Page - 131 - in Clean Water Using Solar and Wind - Outside the Power Grid

Handling variable production 131 10.4.1 Battery capacity A battery’s capacity is the total amount of energy (kWh) that it can store. Capacity is often expressed in ampere-hours (Ah). The voltage is assumed to be given, so the capacity in kWh can be calculated. This is the discharge current a battery can deliver over time. By stacking several batteries, the total capacity can be increased. It should be noted that capacity does not indicate how much power a battery can deliver at a given moment; therefore, its power rating (measured in kW) should be declared. Consequently, a battery with a high capacity and a low power rating would deliver a low amount of electricity for a long time. A battery with a low capacity and a high power rating could run a high load but only for a relatively short time. The electrochemical battery has the advantage over other energy storage devices in that the delivered power stays high during most of the discharge and then drops rapidly as the charge depletes. Usually it is not possible (or advisable) to use all the capacity of a battery. Most batteries need to retain some charge at all times due to their chemical composition. The depth of discharge (DoD) of a battery refers to the amount of a battery’s capacity that has been used. Most batteries have a recommended maximum DoD for best performance. For example, if a 10-kWh battery has a DoD of 90% then only 9 kWh can be used before recharging it. The rating of a battery is usually expressed in amp-hours (Ah). The standard rating is a discharge current taken for 20 hours. Thus, a 100 Ah battery can discharge 5 A for 20 hours, from 100% state-of-charge to the cut-off voltage. It is important to know that this is not a linear relationship. If instead the discharge current is 100 A then the discharge time is less than one hour. Battery capacity can also be expressed in terms of “energy capacity”. This is the total energy (in kWh or Wh), expressed as the discharge power times the discharge time. The time is defined as the time for 100% state-of-charge to the cut-off voltage, which is the voltage that generally defines the “empty” state of the battery. Like capacity, energy decreases with increasing discharge current. Temperature will significantly influence the battery. Since a lead-acid battery is an electrochemical device, an increasing temperature  will accelerate the chemical activity while a colder temperature will slow Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user