Technical Articles & Engineering Insights

How to Match an On-Board Charger to an EV Battery Pack

Learn how to match an EV on-board charger with battery voltage, charging current, BMS CAN control, AC input and thermal limits for reliable vehicle integration.

From an engineering perspective, OBC selection is an operating-window and system-integration problem. The charger must deliver the required voltage, current and power across the battery's charging range while remaining compatible with the BMS control strategy, AC input conditions and thermal limits of the vehicle.

Before OBC hardware and software parameters are finalized, several electrical and system-level factors should be evaluated.

1. Start with the Battery Charging Voltage Window, Not Nominal Voltage

A battery designation such as 72 V, 96 V or 320 V describes the nominal system voltage. It does not directly define the required OBC output voltage.

For charger selection, the more relevant parameters are:

Vbat,min — minimum battery voltage at which charging may begin

Vbat,max — maximum permitted pack charging voltage

Ns — number of cells connected in series

Vcell,max — maximum permitted charging voltage per cell

For a series-connected battery pack:

Vbat,max = Ns × Vcell,max

This is why two battery packs both described as “72 V” may require different OBC output settings.

Battery chemistry alone is also insufficient. Knowing that a battery is LFP or NMC does not fully define its charging voltage. The series-cell configuration and BMS charging limits must also be confirmed.

From the OBC side, the complete battery charging-voltage window should fall within the charger's regulated DC output range with appropriate engineering margin.

2. Check Whether the OBC Can Deliver Rated Power Across the Required Voltage Range

A charger rated at 3.3 kW or 6.6 kW should not automatically be assumed to deliver its rated power at every point within its output-voltage range.

The relationship between voltage, current and output power is:

Pout = Vout × Iout

For example, delivering 3.3 kW at 100 V requires approximately:

Iout = 3300 / 100 = 33 A

At 80 V, maintaining the same 3.3 kW would require:

Iout = 3300 / 80 = 41.25 A

If the OBC's maximum output-current capability is below this value, the charger becomes current-limited at the lower battery voltage and cannot maintain full rated power.

This distinction is important when evaluating a 3.3 kW or 6.6 kW on-board charger for a specific battery system.

Therefore, engineers should evaluate the OBC's voltage-current operating envelope, rather than comparing charger power ratings alone.

A useful specification should make clear both the regulated output-voltage range and the maximum available output current.

3. Battery Charge-Current Limits Must Be Matched to OBC Output Capability

The maximum permissible charging current is determined by the battery pack and its operating conditions, not by the OBC nameplate.

As an initial reference:

Ichg,max = C-rate × Battery Capacity (Ah)

For example, if a 100 Ah battery is permitted to charge at 0.5C, the corresponding charging-current limit would be approximately 50 A.

However, the allowable battery charging current is rarely a fixed value throughout the entire charging process.

The BMS may reduce permissible current according to:

state of charge (SOC);

cell temperature;

pack temperature;

cell-voltage spread;

battery protection status;

battery aging or operating strategy.

This means the OBC must not simply deliver its maximum available current whenever charging is enabled.

In a CAN-controlled system, the effective charging current may need to follow a dynamic limit requested by the BMS.

The selected OBC should therefore satisfy two conditions:

OBC maximum current ≥ required charging current

while

Actual charging current ≤ battery/BMS permitted current

These are different requirements and should not be confused.

The four-number datasheet check

Before working through the full selection process, there is a faster screen. Four numbers decide most of the compatibility question, and they only work when read side by side — one from each datasheet, on the same line.

1. Voltage window against voltage window. The OBC datasheet gives the output range it can regulate. The battery datasheet gives minimum discharge voltage to maximum charge voltage across the state-of-charge range. The OBC range must contain the battery range. A pack that tops out at 420 V behind a charger that caps at 400 V never finishes its charge — and sits at its limit, running hot, for the whole top-up.

2. Battery charge-current limit against OBC output capability. The pack decides what it will accept. Compare the OBC's maximum output current at the relevant voltage against the pack's maximum continuous charge current, at both low and high state of charge, because most BMS profiles taper the limit as the pack fills.

3. Charge curve against regulation mode. Two values must align: the CV threshold — the exact voltage at which the charger switches from constant current to constant voltage — and the taper and termination behaviour. A charger that does not follow the pack's required curve either undercharges it or stresses it.

4. CAN registers against register map. Matching the baud rate proves nothing. Message IDs, signal names, scaling, byte order, the enable command, the setpoint registers, the status feedback and the fault codes all have to line up.

Check	Read on the OBC datasheet	Read on the battery datasheet
Voltage	Regulated output range, min–max	Charge voltage range across SOC
Current	Max output current at that voltage	Max continuous charge current, low and high SOC
Profile	CV threshold, taper, termination	Required charge profile and end condition
CAN	Message IDs, signals, scaling, byte order	BMS register map and expected format
Thermal	Derating curve vs ambient	Permitted current vs cell temperature

Any row that cannot be filled is a risk carried into the prototype.

4. CC/CV Describes Charger Regulation, but Not the Complete Vehicle Charging Strategy

OBC charging is commonly described as CC/CV — constant current followed by constant voltage.

During the constant-current stage, the charger regulates output current while battery voltage increases.

As the pack approaches its upper charging-voltage limit, charging transitions toward constant-voltage operation and current gradually decreases.

However, in a vehicle with active BMS control, the actual charging process may not follow one fixed CC/CV curve from beginning to end.

The BMS may continuously update:

requested charging voltage;

maximum charging current;

charging enable/disable status;

charging termination conditions.

The effective charger output is therefore constrained by several limits simultaneously:

OBC hardware capability Battery/BMS request OBC thermal condition AC input capability System protection limits

The OBC should operate within the most restrictive valid limit at any given time.

This is an important distinction between a basic standalone charger and an OBC integrated into a vehicle-level charging control system.

5. CAN Compatibility Requires More Than Matching the Baud Rate

“CAN supported” does not necessarily mean that an OBC can communicate correctly with a particular vehicle.

Before integration, the charger supplier and vehicle engineering team should confirm the actual CAN protocol requirements, including:

CAN baud rate;

message IDs;

transmit and receive direction;

DLC;

Byte/Bit definitions;

factor and offset;

data range and units;

message cycle time;

charging enable logic;

requested voltage and current;

charger status feedback;

fault definitions;

communication timeout behavior.

Timeout strategy is particularly important.

If the OBC stops receiving a valid charging command from the BMS or VCU, the system needs a defined response. Depending on the vehicle control architecture, this may require output-current reduction, charging shutdown or entry into a communication-fault state.

Communication behavior is therefore part of charging-system safety and control, rather than simply a software-interface detail.

For a more detailed discussion, see How CAN Communication Improves EV Charger Integration with Vehicle Systems.

6. Charging Power Should Be Evaluated Against Battery Energy and Required Charging Time

OBC power should be selected according to both the battery and the vehicle's required charging time.

A first-order estimate is:

Charging time ≈ Battery energy to be replenished / Average battery-side charging power

However, dividing battery capacity directly by the OBC's rated power normally gives an optimistic result.

Actual battery-side charging power is affected by:

charger conversion efficiency;

battery voltage during charging;

battery charge-current limits;

BMS power reduction;

AC input limitations;

thermal derating;

current tapering near the end of charge;

auxiliary loads operating during charging.

For this reason, a 6.6 kW OBC does not necessarily deliver 6.6 kW to the battery continuously throughout the complete charging cycle.

When charging time is a vehicle-level requirement, engineers should evaluate the expected charging profile, rather than relying only on OBC nameplate power.

7. AC Input Conditions Can Limit Battery-Side Charging Power

OBC-to-battery matching should not be evaluated only from the DC output side.

The AC source must also provide sufficient input power for the requested battery-side charging power.

Approximately:

Pin = Pout / η

where:

Pin = AC-side input power Pout = battery-side OBC output power η = OBC conversion efficiency

The actual AC input current will then depend on input voltage, power factor and operating conditions.

This becomes important when the same vehicle platform is intended for markets with different AC supply conditions.

If the AC source, inlet, cable or upstream protection cannot support the required input current, the requested battery charging power cannot be sustained even if the OBC itself is capable of the required DC output.

OBC selection should therefore consider the complete energy path:

AC source → OBC input stage → power conversion → DC output → battery pack

8. Thermal Derating Must Be Included in OBC Selection

A charger can satisfy the required voltage and current specifications on paper but still fail to maintain the expected charging power under actual vehicle conditions.

Power conversion generates losses, and those losses become heat.

At high ambient temperature or under restricted cooling conditions, internal component temperatures may approach their allowable limits. The OBC may then reduce output power to protect the power stage and other temperature-sensitive components.

This means:

Rated power ≠ guaranteed continuous power under every thermal condition.

For commercial and specialty EV applications, engineers should confirm:

rated ambient-temperature range;

cooling method;

installation orientation;

required airflow;

thermal derating threshold;

output behavior after derating begins;

continuous-load capability under the intended installation condition.

For more detail on this issue, see How Heat Dissipation Affects On-Board Charger Performance Under Continuous Load.

This is particularly relevant for utility vehicles, agricultural vehicles, GSE and other EVs where the charger may operate in enclosed installation spaces or under demanding ambient conditions.

9. Protection Thresholds Should Be Coordinated with the Battery and Vehicle

Both the BMS and OBC provide protection functions, but their thresholds and control responses should be coordinated.

Typical OBC protections may include:

input overvoltage and undervoltage;

output overvoltage;

output overcurrent;

short-circuit protection;

overtemperature protection;

communication fault protection.

The BMS may independently monitor:

individual cell voltage;

pack voltage;

charging current;

cell and pack temperature;

insulation or other battery-system conditions.

These protections should operate as coordinated layers.

For example, the normal maximum charging-voltage command should remain below the OBC's hardware overvoltage-protection threshold with suitable margin.

Similarly, normal BMS current regulation should occur before the system repeatedly reaches the OBC's hardware overcurrent protection.

Protection trips should remain fault responses, not become part of normal charging regulation.

10. Define the OBC–BMS Interface Before Prototype Integration

Before a charger sample is installed in the vehicle, it is useful to define clearly which system has authority over each charging function.

For example:

Function	Typical Control Consideration
Charging enable	BMS / VCU command or defined charger logic
Target voltage	Battery requirement / BMS command
Current limit	BMS request + OBC hardware limit
Charge termination	BMS / vehicle charging strategy
Fault handling	OBC protection + BMS/VCU response
CAN timeout	Defined safe-state behavior
Thermal derating	OBC internal control with status feedback

This interface definition helps avoid a common prototype-stage problem: the charger and BMS are electrically compatible, but their control logic does not behave as expected when integrated into the vehicle.

11. What Information Should Be Provided to the OBC Supplier?

Before hardware selection or software calibration, the vehicle manufacturer should ideally provide a charging requirement sheet containing the following information:

Parameter	Information Required
Battery chemistry	LFP / NMC / other
Cell configuration	Number of cells in series
Nominal pack voltage	V
Battery operating-voltage range	V
Maximum charging voltage	V
Battery capacity	Ah / kWh
Maximum charging current	A
Target charging power	kW
AC input	Voltage / frequency
Communication	CAN protocol / message definition
Charging control	Enable and command logic
Thermal environment	Ambient temperature / airflow
Vehicle application	Utility EV / GSE / LSV / commercial EV
Installation constraints	Space / mounting / connector requirements

Providing these parameters before sample production allows the OBC supplier to determine whether an existing charger operating envelope is suitable and what parameters may require configuration.

For vehicle architectures that require both battery charging and low-voltage power conversion, an OBC + DC/DC integrated solution can also be evaluated during the system architecture stage.

This evaluation should be made at the architecture stage because integration affects not only packaging, but also high-voltage distribution, low-voltage output requirements, communication and thermal design.

When the numbers do not line up

Two failures are common, and both are resolved on paper rather than in the vehicle.

Either the charger is oversized for the pack: the BMS limits charge current, the extra charger power is never delivered, and the money went into heat and weight. The fix is a lower power level, or a pack with a higher charge limit.

Or the charger is undersized for the duty cycle: the pack would accept more, the charger cannot deliver it, and every session runs long. The fix is to step up a power level.

Neither problem is visible from a nominal voltage label. Both are obvious from the five-row table above.

OBC Selection Should Be Verified as an Operating Envelope

The most reliable way to match an OBC to an EV battery pack is to compare the operating envelopes of the battery, charger and vehicle system rather than matching nominal voltage labels.

The engineering review should verify:

Battery voltage window → within the regulated OBC output range

Battery allowable charging current → compatible with OBC current capability

Required charging power → achievable across the relevant battery-voltage range

BMS charging commands → compatible with OBC control and CAN strategy

AC input capability → sufficient for the required battery-side charging power

Thermal environment → allows the required continuous output without unacceptable derating

Protection and fault logic → coordinated between OBC, BMS and vehicle controller

Once these conditions are aligned, prototype testing can focus on validating the complete charging system rather than correcting fundamental specification mismatches after vehicle integration.