LiFePO4 Lithium Iron Phosphate vs. Conventional Lead-Acid Batteries
By now you’ve probably heard about Lithium batteries, more specifically LiFePO4 Lithium Iron Phosphate batteries outperforming conventional Lead-Acid batteries. Our goal is to answer the following important questions to help you make the right choice for your application:
• Is lithium really better?
• Are lithium batteries safer?
• What are the main differences between conventional and LiFePO4 batteries?
• Why are lithium batteries more expensive and is it worth the investment?
For this post, when we use the term Lithium we are referring to Lithium Iron Phosphate batteries, a type of lithium-ion battery often spelled LiFePO4 or LFP. We will also use the term Lead-Acid that refers to conventional flooded, SLA, AGM and gel batteries.
Structural & Materials Differences
Lead-acid and lithium batteries work on the same principle but with different materials:
12 Volt lead-acid batteries contain six 1.2V cells in series that reach 12.6V. Lead is used as the anode, lead oxide is used as the cathode and sulphuric acid as the electrolyte.
12 Volt lithium batteries contain four 3.2V cells in series that reach 12.8V. Carbon is used as the anode and lithium iron phosphate is used as the cathode.
Charging and Discharging Efficiency
One of the major advantages lithium has over lead-acid batteries is the efficiency at which it can both charge and discharge. Battery efficiency is best described as the energy required to charge the battery relative to the energy available during discharge.
Lead acid batteries have an efficiency rating of about 75%-85%. Meaning that for every 100 watts of energy used to charge the battery, only about 75-85 watts will remain for discharging purposes.
Lithium batteries have an efficiency rating of about 90%-99%. Meaning that for every 100 watts of energy used to charge the battery, almost all 90-99 watts will remain for discharging purposes.
Another important point of contrast between the two is the charging times. Lithium batteries can accept a much faster rate of current compared to lead acid, meaning it can charge at a fraction of the time. For example, a 12V 100Ah lithium battery can charge in as little as 2 hours compared to about 8 hours it would take a lead-acid battery.
Depth of Discharge (DoD)
Depth of discharge is the maximum amount of energy that can be discharged from the battery without having to recharge it, otherwise it would cause damage to the lifespan of the battery.
Lead acid batteries can safely be discharge to 50% of its capacity. Anything more than that would damage the battery's life cycles and severely shorten its life.
Lithium batteries have a much higher depth of discharge of about 80%-85% of its capacity without having to charge. As a result, lithium batteries can be used for longer periods of time without having to maintenance. This make them the ideal option for off-grid, trolling motor and other applications.
Dimensions and Weight
Dimensions and weight are one of the most significant factors in deciding which battery best suits your application. Energy density is the ratio of battery capacity to battery weight. The energy density of a battery can be calculated using the following formula:
Nominal Battery Voltage (V) x Rated Battery Capacity (Ah) / Battery Weight (kg) = Energy Density (Wh/kg)
Lead acid batteries tend to have lower energy density of about 25-35 Wh/kg, resulting in bulky and very heavy batteries. As an example, a 12V 100Ah lead acid battery weighs about 70 pounds
Lithium batteries on the other hand have much higher energy density of about 90-160 Wh/kg, resulting in a very lightweight battery. As an example, a 12V 100Ah lithium iron phosphate battery weighs about 25 pounds.
Although the costs associated with lithium battery manufacturing and materials has come down, the price of a lithium battery can sometimes be two to three times higher than a lead acid battery. No need to panic or look the other way, there is a very simple explanation. When comparing the lifespan and life cycles of both batteries, lithium batteries last much longer than lead acid batteries. This offsets the upfront cost of the more expensive lithium battery since it will last you much longer than the lead acid.
Battery life is measured in battery cycles, or the number of charge and discharge cycles performed on a battery before performance becomes compromised. All batteries regardless of their chemical composition will lose capacity and potency over time. Completely discharging the battery at 100% DoD will affect the battery life cycles.
Lead acid batteries usually have about 300 - 500 charge cycles over their lifespan.
Lithium batteries usually have 2,000 - 5,000 charge cycles over their lifespan, which is 5-10 times the amount of cycles compared to lead acid.