Types of solar batteries: Chemistry and coupling

Just like there are different types of batteries for home appliances and gadgets–you wouldn't put double A batteries in your watch or cellphone, would you?–there are different types of batteries for solar-plus-storage applications. The two primary differences to remember are the battery's chemistry and whether the battery is AC or DC-coupled. Which battery is right for you depends on when you're installing the battery (as a retrofit to an existing solar panel system or as a part of a new one), how often you plan on running it (are you charging and discharging every day or every week), and how much space you have (a whole garage/shed or just a bit of wall space).

Energy storage products come in all shapes and sizes and use various chemistries to store electricity. As explained in greater depth in our article about how batteries work, batteries store electricity by pulling ions from one compound to another, and discharge electricity by reversing this flow through an external circuit. That's a hefty sentence to digest, but for now, what you need to know is that when we refer to the 'chemistry' of a battery, we're talking about the compound that's used to store electricity.

When most people talk about the different solar battery types, they usually refer to battery chemistry. Different types of battery chemistries vary primarily in their power density, i.e., how much electricity they store in a certain space. The main chemistries you'll see in home batteries are:

• Lead-acid batteries

• Lithium-ion batteries

• Flow batteries 

Lead acid batteries

Lead acid batteries are the oldest type on this list: they've been around since the 19th century! Lead acid batteries use different lead compounds at the two separate electrodes (positive & negative) and an acidic electrolyte–hence, "lead acid." These batteries are not particularly power dense (they take up a lot of space) and are not designed to be discharged fully all the time (i.e., only a 50% depth of discharge). However, lead-acid batteries have long been used in cars as the primary power source to provide the power surge necessary to start your gasoline engine. 

Lithium-ion batteries

If you have a solar battery at your home or business, it is almost certainly a lithium-ion battery. Lithium-ion is the main chemistry used in batteries offered by the primary players in today's solar-paired storage market, such as Tesla, LG Chem, Generac, Panasonic, and many more. These batteries use lithium compounds for an electrode and are called lithium-ion batteries because they utilize the flow of ions away from a lithium compound to store energy. The category of lithium-ion batteries covers several different chemistries, each of which has slightly different characteristics. The two most popular lithium-ion batteries are lithium nickel manganese cobalt oxide, or NMC, batteries and lithium iron phosphate, or LFP (for iron's chemical sign of Fe). NMC batteries tend to be more power-dense (i.e., smaller for the same storage capacity), while LFP batteries tend to have longer lifetimes.

Flow batteries

As opposed to passing ions from a metal compound to another metal compound through an electrolyte like lead-acid and lithium-ion batteries, a flow battery passes ions from one tank of liquid to another, and then back again. These types of batteries–which are also called redox flow batteries due to their use of reduction/oxidation as a way to pull ions from one liquid and give them to another–are not particularly power dense (they can be quite big), but can store much more energy than a typical lithium-ion home battery. While uncommon, they could become very popular in the coming decade.

Solar panels produce direct current (DC) electricity, and batteries store DC electricity. However, we use alternating current (AC) electricity to run our homes and businesses. This means that for you to use either the electricity from your solar panels or battery, it will have to be inverted from DC electricity to AC electricity. 

Batteries can either be AC coupled or DC coupled, a distinction that may sound esoteric but is essential because both have pros and cons. An AC-coupled battery takes AC electricity, inverts it to DC electricity, stores it, and then inverts it back to AC electricity for your home or business. This is the most common setup for a solar-plus-storage system and is almost certainly what you'll get if you already have a solar panel system installed and are adding storage later.

Unfortunately, electricity is lost whenever you invert electricity from DC to AC or back. And with an AC-coupled system, your electricity will be inverted many times, and the electrical losses can add up. For instance, if you have an AC-coupled solar-plus-storage system, the DC electricity produced by your solar panels is inverted once by your solar inverters, and then from AC to DC electricity by your storage inverter to store it, and then a third time from being stored in your battery to being used in your home.

On the other hand, a DC-coupled battery bypasses the need for multiple inversions. A DC-coupled solar-plus-storage system charges your battery straight from your solar panels, without first inverting it to AC electricity or back. To do this, a DC-coupled storage system uses a hybrid inverter that can work as both a solar and storage inverter. 

However, hybrid inverters are not always quite as proficient as standard solar inverters at turning the energy from your solar panels into usable AC electricity. In other words, a DC-coupled solar-plus-storage system is sometimes a compromise between avoiding losses from multiple inversions and slightly worse performance as a solar inverter. 

To learn more about the differences between AC and DC-coupled batteries, check out this article.

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