An Introduction to Today’s Battery Options
By George Harvey
We use batteries for all sorts of things, and most of us probably take them for granted. It is an easy thing to do. If we need AAA batteries, we often do not think beyond that fact. However, when we buy batteries for an off-grid house or backup power, things can get very complicated.
Designing a system requires a good deal of understanding. There are many kinds of batteries out there, and there is no such thing as a best battery for all applications. In fact, there may not be a single best type of battery for a specific application, because different kinds have different advantages and disadvantages, making the choice a trade-off.
For example, one way to measure cost of a battery is by the price per kilowatt-hour (kWh) of electricity the battery can store. This is not as straight forward as it might seem. A lead-acid battery that can store 2.6 kWh might cost half as much as a sodium-ion battery of the same capacity. The problem with the comparison, however, is that lead-acid batteries should not be discharged to less than 45% to 80% of their capacity, depending on design, while a sodium-ion battery might be able to be 100% discharged. That means while only about a quarter to a half of a lead-acid battery’s capacity is actually available, all of the sodium-ion battery’s capacity might be.
Cost comparisons also should include life expectancies of batteries. These are impacted by maintenance requirements, and tolerance for abuse such as overcharging or temperature swings. Also, some batteries are designed to maintain backup power when attached to the grid, while others are designed for off-grid applications, and installing a poorly chosen battery can reduce its life.
Another thing that can reduce battery life is poor charging practice. Some batteries are damaged by overcharging, and some are damaged by undercharging.
Different kinds of batteries have very different storage requirements. Flooded lead-acid batteries are not safe to use inside a building unless they are contained in a sealed box with ventilation to the outside, because they can generate hydrogen, which can be explosive when it is mixed with air. Some kinds of batteries can be stored inside without ventilation.
Many batteries have toxic chemicals in them, but some do not. Many are difficult to recycle, but some, including lead-acid batteries, can be nearly 100% recycled.
Some batteries store electricity more efficiently than others. But if the energy is free, such as when there is a superabundance from a solar array, storage efficiency alone does not tell us much.
Here are a few battery types:
Presently, the most common energy storage systems for grid-tied and off-grid applications are the lead-acid batteries. They come in a wide variety of types, with different requirements for how they are stored, how they are charged and discharged, and how they are maintained. They are well-known, and they are among the least expensive technologies.
It is imperative that users of lead-acid batteries understand their limitations and maintenance. They are rated for 20% to 55% discharge, so you should get batteries sufficient to store a minimum of twice the energy you actually need and possibly five times as much. They are, however, the best known batteries we have, and if the right battery is chosen for a job, they can perform for many years (generally they are warrantied for seven to ten years; but with proper care can last much longer), inexpensively. They deserve to be covered in articles focusing on them alone.
A number of different companies make a number of different types of lead-acid batteries. It is important for home and business energy storage that appropriate deep-cycle batteries be used. These cost more than automotive batteries, but you get what you pay for. Two reputable battery brands are Trojan and Rolls (Surrette).
Nickel-iron batteries were designed in early 20th century, and had some really superior characteristics. They are a really good choice for batteries that will be discharged and recharged every day, such as in off-grid applications.
While they cost more than lead-acid batteries, they are also very forgiving of abuse and last for a long time. Their very long service life can make them a better investment than lead-acid batteries in the long term, despite a higher price.
It is said that nickel-iron batteries may wind up being used by a grandchild of their original purchaser. They are rated at 10,000 cycles to 80% discharge, which implies that you should buy sufficient storage to cover about 1.25 times as much power as you need.
Iron Edison is the only company we know of that is offering nickel-iron batteries at present.
Anyone who reads Green Energy Times should know about the lithium-ion batteries offered by Tesla. They are intended for daily charging and discharging, for off-grid applications, and for weekly storage, which is for grid-tied backup. They are a good deal more expensive than lead-acid batteries, but require little to no maintenance and are long lasting. They can be deeply discharged, but not 100%.
These are coming to the market slowly, and getting them from Tesla might entail a wait of over a year for most people. Green Mountain Power does have a supply, however, and its customers might find they are able to get batteries quickly. It also happens that Iron Edison is also building lithium-ion batteries, and orders for these are being fulfilled much faster than those from Tesla. Though their chemistry is not identical to Tesla’s, it has its own advantages.
Sodium-ion batteries, made by a company called Aquion, came to market over a year ago. Their components consist of carbon, magnesium oxide, and sodium sulphate. These are about as non-toxic as chemicals can be, and they have the added advantage of being commonly available.
Aquion’s batteries can be 100% discharged. They should be good for 7000 cycles of charging and discharging and require less maintenance. A notable drawback is that they have a slow discharge rate, and so the battery bank should be somewhat over-sized for this reason.
One dealer of Aquion batteries, altE Store in Boxborough, Massachusetts, says they have sold about 200 of the batteries, with a capacity of 2.6 kilowatt-hours each, in the past year. They are so successful that the store currently has about 500 on order from the manufacturer.
We will continue looking deeper into these and other types of batteries in future issues of Green Energy Times.
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