LFP or NMC? Battery chemistry, and how much it actually matters

What is actually in a home battery

The government’s review of domestic storage safety, carried out by Intertek for the Office for Product Safety and Standards and published in September 2020, surveyed the market. Its finding on chemistry is blunt:

The chemistries of the cells are often Lithium Nickel Manganese Cobalt Oxide (NMC) or Lithium Iron Phosphate (LiFePO4).

Both are lithium-ion. The difference is the cathode. NMC uses lithiated nickel manganese cobalt oxide; LFP uses lithiated iron phosphate. The review lists NCA and LCO as other cathode types in the wider lithium-ion family, and notes that while graphite is the usual anode, hard carbon and titanate are alternatives.

You will see LFP written as LiFePO4. It is the same thing.

The trade-off, stated once

LFP is less energy dense. That is not a defect the industry is working around; it is the property that produces the safety difference. Less energy packed into the same volume means less energy available to release in a fire — and it means the battery is bigger and heavier for the same usable capacity.

That has practical consequences. A wall-mounted LFP unit weighs more, which matters for what it can be fixed to. It takes more space, which matters in a small garage or utility room. And the difference is one of the reasons a compact product may not be LFP.

What the safety difference actually is

Here is the sentence, in full, because the two halves are usually separated:

cells with LiFePO4 and Titanate had a lower HRR and lower flammability (due to lower power density) compared to other chemistries but in the context of a fire of a battery pack it did not have a significant impact.

HRR is heat release rate: how fast a fire puts out energy. So LFP burns less fiercely at cell level. At pack level — which is the level a home battery exists at — the review found that did not translate into a significant difference.

State of charge matters more than most buyers realise

Two findings from the same review, both about how full the battery is rather than what it is made of:

• a higher state of charge increases the likelihood of a violent thermal event
• gas volume increases with increased state of charge

A battery that spends its life cycling between roughly a third and roughly full is in a different condition from one held at 100% waiting for a power cut. This is one of the few places where how you use a battery interacts with its risk, and it is worth asking your installer how the system is configured — particularly if backup is one of the reasons you are buying it.

The gas, and why the siting rules read as they do

The review measured hydrogen fluoride released in fire tests across a broad range of commercial lithium-ion cells:

The amount of HF measured in the fire test ranged between 20 and 200 mg/Wh of nominal battery capacity

That is per watt-hour. A 5 kWh battery is 5,000 watt-hours, so the range scales to something in the order of 100 to 1,000 grams. Hydrogen fluoride is acutely toxic at low concentrations.

We give the figure not to alarm but because it explains the rules. When PAS 63100:2024 excludes bedrooms, escape routes, protected stairwells and lofts, this is what those exclusions are about: not the probability of a fire, but what happens to the people in the building if there is one and the battery is between them and the door.

What to ask, and what not to bother asking

Worth asking

• Which chemistry, in writing, on the datasheet — not “lithium”.
• Usable capacity, weight and dimensions, because the chemistry drives all three.
• Which product standards the cells and the system are certified to, and by whom.
• Where it will be sited, and whether that location meets PAS 63100:2024.
• How the system manages state of charge, and whether that changes if backup is configured.

Not worth much

• Comparisons of one manufacturer’s LFP against another’s LFP on safety grounds. The chemistry is the same; the pack design, the battery management system and the installation are where the differences live, and none of those are visible from a chemistry acronym.
• Any claim that a chemistry removes the need for the location requirements. It does not, and the standard does not offer an exemption for it.

Where the real difference is

Between two lithium-ion products, the things that most affect how safely one ends up operating in your house are, in rough order: where it is installed, whether the installation meets PAS 63100:2024, the quality of the battery management system, and only then the chemistry.

That ordering is unsatisfying, because chemistry is the one you can read off a brochure and the other three take a conversation. It is still the ordering.