Battery Voltage Drops Suddenly Under Load

Problem Description

• Case #010
• Fault Name: Battery Voltage Drops Suddenly Under Load
• The battery shows a healthy SOC (e.g., 60%). However, the moment a heavy load (like a water pump, air conditioner, or microwave) turns on, the overall battery voltage sags dramatically. This sudden voltage drop often causes the inverter to trigger a "Low Battery Voltage" alarm or even shut down the system entirely.

Symptoms

• Inverter screen shows battery voltage dropping instantly from 52.0V to 47.0V as soon as a heavy load kicks in.
• Lights flicker severely when appliances start.
• Once the heavy load turns off, the battery voltage instantly "bounces back" to 52.0V.
• The BMS may trigger a temporary "Under-Voltage" alarm that disappears when the load stops.

Possible Causes

• Loose DC Terminals (High Contact Resistance): A loose nut on the battery terminal or inverter input is creating a bottleneck.
• Undersized DC Cables: The cables connecting the battery to the inverter are too thin for the high discharge current (Ohm's Law: Voltage Drop = Current × Resistance).
• High Battery Internal Resistance (IR): The lithium cells are heavily degraded/aged, or the battery's C-rate is too low to support such a massive surge current (e.g., pulling 150A from a single 100Ah battery).
• BMS MOSFET Resistance: The internal switching components of the BMS are overheating or failing, adding resistance to the circuit.

How to Verify

You must trace the voltage drop under active load. Have someone turn on the heavy appliance. Use a multimeter to measure the voltage at the inverter terminals, then measure at the battery terminals. Finally, check the individual cell voltages in the BMS software to see if one specific cell is sagging, or if the whole pack is sagging.

Required Tools

• Digital Multimeter
• Thermal Imaging Camera / Infrared Temp Gun
• Torque Wrench
• BMS PC Software

Normal Values

• Voltage sag on a healthy LFP battery under 0.5C load: < 1.0V to 1.5V
• Voltage drop across DC cables (between battery and inverter): < 0.5V
• DC Cable/Terminal Temperature: Warm, but < 45°C
• Inverter terminals measure 46.0V, while battery terminals simultaneously measure 50.0V (Massive 4V loss in the cables!).
• One specific battery terminal is glowing red on the thermal camera (> 80°C).
• BMS shows one cell dropping to 2.5V under load while others remain at 3.2V.

Diagnostic Procedure

• Step 1: Do the "Thermal Check". Turn on the heavy load and scan the DC cables, battery terminals, and breakers with a thermal gun. A hot spot instantly points to a loose connection (High Resistance).
• Step 2: Do the "Voltage Delta Check". Measure voltage at the Inverter VS. Voltage at the Battery while the load is running. If the gap is >1.0V, your cables are too thin, too long, or poorly crimped.
• Step 3: If cables are fine and terminals are cool, log into the BMS. Check the Cell Voltages under load. If all cells sag equally, the battery capacity is simply too small for the load. If ONE cell sags deeply, that specific cell is degraded (High Internal Resistance).

Root Cause

Ohm’s Law dictates that Voltage Drop = Current × Resistance (V=IR). During a heavy load surge (e.g., 100A), even a tiny resistance of 0.05 Ohms from a loose nut will cause a 5-Volt drop! The inverter sees 46V instead of 51V and panics, shutting down the system. If it's not the cables, then the cells themselves have developed high Internal Resistance (IR) due to aging, meaning the voltage naturally collapses when high current is drawn.

Corrective Actions

• Tighten & Clean: Power off the system. Unbolt all DC connections. Clean any oxidation, and re-torque the nuts to the manufacturer's specification using a torque wrench.
• Upgrade DC Cables: Replace undersized cables (e.g., 4 AWG) with appropriately sized ones (e.g., 2/0 AWG or 4/0 AWG) and ensure terminal lugs are hydraulically crimped.
• Expand Capacity: If the battery cells are simply overloaded by the surge, the only solution is to install a "Soft Starter" on the heavy appliance or add another battery module in parallel to share the current load.