Battery State of Health (SOH): Monitoring Degradation

What is State of Health?

State of Health (SOH) is a percentage representing your battery's current maximum capacity compared to its original capacity. A new battery starts at 100% SOH. Over time, chemical processes within the cells gradually reduce the amount of energy the battery can store.

A 10kWh battery at 85% SOH can effectively store 8.5kWh. It still functions perfectly — it just holds less energy per charge cycle.

SOH is different from State of Charge (SOC), which shows how full the battery is right now. SOC fluctuates constantly as the battery charges and discharges. SOH changes slowly over months and years.

What's normal?

For modern lithium iron phosphate (LFP) batteries commonly used in UK home storage:

• Year 1: 98–100% SOH (minimal degradation)
• Year 3: 95–98% SOH
• Year 5: 90–95% SOH
• Year 10: 80–90% SOH
• Year 15: 70–85% SOH

These ranges assume normal residential use (one cycle per day, moderate temperatures, reasonable depth of discharge). Aggressive use (multiple full cycles daily, temperature extremes) accelerates degradation.

Most manufacturers warrant 60–70% SOH at 10 years. If your battery drops below the warranted SOH within the warranty period, you have a warranty claim.

What causes degradation

Calendar ageing

Batteries degrade simply with time, regardless of use. Chemical reactions within the cells slowly reduce capacity. You can't prevent this — it's inherent to the chemistry.

Cycle ageing

Every charge-discharge cycle causes incremental capacity loss. The depth of each cycle matters:

• Shallow cycles (20–80% SOC range): minimal degradation per cycle
• Deep cycles (0–100% SOC range): more degradation per cycle

This is why many inverters default to keeping 5–10% reserve and not charging beyond 95–98%.

Temperature

Battery degradation accelerates at high temperatures. Every 10°C above 25°C roughly doubles the degradation rate. This is why battery placement matters — avoid south-facing walls in direct sunlight, unventilated cupboards, or rooms that overheat in summer.

Cold temperatures (below 5°C) don't cause permanent damage to LFP batteries, but they temporarily reduce performance and charging speed. Repeated charging in very cold conditions can cause some degradation in certain chemistries.

High state of charge

Keeping a battery at 100% SOC for extended periods accelerates degradation. This is why some systems limit the maximum charge level to 95% unless you specifically override it.

High charge/discharge rates

Consistently charging or discharging at the maximum rate generates more internal heat and stress, accelerating degradation slightly. Normal residential use rarely pushes these limits.

How to check SOH

Through your inverter app

Most hybrid inverter monitoring platforms display battery SOH:

• GivEnergy: Visible in the portal under battery details
• Sunsynk: Available in the SunSynk Connect app
• Tesla Powerwall: Shown in the Tesla app
• Fox ESS: Available in Fox Cloud monitoring
• Solis: Accessible through SolisCloud

Check SOH quarterly and note the reading. A spreadsheet tracking SOH over time helps you spot abnormal degradation early.

Through the BMS

The Battery Management System (BMS) in your battery monitors individual cell voltages, temperatures, and overall health. Some BMS units provide more detailed SOH data than the inverter app, though accessing it may require specialist tools.

Through third-party monitoring

Home Assistant users can pull battery SOH data via inverter integrations and track it automatically with graphs and alerts.

When to worry

Normal (no action needed)

• SOH declining 1–3% per year in early years, slowing over time
• Occasional small fluctuations in reported SOH
• SOH above manufacturer's warranty curve

Worth investigating

• SOH declining faster than 5% per year consistently
• Individual cell voltage imbalances (visible in detailed BMS data)
• Battery capacity noticeably reduced (you're running out of stored energy earlier in the evening than expected)

Potential warranty claim

• SOH below manufacturer's warranted minimum for the battery's age
• Battery failing to charge or discharge to expected capacity
• BMS errors or faults appearing regularly

Maximising battery longevity

1. Keep the battery cool — install in a cool, ventilated location. A garage is typically ideal in the UK.
2. Don't leave it at 100% for days — if you're away on holiday, let the system cycle normally rather than capping at 100%.
3. Use moderate charge rates — most home systems don't push this limit, but if you're doing aggressive grid charging, consider whether maximum rate is necessary.
4. Keep firmware updated — manufacturers often improve BMS algorithms that optimise for longevity.
5. Don't stress about it — modern LFP batteries are robust. Normal residential use is well within their design parameters. Use your battery to its full potential rather than babying it into a longer but less useful life.

For a battery with excellent built-in SOH monitoring via the GivEnergy portal:

GivEnergy All-in-One 9.5kWh Battery

£5,500

capacity kwh

9.5

usable capacity kwh

8.6

chemistry

LFP

cycles

6000

View on Amazon

Affiliate link — we may earn a small commission at no extra cost to you

The Tesla Powerwall 3 also provides outstanding health tracking through the Tesla app:

Tesla Powerwall 3

£8,500

capacity kwh

13.5

usable capacity kwh

13.5

chemistry

LFP

cycles

4000

View on Amazon

Affiliate link — we may earn a small commission at no extra cost to you

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