I bought my first solar panel in 1989, a 50-watt Solarex photovoltaic module. Along with an old motorcycle battery, some DC car lights, and a small radio, I created an off-grid room in the house I was renting. When the power was out, and even when it wasn’t, I had lights and music operating on stored solar energy.
When I built my home in the 1990s, it was far enough from utility power that an off-grid system made economic sense, in part because I could do the work myself. Solar panels were more expensive back then, and the only accessible, affordable battery technology available was lead-acid. I’ve had to replace my battery bank about every seven years since, despite different brands, sizes, and configurations. Meanwhile, improvements in chargers, inverters, and PV panels have made lead-acid technology feel more antiquated.
This article compares some of the qualities of lead-acid and lithium iron battery technologies for off-grid applications.
Breaking Down Batteries
Lead-acid batteries haven’t changed much in 100 years, and they have a long history of dependability and affordability. They’re typically named according to the electrode material used, and further defined by the electrolyte used. For example, “flooded” refers to liquid electrolyte that needs to be replenished periodically by adding distilled water. Various types of maintenance-free, sealed, or valve-regulated lead-acid batteries exist.
Batteries are designed for specific purposes. Starting, lighting, and ignition (SLI)batteries used in cars are engineered to release lots of energy in a short period of time, and then quickly
receive a recharge. Deep-cycle batteries have thicker plates, allowing for greater energy storage capacity and deeper discharging. In this article, I’ll focus on deep-cycle, flooded lead-acid (FLA) batteries, because they’re the most likely lead-acid batteries to be used in off-grid applications.
A relatively new technology, lithium-based batteries are engineered for performance and represent advanced energy storage. Several configurations are in use today, each with different electrode and electrolyte chemistry, charge and discharge characteristics, specific energy, costs, and safety factors. For this comparison with deep-cycle FLA batteries, I’ll focus on lithium iron phosphate batteries (LFP). Lithium iron phosphate (LiFePO4) technology, also called “lithium ferro phosphate,” is named after its cathode material. LFP batteries are gaining popularity for automotive, offshore, and off-grid use as a direct replacement for lead-acid batteries.
Safety is a widely publicized concern with lithium batteries. Lithium-ion batteries are under internal pressure and contain a flammable electrolyte, so they need to be durably packaged, and handled and charged with extreme care. Lithium-ion is not the same as lithium iron phosphate, the LFP batteries referred to in this article. LFP uses an inherently safer, more stable design based on different electrode alloys and electrolyte.
Lifetime. Battery lifetime is expressed as the number of charge and discharge cycles a battery can survive before losing a certain percentage of capacity. Deeper discharge results in fewer total cycles and overall reduced lifetime. Deep-cycle batteries are considered durable enough to withstand repeated discharging of 80 percent of their total capacity, or 80 percent “depth of discharge” (DOD).
Batteries used in off-grid applications are often heavily cycled at partial states of charge, meaning that energy is drawn out of them over the course of a day or more, followed by some recharging, but it may be several days, or even weeks, before they’re fully recharged. This is especially hard on lead-based batteries, and also makes it difficult to interpret the manufacturer’s battery cycle life chart to predict longevity. It may be tempting to predict that my average seven-year battery life expectancy (2,555 cycles) translates to an average DOD of 35 to 40 percent, but many factors are at play. Ultimately, lead will shed from the electrodes, depleting them quicker during periods of high discharge rates, partial charge levels, or elevated heat.
LFP batteries have a considerably greater lifetime than FLA batteries, with far more charge and discharge cycles available, and less dramatic partial charge degradation.
Capacity. Battery capacity is typically expressed in terms of how many amperes (amps) of current can be delivered over a period of time. For example, a common deep-cycle lead-acid battery indicates that it can deliver 75 amps for 115 minutes. A more meaningful capacity term is the amp-hour (Ah), which expresses how many hours a battery can deliver energy until its specified end-point voltage is reached and the battery dies. Battery spec sheets provide ratings that reflect the capacity at different discharge rates over time. These ratings are expressed as a number followed by the letter C. For example, a “20C” rate indicates that a 6-volt battery can deliver 225 Ah when discharged at a consistent rate over 20 hours. Doing the math reveals that the 20C rate of this battery is 11.25 amps (225 Ah / 20 hours = 11.25 amps). That same battery used at a higher discharge rate might have a capacity of 146 Ah, indicating that less energy is available from the battery at higher discharge rates.
Ah tells us how many amps a battery can deliver over time at a certain rate, but it doesn’t fully quantify the energy stored in a battery. Energy is power (watts) delivered over time (hours). Watts are calculated by multiplying volts by amps. The 225-Ah, 6-volt example above can be wired in series to achieve the required battery bank voltage. Since Ah already has a time factor built in, all we have to do is multiply 6 volts by 225 Ah to get 1,350 watt-hours (Wh). Divide Wh by 1,000 to express energy storage capacity in the more familiar term of kilowatt-hours (kWh), which is how electric companies bill us for the electricity we consume. For perspective, a 100-watt lightbulb that’s lit for 10 hours consumes 1,000 Wh, or 1 kWh.
A wide variety of sizes and capacities are available for LFP batteries. Case configurations make direct replacement of FLA possible. Because of the higher charge and discharge rates possible with LFP, you may see fractional C-values (such as 0.5C), which indicate a charge or discharge period of under an hour.
Specific energy. Sometimes called “energy density,” specific energy is energy per unit of weight. Our example FLA battery stores 1,350 Wh and weighs 67 pounds, so it has a specific energy of 20.1 Wh per pound (1,350 ÷ 67 ). For perspective, a gallon of gasoline weighs 6.3 pounds and stores the equivalent of approximately 36,000 Wh, with a specific energy of about 5,714 Wh per pound.
The specific energy of LFP batteries is over 40 Wh per pound, twice that of FLA.
Charge and discharge dynamics. Charge and discharge dynamics in FLA batteries are driven by lead-based electrode plates immersed in a sulfuric acid and water electrolyte solution. As the battery is charged, lead oxide builds up on the positive plates, and the electrolyte becomes stronger. During discharge, the electrolyte solution grows weaker as both electrodes become lead sulfide, having absorbed sulfuric acid from the electrolyte. During discharge, the voltage drops predictably. The state of charge of an FLA battery can be determined by reading the resting voltage of the battery. Battery voltage in conjunction with a hydrometer reading of the specific gravity of the electrolyte in each cell can reveal a lot about the state of charge and overall health of an FLA battery.
The charge and discharge dynamics in LFP batteries differ from FLA in terms of the electrochemical process, but can be described in a similar way. During discharge, positively charged lithium ions move within lithium salt electrolyte from the negative electrode to the positive electrode. Electrons are carried from the negative electrode, through the electric circuit, and back to the positive electrode. One big difference between FLA and LFP batteries’ reactions is that FLA chemistry happens on the surface of the lead electrodes, while ions in the nonliquid electrolyte of LFP batteries are fully absorbed into the crystalline structure of the electrodes.
You can’t easily determine the state of charge of LFP batteries with a voltmeter, because the battery voltage remains fairly constant over a wide range of discharge depths. Those who switch from FLA to LFP may find this, as I did, a bit flummoxing. When the voltage finally drops, it means the battery needs to recharge. The best way to understand and manage charge cycling is to use a properly calibrated monitor that tracks energy going in and out of the battery bank. Because of the sensitive nature of LFP chemistry and tight limits on charge and discharge parameters, manufacturers provide an electronic battery management system (BMS) to help prevent charge- and discharge-related battery damage. The BMS may also include a capacity monitor.
Charging. Charging must be performed using the proper “charge profile,” meaning the correct voltage and current for a prescribed amount of time to rejuvenate a battery’s chemistry, ensure maximum lifetime, and avoid potentially catastrophic failure. Voltage and current requirements change over the course of the charge cycle. As a battery fills up, the charge current needs to decrease to avoid overheating. If the current is too high, the battery heats up. When the battery heats up, it accepts a charge faster and heats up even more. Soon, the battery is in “thermal runaway,” and its life will be significantly reduced in a very short time.
Typically, FLA charge current shouldn’t be much more than 10 percent of the capacity rating, though higher current can be tolerated if the temperature is closely monitored. Periodically, FLA batteries should be “equalized,” which is a controlled overcharge that helps reverse sulfation (lead sulfate crystal growth) on the plates. Most modern battery chargers are sophisticated enough to manage a complex three-stage charge profile automatically.
In LFP batteries, charging is the reverse of discharging in terms of ion and electron transfer. Most modern off-grid battery chargers (solar and inverter-integrated) are adjustable to accommodate the specific LFP charge profile. This is essentially a two-stage charge consisting of bulk and float charges. Conventional FLA charging schemes don’t belong in the LFP world. That is, no equalizing, trickle charging, or temperature compensation should be used. LFP batteries have a very low internal resistance and can accept very high charge currents, resulting in faster recharge times. An LFP battery’s BMS is designed to prevent damage due to overcharging and over-temperature operation.
Temperature. Temperature affects the chemical reactions within a battery. Higher temperatures mean faster charge and discharge capability. However, if the temperature is too high, the battery will deteriorate rapidly. Lead-acid batteries operate best around 80 degrees Fahrenheit. Much colder than 70 degrees, and capacity is noticeably reduced; much warmer than 90 degrees, and longevity is negatively affected.
Temperature consistency is important for LFP battery operation, but not to the same degree as FLA batteries. The operational temperature range for charging is between 32 and 113 degrees, though colder temperatures are acceptable for discharge and storage.
Efficiency. FLA batteries average about 75 percent efficiency, meaning 25 percent more energy needs to be put into an FLA battery than was taken out to fully recharge it.
LFP efficiency is in the range of 95 to 98 percent, resulting in shorter charging times and higher current delivery to the load with much less heat. More efficient charging means a smaller, less costly PV array, fewer hours on the generator, and lower energy cost if you charge batteries from the grid.
Off-gassing. Off-gassing occurs when lead-acid batteries release hydrogen gas at the negative plates and oxygen at the positive plates during charging and heavy discharge. This is a potentially explosive combination, which can be exacerbated by the production of hydrogen sulfide gas boiling off the electrolyte while the battery is charging. Storage batteries are often enclosed in boxes in the basement or utility room of an off-grid home; it’s important to seal and ventilate the battery box with a fan ducted to the outdoors while drawing fresh air into the battery box.
Off-gassing is nonexistent in LFP batteries, because they’re sealed and the electrolyte doesn’t boil off.
Maintenance. In addition to proper charge and temperature management, FLA batteries need to have their electrolyte checked and refilled monthly. You’ll probably need to clean corrosion off some of the terminal connections as well; applying anti-corrosion grease or spray will help. Be aware that when FLA batteries are in a partially discharged state for any length of time, or if the electrolyte dips low enough to expose the plates to air, sulfation will occur rapidly, insulating the plates against the required chemical reactions. Sulfation is difficult, if not impossible, to reverse if the battery isn’t properly maintained.
Maintenance for LFP batteries amounts to checking and tightening connections as needed. There’s no electrolyte to check, and corrosion isn’t a problem for LFP batteries.
On the topic of maintenance, one thing to consider is that lead-acid batteries have been around a long time, and technicians have plenty of experience with them, but I can’t say the same for relatively new lithium-based batteries.
Recycling. Recycling lead-acid batteries is fairly straightforward and extremely important. Lead is a toxic material that needs to be handled safely and kept out of landfills. Take the old battery to an auto parts store or salvage yard for recycling.
LFP batteries should be recycled, though they’re considered nontoxic and both landfill and incinerator friendly.
Cost and performance. I found disparity in how LFP battery performance is reported among manufacturers, and even by the same manufacturer, especially concerning the relationship between capacity and DOD. Most LFP manufacturers provide a cycle life cost analysis that, not surprisingly, favors LFP over FLA. Their analyses are fair in that they put values on FLA maintenance labor and grid charging costs, but the data used doesn’t always compare apples to apples. So I’ve decided to present a comparison of one FLA battery to one LFP battery of similar size and capacity made by the same manufacturer to eliminate some uncertainty.
I’ve compiled the information into comparison tables. These tables don’t include costs to maintain and replace FLA battery banks, as most off-gridders actively participate in these activities. They also don’t factor in the cost to buy grid power or to operate a fossil fuel generator to charge batteries. Unless you rely 100 percent on renewable energy for charging, this can be an important factor in determining lifetime cost-effectiveness.
Note that these figures reflect ideal conditions of charge and discharge regime, temperature, and maintenance.
Based on my experience, lead-acid batteries in off-grid homes offer affordable storage with acceptable performance. However, I can’t say I ever look forward to adding water, cleaning terminals, exposing myself to toxic and explosive gases, or lifting and transporting 100-pound anchors every seven years, and I’ve certainly lost my share of clothing over the years to dripping electrolyte.
If you’re considering LFP batteries, ask the dealer, installer, or energy consultant plenty of questions to make sure apples are compared to apples and you get the evaluation you need. And regardless of which battery type you choose, make sure you fully understand the warranty, which is typically prorated based on age for both FLA and LFP, but may change based on how the batteries are used.
I’m due for a battery bank replacement this summer, and haven’t yet decided which technology I’ll end up with. If upfront cost weren’t an issue, lithium would be the winner based solely on longevity and lack of maintenance. Each battery type has pros and cons; take some time to thoroughly research both, and consider your energy needs and personal preferences before making a decision.