Battery Chemistry

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When the clouds roll in or the sun sets, solar energy stored in batteries can supply the needed power.
Anatomy of a lead-acid battery
Lead-acid chemical reaction
Flooded lead-acid is the least expensive technology, but requires watering. These HuP Solar-One batteries are available in large amp-hour capacity models (in 12, 24, and 48 VDC) that allow simple series-string configurations.
Gel-cell lead-acid batteries don’t normally release gas or need water replacement. They may have sensitive charge regimes. This 12 VDC gel battery from Trojan is rated for 105 Ah (at the 20-hour rate), providing 1.26 kWh of energy storage.
“Maintenance-free” AGM batteries require no watering or topping off of the electrolyte. This 6 VDC, L16 battery from U.S. Battery is rated for 390 Ah (at a 20-hour rate), providing 2.34 kWh of energy storage.
Lithium iron phosphate batteries don’t gas, are extremely stable, and can handle high charge/discharge rates. These batteries from Iron Edison are offered in 12, 24, and 48 VDC, in capacities from 2.3 to 291 kWh. Pictured is a 48 VDC, 18.7 kWh bank.
Lithium iron phosphate battery reaction illo.
Lithium nickel manganese cobalt batteries have exceptional energy density and, like LFP batteries, have a long cycle-life and do not require maintenance. This Tesla Powerwall 2 has 14 kWh of storage and an integrated inverter.
SimpliPhi offers 24 and 48 VDC LFP batteries with 2.6 and 3.4 kWh capacities. They are designed to be a drop-in replacement for LA batteries.
An LFP battery with 1.2 kWh capacity, the Battle Born battery is designed as a drop-in replacement for a 12 VDC lead-acid battery.
Nickel-iron batteries have exceptional durability and longevity, but require frequent maintenance.
Nickel-iron battery, chemical reaction.

The intermittent nature of solar and wind resources works well with energy storage so energy can be tapped when the sun is not shining and the wind not blowing. This is true for home-scale renewable energy (RE) systems through utility-scale. Energy storage is “the missing link” for RE to become a large portion of our energy mix. This article is an overview of the different battery chemistries used for storing renewable energy.

Off-grid RE systems require energy storage, as there is no utility to rely on at night or during cloudy weather—as do on-grid RE systems that include outage backup. Recent changes in utility interconnection requirements and some net-metering programs are spurring more grid-tied RE system owners to include energy storage.

Battery storage has been around for more than 200 years. But recent price drops in lithium batteries (i.e., they are now about one-half the cost they were in 2014), primarily due to the increasing electric vehicle (EV) market, have propelled the energy storage topic to the media forefront. While most battery types operate under similar principles, there are significant differences that are worth understanding as energy storage options are considered.

Fundamentals

RE storage batteries are made up of cells, each with two electrodes—a cathode (positive plate) and an anode (negative plate). The electrodes are submerged in an electrolyte, with a physical separator between the anode and cathode that allows ions, but not electrons, to flow through. Under charge and discharge, a chemical reaction occurs where ions flow though the battery’s electrolyte between electrodes, while electrons flow through the external circuit placed on the battery posts. The direction of this electron and ion flow is dependent on whether the battery is discharging or charging.

Lead-Acid Batteries

Lead-acid (LA) batteries were invented more than 150 years ago, and became the first commercially available rechargeable battery. LA batteries are the dominant battery type in home-scale RE systems, primarily due to price, availability, robustness (overcharge tolerance), and familiarity.

LA battery cells have lead (Pb) and lead dioxide (PbO2) plates submerged in an electrolyte made up of sulfuric acid and water. When a load is placed on the battery (discharging), electrons are released from the negative anode (Pb plate) to the positive cathode (PbO2 plate) stemming from the electrochemical redox reaction (see "Back Page Basics" in this issue) between the lead plates and the electrolyte. The sulfuric acid (H2SO4) is broken into positive hydrogen ions (H+) and negative sulfate ions (SO42-). The sulfate ions are drawn to both the anode and the cathode, while the hydrogen ions are pulled to the cathode, resulting in two electrons being released at the anode and two being pulled in at the cathode per reaction. During this process, lead sulfate (PbSO4) is created and proceeds to cover both plates until there is no more surface area available for the chemical reaction to take place. At this point, the battery is fully discharged. Because sulfate ions are pulled out of the solution, a discharged battery’s electrolyte has a higher concentration of water to sulfuric acid so specific gravity (a measure of liquid density, which reveals the acid-to-water ratio) can indicate a battery cell’s state of charge (SOC).

When an LA battery is charging, the process is reversed—electrons are driven into the Pb plate and pulled from the PbO2 plate. This process breaks the chemical bond between the lead and the sulfate ions, releasing that sulfate (SO42-) from the electrodes back into the solution, resulting in a higher concentration of sulfuric acid to water. During the charging process, some electrolysis takes place, which splits water into hydrogen and oxygen gas. For flooded LA (FLA) batteries, this must be vented and distilled water be periodically added to make sure the electrolyte always covers the plates.

An LA battery cell’s nominal voltage is 2 volts. To reach a useful voltage, several cells are wired in series. For example, a 12 V LA battery will have six individual battery cells. While 2 V per cell is consistent for all LA batteries, the storage capacity (measured in amp-hours) of the battery is dependent on how large the battery cell is. Because larger battery electrodes have more surface area for the chemical reaction to take place, they also yield a higher rate of electron flow (amps) and can store more amp-hours.

FLA batteries generally have the lowest initial cost, but require regularly adding water. The water is their weak point—discharged FLAs can freeze, possibly causing the battery case to crack or the plates to warp, and thus need to be housed in a freeze-protected enclosure.

Valve-regulated lead-acid batteries (VRLAs) are more tolerant to freezing temperatures and are nonspillable. They are designed to recombine minimal hydrogen and oxygen gassing back into water within the battery and do not have to be watered. While VRLA batteries have pressure valves that can let gas escape if overcharging occurs, the lost electrolyte cannot be replaced, and the battery’s capacity will be reduced, and can cause premature failure.

VRLAs come in two types—gel and absorbed glass mat (AGM). Gel cells have electrolyte thickened with silicon, so it’s not very liquid. Because the electrolyte in AGMs is liquid within a fiberglass mat between plates, the acid is more readily available to react with the lead plates, and AGMs can be charged and discharged faster than gel-cell VRLAs.

LA Pros

  • Lower upfront cost (FLAs)
  • Good durability
  • Readily available
  • Recyclable
  •  Familiar technology
  •  Low to moderate self-discharge (5%–15% per month)

LA Cons

  •  Limited depth-of-discharge (DOD), ~50%–80% recommended
  • Fewer cycles than other chemistries
  • Maintenance required (FLAs)
  • Moderate efficiency—the ratio of energy pulled out to energy put in. (FLAs: 80%-85%; higher efficiency for VRLAs: +90%)
  • Higher weight per capacity
  • Vented battery enclosure required because of outgassing
  • Made with toxic materials (lead and sulfuric acid)
  • Discharged FLAs can freeze

Lithium Batteries

Lithium rechargeable batteries became available in the 1980s, but a large recall of metal lithium batteries happened in 1991 in Japan when a mobile phone released flaming gases and inflicted burns. Cycling of this battery type produced dendrites on the anode that penetrated the separator and caused the cell to short-circuit. This spurred the lithium-ion (Li-ion) battery, which uses graphite (carbon) anodes rather than lithium, and does not have this dendrite issue.

While LA battery cells store and release energy via a redox reaction, Li-ion batteries use intercalation—inserting lithium ions into the electrodes’ crystal lattice structure without changing their structure. Unlike LA batteries, this process doesn’t create new compounds.

Cathodes used in lithium batteries have a lithium metal oxide base. Ones most commonly used in RE systems include lithium nickel manganese cobalt (NMC) and lithium iron phosphate (LFP). Li-ion batteries have a lithium (non-aqueous) salt electrolyte and a polymer separator that allows the lithium ions, but not electrons, to flow through it. During discharging, positive lithium ions flow into the cathode, while electrons are released at the anode’s external circuit and also flow to the cathode.

During charging, the process reverses—the lithium ions flow to the anode internally, and the electrons flow from the cathode to the anode externally. Because there is a non-aqueous electrolyte, no hydrogen or oxygen gas is created in the reaction, and there’s no need to ventilate an Li-ion battery. NMC Li-ion batteries produce 3.7 V per cell, while LFP versions average 3.2 V per cell.

Li-ion Pros

  • High energy density. Li-ion batteries are smaller and lighter for the same capacity. This can be advantageous in homes with limited space for housing batteries.
  • Higher voltage, fast recharge, deep discharge OK—80% is normal and 100% is possible
  • Long cycle life (1,000 to 3,000 cycles)
  • High battery efficiency (99%)
  • No maintenance
  • No voltage sag—a Li-ion battery at 20% DOD has the same voltage as when it’s at 80% DOD.
  • Holds capacity better at low temperatures than LAs. At -20°C, Li-ions will still have 80% capacity, while LAs will only retain 40% of their capacity.
  • Low self-discharge (<5% per month)
  • LFP is extremely stable

Li-ion Cons

  • Expensive—while large-scale Li-ion pricing is rapidly declining, upfront costs for those used in home-scale storage are still comparatively high.
  • Extremely sensitive to overcharge and needs  battery management system (BMS) per cell. Newer Li-ion battery packages usually incorporate BMSs.
  • Should not be charged at low temperatures (<32°F). Some sources say a slow charge at low temperatures is OK.
  • Can be prone to thermal runaway (NMCs)
  • Some have toxic and hard-to-source materials, such as cobalt

Nickel-Iron Batteries

Nickel-iron (NiFe) battery technology was introduced around 1900 by Thomas Edison, so is referred to as the “Edison cell.” These batteries were originally intended to power electric vehicles but were also used for backup power for mining and railroad operations.

In NiFe batteries, the cathode is nickel oxide hydroxide [NiO(OH)], and the anode is made of iron (Fe). The electrolyte is a solution of potassium hydroxide (KOH), a little lithium hydroxide (LiOH), and water.

When discharging, the active material of the positive plate changes from nickel oxide hydroxide to Ni(OH)2, and the negative plate changes from Fe to ferrous hydroxide [Fe(OH)2], with two electrons being released at the anode and pulled in at the cathode via the external circuit for each reaction. The electrolyte is only used as a medium for the hydroxide (OH-) ions to flow through.

The process is reversed during charging. Electrons are pulled from the nickel hydroxide electrode and driven into the ferrous hydroxide electrode. The potassium hydroxide and lithium hydroxide are not reflected in the chemical equations because the electrolyte is only a catalyst and does not participate in a chemical change with the active materials during charging or discharging. Unlike an LA battery, then, the electrolyte’s specific gravity does not indicate the battery state of charge. Instead, SOC is usually measured by voltage while the battery is at rest. The nominal battery voltage is 1.2 V per cell. Nickel-iron batteries do gas during the entire charge cycle, and must be adequately vented and routinely watered.

NiFe Pros

  • Long life (10,000+ cycles)
  • Robust (can be overcharged and deeply discharged)
  • Freeze-resistant, even if discharged
  • Large acceptable operating temperature range (approximately -20°F to 190°F)
  • Best in systems with daily discharge/charge (not floating)

NiFe Cons

  • High cost
  • Moderate efficiency (80%)
  • Need a lot of watering and venting
  • High rate of self-discharge (30% per month)

Choosing Your Battery

Regardless of the type of battery, energy storage opens up new options for RE systems. For both residential and commercial, energy storage allows homes and businesses to operate off-grid, provides backup energy for grid-tied systems, and can change the financial equation for grid-tied systems in regions undergoing changes to their net-metering programs. At the utility scale, energy storage provides many potential services, from stabilizing the ebb and flow of solar and wind power plants to frequency regulation and voltage support. All of these energy storage services are widening the pathway for RE to become a much larger portion of our worldwide energy mix.

Web Extras
“Net Metering & Beyond” by Christopher Freitas & Carol Weis in HP177 homepower.com/177.44

“Maximizing Solar Self Consumption” by Carol Weis & Christopher Freitas in HP178 homepower.com/178.46

Comments (18)

Jim@IronEdison.com's picture

Nickel Iron is a battery that can offer 11,000+ cycles with watering using distilled or deionized water with electrolyte exchange every ten +/- years of use. The temperature range in which it can be charged/discharge is -22F to 140F without shortened battery life. You can use as much as 80% of the battery capacity everyday without shortened battery life as well.

When sizing for a Nickel Iron battery you can plan to use as much as 80% dod in comparative to 30%-50% dod of a traditional lead based battery, so make sure to compare the amount of available kilowatt hours of available power (so don't compare a 500Ah Lead battery to a 500Ah Nickel Iron battery; instead compare a 500Ah 48V Lead battery at 12kWh of usable power at 50% dod for max of 2,500 cycles to a 300Ah 48V Nickel Iron battery at 11.52kWh of usable power at 80% dod for 11,000+ cycles).

With a side by side comparison of 'usable' capacity / cycle life shows how the Nickel Iron battery is the lower cost of ownership battery that will last 3-5 times as long as a traditional lead battery. Price compare at 'usable' kW hours of power / cycle life = cost per kilowatt hour over time (Nickel Iron will price out at about 7 cents per usable kilowatt hour over time).

The Thomas Edison Nickel Iron Battery chemistry has been proven to work successfully in harsh off-grid applications for over 100 years. Nickel Iron is a eco friendly chemistry that last for years to support off-grid living for the most demanding of load profiles (example 800Ah 48V Nickel Iron can support up to 30kW hours of usable power for daily use, and can deliver power for heavy loads of up to 400Ah of instantaneous load demand).

Greg Smith_0_0's picture

People still use nickel iron? I thought it was being pushed out for other types of rechargeable batteries since it costs so much to manufacture and can't really hold a charge.I don't know of anyone in any of my classes that ever used this chemistry.

Jim@IronEdison.com's picture

Nickel Iron is a favorite among off-grid home owners for years. There are also grid-tied customers that use the Nickel Iron as well because of its extended life span to cost factor. The key point is for self-consuming battery based solar/wind/hydro systems for daily use (the Nickel Iron battery has a self-discharge rate of less than 1% per day). We recommend Lithium Iron Phosphate for backup batteries as they hold the charge for an extended period of time in a standby mode (the Lithium Iron battery has a self-discharge rate of less than 1% per month). Currently both our Nickel Iron to Lithium Iron sales continue to increase with each year we offer them to both grid-tied and off-grid customers. Seven+ years of offering Nickel Iron and two+ years of offering Lithium Iron Phosphate batteries to home owners and commercial customers. Serving customers that need support from 7.5kW hours to 500kW hours of battery storage. So, to answer the question, Nickel Iron battery's are very much still a very viable battery option that will cost less over time!

Greg Smith_0_0's picture

I'm with you there. Not a fan of Li-ion in off grid applications.

William Philipson's picture

Whoops. Wrong.

Escape Artist's picture

SILICON SALT BATTERIES NOT MENTIONED. TRYING THEM NOW.

We are on our third set of batteries in 15 years of PV/wind off-grid living. Our first set of sixteen L-16 lead acids by MK Battery lasted 10 years. Our second set of sixteen L-16 leads acids by Trojan lasted us 5 years and has maybe another 2 years left in it. Anticipating its failure, given deteriorating charge and discharge performance despite meticulous maintenance, we just purchased sixteen 230 amp-hour 6-volt silicon salt batteries (total bank capacity: 22kWh).

These represent a new battery technology not mentioned in the Home Power battery article. They are made by a Canadian company and sold in the US by their distributor, Backwoods Solar, for $269/each (model SSW230-6). Each battery weighs 78 pounds and measures 7.00 x 10.25 x 10.50 inches, allowing us to put them into the same racks that we had constructed for our L-16 banks. (We have no financial interest in any of these companies.)

We initially hesitated until we obtained data and feedback from a Canadian renewable installer who provided us with two years of remote unmanned use in British Columbia that demonstrated good performance even at sub-zero temperatures.

In choosing our replacement bank, we also considered lithium batteries and the new saltwater batteries (not covered in this article) sold by Aquion and distributed by Altestore.com. We rejected lithium batteries due to high cost, temperature requirements, and ongoing safety considerations. We rejected the saltwater batteries because of their high cost (more than double the silicon salt batteries) and inability to withstand our sub-zero Maine winters (low end of operating range 23F). The company, Aquion, also has had problems and, two months ago, declared bankruptcy in March 2017.

But back to the silicon salt batteries. To match the pro/con lists from the original article:

SILICON SALT PROS
♣ Low upfront cost
♣ Functions at temperature extremes (-35F to 131F)
♣ Can be repeatedly discharged to 60% capacity without affecting battery life
♣ Can occasionally be discharged to 0% capacity without harm
♣ Manufacturer claims 15-year use lifespan (>double lead acid)
♣ Low self-discharge (<1% per month)
♣ No venting required
♣ No watering required
♣ No memory effect
♣ Nontoxic electrolyte that “can be used as organic fertilizer”
♣ Three-year manufacturer warranty

SILICON SALT CONS
♣ New technology with limited real-world longevity data
♣ Sole manufacturer, longevity unknown
♣ Charging requirements significantly different than for lead acids so charge controllers must be adaptable to shorter bulk charge periods at lower maximum bulk voltages

An off-grid neighbor installed these batteries last season and reported that his batteries were discharged to 0% after snow covered his PV panels and an unknown phantom load depleted his bank. After tracking down and disconnecting the phantom and uncovering his panels, the batteries charged and functioned normally. In comparison, our original L-16 bank did not tolerate deep discharges or subzero temperatures. Several years ago we lost three when they froze in -5F weather.

We still don’t how the silicon salt bank will ultimately perform in real life use. But given their low cost and unique ability to withstand severe cold and deep discharges, we are hopeful they will outlast and outperform our prior lead acid banks.

James Li
Off the coast of Maine

Ed Kelly_3's picture

James
Thanks very much for this information

Marc Fontana's picture

James, Thanks for sharing that. Your experience with L16's (MK's lasting 10 years, and Trojan's not as long) mirrors our own experience. We're currently using refurbished Hawker 2v batteries, they seem pretty rugged so far. Do these new Silicon Salt batteries have a rated number of cycles? What is the electrolyte?

Marc Fontana's picture

I understand that a battery bank should have as few parallel strings as possible for more uniform charging of individual batteries. What are the pros and cons of using 2v vs 6v vs 12v batteries in a string for, say, a 24v battery bank? (assuming the same Ah rating).

Eric Gallant's picture

Marc,
As William mentions, when multiple strings of lead acid batteries are used in parallel there is a risk of cell imbalance. However, this is usually easily solvable with a periodic equalization charge. Most inverters (Schneider, Outback, SMA etc..) can be configured to equalize on a regular basis.

william von novak's picture
william von novak (not verified)

"As few parallel strings as possible" applies to lead-acid batteries; it is chemistry specific. In general, use the largest cells you can get easily to accomplish this. For larger systems (>500ah) 2V cells work well.

william von novak's picture
william von novak (not verified)

A few notes:

"Lithium rechargeable batteries became available in the 1980s, but a large recall of metal lithium batteries happened in 1991 in Japan when a mobile phone released flaming gases and inflicted burns. Cycling of this battery type produced dendrites on the anode that penetrated the separator and caused the cell to short-circuit. This spurred the lithium-ion (Li-ion) battery, which uses graphite (carbon) anodes rather than lithium, and does not have this dendrite issue."

Dendrite formation is a problem during overcharge of any standard lithium ION chemistry; that's one reason protection circuitry is required. Lithium iron phosphate avoids this, and LiFePO4 are thus considerably safer.

Systems like the Sonnen systems use lithium iron phosphate. The Tesla system (not available yet) and the LG Chem system uses lithium ion.

Also, nickel iron have a very high internal impedance. This means either inability to start heavy loads (like refrigerator compressors) or a larger battery bank than you would otherwise need.

Ed Kelly_3's picture

Eric
I checked your website, which battery are you referring too. I could not find it. We do lots of battery backup and independent units.
email me with pricing shenengsvcs@gmail.com

Eric Gallant's picture

Ed,
I just sent you .pdf versions of the spec sheets for our lead-carbon batteries. I also included a link to our product configured for 48-volt system on our distributor's website, Please let me know if you have any questions.

Eric Gallant's picture

Advanced carbon nano-materials have dramatically improved the performance of lead-acid batteries for renewable energy applications. Our SLR lead-carbon battery is rated for 5000 cycles at 70% DOD and warrantied for 10 years. Similar performance to lithium ion at lower price, better safety case and 100% recyclability. See GS Battery for more info.

Greg Smith_0_0's picture

Each battery has it's strengths and weaknesses and the "best" battery varies depending on application. While Li-ion batteries may be more expensive, they are cheaper in the long run since they will still retain much of their capacity after 20 years as compared to lead acid, which means fewer replacements. LFP chemistry is the safest, as compared to NMC (Tesla) and thermal runaway is not an issue. LFP will also provide more cycles (>7000), with some providing up to 10,000. However, for larger applications, Li-ion may not have the capacities required. It all depends on the application. When someone asks, "What's the best type of PV inverter?", you have the same conversation. There isn't a "best" for all applications.

Raymond Rieger's picture

I agree with Kenneth, disappointed that article did not answer the question. Are there new technologies available that are better than traditional lead acid batteries? Until something better comes along, I'll stick to my bank of L-16 batteries.

Kenneth Grant's picture

This article did not articulate which battery was the best, it stated the construction and chemistry

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