Types of Batteries (Chemistry)
When it comes to storage for energy for solar the best option is in chemical storage, in the form of a battery, while there are other methods out there but not as efficient as this. The chemistry behind batteries goes into electrochemistry using galvanic cells that transform stored chemical energy into electrical energy when its discharged, and converted the DC electricity energy into chemical energy to store for later (Recharging). The listed batteries have different chemical species involved but follow the same electrochemistry method to produce electricity and capture it. Redox-Oxidation reactions occur to produce the electric current to flow through the anode plate (negative terminal) to the cathode plate (positive terminal) which is divided by a separator plate. This separator plate helps separate the oxidation and redox products from each other, but for most battery storages they both share the same electrolyte which is normally an aqueous solution which requires no salt bridge or connecter for the two species.
Capacity and Power of the Battery
Capacity is the total amount of electricity that a solar battery can store, measure in kilowatts hours (kWh). If you wanted to get extra capacity it is possible to include multiple batteries connected to your building. Going into capacity you must think about the power flow rating which is measured in kilowatts (kilo-Joules/sec) that tells you how much power is flowing through the battery to produce electricity to the building. Having a high-power rating is good to power the building with also having a high capacity for longer period of functioning at night.
Depth of Discharge
When it comes to using the battery to power the building its best not to fully use the battery up, if doing so will result to in a decrease in the battery life span. Most solar batteries need to retain some charge always due to their chemical compositions. Referring to the title of the section, the depth of discharge (DoD) is the amount of a battery capacity that has been used. The below figure will illustrate the definition of DoD.
If we have a 20-kWh battery that has a DoD of 90% that means that battery can use about 18kWh of the battery leaving 2 kWh in the battery. Once you hit the DoD you can recharge the battery back to its full capacity of 20 kWh and repeat the cycle.
18 KWh can be used
Charged back to 20KWh
Figure 1 - Depth of Discharge Flow
Higher DoD’s are great for the building plans due to the fact we can use more of the battery’s capacity without greatly damaging the battery in the long run. Different manufacture batteries have different DoD’s and choosing the highest one will help on choosing what battery to use for the storage. There is other important info we need to know about the battery though to get the conclusion.
Round Trip Efficiency
We figured out the depth of discharge is great to know for the battery life, cycles it can go for, and amount of power we can use of the battery. Another helpful thing to look at is the round-trip efficiency which tells us the amount of energy that can be used as a percentage of the amount of energy it took to store it.
Now let’s look further back to the flow chart given below
Figure 2 - Round trip flowchart
In figure 1.3 we are shown 10 kWh of electricity flowing (Power rating) into the battery storage and leaving out is 8 kWh of useful electricity we can use back. If divide the outlet by the inlet electricity stream and multiply by 100% we get 80%. That 80% signifies that our battery has an 80% round trip efficiency, the higher the round-trip efficiency means more economic value for your battery. That’s another perk into looking for the right battery for the garage building, the more efficient it is the more useful power we can use to power the building and save money on generating less energy.
Battery life and Warranty
With all things in life there comes a time where the battery will start to lose more and more of its capacity through long charge/discharge cycles through the years. This goes back to not overdoing the DOD based on the manufacturer's specifications which can shorten the span life of the battery. There is also another consideration to think about is the inner reactions undergoing in the battery that can sometimes generate alternative reactions that degrade battery performances as well.
Warranty is a major factor into looking for a good battery, based on the manufacture specifications of the battery’s lifespan should be or many cycles it can achieve of useful life. If a battery had a warranty for 9 years or 4,000 cycles of charge/discharge at 75% its original capacity, meaning you would have lost no more than 25% its original ability to store energy.
Going back to the battery life, sometimes it can be an easy fix by filling the battery up with more aqueous solutions (mostly water) into the battery to give it the right composition. Then there is sometimes where the manufacturer can
Lead Acid Battery
Figure 3 - Lead Acid Batthery
Lead acid batteries is the oldest type of rechargeable battery going back to 1859 of its first production by French physicist Gaston Plantè. Without going too far into the chemistry side of this, aqueous sulfuric acid plays a huge role in the battery when the battery is charging it acts like a water splitter where the chemical energy is stored in the sulfuric acid, and vice versa when the battery starts to discharge sulfuric acid breaks it bond to release chemical energy to electrical energy. The above figure shows the transmissions I mentioned in the following text. Lead acid is still popular today and still used on some solar energy storage batteries, despite it having a low energy to weight ratio and a low energy to volume ratio, but its ability to produce high current which means it does have a large power to weight ratio. This goes back to Power = IV where (I) is the current and (V) is the voltage drop, and having a higher current will generate more electricity from the battery. Lead batteries are cost effective than the new generation of rechargeable batteries which is good for the consumer, a minor downside which isn’t too effective of the product is the weight of lead batteries which range 30 to 32 pounds.
Lithium ion Battery
Figure 2 - Lithium ion Battery
Lithium ion Batteries still follow the same electrochemistry to conduct electricity from chemical energy by using an anode (negative electrode), a cathode (Positive electrode), and electrolyte as a conductor. The anode consists of porous carbon, the cathode would be lithium metal oxide. The flow of electricity flows from anode to cathode when the battery is discharged, and the opposite direction when the battery is being charged. One major advantage of the lithium ion battery is the cathode can be changed out to increase capacity and power delivery which has made the lithium ion battery well to begin with. Different cathodes involve Lithium cobalt oxide, Lithium manganese oxide, Lithium iron phosphate, Lithium Nickel Manganese Cobalt (NMC). Lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest specific energy per weight.
Lithium Vs Lead
As mentioned lead-acid batteries are heavy, and lithium ion batteries are very light. This doesn’t affect the choice as much since the structure can handle the different weight limits.
In further research lead acid batteries were found to have a highly inefficient with the charging/ discharge life where you see a loss in amps while charging and high voltage drop when discharge that decreases your capacity of your battery. That goes back to the round-trip efficiency mentioned earlier where it will feed the battery and due to inefficiency, you lose that store power and get less useful electricity to use back. On the other hand, lithium ion has a high efficiency when it comes to charging/ discharging has a high round trip efficiency making it more economic value for your battery.
Going back on the discharge life for lead battery, most people notice no more than 50% of depth of discharge for lead batteries, for example on the Trojan battery website flooded batteries contain a 50% DoD for their batteries.
On the pveducation.org website it lists the general cycle life for lead acid and lithium ion battery. Going to compare the lead battery with the lithium iron phosphate since that’s the safest of the four I mentioned above. Lead acid has a cycle life of 200 to 300, while lithium iron phosphate has 1,000 to 2,000. This cycle life can accelerate faster by over doing the depth of discharge on the lead battery shortening it life, and lithium ion is hardly affected by the discharge level.
When a battery is discharged, the voltage starts to drop at a negative exponential rate until there is no voltage left or until the battery switch is flipped back on for charging. For lithium ion batteries there tends to be a theoretical constant voltage during time of discharge, while lead acid battery experience a quick voltage drop. This is where being connected to the grid will come in handy to make sure there is enough energy being out in the building during night.
When you hear lead acid battery the first thing to come to mind is the cost of it being inexpensive making it a bit competitive to the lithium ion. Lithium ion is newer technology meaning it’s not inexpensive, but in the end, it will be a better investment. Think about it the normal lifespan of lead acid is 300 cycles the max, and lithium ion highest is 2000 life cycles. That’s about 6.67 life cycles greater than lead and having to replace it 7 times will begin to add up in cost expenses where you were better off investing into lithium and saving money.
Lead acid batteries release noxious acidic gases when charging, and must be contained in a sealed battery box that is vented to the outside. Lithium ion on the other hand doesn’t need to be vented and can be placed anywhere you see fit. This also tackles the fact that lead acid batteries aren’t environmentally safety compared to lithium ion batteries.