Discharge Voltage & Impedance. LA batteries’ discharge voltage tapers significantly as state-of-charge decreases, whereas LFP batteries’ voltage remains fairly steady until they are close to being fully discharged. LFPs have about one-quarter the internal resistance (impedance) of LA batteries, which reduces battery energy lost to heat. These both combine to improve system efficiency and prevent DC voltage sags that can affect voltage-sensitive equipment.
Charge & Discharge Current. LFPs can be safely charged and discharged at a higher current than LA batteries. But the relatively low current in RE applications (compared to EV applications) makes this aspect not very useful.
Self-Discharge. At room temperature, idle (stored or disconnected) LA batteries lose 5% to 15% of their electrical capacity per month, compared to 1% to 3% for LFPs. In RE applications where energy is used only occasionally (such as pleasure boats, RVs, or vacation cabins), or during periods of low RE source, this can be a useful attribute.
Maintenance. Wet LA batteries, if not watered when needed, will have a greatly shortened life. LFPs require no additional liquid to maintain their electrolyte levels. Sealed LA batteries, which have a much lower up-front cost than LFP batteries, compare better than FLA but their lifetime cost per kWh is greater than either LFP or wet LA batteries.
Lifetime. While longevity can vary widely depending on factors such as daily depth of discharge and LA battery type (marine, golf cart, AGM, industrial, etc.), regularly used and properly maintained common deep-cycle LA batteries have an average lifespan of about five years; LFP batteries have an estimated longevity of 10 years—half the frequency of LA battery replacement. In both cases, natural aging of the battery chemicals can impair batteries before their cycle life is used if they are cycled infrequently. When used up, both types of batteries can and should be recycled by returning them to a dealer, although due to the long history of LAs, there are presently more recyclers for LAs than LFPs.
Cost. Although LA batteries are cheaper up-front than LFPs, their lifetime price per kWh can be higher. This assumes that you can use most of the lifetime capacity (usable capacity multiplied by cycle life) prior to the battery failing due to age. With a 3,000-cycle or 10-year life (whichever comes first), one would need to cycle LFP batteries nearly daily to optimize the payback. (Note: End-of-battery life is generally considered to be when the battery can maintain only 70% to 80% of its original capacity.)
The biggest disadvantage of any Li-ion battery is needing a battery management system (BMS). The job of a BMS is to monitor the voltage and temperature of each individual cell and protect from excessive charging and discharging. While any battery system, whether it be LA or LFP, can be improved with a BMS, a BMS is not typically required of LA cells. As long as all the batteries in a pack are of the same model and age (ideally from the same manufactured batch) and have been treated equally, the individual cells tend to behave the same while being charged. However, LFP battery cells, even in the same manufactured batch, can have variations in capacity. When charging, cells with lower capacity can become full much sooner than cells with higher capacity, which can lead to dangerously elevated voltages on the full cells as the others continue charging.
While LA cells tolerate brief periods of overvoltage (in fact, periodically elevating charge voltage to perform an equalization charge is recommended), even a fraction of an hour at elevated voltage can damage an LFP cell. A BMS protects individual cells from overvoltage by shunting current around the full cells when they reach their recommended “full” voltage. This allows the remaining cells to continue charging. A good BMS can detect when a cell is beginning to overheat (another sign of pending danger to cells) and shut off charging to the pack to protect all cells.