Solar Battery Types: Lead-Acid, AGM, and Lithium

Choosing a battery chemistry for a solar installation is an engineering decision that follows from arithmetic and equipment limits, not from preference. The three chemistries in common use—flooded lead-acid, AGM (absorbent glass mat), and lithium iron phosphate (LiFePO4)—differ in usable depth of discharge, cycle life, charging behaviour, and response to mistreatment. Each has a set of specifications that must be satisfied by the rest of the system, and the right choice depends on how those specifications align with the load profile and the environment.

This page compares the properties of the chemistries themselves. It does not recommend a brand or a product. The numbers given are typical values for the technology, not for a particular unit. Always check the datasheet of the specific battery under consideration, because manufacturers rate cycle life at different depths of discharge and temperatures.

Chemistry Basics

Flooded lead-acid is the oldest battery technology still in common use. The electrodes are lead and lead dioxide, and the electrolyte is a liquid sulfuric acid solution. The cells vent gas during charging, so they require regular maintenance: checking electrolyte levels, adding distilled water, and keeping the terminals clean. They are the least expensive per kilowatt-hour of storage, but they have the shortest cycle life and the lowest usable depth of discharge.

AGM (absorbent glass mat) is a type of lead-acid battery where the electrolyte is absorbed into a fiberglass mat between the plates. This makes the battery spill-proof and allows it to be mounted in any orientation. AGM batteries are sealed, so they do not require watering, and they have a lower self-discharge rate than flooded batteries. They are more expensive than flooded, but they offer a longer cycle life and better performance at low temperatures.

Lithium iron phosphate (LiFePO4) is a lithium-ion chemistry that uses iron phosphate as the cathode material. It is the most common lithium chemistry for solar storage because it is safer and has a longer cycle life than other lithium-ion variants. LiFePO4 batteries have a higher energy density, a higher usable depth of discharge, and a longer cycle life than lead-acid batteries, but they are also more expensive upfront. They require a battery management system (BMS) to protect the cells from overcharge, over-discharge, and over-temperature.

Key Specifications

Note: Cycle life figures are approximate and vary by manufacturer and temperature. Depth of discharge is the percentage of the battery’s capacity that is used before recharging.

Usable Depth of Discharge

The most important difference between the chemistries is the usable depth of discharge (DoD). Lead-acid batteries (both flooded and AGM) suffer accelerated aging if they are discharged beyond about 50% of their rated capacity. Discharging deeper reduces the number of cycles the battery can deliver. For example, a lead-acid battery rated for 500 cycles at 50% DoD might only deliver 200 cycles if regularly discharged to 80%. In contrast, LiFePO4 batteries can be discharged to 80–90% without significantly reducing their cycle life. This means that for a given usable capacity, a lithium battery can be smaller and lighter than a lead-acid battery.

Implication for sizing: To get 10 kWh of usable energy from a lead-acid battery, you need a battery with a rated capacity of 20 kWh (because you can only use half). With LiFePO4, you can use 80% of the rated capacity, so you need only 12.5 kWh. This affects the physical footprint, weight, and cost of the battery bank.

Cycle Life

Cycle life is the number of charge/discharge cycles a battery can deliver before its capacity drops to a specified percentage of its original (often 80%). Lead-acid batteries typically last 300–600 cycles at 50% DoD, while LiFePO4 batteries can last 3000–5000 cycles at the same DoD. If a battery is cycled daily, a lead-acid battery might last 1–2 years, whereas a LiFePO4 battery could last 8–13 years. Cycle life is also affected by temperature: high temperatures accelerate degradation, and low temperatures reduce capacity temporarily.

Implication for total cost: Although LiFePO4 has a higher upfront cost, its longer cycle life can make it more economical over the life of the system, especially if the battery is cycled frequently. However, the break-even point depends on the number of cycles per year and the cost difference, which varies by market.

Charging Behaviour

Lead-acid batteries require a specific charging profile: bulk, absorption, and float. The bulk phase delivers constant current until the battery reaches a certain voltage, then the absorption phase holds the voltage constant while the current tapers, and finally the float phase maintains a lower voltage to keep the battery topped off. Overcharging a lead-acid battery causes gassing and water loss (in flooded) or pressure build-up (in AGM, which has a pressure relief valve). Undercharging causes sulfation, a buildup of lead sulfate crystals that reduces capacity and eventually destroys the battery.

LiFePO4 batteries have a different charging profile. They are charged with a constant current/constant voltage (CC/CV) method, but they do not require a float charge. In fact, holding a LiFePO4 battery at float voltage is harmful because it stresses the cells. The BMS typically disconnects the battery when it is full. The absorption voltage for LiFePO4 is higher per cell (3.6–3.65 V) compared to lead-acid (2.40–2.45 V).

Implication for charge controllers: A charge controller must be programmed for the correct battery type. Many charge controllers have preset profiles for flooded, AGM, and lithium. Using the wrong profile can damage the battery. For example, applying a lead-acid float voltage to a LiFePO4 battery will not fully charge it, and applying a lithium absorption voltage to a lead-acid battery will overcharge it.

Temperature Effects

All batteries are affected by temperature, but in different ways. Lead-acid batteries lose capacity in cold temperatures: at 0°C, a lead-acid battery may deliver only 70–80% of its rated capacity. They also require a higher charging voltage in cold temperatures to avoid undercharging. Conversely, high temperatures increase the rate of chemical reactions, which can lead to overcharging and accelerated grid corrosion in lead-acid batteries.

LiFePO4 batteries also lose capacity in cold temperatures, but they have a more serious limitation: they cannot be charged at temperatures below 0°C (32°F) without causing lithium plating, which permanently damages the cells. Many LiFePO4 batteries have a built-in heating pad or the BMS disables charging below a certain temperature. If the battery is in an unheated space, this can be a significant issue.

Implication for cold climates: If the battery is installed in a location where temperatures drop below freezing, lead-acid batteries may be more forgiving, but they still need temperature-compensated charging. LiFePO4 batteries may need to be kept in a heated enclosure or have a heating system. The decision depends on the specific temperature range and the design of the battery enclosure.

What People Get Wrong

A common mistake is to assume that because LiFePO4 has a higher usable depth of discharge, it is always the best choice. That overlooks the cost and the charging requirements. For a small system that is cycled rarely, a flooded lead-acid battery may be perfectly adequate and much cheaper. Another mistake is to use a lead-acid charge controller profile on a lithium battery, or vice versa. This is a natural error because many charge controllers have a generic “battery” setting, and the user may not realize that the charging voltages are different.

Another frequent error is to size a lead-acid battery bank based on the rated capacity without accounting for the 50% DoD limit. This leads to a system that is undersized and the battery is regularly discharged too deeply, shortening its life. The same mistake occurs in reverse: sizing a LiFePO4 battery too large, which is unnecessary and wastes money.

Temperature compensation is often overlooked. Lead-acid batteries need a higher charging voltage in cold weather and a lower voltage in hot weather. If the charge controller does not have temperature compensation, the battery may be undercharged in winter and overcharged in summer. For LiFePO4, the cold-temperature charging restriction is a safety issue that must be addressed.

Finally, some people assume that all lithium batteries are the same. LiFePO4 is one of several lithium chemistries, and it is chosen for its safety and cycle life, but it has a lower energy density than some other lithium-ion types. The term “lithium” on a battery does not guarantee it is LiFePO4; it could be lithium nickel manganese cobalt (NMC) or lithium titanate (LTO), which have different characteristics.

Choosing the Right Chemistry

The choice between lead-acid, AGM, and LiFePO4 comes down to the following factors:

• Depth of discharge required: If the system is designed to use more than 50% of the battery capacity regularly, LiFePO4 is the only option that will not suffer accelerated aging.
• Cycle frequency: If the battery is cycled daily, LiFePO4’s longer cycle life is likely to be worth the higher upfront cost. If the battery is used only occasionally, lead-acid may be more economical.
• Temperature environment: If the battery is in a cold location and cannot be heated, lead-acid may be more practical, provided the charge controller has temperature compensation.
• Maintenance: If the battery is in a hard-to-reach location, AGM or LiFePO4 (both sealed) are preferable to flooded lead-acid, which requires periodic watering.
• Upfront cost vs. lifetime cost: Lead-acid has a lower upfront cost, but LiFePO4 may have a lower cost per cycle over its lifetime. The break-even point depends on the number of cycles and the price difference.

There is no single “best” chemistry. The right choice is the one that satisfies the system’s requirements at the lowest total cost over the battery’s life.

Installation and Safety

All battery chemistries have safety considerations. Lead-acid batteries can emit hydrogen gas during charging, so they must be installed in a ventilated area. AGM batteries are sealed but still have pressure relief valves, and they should be installed in a location where they are not exposed to extreme heat. LiFePO4 batteries are generally safer than other lithium chemistries, but they can still catch fire if punctured or overcharged. They must be installed with a BMS that protects against overcurrent, overvoltage, and over-temperature.

Important: A PV array produces dangerous DC voltage whenever there is light on it, and it cannot be switched off at the panel. Never open a combiner box, probe a live string, or disconnect an energised connector. Work on the DC side should be performed by a qualified person. Roof work and mains connection are licensed trades.

Local Regulations

Net metering, interconnection, export compensation, permitting, and incentives are set by state and by utility, and they change. Check with the local utility and the authority having jurisdiction for the rules that apply to a specific installation. Do not assume that a rule from one jurisdiction applies elsewhere.

Summary

• Lead-acid batteries are inexpensive but have a limited usable depth of discharge (50%) and a shorter cycle life.
• AGM batteries are a sealed lead-acid option that requires no maintenance but still has the same DoD and cycle life limitations.
• LiFePO4 batteries have a higher usable DoD (80–90%), a longer cycle life, and higher efficiency, but they cost more and have a cold-temperature charging restriction.
• The correct charging profile must be used for each chemistry, and temperature compensation is essential for lead-acid.
• The choice depends on the load profile, temperature, and cost over the battery’s life.