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The application of Appropriate Technology

Articles for Keyword "battery charging"

Part 3: Charging

Posted on Jun 4, 2011

Charge State There are two main methods for determining the state of charge for lead-acid batteries: Terminal Voltage – The open circuit voltage (no current flowing) of a fully charged cell depends on its type but will be 2.1V to 2.3V (12.6V to 13.8V for a 12V battery). If the voltage is measured with the charging current flowing it will be increased by the voltage drop across the internal resistance. If discharging the measured voltage will drop due to the internal resistance of the cell. Table 1 gives the approximate battery and cell voltages for various states of charge. Specific Gravity – This is the recommended method if the battery is not sealed and a hydrometer can get into the battery. For a flood-type battery in good condition the specific gravity should vary in the region of 1.25 for a fully charged battery to 1.17 for a fully discharged battery. These figures vary slightly depending on the battery type and the temperature: 0.0007 should be added to these values for each degree above 15°C. Table 2 gives the specific gravity values for several lead-acid batteries. Table 1: The approximate battery and cell voltages for various states of charge. State of Charge (approx.) 12 Volt Battery Volts per Cell 100% 12.70 2.12 90% 12.50 2.08 80% 12.42 2.07 70% 12.32 2.05 60% 12.20 2.03 50% 12.06 2.01 40% 11.90 1.98 30% 11.75 1.96 20% 11.58 1.93 10% 11.31 1.89 0% 10.50 1.75 Table 2: The approximate specific gravity values for several lead-acid batteries in various states of charge. * SG = specific gravity at 25°C. ** OCV open circuit voltage per 2V cell. State of Charge (approx) Apex Suncycle PVStar SG* OCV** SG* OCV** SG* OCV** 100% 1.277 2.12 1.240 2.0866 1.225 2.0950 90% 1.258 2.10 1.230 2.077 1.216 2.0775 80% 1.238 2.08 1.220 2.067 1.207 2.0600 70% 1.217 2.06 1.210 2.058 1.198 2.0425 60% 1.195 2.04 1.200 2.048 1.189 2.0250 50% 1.172 2.02 1.190 2.040 1.179 2.0075 40% 1.148 2.00 1.180 2.031 1.171 1.9900 30% 1.124 1.98 1.170 2.022 1.163 1.9725 20% 1.098 1.95 1.160 2.013 1.153 1.9550 10% 1.073 1.93 1.150 2.005 1.145 1.9375 0% 1.048 1.91 1.140 1.996 1.135 1.9200 Charging The charging voltage must be higher than the battery voltage for current to flow into the battery. There are two basic ways to charge a lead-acid battery from an uninterrupted supply (e.g. mains or a generator): Constant-voltage charge – A constant voltage is applied across the battery terminals. As the voltage of the battery increases the charging current tapers off. This method requires simple equipment but it not recommended. Constant-current charge – An adjustable voltage source or a variable resistor maintains a constant current flows into the battery. Thus requires a sophisticated charge controller. From uninterrupted power supplies lead-acid batteries are normally recharged using the constant-current technique; the manufacturer’s data should be checked to find an appropriate charging rate. A common rule of thumb used to calculate a suitable charging current is that it should be...

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