Runtime Much Shorter Than Expected

Problem Description

• Case #009
• Fault Name: Runtime Much Shorter Than Expected
• The customer complains that the battery's backup runtime is significantly shorter than what was promised or calculated. For example, a 10kWh battery is expected to power the house for 10 hours at a 1kW load, but the system shuts down or switches back to the grid after only 5 or 6 hours. The customer suspects the battery has "fake capacity" or is defective.

Symptoms

• The SOC percentage drops rapidly during discharge.
• The system triggers the "Discharge Cut-off" prematurely.
• The overall battery voltage is relatively high when it stops (e.g., stopping at 50V instead of 46V).
• No fault alarms on the BMS; it simply acts as if it is "empty".

Possible Causes

• Grossly Underestimated Loads: The customer added hidden high-draw appliances (e.g., older non-inverter aircons, water heaters) that draw much more power than estimated.
• Nominal vs. Usable Capacity (DOD Limit): A 10kWh battery with a 20% Inverter Cut-off SOC setting only has 8kWh of usable DC capacity.
• Inverter Self-Consumption (Tare Loss): Large hybrid inverters consume 70W-150W of power just staying awake. Over a 10-hour night, the inverter itself eats 1.0kWh to 1.5kWh of battery energy silently!
• Severe Cell Imbalance: One weak cell hits the low-voltage cutoff (e.g., 2.8V) early, forcing the BMS to stop the entire pack (See Case #011).

How to Verify

Do NOT guess. Log into the inverter's cloud portal or check the LCD for the "Daily Discharged Energy" (in kWh). Add this number to the energy still left in the battery (e.g., remaining 20% SOC). If the total roughly equals the battery's nominal capacity minus inverter efficiency loss, the battery is perfectly healthy—the load is simply too big.

Required Tools

• Inverter Cloud Monitoring App (to pull historical kWh data)
• Clamp Meter (to measure actual discharge current)
• BMS PC Software

Normal Values

• Inverter conversion efficiency: ~92% to 95% (DC to AC).
• System Usable AC Energy: A 10kWh battery set to 15% cutoff usually delivers about 7.5kWh to 8.0kWh of actual AC power to the house.
• The inverter log shows only 3.5 kWh was discharged, yet a 10kWh battery went from 100% to 10% and triggered Under-Voltage protection.

Diagnostic Procedure

• Step 1: Pull the Inverter data. How many kWh were actually pushed to the load before shutdown?
• Step 2: Check Inverter Settings. Is the "Discharge Cut-off SOC" set too conservatively (e.g., 30%)? If so, the customer is only using 70% of the battery.
• Step 3: Perform a BMS check. Connect to the battery when it hits the "empty" mark. Is one cell significantly lower than the rest (e.g., <2.8V while others are >3.2V)? If yes, balancing is required.
• Step 4: Investigate the loads. Put a clamp meter on the AC output. Did the customer turn on an old 2HP non-inverter air conditioner that draws a constant 1800W?

Root Cause

• A severe mismatch of expectations versus engineering reality. Customers calculate runtime using the formula: 10kWh ÷ 1kW load = 10 hours. They completely ignore three immutable facts:
• You cannot drain LFP to 0% (usually limited to 10-20% DOD).
• The inverter loses 5-8% in DC-AC heat conversion.
• The inverter's own motherboard and fans constantly draw ~100W.
• Unless the BMS reveals a severe cell imbalance, a "short runtime" is almost always an energy math reality check.

Corrective Actions

• Show the Data (Education): Open the app with the customer. Show them that the battery delivered 8kWh, which is exactly its usable capacity.
• Adjust DOD Limits: If the customer wants more runtime and understands the lifespan trade-off, cautiously lower the Discharge Cut-off SOC from 20% to 10%.
• Load Management: Identify the energy vampires (e.g., thermal pots, old fridges) and put them on non-backup circuits.
• Fix Imbalance: If the issue is false capacity due to cell imbalance, perform a full Grid Charge to 100% to let the BMS balance the cells (Refer to Case #010).