Off-Grid Batteries: LiFePO4 vs Lead-Acid

The battery bank is usually the most expensive part of an off-grid power system to own over time, and the part most often ruined by poor sizing, cold or neglect. This guide compares flooded lead-acid, AGM, gel and lithium iron phosphate (LiFePO4) batteries for cabins, homesteads and rural properties in Canada and the northern United States, and shows how to size, wire, house, maintain and eventually recycle a bank.

The four common chemistries

Flooded lead-acid

The oldest and cheapest option per nominal kilowatt-hour. Each cell is 2 V nominal, with plates immersed in liquid sulphuric acid electrolyte. Flooded batteries tolerate overcharging better than any other type, can be equalized to recover from sulphation, and are fully recyclable. They release hydrogen during charging, need regular watering, and lose capacity quickly if left partly charged for long periods. Deep-cycle 6 V golf-cart batteries, 6 V L16s and 2 V industrial cells are the usual off-grid choices. Automotive starting batteries are not suitable.

AGM

Absorbent glass mat batteries hold the electrolyte in fibreglass separators. They are sealed and valve-regulated, need no watering, do not spill, and release very little gas in normal use. They cost more than flooded batteries, generally deliver fewer cycles, and are damaged by overcharging and by repeated partial charging.

Gel

Gel batteries immobilize the electrolyte with silica. They handle deep discharge somewhat better than AGM but need lower charging voltages; charging a gel battery at flooded or AGM settings damages it permanently. They have largely been displaced by AGM and lithium in off-grid use.

LiFePO4

Lithium iron phosphate cells are 3.2 V nominal, so a 12 V battery is four cells in series (12.8 V), a 24 V battery is eight (25.6 V) and a 48 V battery is sixteen (51.2 V). LiFePO4 is the most thermally stable common lithium chemistry and is the standard lithium choice for stationary storage. It offers several times the cycle life of lead-acid, about 95% round-trip efficiency, no maintenance, no gassing in normal use, and roughly a third of the weight. It requires a battery management system (BMS) and must not be charged below 0 °C (32 °F).

Ranges are typical; the manufacturer's datasheet governs. Multiply 12 V figures by 2 for 24 V and by 4 for 48 V. Cycle life for lead-acid is usually quoted to 80% of original capacity; for LiFePO4, to 70–80%.

Usable capacity and depth of discharge

A battery's nominal energy in watt-hours is its voltage multiplied by its amp-hour rating. A 12 V, 100 Ah lead-acid battery holds 1,200 Wh nominal. A 12.8 V, 100 Ah LiFePO4 battery holds 1,280 Wh nominal.

Usable energy is what you can take out without shortening the battery's life unreasonably. Depth of discharge (DoD) is the fraction removed: 50% DoD means the battery is drawn down to half full.

• 12 V 100 Ah lead-acid at 50% DoD: 1,200 × 0.50 = 600 Wh usable.
• 12.8 V 100 Ah LiFePO4 at 80% DoD: 1,280 × 0.80 = 1,024 Wh usable.

The lithium battery delivers about 1.7 times the usable energy from the same amp-hour rating.

Lead-acid capacity also depends on discharge rate. Ratings are normally given at the 20-hour rate (C/20). Discharged faster, a lead-acid battery delivers noticeably less than its rating. LiFePO4 delivers close to its rated capacity across normal discharge rates.

Cycle life and depth of discharge

Shallower cycles extend life for every chemistry. A lead-acid battery that gives 1,200 cycles at 50% DoD may give only about 500 at 80%. Routinely discharging lead-acid below 50% is the most common way off-grid owners destroy a bank in two or three years.

Lead-acid also has to be recharged fully and regularly. Left at partial state of charge, hard lead sulphate crystals form on the plates and capacity is lost permanently. This is a real problem in Canadian winters, when short days can leave a lead-acid bank undercharged for weeks. Plan for a generator to bring lead-acid to full at least weekly in winter. LiFePO4 is not harmed by sitting partly charged and does not need a regular full charge, beyond an occasional one to let the BMS balance the cells.

Cold weather

Lead-acid in the cold

Lead-acid batteries can be charged below freezing, but cold reduces available capacity and raises the voltage needed to charge. A charge controller with a battery temperature sensor adjusts charging voltage automatically, typically by about −3 to −5 mV per °C per cell (about −0.018 to −0.030 V per °C for a 12 V battery) relative to 25 °C. Without compensation, a cold lead-acid battery is chronically undercharged.

A lead-acid battery at about 5 °C (41 °F) delivers roughly 75–80% of its rated capacity; at −20 °C (−4 °F) it may deliver only about half.

The electrolyte in a discharged battery can freeze, which can crack the case and buckle the plates. Approximate freezing points:

• Fully charged: below −50 °C (−58 °F).
• 50% charged: about −24 °C (−11 °F).
• Fully discharged: about −8 °C (18 °F).

A charged lead-acid bank survives a cold, unoccupied cabin. A discharged one may not.

LiFePO4 in the cold

LiFePO4 can discharge well below freezing, typically to −20 °C (−4 °F), with reduced capacity. It must not be charged below 0 °C (32 °F). Charging a frozen lithium cell causes lithium metal to plate onto the anode, which permanently reduces capacity and can create internal short circuits. Many manufacturers also require reduced charging current between 0 °C and about 10 °C (50 °F).

Options for cold locations:

• Install the batteries inside the heated part of the building.
• Use batteries with internal heating pads that draw from the charging source to warm the cells above freezing before charging begins.
• Place the bank in an insulated box with a thermostatically controlled heating pad.
• For an unoccupied winter cabin, disconnect charging sources and let the bank sit; LiFePO4 can usually be stored below freezing as long as it is not charged.

The battery management system

Every LiFePO4 battery needs a BMS. Drop-in batteries have one built in; larger rack-mounted and DIY banks use a separate unit. The BMS:

• Disconnects charging if any cell exceeds its maximum voltage, typically about 3.65 V.
• Disconnects discharge if any cell falls below its minimum, typically about 2.5 V.
• Limits charge and discharge current.
• Blocks charging below 0 °C and both charging and discharging at high temperatures.
• Balances cells so they stay at equal state of charge.

The BMS is a last line of defence, not a charge controller. Set the charge controller and inverter-charger to voltages the battery manufacturer specifies so the BMS rarely has to act. Many current batteries communicate with inverter-chargers over a data cable, letting the BMS set charge voltage and current directly; this is the most reliable arrangement where equipment supports it.

Because LiFePO4 voltage stays nearly flat from about 20% to 90% state of charge, voltage is a poor fuel gauge. Install a shunt-based battery monitor, which counts amp-hours in and out, for any battery type.

Sizing the bank

Start with daily energy drawn from the battery, measured at the battery terminals. If you have a load audit of AC appliances, divide by inverter efficiency, about 0.90–0.92.

Next choose days of autonomy: how many days the battery must carry the loads with no charging. With a generator on site, two to three days is typical. Without one, northern sites often need five or more, which quickly becomes expensive; adding array capacity or a generator is usually cheaper than adding battery.

Nominal capacity (Wh) = daily Wh × days of autonomy ÷ DoD × temperature factor

Divide by system voltage to get amp-hours. For lead-acid in an unheated space, include a temperature factor; about 1.3 at 5 °C (41 °F) is common. For LiFePO4 in a heated space, use 1.0.

Series and parallel wiring

Series wiring, positive of one battery to negative of the next, adds voltage while amp-hours stay the same. Four 6 V, 390 Ah batteries in series make 24 V, 390 Ah.

Parallel wiring, positive to positive and negative to negative, adds amp-hours while voltage stays the same. Two 24 V, 390 Ah strings in parallel make 24 V, 780 Ah.

Rules for a healthy bank:

• Use batteries of the same model, capacity and age. Never add new batteries to an old lead-acid bank; the new ones are dragged down to the condition of the old.
• Reach the target voltage with as few parallel strings as possible. For lead-acid, keep to three parallel strings or fewer. Higher-capacity cells in a single string are better than many small batteries in parallel.
• Make every current path equal. Connect the main positive to one end of the bank and the main negative to the opposite end, or use positive and negative busbars with equal-length cables to each string. Otherwise the batteries nearest the connections work harder and fail first.
• For LiFePO4 drop-in batteries, check the manufacturer's limits; many allow only a few in series, and some do not allow series connection at all. Fully charge each battery individually before connecting them.
• Fuse each parallel string, and fit a main fuse at the bank. Torque terminals to the manufacturer's specification.

Ventilation and hydrogen

Flooded lead-acid batteries release hydrogen and oxygen during charging, most heavily during absorption and equalization. Hydrogen is flammable in air at concentrations from 4% to 75% by volume and is lighter than air, so it collects at the ceiling of an enclosed space. AGM and gel batteries release far less, but can vent hydrogen when overcharged.

Flooded batteries belong in a dedicated, vented enclosure or room, with an inlet low and an outlet high, venting to the outdoors. Keep inverters, chargers, switches, relays and anything else that can spark out of the battery enclosure.

In Canada, the CEC covers storage batteries in Section 26 and renewable systems in Section 64, as adopted by each province and territory. In the US, NEC Articles 480 and 706 apply. Battery installations may require a permit and inspection; check with your local authority. This guide is not legal advice.

Maintenance

Flooded lead-acid

• Check electrolyte levels monthly. Top up with distilled water only, after charging, to the level the manufacturer specifies. Never let plates become exposed.
• Measure specific gravity with a hydrometer. A fully charged cell typically reads 1.265–1.280 at 27 °C (80 °F); add 0.004 for every 5.6 °C (10 °F) above that, and subtract 0.004 for every 5.6 °C below.
• Equalize when cells differ by more than about 0.030 in specific gravity, or on the manufacturer's schedule. Equalization is a controlled overcharge, typically at about 2.58 V per cell (15.5 V for 12 V), for a few hours.
• Clean terminals and tops with a baking soda solution, and keep connections tight.

AGM and gel

• Keep charge voltages within specification; overcharge dries them out and cannot be reversed.
• Check terminal tightness and look for swelling, which indicates overcharge or overheating.

LiFePO4

• Check cell voltages and BMS status periodically through the monitoring app or display.
• Allow an occasional full charge so the BMS can balance the cells.
• Check terminal torque once a year.

Cost per usable kWh over the battery's life

Purchase price per nominal kWh is misleading. The useful comparison is the cost of each kWh the battery delivers over its life:

Lifetime cost per kWh = price per nominal kWh ÷ (DoD × cycle life)

Prices are illustrative and change frequently; substitute current quotes. The comparison still understates lithium's advantage, because lead-acid's lower efficiency requires more solar or generator energy for every kWh delivered, and flooded batteries add water, labour and ventilation costs.

Calendar life also limits every battery. A full-time home cycling about 300 times a year uses 1,500 cycles in 5 years and 4,000 in about 13. A seasonal cabin cycling 100 times a year will see a lead-acid bank age out after perhaps 8–10 years with good care, long before it reaches its cycle rating, which raises its real cost per kWh. LiFePO4 calendar life is commonly 10–15 years or more.

Storage and disposal

Seasonal storage

• Lead-acid: store fully charged, disconnected from all loads. Self-discharge is a few percent per month and faster in warmth. Recharge every one to three months, or leave a correctly set solar charge controller connected to maintain it.
• LiFePO4: store at about 40–60% state of charge, with loads and charging disconnected. Self-discharge is low, but some BMS units draw a small current continuously; check the manufacturer's maximum storage period before leaving a battery for a full winter.

Recycling

Lead-acid batteries are among the most recycled products in North America; nearly all the lead and plastic is recovered. Return them to a battery retailer, scrap metal dealer or municipal hazardous waste depot. Many retailers charge a core deposit that is refunded when the old battery is returned.

Lithium batteries must not go into household garbage or scrap metal bins, where damage can start a fire. Take them to a provincial or municipal battery recycling program or a lithium battery recycler. Before transporting any battery, tape or cap the terminals, keep it upright and secured, and do not transport visibly damaged or swollen lithium batteries without first asking the recycler how to handle them. Both chemistries are regulated as dangerous goods for commercial transport.

Key points

• Lead-acid gives about 50% of its rating as usable energy; LiFePO4 gives 80–90%.
• LiFePO4 costs more up front but usually costs far less per delivered kWh over its life.
• Never charge LiFePO4 below 0 °C without cell heating; keep lead-acid charged so it does not freeze.
• Size the bank from daily battery draw, days of autonomy, depth of discharge and temperature.
• Match batteries, minimize parallel strings, and equalize current paths.
• Vent flooded batteries to the outdoors and keep spark sources out of the enclosure.
• Recycle every battery through an approved program.

This guide is general information. Codes, permits and safety requirements vary by province, state and municipality. Check local rules and use a qualified professional for electrical, gas, chimney and septic work where required.

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