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Battery losing capacity quickly

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What you’re seeing (symptoms)

You notice that your battery percentage (State of Charge) is dropping much faster than it used to for the same house load. You may find that it reaches its minimum percentage (e.g. 10%) by 9pm when it used to last until midnight.

What’s normal vs not normal

Normal:

  • Gradual Degradation: Lithium batteries naturally lose about 1-2% of their total capacity every year.
  • Seasonal Variation: In winter, you may use more power for lighting and heating, making it seem like the battery is dying faster.

Not normal:

  • A sudden 10-20% drop in capacity over a single month.
  • The battery percentage “jumping” (e.g. dropping from 40% to 10% in a few minutes).
  • The battery never reaching 100% even after a full day of solar generation.

Likely causes (ranked)

  1. Imbalanced cells: The individual battery cells inside the unit have different voltages, causing the BMS to shut down early to protect the “weakest” cell.
  2. Incorrect ‘Depth of Discharge’ (DoD): Your settings have changed, and the battery is now stopping at 20% instead of 10%, giving you less usable power.
  3. Self-discharge fault: An internal component is consuming power even when the battery is supposed to be idle.
  4. Temperature extremes: The battery is installed in a very cold garage or very hot loft, which temporarily reduces its usable capacity.
  5. BMS software error: The software is miscalculating the remaining capacity (known as “SOH” or State of Health).

Quick checks you can do safely

  • Check the DoD setting: Go into your app’s “Advanced Settings.” What is the “Minimum State of Charge” or “Discharge Cut-off”? Ensure it matches your original installation spec.
  • Force a 100% charge: If safe and on a suitable tariff, force the battery to charge to 100% from the grid. Leave it at 100% for a few hours to allow the cells to “balance.”
  • Check the temperature: If the battery feels very cold or very hot, this is likely the cause. Try to improve the insulation or ventilation of the area.
  • Audit your house load: Have you recently installed a new appliance (like a hot tub or electric heater) that is draining the battery faster than you realized?

When to call an installer

  • If the battery percentage “jumps” by more than 5% suddenly.
  • If your app shows a “Cell Voltage Imbalance” or “BMS Internal Fault.”
  • If you have performed a grid-charge to 100% but the capacity issue persists.

Questions to ask your installer

  • “What is the current ‘State of Health’ (SOH) percentage reported by the BMS?”
  • “Is the capacity loss within the manufacturer’s performance warranty (usually 70-80% after 10 years)?”
  • “Does the battery firmware need an update to improve cell balancing logic?”

Related NetZeroNow tools & guides

FAQs

What is SOH? State of Health (SOH) is a percentage that compares your battery’s current capacity to its original “as new” capacity. 100% is perfect; 80% means it has lost 20% of its storage. Can I fix it? If the issue is “cell imbalance,” a full 100% charge and a few hours of “soaking” at that level can often fix it. If it’s physical degradation, the battery cannot be “repaired,” only replaced. Is it covered by warranty? Most UK battery warranties (like Tesla, GivEnergy, Pylontech) guarantee at least 70% capacity after 10 years. If your SOH is 60% after 3 years, you likely have a valid claim. Does cold weather help? No. Lithium batteries hate the cold. While it won’t permanently damage them, it will make them much less efficient and they will seem to “empty” faster. Should I keep it at 100%? For daily use, yes. However, if you are going away for a month, it’s often better to leave the battery at around 50-60% to reduce stress on the cells.\n

Written by NetZeroNow | Last updated on 2026-03-02