March 11, 2018 7 min read

What are the best deep cycle batteries for solar energy storage?

If you want to power your home entirely off of solar power, you have two options:

  1. Live in the dark after the sun goes down.
  2. Store extra solar energy in a battery.

Here at Ohmstead, we prefer option #2. There are many cost effective options for solar batteries, each with their unique pros and cons. Read on to learn about all of the viable options for solar energy storage in the market today.

 

Terminology

Battery: A device that is able to store energy as chemical energy and then make it available to use as electricity. Different types chemical batteries are usually referred to by their chemistries, such as "lead acid" or the mouthful "lithium nickel manganese cobalt oxide".

Battery bank: Multiple batteries wired together into one large battery. More common in lead acid and nickel iron battery systems, less common in lithium ion systems.

Capacity: The total amount of energy a battery can store, measured in kilowatt-hours.

Kilowatt-hours (kWh): A measure of energy. For example, 1kWh is the amount of energy needed to run a compact microwave for one hour. The average home in the U.S. home uses 30 kWh per day1.

Cycle: A complete discharge of a battery. For example, say that you use 75% of your computer's battery today, charge it back to 100% at night, and then use another 25% tomorrow. In this example, the battery will have underwent one charge cycle.

Cycle life: Almost all batteries lose storage capacity through normal use. A battery's cycle life is the number of cycles a battery can withstand before its original storage capacity is reduced significantly – usually to 80% of its original capacity.

Depth of discharge (DoD): Many batteries are damaged if they are discharged past a certain point, like car batteries. Discharging a battery too far will reduce its cycle life. In order to avoid this, a recommended depth of discharge is provided for each battery by the manufacturer which states how far it can be drained before it's significantly damaged. This is stated in terms of a percentage of the batteries capacity, such as 50%.

Cycle life and depth of discharge example: A 1 kWh flooded lead acid battery has a cycle life of 500 cycles when discharged at 50% DoD. This means that we can discharge it to 50% capacity and recharge it back to 100% capacity one thousand times. At this point, the battery is still usable, but it's maximum capacity has been reduced to 0.8 kWh.

Usable storage: A battery's stated capacity multiplied by its depth of discharge. For example, a 1 kWh lead acid battery with a 50% DoD has a usable storage of 0.5 kWh.

Lifetime storage: A battery's stated capacity multiplied by its cycle life. For example, a 1 kWh lead acid battery with a cycle life of 500 cycles has a lifetime storage of 500 kWh.

Deep cycle battery: A rechargeable battery with a high depth of discharge, as opposed to a non-rechargeable battery or a rechargeable battery with a shallow depth of discharge, like a car's starter battery.

Charge efficiency: How efficient a battery is at converting electricity into stored chemical energy. A perfect charge efficiency of 100% means that every kWh sent to the battery is stored for later use. Batteries with lower charge efficiencies require more power to charge. This can be an issue if the battery is being charged directly from expensive grid energy.

Self-discharge rate: The amount of charge that a battery loses over specified period of time.

The grid: The electrical grid maintained by utility companies. If you get electricity from the grid, you're on grid. If you don't, you're off grid. If you're interested in living off the grid, check out our profile on Colorado's most energy efficient home.

 

Introduction to Deep Cycle Battery Chemistries

The state-of-the-art in battery technology is changing quickly. New chemistries are coming to market, and older chemistries are being refreshed by improved manufacturing techniques. These are the most common battery chemistries you can expect to find for solar energy storage.

Lithium ion (metal oxides)

Tesla Powerwall 2

Best in class: Tesla Powerwall 2
Where it shines: On grid storage

Lithium batteries are the best batteries for many reasons, such as reliability, ability to handle extreme temperatures, slow self discharge rate, and overall system usability. They make the most sense for on grid storage due to their superior efficiency, which is about 99% in most cases, but be prepared to wait on a list to get one – many lithium ion systems are backordered for months. This class of lithium ion chemistries includes the kind of batteries found in computers and electric cars. Our pick for this chemistry is the Tesla Powerwall 2 due to it's low cost per usable kWh. It's available for purchase directly through Tesla.

 

Lithium ion (iron phosphate)

SimpliPhi Phi 3.5
Best in class: SimpliPhi Phi 3.5
Where it shines: Everywhere

Our recommended pick for most solar storage applications. Lithium iron batteries are similar to lithium ion metal oxide batteries in many ways, but they trade off a little bit of power density for a lot of longevity. As an additional benefit, they're much less likely to suffer from "thermal runaway" (aka "explosions"). Check out the SimpliPhi Phi at the AltE Store. It's a small, modular battery that makes sense for both small and large installations. 

 

Lead acid (flooded)

Rolls 4000 Series
Best in class: Rolls 4000 Series
Where it shines: Off grid storage

"Old reliable." Inexpensive solution, but requires regular maintenance. Immense number of resources online to DIY. As with most battery chemistries, improved manufacturing processes have breathed new life into this chemistry in the form of a greater depth of discharge and longer cycle life. Spills toxic electrolyte solution when knocked over. Avoid this by simply not knocking it over. You're welcome. As for the toxic fumes emitted when charging them, you'll have to make sure they have sufficient ventilation wherever they're housed. The Rolls 4000 Series batteries have a higher cycle life than other flooded lead acid batteries, making them more cost effective over their lifetime despite their premium price tag. They are available for purchase over at Wholesale Solar.

 

Lead acid (sealed)

Firefly Oasis L15+
Best in class: Firefly Oasis L15+
Where it shines: RVs, boats

Like above, but maintenance is discouraged, so the system doesn't last as long. Better than flooded lead acid for non-stationary applications because it doesn't ooze toxic liquid. They're also known as valve-regulated lead acid (VRLA) batteries or absorbent glass mat (AGM) batteries. The Firefly Oasis boasts an uncharacteristically high cycle life for this class of batteries. It's available for purchase at eMarine Systems.

 

Nickel iron

Iron Edison NiFe
Best in class: Iron Edison NiFe
Where it shines: Off grid storage

Nickel iron batteries make for a big, heavy battery bank, but they're nearly indestructible. We highly recommend them for stationary and off grid applications. They feature an extremely low lifetime cost of ownership due to their durability. Unfortunately, there are very few manufacturers of this chemistry – we've only been able to find one manufacturer in China, and a few resellers who put their own branding on it. They're available for purchase through Iron Edison.

 

Flow (zinc bromine)

RedFlow ZCell
Best in class: RedFlow ZCell
Where it shines: Off grid storage

The research behind flow batteries suggests that this new battery chemistry will last longer than most other widely available chemistries, but it's early days for proving it in the field. Flow batteries have their own set of unique behaviors: the battery unit must shut itself off every few cycles to perform regular self-maintenance, and they make a distinctive gurgling sound when operating. The RedFlow ZCell is only available in Australia, but you can call the company to register your interest where you live.

 

Salt water

Aquion Aspen
Best in class: Aquion Aspen
Where it shines: Off grid storage

Like nickel iron, salt water batteries are another heavy, large battery type. Aquion was the only manufacturer of these batteries for the residential market until they went bankrupt in 2017. Even though they emerged from bankruptcy later that year, there's been speculation that they may no longer sell their product to the residential market. Aquion was able to have their batteries Cradle to Cradle certified, but the low charge efficiency of the chemistry makes it worse for the environment than lithium ion chemistries when charging straight from the grid.

 

 

Deep Cycle Battery Cost of Ownership

This table summarizes data from the battery specification sheets provided by the manufacturers. We chose to use manufacturer data instead of third party studies because we want to give you a sense of what you can expect from the best examples of each chemistry. Also, due to improvements in manufacturing processes, lead acid batteries have come a long way in cost effectiveness; these improvements are not captured in studies.

That being said, research and educated guesses are used wherever manufacturers omit data or make extraordinary (and unverified) claims.

Battery Price Capacity DoD Cycle Life Price / Usable kWh Price / Lifetime kWh

Tesla Powerwall 2

$5,900

13.5
100% 1
2,700 2

$437

$0.16

SimpliPhi Phi 3.5

$3,295

3.5

80% 10,000

$1,177

$0.09

Rolls 4000 Series

$410 2.5 50% 1,350

$328

$0.12

Firefly L15+

$659 1.8 50% 2,190 3

$732

$0.17

Iron Edison NiFe

$970 1.44 80% 11,000

$792

$0.06

Redflow ZCell

$4,000 10 100%

2,000 4

$400

$0.20

Aquion Aspen

$2,200 5

2.2 100% 3,000

$1,000

$0.33


1: The Tesla Powerwall 2 boasts a 100% depth of discharge likely due to containing more than 13.5 kWh of storage but only allowing the user to discharge up to 80% or 85% of capacity.

2: Lithium ion battery manufacturers are wary to make claims about cycle life, and Tesla is no exception. Cycles are estimated using the Powerwall 2 warranty which states that they will replace the unit if it falls below 70% of initial capacity in whichever comes first: 10 years or 37.8MWh of aggregate throughput. 37.8MWh / 14kWh = 2,700 cycles.

3: Cycles are estimated using the Firefly Oasis warranty, which provides prorated coverage for six years of the product's life. Six years at one cycle per day gives us 2,190 cycles. Firefly does claim in their data sheet for this product that it will last 4,650-5,200 cycles (13 years at one cycle per day!), but we believe such a strong claim should be backed by a strong warranty.

4: Redflow does not disclose the cycle life of the ZCell, so this cycle life is estimated using third party research.

5: The Aquion Aspen is currently unavailable for purchase, and there's some doubt whether Aquion will continue to sell batteries to residential customers having recently emerged from backruptcy.

Source data (March 2018). Prices in USD. Capacities in kWh.

 

 

Next Up: Comparing Solar Panels

We'll be updating this page periodically to keep it fresh and relevant. A big thank you goes out to everybody who sent in suggestions for improvements – we love the feedback! Thanks to one of your suggestions, we're currently researching the pros and cons of different types and brands of solar panels for a similar reference. Stay tuned!


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