VDA 355 Module Balancing: Safe String Connection
Short answer: Before connecting VDA 355 modules into a string or adding a parallel string, bring all modules to a comparable state of charge and verify that open-circuit voltages are within the tolerance given in the datasheet or the pack design. A BMS balances cells during cycling; it does not substitute for correct starting conditions. Skipping voltage matching risks damaging inrush currents.
Connecting battery modules into a pack is a series of engineering decisions, not a single wiring step. The VDA 355 module, a prismatic automotive module measuring approximately 355 x 151 x 108 mm, is defined by its outer dimensions, but electrical compatibility still depends on the integrator's commissioning procedure. This guide covers what to check, in what order, and why the numbers matter.
Why Series and Parallel Connections Behave Differently
In a series string, every module carries the same current. The string voltage is the sum of all module voltages, and any module with a lower capacity than its neighbours will reach its cut-off voltage first, reducing the usable capacity of the entire string. The BMS will then terminate discharge to protect that weakest module, leaving energy stranded in the others.
Parallel connection introduces a different risk: if two strings or two modules at different voltages are connected directly, current flows instantly to equalise the potentials. The magnitude depends only on the voltage difference and the loop resistance of the circuit.
Illustrative Calculation (Hypothetical Values)
This is a worked example with invented figures to illustrate the principle; treat it as an order-of-magnitude check, not a specification.
Assume two strings with an open-circuit voltage difference of 2 V, and a total loop resistance (cabling, connectors, busbar) of 10 milliohms:
I = U / R = 2 V / 0.010 ohm = 200 A
That initial inrush current flows the moment the circuit closes. A current of that size through connectors and busbars sized for less can stress insulation and contact surfaces. The remedy is not a faster fuse; it is eliminating the voltage difference before closing the circuit.
What Cell Balancing Actually Does
A battery management system running passive or active balancing redistributes charge between individual cells within a module, or between modules within a string, during normal operation. This corrects the small divergences that accumulate over hundreds of cycles as cells age at slightly different rates.
Balancing is a maintenance function. It is not designed to absorb the kind of energy differential that exists between a module at 60 % state of charge and one at 90 %. Expecting the BMS to compensate for a poor starting condition is a misuse of the function and typically results in balancing currents that are too small to close the gap in a reasonable time, or in BMS protection trips that prevent operation entirely.
The correct sequence is: pre-condition first, then connect, then allow the BMS to manage the fine-level variation that arises during cycling.
Voltage Matching Before Parallel Connection
The acceptable voltage window for connecting modules or strings in parallel is defined by the module datasheet and the pack design specification. No generic figure applies across all configurations; a 1P4S module and a 2P6S module have entirely different nominal voltage ranges. If the datasheet does not state a parallel-connection tolerance explicitly, treat it as a request to the module supplier before proceeding.
Practical steps:
- Charge or discharge each module or string to a common target state of charge using a calibrated charger or load.
- Allow a rest period, typically one to several hours, so surface charge dissipates and the open-circuit voltage stabilises.
- Measure open-circuit voltage with a calibrated meter; do not rely on an estimated SoC alone.
- Compare measured voltages against the tolerance in the datasheet or pack specification.
- If any module or string is outside tolerance, repeat conditioning before proceeding.
Pre-Charge Circuits When Connecting to a DC Bus
When a battery string is connected to a DC bus, a common topology in vehicle packs and in DC-coupled stationary systems, the bus capacitance draws an inrush current at the moment of connection. The larger the bus capacitance and the greater the voltage difference, the larger the spike.
A pre-charge circuit addresses this by connecting the battery through a current-limiting resistor for a controlled period, allowing bus voltage to ramp up to within a few volts of battery voltage before the main contactor closes. The resistor value, pre-charge duration and acceptable voltage delta before main contactor closure come from the system design or the inverter/PDU specification, not from a generic recommendation. The module datasheet gives the module's electrical characteristics; the pre-charge parameters are derived from the full system.
Transport of cells and modules is covered by UN 38.3 (unece.org), which sets testing requirements for cells and batteries before shipment.
Commissioning Checklist
The table below summarises the work sequence for connecting VDA 355 modules into a string or adding a parallel string. Specific values, torque, voltage tolerance, insulation resistance threshold, must come from the module datasheet and the pack design document.
| Step | What to Check | Why It Matters |
|---|---|---|
| 1. Documentation review | Module datasheet, pack design spec, wiring diagram | Provides the numeric limits for every subsequent step |
| 2. Isolation check | Insulation resistance between each terminal and chassis/enclosure | Confirms no internal fault before energisation |
| 3. Visual inspection | Mechanical damage, terminal condition, correct polarity orientation | Prevents connection errors and contact resistance issues |
| 4. SoC conditioning | Charge or discharge each module/string to target SoC | Reduces voltage delta before parallel connection |
| 5. Rest and OCV measurement | Open-circuit voltage after rest period, per module or string | Verifies actual voltage, not estimated SoC |
| 6. Voltage delta check | Compare OCV readings against datasheet tolerance | Prevents damaging inrush current |
| 7. Pre-charge (if DC bus) | Connect through pre-charge resistor; confirm bus voltage ramps | Protects capacitive loads and contactors |
| 8. Main contactor close | Bus voltage within tolerance of string voltage | Safe final energisation |
| 9. BMS handshake | BMS reads all cell voltages, temperatures and confirms no faults | System ready for operation |
| 10. Initial cycle check | Observe balancing activity and cell voltage spread over first cycles | Confirms pack is operating within design envelope |
Series Strings: Keeping Cell Voltages Aligned
Once a string is in service, the BMS monitors individual cell voltages continuously. A widening spread between the highest and lowest cell in a module is the first indicator that a cell or module is diverging from the group. Integrators should log this spread over time rather than only checking it during commissioning; gradual divergence often signals a cell ageing faster than its neighbours, a connection resistance issue, or a temperature gradient in the pack.
The safety considerations for VDA 355 modules depend on the BMS having accurate and continuous cell-level data. That requires both a correctly specified cell supervision circuit and a thermal management arrangement that keeps the temperature spread across modules within the BMS calibration range.
For a detailed look at how module configurations affect voltage and energy at the string level, the VDA 355 module dimensions and specifications page provides configuration-level data as a reference.
Integrators sourcing modules should review the available configurations and associated documentation at VDA and MEB battery modules from Avantis Energy before finalising the pack architecture.
Frequently asked questions
What is VDA 355 module balancing and why is it needed?
VDA 355 module balancing is the process by which a battery management system equalises individual cell voltages within a module or across modules in a string. It compensates for gradual cell divergence during cycling. It is not a substitute for matching state of charge and voltage before initial connection.
How do I match voltage when connecting battery modules in parallel?
Charge or discharge each module or string to a common target state of charge, allow open-circuit voltage to stabilise after a rest period, then measure OCV with a calibrated meter. Only connect modules or strings in parallel if the voltage difference is within the tolerance stated in the module datasheet or pack design specification.
Why is a pre-charge circuit used when connecting a battery pack to a DC bus?
Bus capacitance draws a large inrush current the moment a battery is connected. A pre-charge circuit routes current through a resistor first, ramping bus voltage up gradually before the main contactor closes. The resistor value and pre-charge duration come from the system design, not from a module datasheet.
What happens if two strings at different voltages are connected directly in parallel?
Current flows immediately to equalise the voltage difference. Using the relationship I = U / R: a 2 V difference across a 10 milliohm loop resistance produces an initial current of approximately 200 A. Even briefly, that level of inrush can stress connectors, contactors and cell terminals beyond their ratings.
Where do I find the correct voltage tolerance and torque values for VDA 355 module commissioning?
These values are specific to each module configuration and come from the module datasheet supplied with the product and from the pack design document. No single generic figure applies across all VDA 355 configurations; always refer to the documentation for the exact module being installed.
Plan the connection before you close the string
Safe commissioning starts with the datasheet of the exact module configuration you are installing. Discuss your pack requirements with Avantis Energy before you close the string.