Lithium-Ion Batteries for Off-Grid Systems

For decades, lead-acid battery technology has been the mainstay of battery-based renewable energy systems, providing reliable storage and ample energy capacity. The most common battery used—flooded lead-acid (FLA)—requires regular watering to maintain electrolyte levels and venting to avoid the buildup of hydrogen and sulfuric gases. Additionally, FLAs are large and heavy, making battery replacement a challenging task for some systems.

With all of the recent action in the electric vehicle and personal electronics industries, lithium-ion (Li-ion) batteries have gained much attention. Here, we examine Li-ion battery pros and cons, and discover why most system owners won’t be swapping out their FLA batteries anytime soon.

What’s Behind Li-Ion?

“Lithium-ion” refers to a variety of lithium-based battery chemistries. Each chemistry has its strong and weak points, which means certain types of chemistries are better-suited for particular applications. There continues to be new lithium-based chemistries being developed (such as lithium-air), but it is too early to tell which will become commercially viable. See the “Lithium Battery Technologies” table for details on a few of the more common types of Li-ion chemistries.

Li-ion batteries typically come in one of three formats: pouch, cylindrical, and prismatic (rectangular-cubic). Pouch types tend to be used in small portable devices, such as smart phones and tablet PCs, or in devices where low weight is important, such as hobbyist remote control vehicles. Cylindrical forms lend themselves to powering medium-sized portable devices, such as power tools. Prismatic are generally the largest, and are typically used in electric vehicles. Prismatic types are also favored in applications that were previously powered by lead-acid batteries, such as backup or off-grid telecommunication systems. Prismatic types usually have hard corrugated sides, which creates air gaps between adjacent cells—an aid to cooling.

Suitable for Renewables?

If Li-ion has an application for residential RE storage, the best candidate is the large-format prismatic lithium iron phosphate (LiFePO₄; LFP) battery. But how do they compare with lead-acid technologies?

Weight. Comparing weight versus available energy storage, an LFP is about one-third the weight of a lead-acid (LA) battery. This is a great advantage for mobile applications, such as boats/RVs, but for stationary RE applications, weight is usually a consideration only during battery change-out.

Space. At about half the volume of an LA battery with equivalent energy storage, LFPs take up far less space. This may be an advantage for mobile applications, but for stationary RE applications, size or volume is typically not a deciding factor.

Low-Temperature Capacity. The storage capacity of LAs drops by 50% at -4°F, compared to 8% with LFP. Keeping lead-acid batteries warm so that they maintain reasonable capacity in cold climates can be challenging, giving LFPs an advantage. However, LFP batteries generally should be charged at a slower rate when cold—usually no more than a C/10 at ambient temperatures below 32°F. (For example, if you have a 200 Ah battery, a C/10 charge rate will be 20 A.)