Causes and Solutions for Voltage Drop at Inverter Startup

A voltage drop when switching on the inverter is one of the most common and frequently misinterpreted issues in real-world use with lithium batteries. Many users quickly attribute this phenomenon to statements such as “the battery is too weak”, “the voltage is not sufficient”, or “the power cannot be supplied”. From a technical perspective, however, this explanation is not precise.

The voltage drop during inverter start-up is not a problem caused by an incorrect voltage parameter, but the result of the interaction between transient power supply capability and system impedance. Anyone who truly wants to understand and solve this problem must consider the inverter’s start-up characteristics, the battery’s transient discharge capability, and the electrical properties of the entire DC side together.

1. Inverter Start-Up Moment and Steady-State Operation

Before discussing voltage drop, one basic fact must first be clear: the start-up moment of an inverter and its stable operation at rated power are two completely different operating states.

1.1 Current Characteristics During Start-Up

When the inverter is switched on, several processes must take place at the same time:

• Rapid charging of the internal capacitors
• Switching of the power semiconductors
• Building up a stable AC output voltage

During this process, a very high inrush current occurs briefly on the DC input side.

This current is usually 2 to 5 times higher than the inverter’s steady-state operating current and may be even higher under certain load conditions.

That is exactly why an inverter that can operate without problems in steady-state conditions may still cause a voltage drop or protection shutdown during the start-up moment.

2. Direct Cause: Transient Voltage Drop

The voltage drop during the inverter’s start-up moment is not a random phenomenon, but can be clearly explained using the fundamentals of electrical engineering.

2.1 Mechanism Behind Transient Voltage Drop

During the inverter’s start-up moment, the battery voltage can be simplified as follows:

The total system impedance includes not only the battery itself, but consists of several parts:

• The internal resistance of the battery
• The on-resistance inside the BMS
• The resistance of the connecting cables
• The contact resistance of terminals, fuses, and connectors

When the inrush current rises suddenly, even seemingly very small resistances can cause a clearly measurable voltage drop during this brief moment.

2.2 Why No Problem Is Often Visible at No Load or Under Light Load

At no load or under light load:

• The current is very small.
• The voltage drop is practically negligible.

In this state, different batteries and different systems behave very similarly at the terminals. However, this is exactly what hides the real performance differences of the system under extreme conditions.

3. BMS Protection Mechanism During Start-Up Voltage Drop

When discussing voltage drop during inverter start-up, the BMS is often mistakenly seen as the cause. However, this view is not correct.

3.1 The Nature of the BMS

The main task of a BMS is safety control, not performance optimization. It essentially evaluates only two things:

• Whether the current exceeds the safe limit
• Whether the voltage falls below the safe threshold

As soon as one of these conditions is met, the BMS follows its protection logic and limits current or shuts down the output.

3.2 Typical Trigger Path During Start-Up

During the inverter start-up process, the trigger sequence often occurs as follows in practice:

• The inrush current rises sharply within a very short time.
• The total system impedance causes a momentary voltage drop at the terminal.
• The terminal voltage falls below the undervoltage threshold set in the BMS.
• The BMS detects an abnormal condition and disconnects the output.

From the user’s perspective, this appears as a sudden battery shutdown. From a technical perspective, however, the BMS is simply carrying out the correct protection function according to its safety logic.

4. Why 12V Lithium Batteries React Differently

The differences between various 12V lithium batteries become especially clear during inverter start-up. The reason is not the nominal voltage, but several decisive technical factors.

4.1 Differences in Internal Resistance

The lower the internal resistance of the battery, the smaller the voltage drop at the same inrush current. Internal resistance is influenced by several factors:

• Cell material and manufacturing process
• Cell capacity and cell size
• Series/parallel design and internal connection structures

Under highly dynamic discharge conditions, this difference is directly reflected in voltage stability at the terminals.

4.2 Differences in BMS Parameters

The settings of different BMS systems are not identical. Differences may include, for example:

• The trigger point of the undervoltage protection function
• The threshold of overcurrent protection
• The permitted duration of inrush current peaks

Even if two batteries have similar cell performance, their start-up behavior can differ significantly due solely to different BMS strategies.

4.3 Amplifying Effect of System Connection Impedance

Under transient high-current conditions, the influence of the connection path must not be underestimated. The following factors increase total impedance:

• Cables that are too long
• Cable cross-section that is too small
• Asymmetrical wiring

These factors further increase the voltage drop during start-up.

5. Why More Capacity Does Not Automatically Solve the Problem

A common misconception is: if you simply use a battery with a larger capacity, the voltage drop during inverter start-up will disappear. From a technical perspective, this assumption is only partially correct.

5.1 Relationship Between Capacity and Transient Power Capability

Battery capacity in Ah primarily describes:

• How long the battery can deliver energy at a lower C-rate

The inverter start-up problem, however, concerns something different:

• Whether a sufficiently high transient current can be delivered within a very short time
• Whether the terminal voltage remains above the protection threshold while doing so

These two aspects do not have a simple one-to-one relationship.

5.2 Limits of a Pure Capacity Upgrade

If only the capacity is increased without also changing the following:

• The cell structure
• The internal resistance level
• The BMS protection strategy

then the transient behavior of the system during start-up often does not fundamentally improve.

6. Solutions for Voltage Drop During Start-Up

Truly effective solutions must always start at the system level, not just at a single component.

6.1 Reduce the Total Impedance of the System

This is the most direct and fundamental measure. It includes:

• Using shorter and thicker DC cables
• Designing positive and negative cable routing symmetrically
• Reducing unnecessary transitions, connectors, and connection points

Under transient high-current conditions, these improvements are often more effective than simply replacing the battery.

6.2 Match the Inverter and Battery in Terms of Start-Up Characteristics

Already during the selection phase, attention should be paid not only to the inverter’s rated power, but above all to:

• The characteristics of the inrush current
• The allowable input voltage sag during start-up

The degree of matching between inverter and battery directly determines the stability of the system during the start-up moment.

6.3 Choose Battery Designs with Higher Transient Discharge Capability

In practice, the decisive factors are primarily:

• Transient discharge capability
• The level of internal resistance control
• The BMS strategy for handling inrush current peaks

These factors are often much more important than simply looking at Ah capacity or rated discharge current.