Battery Chemistry


Inside this Article

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.


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.

Comments (18)'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.'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


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 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:

♣ 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

♣ 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

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

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

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

Eric Gallant's picture

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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