Why this chemistry, and what it costs you
Lithium iron phosphate is the right battery for a household store, and it is worth saying why before the warnings start.
- You can use 80 to 100% of the capacity rather than the 50% lead acid allows, so a 100 Ah LiFePO4 does the work of a 200 Ah lead acid bank.
- Typically 3,000 to 5,000 full cycles, against a few hundred for lead acid. Some packs are rated 3,000 to 7,000 cycles to around 70% remaining capacity at 25°C. That is 8 to 15 years of ordinary use.
- It does not mind being left partly charged. Lead acid sulphates and dies if you do that. LiFePO4 is happier there than it is full.
- No venting, no watering, no acid.
- It holds voltage almost flat through the discharge, so lights do not dim as it empties.
What it costs you, honestly:
- More money up front, though far less per usable kilowatt hour over its life.
- A hard temperature rule that lead acid does not have, and which is the subject of most of this page.
- A flat discharge curve means voltage tells you very little about how full it is, which is a genuine practical nuisance.
The cold charging rule
Why this matters so much in this scenario specifically: an unheated garage, outbuilding or garden store in a British winter sits below freezing for long stretches. The morning sun comes up, the panel starts producing, and the controller starts pushing current into a battery at minus two. Nothing announces it. That is how a bank bought for a decade is quietly ruined in its first February.
Cell temperature, not air temperature. A large bank has thermal mass and lags the air by hours. A battery that was at minus one overnight is still near freezing at mid morning even if the air has climbed to five, and mid morning is exactly when the panel starts producing.
Discharging in the cold is a different question
The rule is asymmetric, which surprises people, and it is genuinely useful to know.
- Charging below 0°C: no.
- Discharging: acceptable down to about -20°C, at roughly 40 to 50% of rated capacity.
So a battery in a cold shed can keep running your lights all winter perfectly safely. It is only putting energy back in that is the problem. A bank that gets cold is not ruined. A bank that gets charged while cold is.
This is worth knowing because it changes what you do in an emergency. If it is freezing and you need light, use the battery. Then work out how to warm it before you charge it.
What the BMS does and does not protect you from
Every sensible LiFePO4 pack has a battery management system built in, and most have a low temperature charge cutoff, typically around 0°C for charge and around -20°C for discharge.
Do not assume yours has one. Cheap packs and self-built banks may not. This is the single specification to check before buying, ahead of capacity or price.
Where the cutoff is not enough:
- It measures at one point. In a large or multi-cell bank, a cell at the cold end can be below the sensor's reading.
- It cuts off abruptly, which can look to a charge controller like a disconnected battery. Some controllers respond badly to that.
- It protects the battery, not the system. A bank that disconnects itself mid charge on a cold morning leaves your loads on a controller with no battery, which some controllers do not survive.
- It is a last line of defence, not a design. If your BMS is regularly cutting in, the installation is wrong.
The better answers, in order of preference:
- Put the battery somewhere that does not freeze. Inside the insulated envelope of the house is the correct answer, and LiFePO4 is safe to do that with in a way that flooded lead acid is not, because it does not vent hydrogen.
- Insulate the battery box, and note that a battery box with no heat in it only slows the fall, it does not stop it.
- A self-heating battery, which contains a small internal heater fed from the charge current and warms the cells before allowing charge. These exist and are worth the premium if the bank genuinely has to live outside.
- Manual discipline, meaning a thermometer on the battery and disabling charge overnight in winter. This works but relies on a person, and people are asleep at the relevant time.
Charging settings
If your controller has a lithium profile, use it. If you are setting it by hand:
Three of those need explaining, because they are the ones people get wrong by carrying lead acid habits across.
Float is unnecessary and mildly harmful. Lead acid self-discharges and needs topping up. LiFePO4 barely self-discharges, so holding it at a float voltage keeps it at full charge permanently, and permanent full charge is the condition that ages it fastest. If the controller lets you disable float, disable it.
Equalisation will destroy it. Equalisation is a deliberate controlled overcharge that stirs up a lead acid electrolyte and reverses sulphation. Applied to lithium it is simply an overvoltage. If your controller has a lead acid profile with equalisation enabled and you attach a lithium bank, you can damage it in a single cycle. Check this setting explicitly. It is often on by default.
Temperature compensation is also a lead acid feature, which raises charge voltage as the battery gets colder. That is exactly the wrong instruction for lithium, where cold means charge less, not harder.
Where to put it
- Inside the heated part of the building if at all possible, which solves the cold problem outright.
- Not in a sealed cupboard with no airflow. It does not vent in normal use, but heat needs somewhere to go.
- Where you can reach the terminals to check they are tight, and where you can see it.
- Off the floor, especially a concrete floor, which is cold and can flood.
- Not next to a heat source. High temperature ages lithium as surely as cold charging damages it, just more slowly. Sustained heat above about 45°C shortens life noticeably.
- Secured so it cannot move or be knocked, and with the terminals covered so nothing metal can fall across them.
Long-term storage
If a bank is going into storage rather than service:
- Charge to roughly 50 to 70%, not full and not empty. Full storage accelerates ageing; empty storage risks the cells falling below the voltage at which they can be safely recovered.
- Store cool but above freezing. Cool storage genuinely slows ageing.
- Disconnect everything, including the BMS's own small parasitic draw where the pack allows it, because months of parasitic drain is how a stored pack ends up dangerously flat.
- Check it every few months and top back up to half if it has drifted down.
- Write the storage date and the state of charge on the case. You will not remember.
A cell taken very flat may refuse to charge. Many BMS units latch off below a threshold and will not reconnect. Sometimes a specialist charger recovers it; often it does not. The prevention is trivial and the cure frequently does not exist.
Reading the state of charge
Voltage is a poor guide with this chemistry, and this is a real practical problem rather than a technicality. A LiFePO4 pack sits near 13.2 to 13.3 V across most of its working range, so the difference between 70% full and 30% full is a few hundredths of a volt. Lead acid gives you a usable sloping curve. Lithium does not.
Fit a shunt-based battery monitor. It counts amp hours in and out, which is the only honest measurement. It is not an expensive item and it turns guesswork into a number.
Failing that, the ends of the curve do tell you something: near 14 V under charge means nearly full, and a sharp drop below about 12.8 V at rest means genuinely low and falling fast. The middle tells you nothing useful.
Failure and fire
LiFePO4 is the safest common lithium chemistry, and this deserves stating plainly because lithium battery fires are frightening and the fear is usually attached to the wrong chemistry. LiFePO4 is markedly more thermally stable than the lithium cobalt cells in phones and laptops, does not readily go into thermal runaway, and does not produce the violent fires those do.
It is not immune. Physical damage, an internal short from dendrites grown by cold charging, or a serious overcharge can still cause a fire.
- A damaged pack that has been dropped, crushed or punctured should be treated as suspect, isolated outside, and not charged.
- Swelling, heat, or an unusual smell means disconnect and get it outside if you safely can.
- Water is the correct extinguishing agent on a lithium battery fire, in quantity, because the aim is cooling rather than smothering. This is the opposite of the instinct for an electrical fire, and it applies once the battery itself is burning.
- Do not attempt to fight a well developed lithium fire indoors. Get people out and shut the door behind you. The rule that applies to every fire applies here: if it is bigger than a waste bin, or if you would have to turn your back on your exit to reach it, you are past the point of fighting it.
- The smoke is the danger before the flame is. Burning lithium cells give off hydrogen fluoride among other things, which is severely irritant to the lungs. Do not stand over a venting pack to watch it.