongest and weakest groups limit the pack:
During charge, the highest-voltage group reaches the upper limit first and stops the pack from charging further.
During discharge, the lowest-voltage group reaches the cutoff first and stops the pack from delivering more energy.
The result is lost usable capacity. The pack may still contain energy, but the BMS must stop because one group has reached a limit.
There are two main balancing approaches.
Passive balancing is the most common approach for small and mid-size OEM packs. It bleeds a small amount of current from higher-voltage cell groups through resistors, usually near the top of charge. It is simple, low cost, reliable, and easy to validate. The tradeoff is that the excess energy is dissipated as heat, and balancing currents are usually modest.
As a practical guide: a 1S pack does not need balancing; a 4S pack may need basic passive balancing; and a 7S, 10S, 13S, or higher pack should treat balancing as a normal part of the design unless there is a clear reason not to. What drives the need is the series count and the cell-to-cell spread, not the pack voltage on its own.
Job 3: Fuel gauging - SoC, SoH, and available power
A protection-only BMS can keep a pack safe, but it may not tell your product how much energy is left. That is the job of fuel gauging.
The most common BMS estimates are:
State of Charge (SoC): how full the battery is now.
State of Health (SoH): how much capacity and power capability remain compared with a new pack.
State of Power (SOP) or power limits: how much current or power the pack can safely deliver or accept right now, based on temperature, voltage, SoC, and resistance.
SoC cannot be measured directly. It must be estimated from voltage, current, temperature, and a cell model.
Common SoC methods include:
Voltage-based estimation. Simple and low cost, but inaccurate under load because voltage sag makes the pack look emptier than it is. It works best after the pack has rested.
Coulomb counting. The BMS integrates current in and out of the pack. This is good during dynamic use, but sensor offset creates drift over time, so it needs correction.
Model-based estimation. A fuel-gauge IC or BMS algorithm combines voltage, current, temperature, and a model of the cell. This can be very accurate when the cell is well characterized, but it needs more development and validation.
The right level depends on consequence. A simple handheld product may only need LEDs. A medical device, robot, or industrial product that plans work around remaining runtime usually needs a real percentage, fault reporting, and possibly SoH for service or warranty.
Do not promise fuel-gauge accuracy unless the cell, temperature range, load profile, and algorithm have been validated together. A pack can show perfect SoC on the bench and still be wrong in the field if the cell model does not match the real application.
Job 4: Communication - matching the host system
Communication is where many BMS projects fail late. The pack may charge, discharge, and protect correctly, but if it cannot talk to the host product in the expected format, the product still cannot ship.
Common communication levels include:
No communication. The BMS only protects the pack. The product sees pack voltage and maybe an LED indicator.
Simple signals. Thermistor output, low-battery signal, charge-enable line, or basic fuel-gauge LEDs.
SMBus or I2C. Common in smart battery packs, portable instruments, and some medical or industrial devices.
UART. Useful for simple embedded products and diagnostics.
CAN bus. Common in robotics, AGVs, AMRs, industrial equipment, and higher-power systems.
RS-485. Useful in noisy industrial environments or longer cable runs.
Bluetooth. Useful for service apps, setup, and diagnostics, but it should not be the only safety mechanism.
The protocol is only the first layer. The host also needs the right message map, units, update rate, fault definitions, charge/discharge enable behavior, and wake/sleep behavior. For smart packs, authentication or approved-pack logic may also be required.
This should be defined early. Changing a connector is easy compared with changing BMS firmware, host firmware, charger behavior, and certification samples late in the project.
How much BMS does your product need?
Use the table below as a practical starting point.
The key rule is simple: do not under-spec safety, and do not over-spec intelligence. A robot that needs mission planning should not use a bare protection board. A simple device that only needs a low-battery LED should not pay for a complex CAN-enabled BMS with unused firmware features.
BMS requirements checklist for an OEM quote
Before requesting a custom battery pack quote, prepare these BMS requirements if possible:
Prepare as many of these as you can before requesting a quote: cell chemistry preference (NMC, NCA, LFP, LiPo, or unsure); series/parallel target or product voltage limits; continuous current, peak current, peak duration, and repetition; charge current and charger type; temperature range for charge, discharge, and storage; required protection events and recovery behaviour; whether balancing is needed and expected charge behaviour; fuel-gauge need (LEDs, percentage SoC, SoH, runtime estimate, or service data); communication protocol; connector and pinout expectations; certification targets and markets; and prototype quantity and annual production volume. You do not need every answer to start, but each one you provide removes a round of redesign risk.
You do not need every answer before starting. But the more of these you provide, the faster a pack designer can select the right BMS architecture and avoid expensive redesign.
Safety and compliance note
The BMS and certification path are connected. Standards and test programs evaluate whether the pack responds safely to abuse conditions such as overcharge, short circuit, over-discharge, temperature extremes, and abnormal operation. UN 38.3 is transport-focused; IEC 62133-2, UL 2054, and others address different safety scopes depending on product and market (IEC 60601-1 for medical systems, UL 2271/UL 2849 for some e-mobility products, IEC 62619/UL 1973 for industrial and stationary). Confirm the exact set with a compliance specialist for each target market. The practical lesson for OEMs: design the BMS for the required test path from the beginning, because waiting until the pack is built can force changes to thresholds, firmware, wiring, cells, enclosure, or samples.
For OEMs, the practical lesson is this: design the BMS for the required test path from the beginning. Waiting until the pack is already built can force changes to protection thresholds, firmware, wiring, cells, enclosure, or samples.
Internal fact-check list before publishing
Common BMS mistakes
Saying “include a BMS” with no functional detail. Protection, balancing, gauging, and communication are separate decisions.
Using generic protection thresholds. The BMS must match the actual cell, charger, and load profile.
Ignoring pulse duration. Peak current protection is meaningless without a time limit and recovery condition.
Skipping balancing on a multi-series pack. This can reduce usable capacity over time.
Expecting voltage-only SoC to work under dynamic load. Voltage sag can make the gauge misleading.
Defining communication too late. Protocol, message set, and fault behavior must match the host.
Forgetting the charger. BMS behavior and charger behavior must be designed together.
What this means for your custom battery pack project
A BMS is not an accessory. It is part of the pack architecture. It protects the cells, preserves usable capacity, reports the pack’s condition, and connects the battery to the product’s electronics.
For TapRen, the right BMS discussion starts with the product: What voltage range can it tolerate? What are the continuous and peak currents? How bad is an unexpected shutdown? Does the host need SoC, SoH, fault codes, or CAN messages? Which certifications will the final product need?
From there, the BMS can be matched to the pack instead of forced onto it. The protection thresholds are selected for the actual cells. Balancing is chosen based on the series count. Fuel gauging is chosen based on how much the user or system relies on the number. Communication is matched to the host electronics and charger.
With TapRen’s electronics and PCB background, the BMS is not a black box bolted onto the cells. It is part of the custom battery pack design. That matters when the pack has to fit a real product, pass a real test plan, and scale to production without surprise failures.
Ready to define the BMS for your custom battery pack? Request a custom battery pack quote and send whatever you know about your cells, voltage, current, charger, communication, and certification targets. We can help turn that into a practical BMS specification.
Project-specific note: Battery pack design values, achievable performance, test scope, pricing, lead time, and compliance requirements depend heavily on the selected cells, BMS, charger, enclosure, duty cycle, environment, target markets, order volume, and manufacturing process. Consult TapRen for project-specific recommendations, current component availability, and accurate engineering and commercial data.
Editorial Note:
This article was developed with the assistance of AI tools to help organize the content, improve readability, and create supporting visual elements. TapRen engineers reviewed the technical content for accuracy and practical relevance.
Battery design requirements can vary significantly depending on the application, cell selection, operating conditions, certification requirements, and other project-specific factors. If you have a question about anything discussed in this article, notice something that deserves clarification, or would like to share your technical perspective, we welcome your feedback.
Please leave a comment or contact TapRen directly. We appreciate thoughtful questions and technical feedback that help us continue improving the quality and accuracy of our content.
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