VDA 355 Battery Pack Design: Series & Parallel Guide

Short answer: For a 400 V nominal system, 27 × 1P4S modules in series gives 395 V nominal (302–470 V window); 18 × 2P6S gives 396 V nominal. An 800 V system needs 54 × 1P4S in series for 791 V nominal. The minimum inverter voltage, not the nominal target, determines the required module count.

Designing a battery pack around standardised modules reduces development risk and shortens procurement cycles—provided the series and parallel configuration is calculated from the right inputs. This article works through the arithmetic for three NMC VDA 355 module variants available from the Avantis Energy catalogue, then addresses the less obvious constraints: cold-temperature voltage floors, inverter compatibility and the additional balancing demands of parallel strings.

The three module variants: key figures

All figures in the table below are taken from the Avantis Energy catalogue. Chemistry is NMC throughout. Lead times are stated per product on the product pages.

Module	Nominal voltage	Voltage window	Energy	Mass
1P4S	14.64 V	11.2–17.4 V	2.59 kWh	11.85 kg
2P6S	22.02 V	16.5–26.1 V (0 to 55 °C) / 13.2–26.1 V (−30 to 0 °C)	2.55 kWh	12.2 kg
3P4S	14.68 V	11.2–17.4 V	2.59 kWh	12.2 kg

The 1P4S and 3P4S share the same 4S cell series string and therefore the same voltage window, despite the 3P4S carrying three parallel cell groups. The 2P6S uses a 6S string, giving a higher per-module voltage and—critically—a different cold-temperature floor.

For a broader look at how cell configuration and chemistry interact within this format, see LFP vs NMC in a VDA 355 module: chemistry compared.

Why the minimum voltage drives the series calculation

A common mistake in pack design is to size the series string to hit the nominal pack voltage and then verify compliance with the minimum cell voltage. The correct sequence is the opposite:

1. Establish the minimum operating voltage of the inverter or motor controller.
2. Calculate how many modules in series are needed so that the pack voltage at minimum state of charge (the low end of the window) stays above that threshold.
3. Verify that the resulting nominal and maximum voltages fall within the converter's accepted range.

This matters particularly with the 2P6S module. At temperatures below 0 °C, its low-end window drops from 16.5 V to 13.2 V per module. A string sized on the warm-temperature floor may fall below the inverter's minimum in cold conditions unless the BMS applies a tighter discharge limit at low temperature. The 1P4S and 3P4S do not have a separately stated cold floor in the catalogue; the 11.2 V lower limit applies across the operating range.

Worked example: 400 V system

Using the 1P4S module

• Modules in series: 27
• Nominal pack voltage: 27 × 14.64 V = 395 V
• Voltage window: 27 × 11.2 V = 302 V (min) to 27 × 17.4 V = 470 V (max)
• Pack energy (single string): 27 × 2.59 kWh ≈ 70 kWh
• Pack mass (single string): 27 × 11.85 kg ≈ 320 kg

Whether 302 V is enough depends on one number only: the minimum input voltage of the inverter or motor controller you are pairing it with. If that minimum sits higher, add a module in series (28 × 1P4S = 410 V nominal, 314 V minimum) rather than narrowing the usable state-of-charge window.

Using the 2P6S module

• Modules in series: 18
• Nominal pack voltage: 18 × 22.02 V = 396 V
• Voltage window (warm): 18 × 16.5 V = 297 V (min) to 18 × 26.1 V = 470 V (max)
• Voltage window (cold, below 0 °C): 18 × 13.2 V = 238 V (min)

The cold minimum of 238 V is almost 60 V below the warm figure. Check it against the inverter's minimum input voltage: if the inverter cuts out above 238 V, either the BMS restricts discharge depth at low temperature—raising the effective floor—or the string gets more modules in series. For a pack that has to deliver below freezing, this is the number that decides the series count. This is not a catalogue defect; it is a system design parameter that must be resolved at the BMS and thermal management level.

Worked example: 800 V system

Using the 1P4S module

• Modules in series: 54
• Nominal pack voltage: 54 × 14.64 V = 791 V
• Voltage window: 54 × 11.2 V = 605 V (min) to 54 × 17.4 V = 940 V (max)
• Pack energy (single string): 54 × 2.59 kWh ≈ 140 kWh

Verify that the maximum of 940 V is within the converter's absolute maximum rating before adopting this configuration. If the converter's limit is lower, the BMS must enforce a charge voltage limit below 17.4 V per module, at the cost of some usable energy.

Parallel strings: what changes

When a single string does not provide sufficient capacity, two or more identical strings can be connected in parallel. The voltage and energy scale linearly, but several constraints emerge:

• State-of-charge matching at connection: strings must be at near-identical voltage before being connected in parallel, otherwise large equalisation currents flow through the busbars. The BMS architecture must support string-level isolation or pre-charge.
• Current distribution: in a passive parallel arrangement, current sharing between strings depends on the relative internal resistance of each string. As cells age at different rates, imbalance can grow. Monitoring per-string current is recommended.
• Module-level balancing scope: what is included with each module—temperature sensors, a cell supervision circuit, passive balancing electronics—varies by product. Buyers should confirm the monitoring scope per module when specifying a multi-string pack; the VDA 355 module range lists the modules to ask about.
• Protection and fusing: each string typically requires individual overcurrent protection to prevent a faulted string from being back-fed by healthy strings.

For packs outside road vehicles, IEC 62619 (safety requirements for secondary lithium cells and batteries for use in industrial applications) is the standard the system is assessed against; road-vehicle packs follow UN ECE Regulation 100. See iec.ch for the current edition.

Compliance and documentation

EU Battery Regulation 2023/1542 introduces traceability, carbon footprint declaration and a digital battery passport for electric-vehicle batteries and industrial batteries above 2 kWh. These requirements bear on pack-level documentation as much as on cell supply. Designing around a standardised module format simplifies the data aggregation: module-level lot traceability can be linked to a consistent mechanical and electrical envelope. The regulation text is available via EUR-Lex.

For a summary of how EU regulations affect lithium battery procurement, understanding EU regulations for lithium batteries provides a useful reference.

For a comparison of VDA 355 against the larger MEB 590 format—relevant if your pack envelope allows either—see VDA 355 vs MEB 590: which battery module format fits your pack?.

Next steps

Review the full module datasheets and confirm inverter voltage limits against the window figures above before committing to a series count. Browse the VDA and MEB battery modules from Avantis Energy for current catalogue specifications, or contact Avantis Energy with your pack voltage, energy and operating temperature requirements.

Frequently asked questions

How many VDA 355 modules are needed for a 400 V battery pack?
Using the 1P4S NMC module (14.64 V nominal), 27 modules in series gives 395 V nominal and a 302–470 V operating window, with approximately 70 kWh in a single string. The 2P6S module reaches 396 V nominal with 18 modules in series, but its cold-temperature voltage floor requires careful BMS management.

What determines the number of modules in series?
The minimum operating voltage of the inverter or motor controller sets the floor. Calculate the lowest pack voltage—modules in series multiplied by the module's minimum cell voltage—and verify it exceeds the inverter's cutoff. Nominal voltage is a secondary check, not the primary constraint.

How does cold temperature affect the 2P6S module in a 400 V system?
Below 0 °C, the 2P6S lower voltage limit drops from 16.5 V to 13.2 V per module. For an 18-module string, this gives a cold minimum of 238 V against 297 V in the warm band. If the inverter's minimum input voltage lies above 238 V, the BMS must enforce a higher discharge floor at low temperature, or the string needs more modules in series.

What are the considerations when connecting VDA 355 strings in parallel?
Strings must be voltage-matched before connection to avoid equalisation current spikes. Per-string current monitoring is advisable because ageing imbalance affects current sharing. Each string should have individual overcurrent protection. The module-level monitoring scope—sensors, supervision circuits—varies by product and must be confirmed per module.

How many 1P4S VDA 355 modules are needed for an 800 V system?
54 × 1P4S modules in series gives 791 V nominal and a 605–940 V window, with approximately 140 kWh per string. Verify that 940 V does not exceed the inverter's absolute maximum rating; if it does, the BMS must apply a charge voltage limit below 17.4 V per module.