Technical articles · Batteries
Keeping a LiFePO4 battery healthy
How to keep a lithium iron phosphate battery healthy, and which habits picked up with AGM batteries have to be unlearned before they do damage.
Lead habits hurt a lithium battery
An AGM wants to be kept as close to full as possible and suffers from every deep discharge. A LiFePO4 works well precisely in the middle band, and leaving it sitting at a hundred per cent is one of the things that ages it fastest. The two rules are opposites, which is why people moving to lithium treat it badly at first without noticing.
- The comfortable band is between twenty and eighty per cent: that is where it should spend most of its life
- A full zero to hundred cycle achieves nothing except using up a cycle
- Repeated deep discharge sends the protection system into cutoff and stresses the cells
- The quoted cycle count assumes mild temperatures and moderate currents, not August under the roof
What the voltage tells you, and what it does not
Resting voltage is the one measurement that takes thirty seconds with a multimeter, and it tells you more than any warning light. Read it with the battery at rest, neither charging nor under load, because with current flowing the figure shifts and the reading is worthless. The numbers below are the order of magnitude for a LiFePO4 on a nominal 12 V system: the exact ones are in your battery's manual, and those are the ones that count.
- Fully charged: around 13.6 to 13.8 V, measured shortly after charging ends
- Eighty per cent: around 13.2 to 13.4 V, which is also a good level for summer storage
- Half charge: around 12.8 to 13.0 V
- Twenty per cent: around 12.4 to 12.6 V, and time to recharge
- Below 12.0 V: deep discharge, to be avoided and corrected at once
- A sudden drop of half a volt is not consumption: it is the protection system stepping in
Cold is the real limit
Below zero a LiFePO4 must not be charged. Discharging it is fine, charging it is not, and in a motorhome in winter that difference matters more than it sounds: the solar panel starts pushing current in the morning while the cell is still frozen, and if the protection system has no low-temperature charge cutoff the battery takes the damage.
- Check in the manual whether your BMS has a low-temperature charge cutoff: many do, not all
- Without it, in winter it is worth disconnecting the solar when the vehicle stands in the frost
- Batteries with a built-in heater switch it on themselves, but they use energy to do it
- Heat is the other extreme: above 45 to 50 degrees, cut the loads and find shade and airflow
How much to charge, and at what current
Slow charging is what the battery prefers, and in a motorhome it is also what happens by itself, because solar is in no hurry. Fast charging is for when you are leaving in an hour, and using it every day shortens the life of the battery without giving anything back.
- Normal use: stay between twenty and eighty per cent, recharging when convenient
- Slow charge, three to five hours for a full cycle: the kindest, with less thermal stress
- Fast charge: keep it for emergencies, not as a habit
- One full charge every six to twelve months puts the state of charge counter back in line
- If the charger is still the lead-acid one, the lithium will never get where it should: the charge profile is different
Storage, which is where the damage happens
A battery left flat for three months comes back worse, and one left full in the heat does too. Storage takes ten minutes to prepare and then asks for one check a month.
- Winter: leave it around fifty to sixty per cent, somewhere that does not freeze
- Summer: leave it around seventy to eighty per cent, in the shade with air around it
- Disconnect the loads, so that the vehicle's standby consumption does not eat it
- A voltage check once a month, weekly in the heat: below 12 V, recharge
- When you start again, a slow charge to a hundred per cent before taking to the road
The checks, and how often
This is not workshop maintenance: it is looking at five things and measuring one, at intervals you can keep in your head without writing them down.
- Every departure: terminals and connections by eye, thirty seconds
- Every month: housing, cables, air space around the battery and temperature
- Every three months: clean the terminals, with a dry brush and, if needed, diluted bicarbonate, then dry them
- Every three months: dielectric grease on the contacts, which keeps oxidation away
- Every six months: cable size and insulation, fuse and isolator on the positive
- Never: alcohol, vinegar or solvents on the terminals
What not to do
Four mistakes, and they are the ones that turn a ten-year battery into a three-year one.
- Discharging below five per cent regularly: if it happens often you need more capacity, not more patience
- Fast, repeated charge and discharge cycles on the same day, which heat the protection system
- Leaving it in a damp locker or with condensation: the BMS corrodes and the problem becomes electrical
- Keeping it above 50 degrees for days, which is what happens in a closed locker in the sun
Other articles
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