Energy storage system is the backbone for any renewable energy power system. Not only a solar power system but every renewable energy system need an efficient, reliable, cost effective, safe, eco friendly and long lasting storage system. The storage system must be efficient as much as possible and capable to payback every cent paid for it. In this post we will discuss about solar battery options or Battery types suitable for solar storage system.
Performance and durability of a storage system totally depends upon the batteries used for the purpose. A battery converts electrical energy into chemical energy and store this chemical energy inside it. On requirement the battery yield the stored energy after converting again it to electrical energy by reversing the reactions. Selection of suitable batteries ensures the performance and durability of storage system. Let’s discuss the battery types available for renewable energy storage system and their compatibility with an off-grid solar power system on the basis of their performance, cost effectiveness, service life etc. Battery which is intended to be used as solar battery should have high performance characteristics.
Generally batteries are classified in two categories as Starter Batteries and Deep Cycle Batteries.
Starter Batteries are designed to produce huge sum of energy for a short span of time. These batteries are used in those applications where instant short period boost of energy is required, mostly for cranking automobile engines. These batteries are capable to produce only 40-50% of the energy they store. These batteries are not suitable for backup systems due to low efficiency, very low deep discharging capacity, high self discharging behavior and small life span. Lead acidic batteries are commonly used as starter battery in automobiles. Lead acidic batteries came in existence since 150 years ago and are still proving themselves as leading opponents for starter applications.
DEEP CYCLE BATTERIES
Unlike starter batteries deep cycle batteries are capable to produce 80-90% of the energy they store inside. These batteries are applicable for standby and backup storage purpose. Deep cycle batteries yield most of its stored energy over a long duration of time. Slow is the rate of discharge more will be the capacity of battery to produce. These batteries produce more energy with less withdrawal rate.
TERMS RELATED TO A BATTERY
‘C’ rating is the capacity rating of a battery which denotes the capacity of a battery to supply current with a specific rate for a given time period.
As a 12 Volt battery with 200AH C20 capacity.
Here C20 represents the time period in which battery is capable to produce 200AH. This battery is capable to produce 200AH in 20 hours with a discharging rate of 10 amp/hour
10amp x 20 hrs = 200AH
But a same battery can’t be able to produce 200AH in 10 hrs with discharging rate of 20 amp/hour. It will end up soon with loss of energy in the form of heat due to internal resistance.
But the same battery is capable to produce beyond its capacity in some cases if allowed to discharge in 40 hrs or more with a slow rate of discharge as 5 amp/hrs. With this rate it might work a little more time producing 210AH.
DEPTH OF DISCHARGE
Above illustration is presented to understand the C rating. No battery is capable to produce 100% of stored energy, but deep cycle batteries can produce most up to 80-90%. The percentage amount stored energy which a battery produce while discharging denotes the depth of discharge (DOD). If a battery produced half of its stored energy is said to be discharged up to 50% of its full capacity. So the depth of discharge is 50% in the case. Depth of discharge could be 10%, 20%, 30%, and so on up to 90%.
Discharging a battery up to a specific depth and retaining its previous charged state by refilling energy constitute a single cycle. Life span of a battery is denoted by cycle life, number of cycle it is capable to produce without any performance issue.
Let’s discuss different battery types available and their qualities and drawbacks which make them compatible or bit harder to use in a renewable energy storage system.
LEAD ACIDIC BATTERIES
It works on the chemistry of storing energy if the form of chemical energy.It is introduced 150 years ago. Still proving itself as a best storage method for most applications among other available batteries working on different principle like Lithium Ion, NiCd, NiFe etc. These batteries are made for both starter as well as deep cycle applications. Here we are discussing for deep cycle applications. These batteries consist of cells with two electrode plates as positive and negative, submerged in an electrolyte solution. The positive and negative plates are made up of Lead per oxide (PbO2) and pure lead (Pb). The electrolyte is dilute sulfuric acid (H2SO4). The reaction takes place inside as
PbO2 + H2SO4 > 2H + PbSO4 +2H2O
The exchange of electrons between positive and negative electrodes inside the battery cause inequality of electrons between positive and negative plates. While discharging, these two plates balance the equality of electrons which causes the flow of current. Not going so deep. This is general chemistry happens inside a lead acidic battery.
TYPES OF LEAD ACIDIC BATTERIES
FLOODED DEEP CYCLE LEAD ACIDIC BATTERIES
These are the batteries having simple structure with electrode plates immersed in the electrolyte. For deep cycle application these batteries are provided with heavy electrode plates to produce maximum of their capacity. Lead acidic batteries are inexpensive. It is a most economical storage option available among all.
Advantages of Deep Cycle Lead Acidic Batteries
- Inexpensive as compared to other battery types available in the market.
- Easily available with wide range of specification.
- Extended cycle life with low depth of recharge. Up to 3000 cycles with 50% DOD. ( Cycle life may vary with manufacturer and quality of the product )
- No special attention is required for charging and discharging rates. Other batteries require the special chargers and charge controllers for controlling charging rate.
Disadvantages of Flooded Deep Cycle Lead Acidic Batteries
- Require maintenance as water top up periodically.
- Special precautions needed for handling while transportation. Battery contains sulfuric acid as electrolyte inside it which may spill out from upper caps or may leak if battery is punctured or damaged.
- Do not work in tilted position or in other orientations. Should be placed on a flat surface to ensure that electrode plates are uniformly immersed in the electrolyte solution.
- Emission of corrosive gases occur due to overcharging, fast charging and discharging rates. Due to internal resistance of battery heat is generated causing gases to evolve.
- These batteries are damaged if kept in discharged condition for a long period of few months. Do not work with their full capacity while using again after a few month discharge sleep.
- These batteries are not charging efficient only store 80-90% of total energy supplied to them.
VALVE REGULATED LEAD ACIDIC BATTERIES (VRLA/SLA)
Valve regulated lead acidic batteries works on the same chemistry as flooded type lead acidic battery works. The only difference is the construction of the battery. These batteries are also named as SLA (Sealed Lead Acidic Battery) by some manufacturer. These batteries are sealed or semi sealed with a one way relief valve to allow very small amount of evolved gases to escape out, in high pressure conditions. These batteries works on the recombinant technology. The evolved gases recombine inside the battery to regain the electrolyte as there is no way to refill it again. Oxygen and Hydrogen are the two main gases evolve during charging, discharging and overcharging. These gasses again recombine inside the battery and form water. This results in no loss of electrolyte and thus makes them maintenance free. The recombining process is 98% efficient. There are two types of VRLA batteries as
ABSORBENT GLASS MATT (AGM) BATTERIES
These batteries are developed for military aircrafts in mid 80’s to make lead acidic batteries maintenance free, vibration resistant, leak proof, spill proof and to make emission free. Emission of corrosive gasses from batteries can damage the sensitive electronic console panel inside an aircraft. These batteries are sealed with a one way relief valve and works on recombinant principle. Inside the battery there is micro glass fiber matt which is placed between the electrode plates. This glass fiber mate work as sponge and soaks the entire electrolyte by capillarity. This keeps the quantity of electrolyte optimum inside the battery. Due to optimum quantity of electrolyte which is soaked by the glass mat there is no chance of spilling and leakage even if battery is punctured or physically damaged. These are leak proof and spill proof therefore require no special handling precautions while transportation, this character make them suitable for military applications which require to travel through tough terrains. But these batteries are used for all purpose even in automobiles as a starter battery. Closed packed structure inside the battery makes them highly suitable to withstand high vibrations. Due to closed packed structure fiber glass matt supports the plates. The plate materials do not require impurities to increase the strength of the plates for hanging as in flooded battery types. So pure materials are used for making electrode plates, which enhance the power of AGM batteries.
Advantages of AGM Batteries
- Bear high power density as compared to flooded battery types lead acidic batteries. Produce good power with electrolyte having close proximity with electrodes and equal distribution on the surface of the electrode plates.
- These batteries are maintenance free do not require refilling of electrolyte.
- These batteries are spill proof and leak proof. These properties make them suitable for transportation without any special handling precaution. Can be placed in any orientation while operation.
- Less emission of corrosive gases. So can be installed near sensitive electronic devices.
- Less emissive, leak proof, spill proof design makes them environment friendly.
- Able to withstand high vibrations.
- Very low self discharging rate as 2-3% in a month. Are most suitable for standby applications.
- Due to very low internal resistance these batteries bear high charging efficiency up to 95%.
- High depth of discharge can be achieved. Thickness of positive plate and purity of Lead makes the battery more capable to deeply discharge.
- Low internal resistance due to which AGM have highest discharge and recharge rate among other LA batteries.
- Perform well in cold weather conditions in which flooded battery types die.
- Tolerate higher charging and discharging rates.
Disadvantages of AGM Batteries
- Twice expensive than a premium flooded lead acidic battery with same capacity.
- Do not tolerate overcharging. Over charging causes electrolysis of water which results in high rate of formation of gases which get exhausted and cannot be recovered with recombining reaction.
- Relatively low cycle life as compared to flooded type and gel cell.
- No chance of refilling if electrolyte is lost.
This is a type of VRLA or SLA battery. Works on the same recombinant technology as AGM works. Construction is differ from AGM. Do not have a glass matt but, it contains electrolyte in the form of gel. Gelled electrolyte is prepared with the help of silica. Similar to AGM batteries these are also spill proof, leak proof and emit low fumes. These batteries are also well suited for deep cycle application. These batteries are charge sensitive require special precautions while charging, do not tolerate over charging and high charging rates. Charging voltage is low as compared to other lead acidic batteries. These batteries perform well in hot weather conditions. Gel batteries are also known as gel cell.
Advantages of Gel Batteries
- Being a VRLA battery the gel batteries have same advantages as AGM have, in spite these batteries perform well in hot weather conditions.
- Have better cycle life as compared to AGM and flooded batteries.
Disadvantages of Gel Batteries
- These batteries require special chargers and voltage regulator to maintain charging rates and charging voltage.
- Expensive than AGM and too expensive than a premium flooded type lead acidic battery.
Over all we can say deep cycle flooded batteries have more advantages over AGM and Gel for renewable energy storage application under normal temperature conditions. On going with VRLA batteries we have to pay more just for not looking back for maintenance after installation, with same capacity in normal temperature conditions. AGM have low cycle life with more efficiency. VRLA batteries are well known for having properties of being leak proof, spill proof, vibration resistant which is not that much important for a stable home RE storage system. Gel batteries do not accept variable and high charging rates, which can’t be controlled effectively in a solar power system where intensity of sun rays varies number of times in a day. Special voltage regulator is required for gel batteries. Gel cell perform well in hot weather conditions so could be used for the hot places. VRLA batteries have high charging efficiency with expense of extra cost and reduced cycle life.
If anyone wants an economical solution in normal weather conditions then deep cycle flooded type lead acidic batteries are best. But If anyone looking for a storage system under extreme weather conditions and do not want to look over the system for maintenance purpose after installation then VRLA is the best option with some additional cost.
FACTORS WHICH AFFECT THE PERFORMANCE AND LIFE OF A DEEP CYCLE BATTERY
Performance and cycle life of a deep cycle battery depends on many factors like depth of discharge, discharging rate, charging rate, surrounding temperature, electrolyte level in the cells etc. Depth of discharge influences the cycle life of a battery deeply. Reducing depth of discharge from 80% to 50% can result with twice increment in cycle life of a battery (May vary with brand to brand). All batteries perform with their full capacity after 20-30 cycles. Fast discharging rate reduces the capacity of a battery and fast recharging rates reduces the charging efficiency. Fast charging and discharging rates causes heat generation due to internal resistance of the battery. This generated heat is not of any use and considered as energy loss, also termed as thermal runaway sometimes. Therefore a battery slowly discharging produces most of its stored stuff over a long period.
Heat dissipation caused by overcharging, fast charging and discharging rate causes electrolysis of electrolyte, this reduces the level of electrolyte resulting premature failure of battery. By avoiding over charging, fast charging rate, fast discharging rate and by maintaining proper level of distilled water inside the cells improves the battery life and performance. Only distilled water should be used for top up, tap water should not be used as it contain minerals which contaminate the electrolyte. Proper ventilation should be provided for the battery whether it is a flooded type or a VRLA, both require ventilation to release evolved gases during operation. Flooded batteries should not be kept in discharge state for long time (not more than 2-3 months). This idle state causes sulfation, this makes the battery useless. Such batteries won’t hold charge for long time and their efficiency and capacity decreases considerably. Surrounding temperature also play an important role in affecting the performance of a battery. Capacity decreases at low temperature. Manufacturer provides battery performance and specification data which is true only for a certain surrounding temperature (27O in most cases). Change in surrounding temperature affect the performance of battery dramatically. Decrease in temperature reduces the capacity of a battery but increasing temperature may not affect the performance to much but it affects the cycle life of a battery due to heating. A battery with 200AH C20 capacity at 27O is able to produce 10 amp/hour continuously for 20 hours only at 27O surrounding temperature.
10 AMPERE X 20 HOURS = 200AH
Same battery will die soon if surrounding temperature drops to 20O, 10O or more.
LITHIUM ION, NICKEL CADMIUM (NiCd), NICKEL IRON (NiFe)
Lithium Ion, Nickel Cadmium (NiCd), Nickel Iron (NiFe) are also other opponents in the market. But lithium ion battery is a strong opponent with everyday research and development by the maneufacter around the world. Soon will replace the lead acidic batteries especially in renewable energy storage applications. These batteries have some drawbacks and advantages over lead acidic batteries. We have discussed these batteries in article as
BEFORE BUYING A BATTERY
- Ensure the quality of battery you are buying, Always buy branded batteries.
- Ensure the brand and quality of the battery you are buying, Always buy branded batteries even if seller is forcing you for buying a local brand due his high profit margin on it. Never go for a cheap, it is expensive with short service life.
- Clearly understand the warrantee and guarantee terms of manufacturer. Always try to buy an extended warranty product especially for renewable energy storage system.
- Before buying ask the seller for the technical data sheet provided by manufacturer and check all technical details mainly
Cycle life @ at all DOD ( Depth of discharge ),
Capacity of battery at relevant C rating and temperature,
Graphs representing “affect of temperature on performance”,
Water refilling frequency,
Self discharging rate,
Ampere-Hour and Watt-Hour efficiency.
Charging rates with recommended chargers and voltage regulators if buying a gel cell.