Hybrid systemsOff-grid systemsBatteries

Solar batteries to help you survive any power outage

The average U.S. electricity customer experiences at least three major power outages a year. Whatever the reason, that blackout time isn't the best experience ever, not to mention it can knock you off balance for days. Solar batteries can give you a helping hand when dark times come. Luckily, today the market offers different types of solar batteries for every need and budget, with lead-acid, lithium-ion, flow and nickel-cadmium being among the leaders.

Lead-acid batteries: a cost-effective solution for off-grid system owners

Lead-acid batteries are the same type of battery you have in your car. The main difference is that for a solar power system you'll need deep-cycle lead-acid batteries – they can withstand deeper discharges than those used in cars, hence the name.

Deep-cycle lead-acid batteries have four main components:
•  Positive plate covered with a paste of lead dioxide;
•  Negative plate made of sponge lead;
•  Separator – an insulating material between the two plates, needed to enable conduction without the two plates touching;
•  Electrolyte, consisting of water and sulphuric acid, which helps conduct electricity.

These constituents are enclosed in a plastic cell which keeps the electrolyte in. When the battery discharges, lead and lead dioxide react with sulfuric acid in the electrolyte to form lead sulfate. When the battery recharges, lead sulfate reverts back to lead, lead dioxide, and sulfuric acid.

The voltage difference between the two plates is approximately 2 volts. That's why a 12 V battery, for example, has six single cells linked in series.

Lead-acid batteries are classified hazardous, so they should be disposed of accordingly..

Lead-acid batteries fall in three main types according to the arrangement of their main components:

1. Flooded batteries are called so because the liquid electrolyte is free to move within the cell. Since the liquid isn't sealed, these batteries dry out from time to time. That's why they should be monitored and watered every 1 to 3 months. It also explains why flooded lead acid batteries are a better option for full-time off-grid homesteads than for vacation homes or emergency backup power.

2. AGM (Absorbed Glass Mat) batteries have the electrolyte enclosed in thin glass mats instead of freely flooding the plates. Since they are sealed and don't dry out, they will fit both off-grid and hybrid systems. AGMs normally offer higher discharge and recharge efficiency than flooded batteries, which is reflected in their price.

3. Gel batteries use silica gel in which electrolyte is suspended. They normally offer a longer life cycle and greater resistance to extreme temperatures. They are also capable of withstanding very deep discharging, which typically kills flooded and some AGM batteries. That's why gel batteries are popular with off-grid system owners.

1. Low up-front cost
2. Wide range of sizes and capacities available
1. Cycle life of 1,000-3,000 cycles
2. 50-90% charge/discharge efficiency
3. 3-20%/month self-discharge rate
4. Charging takes from several hours to a day
5. Flooded batteries require continuous maintenance
6. Very heavy and bulky
7. The hydrogen gas the batteries release when charging is explosive

Lithium-ion batteries: an excellent choice if money isn't an issue

Lithium-ion batteries are literally everywhere – from laptops and cell phones to hybrids and electric cars. They offer great capacity and current output for a lightweight and small size. Lithium-ion batteries are deep-cycle by origin, so they can be fully charged and discharged, and thus used for solar energy storage

Lithium-ion batteries have four main components:
•  Anode (positive cobalt-oxide electrode) and cathode (negative graphite electrode) which store the lithium;
•  Separator which blocks the flow of electrons inside the battery;
•  Electrolyte carrying positively charged lithium ions from the anode to the cathode and vice versa through the separator;
•  Two current collectors (positive and negative).

While the battery is discharging, the anode releases lithium ions to the cathode, generating a flow of electrons from one side to the other. When the battery is getting charged, lithium ions are released by the cathode and received by the anode. The battery is fully charged when no more ions flow. If all the ions have moved back, it means that the battery is fully discharged.

There are three types of lithium-ion batteries: cylindrical such as in a remote control, pouches such as in smartphones and tablets, and prismatic such as in electric vehicles. The former type often has corrugated sides, which create air gaps between adjacent cells and can aid in cooling. This feature makes them suitable for solar energy storage

Lithium-ion refers to a variety of lithium-based battery chemistries. Lithium iron phosphate (LFP) batteries are considered to be the best solar battery type. Unlike regular lithium ion batteries, LFPs have a higher cell density, which makes them more compact. Moreover, they last longer due to the electrolyte's properties, and are not classified toxic.

1. Lifespan of about 10,000 cycles
2. 80–90% charge/discharge efficiency
3. 0.35-2.5%/month self-discharge rate
4. No maintenance is required
5. Smaller and lighter than lead-acid batteries due to higher storage capacity
6. Fast to recharge (2-3 hours)
7. No off-gassing/ventilation
8. Lithium iron phosphate (LiFePO4) is a nontoxic material
1.High up-front cost

Flow batteries: the way to survive prolonged power outages

Flow batteries are relatively new to the battery storage market, but their use in long duration storage is steadily growing. They boast a 100% depth-of-discharge, greater design flexibility and an ability to last for decades at zero charge.

Flow batteries have four main components:
•  Two electrolyte liquids (positive and negative) enclosed in two tanks;
•  Pumps, which push the electrolyte through two independent loops to the core to generate electricity;
•  Reaction cell, divided into two half cells;
•  Membrane, separating the two half cells to prevent the electrolyte liquids from coming into direct contact with each other

In flow batteries, the charges are stored in the electrolyte liquids, which are contained in external tanks. The charge-carrying electrolytes are pumped through a reaction cell, containing two electrodes (one positive and one negative) separated by an ion-conducting membrane. During discharging, ions are reduced at the positive electrode and oxidised at the negative electrode. When charging, the process reverses.

Since the external tanks have no size limit, the storage capacity of a flow battery can be easily adjusted to the needs by simply adding more electrolyte. That's what makes them ideal for storing large amounts of power, for example, for off-grid systems, which aren't connected to the grid. 

The membrane of flow batteries degrades little over time, which makes them capable of withstanding full discharges, storing power for longer periods (four hours or more), and lasting for decades with almost no maintenance.

1.A vanadium redox flow battery for MW-sized solar storage 2.Zinc-bromide batteries Z Cell for residential solar storage

Flow battery manufacturers offer a range of chemistries including vanadium, iron chromium, zinc bromine, zinc iron and others. Flow batteries can also be redox (reduction oxidation), hybrid and membraneless. The most common types for solar installations are the vanadium redox (for commercial solar systems) and the Zinc-bromide hybrid (for residential solar systems)

1. Cycle life of about 4,000 cycles
2. About 90% charge/discharge efficiency
3. 0.15%/month self-discharge rate
4. No maintenance is required
5. No cycling limitations – can be fully drained without impact on the lifespan
6. Flexible storage capacity
7. No chance of an explosion due to the physical separation of the battery components
1. High up-front cost
2. Low energy density, thus very large

Nickel-cadmium batteries: a solid option for extreme climates

Nickel-cadmium (NiCd) batteries are rechargeable batteries with a high cycle life expectancy. They are typically used in stand-alone solar PV systems where large capacities and high discharge rates are required. Their strongest point is a broad operating temperature range (-4 F to 140 F, compared to around 32 F to 102 F for many lithium-ion batteries).

Nickel-cadmium batteries have four main components:
•  Anode (positive nickel-hydroxide (NiOOH) electrode)
•  Cathode (negative cadmium (Cd) electrode)
•  Separator, to ensure that the anode and cathode do not physically touch
•  Electrolyte (alkaline potassium hydroxide (KOH)), enabling electron transfer

During charging, the negative plates lose oxygen and begin forming metallic cadmium (Cd). The active material of the positive plates, nickel hydroxide, becomes more highly oxidized. When a nickel-cadmium cell is discharged, the nickel hydroxide changes its form (Ni(OH)2) and the cadmium becomes cadmium hydroxide (Cd(OH)2). The concentration of the electrolyte does not change during the reaction.

Nickel-cadmium batteries can be a highly reliable energy storage solution for solar PV systems, especially under extreme weather conditions. In fact, they are the only batteries capable of performing well even at low temperatures (up to -40°F). Due to their robustness and low maintenance, they are favored for off-grid megawatt-sized projects in harsh environments.

1. 70-90% charge/discharge efficiency
2. No ventilation or cooling is required
3. High discharge rates (discharging in one hour or less)
4. Performs well at low temperatures (up to -40°F)
5. Long shelf life (five-year storage is possible)
1. High up-front cost
2. Cycle life of about 1,000-2,000 cycles
3. Low energy density, thus very large
4. 10%/month self-discharge rate
5. Classified as hazardous

To wrap it up, a lithium-ion battery is an absolute leader. It will be a good option for both off-grid and hybrid systems. Lead-acid batteries will suit low-cost solar projects with no space constraints. The other two types of solar batteries - flow and nickel-cadmium - are ideal for large-scale solar installations, but are more difficult to find on the market. Check out what solar batteries we have in our store today.

With a degree in Linguistics, Tatiana uses her vast experience in technical translation to deliver complicated concepts in simple words. She joined the company in 2020 as a contributing writer to become the person to influence Blog’s development.

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