Page - 132 - in Clean Water Using Solar and Wind - Outside the Power Grid

132 Clean Water Using Solar and Wind: Outside the Power Grid it down. For a lead-acid battery an increasing temperature has several consequences: • Increases performance, • Increases internal losses, • Decreases cell voltage for a given charge current, • Shortens the battery life, • Increases maintenance requirement. 10.4.2 Battery sizing Assume that the power requirement is 300 W for 16 hours. This corresponds to an energy requirement of 4,800 Wh or 4.8 kWh. Assuming a battery with 12 V, this means that the required rating of Ah is 4,800/12 = 400 Ah. However, the battery’s losses should be taken into consideration. Assuming 15% losses (85% battery efficiency), this means that 400 Ah will be divided by 0.85 to find the necessary rating. Then the battery cannot be discharged 100%. Assuming an allowable discharge of 60% means that the capacity will be further divided by 0.6 to obtain the required capacity. The final battery rating then has to  be  400/(0.85 ⋅ 0.6) = 784 Ah or around 800 Ah. Typical rating of a lead- acid battery is of the order 100–600 Ah. In summary, the required battery rating is calculated as Battery rating Totalbatteryenergycapacity Batteryeff ( ) ( ) Ah Wh = iciency Depthof discharge Batteryvoltage⋅ ⋅ ( )V 10.4.3 Battery classification Batteries are classified in C- or E-rates, which is a way of showing the discharge current. It its normalised against battery capacity. A C-rate is a measure of the rate at which a battery is discharged compared to its maximum capacity. The rate 1C means that the discharge current will discharge the battery in one hour. For a battery with a capacity of 100 Ah it means that the discharge current will be 100 A. A C/2 rate would be 50 A. Applying a smaller charge/discharge rate will prolong the life of a chemical battery, so a discharge rate of for instance <0.1C is quite favourable for the battery’s life. Similarly, using a battery discharge Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user