A battery is just a battery right? Wrong. When it comes to making a decision about adding batteries to a PV system, it’s important to know what’s inside. Here’s a crash course in battery chemistries.
A battery is basically a chemical reaction that gives off energy in the form of electrons that can then be used to power a device or home. Unlike the batteries in most flashlights that are used until they have too little charge left to be useful, batteries used in homes have to be rechargeable so that they can repeatedly store energy when you need it to and discharge it when it is needed. The battery chemistry refers to the types of chemicals and metals inside the battery that are used to store and release energy.
Lead Acid – This is a typical, old-fashioned battery like those found under the hood of an automobile. This chemistry is not particularly efficient, and and is usually bulky, but the batteries are fairly inexpensive and were widely used in earlier home storage offerings and some off-grid systems.
Lithium-ion– Lithium-ion batteries are the most widely used, and found in most consumer electronics. There are many different flavors of Lithium-ion chemistry, but the two primary types are:
- Lithium-ion with Cobalt– Most Lithium-ion batteries contain cobalt, which can actually make the batteries unstable and at risk of thermal runaway, which basically means that it can catch fire if punctured or crushed and be very dangerous. While not extremely likely in most small devices, this is not something you would want to have happen with a larger scale battery. For instance, this type of battery is used in certain high end electric vehicles and videos of car fires where this sort of battery is involved show dramatic consequences. You may want to avoid having something like this attached to your house.
- Lithium Ferrous Phosphate– Lithium Ferrous Phosphate uses iron instead of cobalt. It is much more stable on its own, and when placed in a prismatic cell it is actually very safe. Crush and puncture tests of Lithium Iron Phosphate batteries show little to no reaction, which you can see in this Eliiy video. The Enphase AC Battery uses this chemistry.
Other technologies – there are other battery technologies beginning to appear, but few are widely available for residential storage yet. These include flow batteries, which have both mechanical and chemical functions that make them more complex and potentially more likely to have maintenance issues, saltwater batteries, which are bulkier but safer and easier to deal with at the battery's end of life.
Battery Packaging Design
Batteries cells come in different shapes, including cylindrical (AA batteries), pouch cells (cell phone) that are rectangular in design, and prismatic, which are also rectangular, but with more protection than pouch cells. Enphase chose a battery with a prismatic packaging design for its stability and better thermal characteristics.
While it's tempting to focus on the cost or design of batteries coming on to the market, another consideration is certainly the stability and the safety of the battery you choose. This is why Enphase uses a prismatic Lithium Iron Phosphate chemistry for its AC Battery, which provides a great combination of stability, safety, and high performance for the Enphase Storage System.