EthosPowerAIThe choice between lithium iron phosphate and valve-regulated lead-acid is usually made on the purchase order, where lead-acid is cheaper. It is often regretted on the maintenance schedule, where lead-acid costs more. Make the comparison below before either document exists. The figures are the manufacturers' published ones and the allowances in the standards. Where a number depends on the application, I say so and do not pick the flattering end of the range.
Weight, size and where it can go
A lithium iron phosphate string weighs roughly a third to a quarter of the lead-acid string it replaces and takes markedly less floor. It can be mounted in any orientation, including inverted. That changes the rack design, the floor loading and sometimes the room. Lead-acid needs a floor rated for its weight, upright mounting and the acid-resistant coating. For an upper-floor installation or a transportable switch-room, the weight alone can decide the chemistry before performance is discussed.
Cycle life and usable capacity
A standard valve-regulated cell gives between 300 and 800 cycles at 50% depth of discharge, and a twenty-year design gives about 1,500. Lithium iron phosphate gives 5,000 and more under the same test. Its discharge curve is flatter, and it holds capacity better at high rates and in the cold. In high-discharge applications it can deliver up to twice the usable capacity of a lead-acid string of the same rating. Twice is the upper limit, and the gain in a float-standby application is smaller.
Lead-acid discharges comfortably at a tenth to a quarter of its capacity per hour. Lithium iron phosphate discharges continuously at ten times its capacity and takes pulses at twenty times. It charges at up to three times its capacity where the charger allows, and few chargers do.
Temperature and safety
Lithium iron phosphate is rated from -40 to 85 °C without significant loss. A lead-acid cell at -20 to -40 may deliver a fifth of its capacity, and every sustained degree above 25 shortens its life. Lithium iron phosphate has no thermal runaway characteristic of its own, which is why an engineer can put this lithium chemistry in a substation. The cabinet still needs the barriers and the detection that the standards require.
Maintenance is the other half of the safety case. Lead-acid needs yearly testing, equalisation charging and a watch for sulphation. Lithium iron phosphate needs very little beyond confirming that the battery management system reports correctly.
The environmental account
Manufacturing a kilowatt-hour of lead-acid emits roughly 35 to 40 kg of CO2. A kilowatt-hour of lithium iron phosphate emits roughly 65 to 80 kg, so the lithium cell starts behind. Its far longer life brings its footprint per cycle down to about a tenth to a third of lead-acid's, provided the cycles are used.
Recycling favours lead-acid. Lead is recovered at rates above 99% in developed countries. Lithium iron phosphate recovery is still below 5% while the infrastructure catches up. A lead-acid string therefore has a known end of life, and a lithium string does not yet.
Cost, and which cost
Lithium iron phosphate costs more to buy and less over the life of the system. The saving comes from maintenance avoided, replacements deferred and a residual value that the lead-acid string does not have. The saving is real only if the system stays in service long enough to collect it. That depends on the site, not the chemistry, so ask about the site first.
Choose lead-acid when
Choose lead-acid when the budget is fixed at purchase, the recycling path matters to the client, the application floats and rarely cycles, and the room holds its temperature. Choose lithium iron phosphate when cycle life is the design driver, when space or weight is limited, when the temperature will swing, when nobody will be sent to maintain it, or when the owner is paying for the life of the system and not only the purchase price.
Both chemistries are sized on ebatt.ai to IEEE 485. To discuss the choice between them, use the contact page.