How home battery storage works chemistry, cycles, and the inverter's role
A battery is not a black box. Understanding how it charges, discharges, and ages helps you configure it correctly and diagnose problems when things go wrong. This guide covers lithium chemistry, the BMS, charge cycles, efficiency loss, and why your system limits SoC.
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Book a diagnostic — £75 → How diagnostics workLithium chemistry and the Battery Management System
Two chemical formulations dominate the UK solar market. Understanding the difference helps you choose the right battery and understand its behaviour.
LFP uses an iron phosphate cathode. This chemistry is thermally stable — harder to trigger thermal runaway (fire) even if the pack is physically damaged or over-charged. LFP batteries last 8,000–10,000 full-depth cycles (100% charge to 0% discharge). Performance in cold weather (5°C +) is excellent. Most UK residential systems today use LFP (GivEnergy, Growatt, Sunsynk, Solis all standard LFP packs).
NMC has a nickel-based cathode. Higher energy density (lighter per kWh) but shorter cycle life: 3,000–5,000 cycles. More prone to thermal runaway if damaged. Cold weather performance weaker. NMC is being phased out in residential solar — you are more likely to encounter it in legacy systems (pre-2023). Not recommended for new installations in the UK.
The BMS is an electronic system embedded in or mounted next to the battery pack. Its job is to monitor and protect:
The inverter continuously communicates with the BMS. If the BMS reports a fault, the inverter immediately stops charging or discharging. This is why you sometimes see a battery error code but the battery still has charge — the BMS has isolated the pack for safety.
State of Charge (SoC) and Depth of Discharge (DoD)
SoC is what your app shows you. DoD is how much of that you actually use each day. Understanding both explains why manufacturers limit your 80% and why this helps your battery last longer.
SoC is shown in your app as a percentage. 100% SoC means the battery is full. 0% SoC means it is empty. The BMS measures this by integrating current flow over time and comparing cell voltages. You cannot have negative SoC — the BMS stops discharge well before the battery is completely empty (typically at 5–10% to protect the cells).
Lithium cells experience stress at high voltage. Charging to 100% every cycle ages the cells faster. By capping at 80%, the system trades 20% capacity availability for measurably extended lifespan — studies show you can add 5+ years to battery life. The last 20% of charge is expensive in terms of cell degradation.
You can reconfigure most systems to charge to 90% or 100% if you need the extra capacity — your installer or engineer can adjust this. But be aware the battery will age faster.
If your 10 kWh battery charges to 80% (8 kWh usable) and you discharge 5 kWh per day, your DoD is 62.5%. Shallow DoD cycles extend battery life dramatically:
If your daily energy use only requires 30% DoD, the battery will outlast the inverter. Most UK residential systems achieve 40–60% DoD on average, which is ideal.
AC coupling vs DC coupling: efficiency and retrofit
The two ways to connect a battery to solar. Each has different efficiency, retrofit implications, and failure modes.
In a DC-coupled system, the solar array, battery, and single inverter share the same DC bus. Solar DC current charges the battery directly, bypassing an AC-to-DC conversion stage. This is the architecture of most new hybrid inverters (GivEnergy, Growatt SPA series, Sunsynk Turbo-X, Solis RHI).
In AC-coupled retrofit systems, the existing solar inverter feeds AC into the house. A separate battery inverter connects to the AC circuit (usually before the consumer unit or via a dedicated circuit) and charges/discharges bidirectionally. The two inverters operate independently but coordinate via communication protocols.
Round-trip efficiency: not all stored energy returns
Energy is lost as heat during charge and discharge. Round-trip efficiency tells you what percentage actually comes back out. This matters for financial modelling.
Round-trip efficiency = (Energy discharged / Energy charged) × 100%. If you charge the battery with 10 kWh and only 9.5 kWh comes back out, round-trip efficiency is 95%. The 0.5 kWh loss is dissipated as heat in the inverter, battery, and wiring.
Three sources of loss during charge-discharge cycle:
If your business case assumes you store 10 kWh daily and use all of it, but round-trip is 95%, you only actually have 9.5 kWh available. Your payback period will be 5% longer. Account for efficiency when calculating ROI. Monitoring portals sometimes show efficiency figures — check the battery health section or ask your installer what the system's round-trip efficiency is (most will know within 0.5%).
Battery degradation: what to expect over 10+ years
Lithium batteries age chemically with every charge cycle and also with calendar time. Understanding degradation helps you plan for replacement and avoid panic when your battery's max SoC drops slightly.
LFP batteries typically lose 0.5–1% capacity per 100 full-depth cycles. If you cycle 300 times per year:
However, shallow cycles (30% DoD) age batteries slower. If you only cycle 30–50% DoD, you can reduce degradation to 0.1–0.3% per 100 cycles, extending usable life to 15+ years.
Lithium cells age chemically over time regardless of use. A battery sitting at 50% SoC in a 25°C room will lose ~2–3% capacity per year just from calendar aging. Hot environments accelerate this (5% per year at 35°C). This is why batteries stored long-term should be kept cool and at 30–50% SoC.
Degradation manifests gradually:
The inverter portal shows battery health as a percentage. Check it quarterly to track degradation rate. If health drops faster than expected (5%+ in one year), investigate: high temperatures, deep cycles, or BMS fault.
Most manufacturers (GivEnergy, Growatt, Sunsynk, Solis) warrant the battery for 10 years or until capacity drops to 70%, whichever comes first. If your battery fails or degrades rapidly before 70%, warranty repair/replacement is available. Degradation to 70% over 10 years is normal wear and not typically covered.
Frequently asked questions
Yes, most systems can be reconfigured to charge to 90% or 100%. The trade-off is accelerated degradation — charging to 100% every cycle ages the battery 30–40% faster. Most installers default to 80% because the extended lifespan is worth the 20% capacity reduction. For seasonal use (winter when solar is limited), you can temporarily increase to 90% or 100% then drop back to 80% in summer. Check with your installer whether your system supports this flexibility.
The BMS will automatically disconnect the battery if internal temperature exceeds ~60°C to protect the cells. Temperatures above 60°C dramatically accelerate chemical degradation and increase thermal runaway risk. If you see temperature warnings in your portal, investigate: is the battery in direct sunlight? Is ventilation blocked? Is the room temperature consistently very high? These are correctable installation issues. Contact your installer to consider relocating the battery if necessary.
100% DoD cycles daily will degrade the battery significantly faster — expect 30–50% capacity loss within 5–7 years instead of 10+. The BMS will cut off discharge around 5–10% SoC to prevent over-discharge damage, so you can't truly reach 0%. If your load genuinely requires deep cycles, you can configure the system to allow it — just accept the shorter battery life. Most UK domestic users don't need deep cycles: daytime solar generation typically meets the load, and overnight use is moderate.
A 3–5% drop in year one is normal — lithium batteries experience burn-in degradation in months 1–6 as cell chemistry stabilises. After that, degradation should settle to roughly 0.5–1% per 100 cycles. If health continues dropping rapidly after year one (5%+ per year), investigate: the battery may be cycling at unusually high depth, running too hot, or the BMS may be misreporting capacity. A remote health check will identify which.
Related guides and problem pages
How the inverter controls battery charging, self-consumption, and export — and the role of MPPT.
How CT clamps give the inverter real-time grid data — essential for battery charge/discharge decisions.
How battery storage affects your SEG earnings and how to configure timed export to maximise both.
Full diagnosis for batteries that won't charge overnight or from solar — settings, CT clamps, firmware.
Why a battery that charged fully won't discharge — minimum SoC settings, inverter modes, and faults.
How zero-export mode interacts with battery storage and what happens when the battery is full.
Battery not performing as described in this guide?
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