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1. PHOTONS ENERGY SOLAR ENERGY

2. Battery is the device that converts chemical energy contained in its active materials directly into electrical energy by means of an electrochemical reaction. The basic purpose of battery is to store charge and deliver the same when used in an application. BATTERY

3. Batteries store energy in direct current form (DC). Since PV systems produce power only when the sun is shinning b atteries are used to provide back-up power. Battery cells allow the electrical energy to be stored as chemical energy within the cell and then converted back to electricity as needed.

4. Battery Types There are varieties of batteries that are available in the market for several types of applications. The type of battery is identified by the chemistry of materials used in making it. The batteries are broadly divided into two categories:

5. Non-rechargeable batteries or primary batteries, and Rechargeable batteries or secondary batteries.

6. Non-rechargeable Batteries (primary battery) In the non-rechargeable batteries, the electrochemical reaction is not reversible. This type of batteries is used for one time and once discharged, they cannot be charged again. These types of battery are portable and are made in various sizes and shapes.

7. The sizes are normally referred as size A, AA, AAA, C, D, etc. and the shapes of such batteries can be of several types like coin, cylindrical, cuboid, etc. These batteries are mainly used in transistors, toys, torches, etc. The 1.5 V (AA) zinc chloride battery.

8. The most common example of non-rechargeable battery is Zinc Chloride battery, Magnesium cells, Aluminum cells, Alkaline-manganese dioxide cells, Mercuric oxide cells, etc.

9. Rechargeable Batteries (Secondary Battery) The batteries in which the conversion of chemical energy into electrical energy (discharging) and the reverse process, that is, conversion of electrical energy into chemical energy (charging) can take place are called the rechargeable battery or secondary battery.

10. The rechargeable batteries are the most widely used batteries in the world. These batteries are used for various applications, such as starting, lighting and ignition (SLI),standby power supply, electronic appliances like DVD player, mobile phones, camera, camcorder, laptops, etc. These batteries are available in a wide range of charge storage capacities in the market and can be easily procured.

11. Note: Rechargeable batteries are the battery used in solar PV systems. Normally used rechargeable batteries are Lead acid batteries and Lithium Ion batteries

12. Lead-acid Batteries Lead-acid closely resemble an automotive battery, however automotive batteries are not recommended for PV applications because they are not deep-cycle. PV system require a battery to discharge small amounts of current over long durations and to recharge under variable conditions. Deep-cycled lead-acid batteries can be discharged down to 45%-80% of its rated capacity. Deep-cycled lead-acid batteries are best suited for PV systems because they are rechargeable, widely available in many sizes, easy to maintain, and relatively inexpensive.

13. Types of Lead-acid batteries Two main types of lead-acid batteries: Vented lead-acid (VLA) Valve regulated (VRLA) VLA , are also known as liquid or flooded batteries which require water to be routinely added to the batteries to maintain the system. VRLA are also known as sealed batteries. It requires less maintenance than the VLA and eliminate the threat of an acid spill due to their sealed design.

14. Lithium Ion (Li-ion) Batteries A newer technology than the lead-acid batteries. They are popular because they are lightweight, have highly reactive elements and they hold their charge, can handle hundreds of charge/discharge cycles. There are different types of Li-ion batteries and below are listed the most common: Lithium iron phosphate (LFP) Lithium nickel manganese cobalt oxide (NMC) Lithium nickel cobalt aluminum oxide (NCA) Lithium manganese oxide (LMO) Lithium titanate (LTO)

15. 5.3kWh lithium iron batteries

16. Comparisons between lead acid and lithium Iron batteries Specification Lead-Acid Battery Lithium Iron Phosphate (LiFePO₄) Depth of Discharge (DoD) 50% (recommended) 80–100% (usable capacity is higher) Cycle Life 300–800 cycles (depends on type/usage) 3,000–8,000 cycles Efficiency 70–85% 95–98% Weight Heavy Lightweight (about 50–70% lighter) Size Larger volume for same capacity More compact Charging Speed Slower (multiple stages, bulk/absorption) Faster charging, constant current Maintenance Regular maintenance (flooded type) Maintenance-free Lifespan (Years) 3–5 years 10–15 years Temperature Sensitivity Performs poorly in extreme temperatures Better performance in a wide range Cost (Initial) Cheaper upfront Higher upfront cost Cost (Long-Term) More replacements needed Lower total cost of ownership Environmental Impact Contains lead and acid – hazardous More environmentally friendly (if managed well) Self-Discharge Rate 5–15% per month <3% per month Recommended Use Budget systems, low-cycle applications Long-term, high-efficiency solar systems

17. Battery Specifications Battery Capacity The capacity of a battery is the capacity to store the charge in the battery. It is the product of current (in amperes) it can deliver for a given time (in hours), i.e., Ampere × Hour (Ah). One ampere-hour (Ah) is the amount of charge delivered when constant current of one ampere (A) is used for one hour (h). Battery capacity is given in terms of Ah. Large size batteries have large Ah capacity

18. Rate of charge and Discharge (C-rating) In order to ensure proper charging/discharging of batteries, manufacturers specify the charging/discharging current rates in terms of C-rating. The C-rating specify in how many hours a given battery should be charged or discharged

19. Depth of Discharge (DOD) In practical applications, all the charge stored in a battery cannot be used for running load. Only some percentage of total charge stored can be used. The percentage of total charge that can be used for running the load is referred as Depth of Discharge (DoD).

20. 50% DoD means that only 50% of the total stored charged can be used. 70% DoD means that only 70% of the total stored charge can be used. In general, we want higher DoD for the batteries which are used in solar PV systems. Manufacturers specify allowable DoD level for their batteries. The battery should not be discharged below manufacturers specified level in order to prevent damage to the battery. The Depth of Discharge indicates the amount of charge (in percentage) that should withdrawn from a battery

21. Life Time Cycles Batteries express their life expectancy in terms of number of cycles (not years) . One charging and discharging of a battery is equivalent to one charge/life cycle. Life expectancy is a function of DOD. The battery that is discharged to a deeper percent of its capacity will not last as many years as a battery that is discharged at a shallower rate.

22. Days of Autonomy Autonomy refers to the number of days a battery system will provide a given load without being recharged by the PV array or another source. Environmental Conditions Batteries are sensitive to temperature. Manufacturers generally rate batteries at 25°C. Battery life expectancy increases with colder temperature and decreases in environments with higher temperatures.

23. State of Charge (SOC) The State of Charge indicates the amount of charge (in percentage) that is still there in the battery. Measuring Battery A voltmeter can be used to measure a battery’s state of charge. Battery should sit at rest for a few hours (disconnected from load). At some point the battery process stops as the battery can no longer deliver power to a load. The current drops to zero and the potential difference no longer exists.

24. Battery Wiring Configuration The batteries in PV systems can be connected in series, parallel and mixed type of connections in order to meet the PV system demand of either higher voltage, high current or both. When batteries are connected together in series , the overall voltage increases but current remains the same. When the batteries are connected together in parallel , the overall current increases but the voltage remains the same.

25. Note: In order to connect batteries in series and parallel, it is recommended that all the batteries that are to be connected together should be of the identical. i.e., same terminal voltage and same capacity.

26. Series Connection of Batteries In a solar PV system, batteries are connected together in series when the required PV system voltage is higher than the individual battery terminal voltage. In series connection, the negative terminal of one battery is connected to the positive terminal of other battery. The positive terminal of the first battery in the series and the negative terminal of the last battery are used to obtain high voltage. In this type of connections, the voltage of each battery gets added. In this way, connecting the batteries in series increases the voltage.

27. Series connection of batteries is shown in Figure below; From the figure above V stand for voltage C stand for current

28. Parallel Connection of a batteries Batteries are connected in parallel when high current is required. In parallel connection, the same type of terminals are connected together at one point, i.e. the positive terminals of all the batteries are connected together as one. Similarly, negative terminals of all the batteries are connected together as one.

29. In this type of connection, the capacity of each battery is additive. So, connecting the batteries in a shunt (parallel) increases the capacity but the voltage remains the same which is equal to the voltage of a single battery. A parallel connection of batteries is shown in Figure below;

30. Mixed Connection of a batteries In mixed type of connections, both the combinations of series and parallel are used together. This type of connection is used when both voltage and current/capacity requirement increases the standard values of available batteries. Depending on the voltage requirement, the calculated numbers of batteries are connected in series and depending upon the current/capacity requirement, the numbers of such series combinations are connected in the parallel combination.

31. The mixed connection of batteries is shown in Figure below;

32. STORAGE BATTERY SIZING Size of storage battery system depend on the following; Storage Energy Demand Days of Autonomy Wiring Efficiency Depth Of Discharge (DOD) System Voltage

33. Mathematical expression for battery storage system; Battery storage (Wh) Where by: NTC- night time consumption DA- days of autonomy DOD- depth of discharge IE – inverter efficiency Number of Battery BS-Battery storage BC- battery capacity  

34. Use the following worksheets to practice series and parallel wiring for 12-,24-48- volt systems. Enter the your answers in the blanks on each page. Draw lines to make connections Instructions Connect the photovoltaic modules(array) either in series or parallel or series/parallel to get the desired system voltage Calculate total module output ; volts and amps Connect the array to charge controller Connect batteries either in series or parallel or series/parallel to get desired system voltage Calculate total battery bank voltage and amp-hour capacity Connect the battery bank to the charge controller Series and Parallel Wiring Exercises

35. Design a 12 V system with four 12 V PV modules Problem 1 Source: Solar Energy International

36. Solution 1

37. Problem 2 Design a 24 V system with four 12 V PV modules

38. Solution 2

39. Problem 3 Design a 48 V system with eight 12 V PV modules

40. Solution 3

41. Problem 4 Design a 48 V system with sixteen 12 V PV modules

42. Solution 4