aamir
Junior Member level 2
In my normal routine and on net, i found that many people are doubtful about charging a lead-acid battery either its automobile's or SLA. Some say that constant voltage must be use, othersay that constant current is important. So I thought to write a few lines about the best method of charging lead-acid batteries for maximum life and efficiency.
After reading many books and articles and experiments, I found that the best method of charging is constant current charging in the 1st stage and constant voltage in 2nd stage when battery is fully charged. Manufacturers and experts recommend that battery must be charged at the rate of 1/10 (1/8 in some cases) of total battery capacity. This means that if we have a 12v battery of 40AH, it should be charged with a constant current of 4-5 amperes and when battery is fully charged, charging voltage must be reduced to 13.5-13.8 volts in order to prevent overcharging and keeping battery in perfect condition. Constant current charging means that charging current through battery remains constant from start to end. Unforunately, it is difficult to control current at high amperes in charging by simple method as current varies with the charging of battery e.g, if a discharge battery is taking 7-8 amperes at start, it will reduce with the increase in battery voltage and in the end current will be about 2 amperes so the battery will take a long time to reach its full charging point. It's very simple to regulate voltage by using a voltage regulator but current regulation is complicated. Many people recommend LM317 I.C for constant current charging but its max output current is 1.25 ampere which means 32 hours for charging a 40 ampere battery. At high amperes, switch mode technique is used for current regulation. A switching transistor is used with a VCO ( Voltage Controlled Oscillator), an error detector (basically an Op-Amp) and a series resistance with the battery. When a discharge battery is connected to the charger, more of the current flows through the series resistance which means more voltage across the resistance. This voltage is monitored by error detector which output is connected to VCO which is connected to switching transistor. As a result, transistor turns on lately and current is set to a pre-set level. Now when battery gains voltage, voltage decreases across series resistor. As a result, transistor turns on early until current reaches to its pre-set level. All this process results in constant flow of current through battery from start to end. Constant current charging is complicated and expensive but if you want your batteries at their best, you must use it
From all this, we conclude that lead-acid batteries should be charged at constant 1/10 or 1/8 of total battery current and when battery reaches to 7.1-7.2v (6v battery) / 14.2-14.4v (12v battery), charging voltage must be reduced by a voltage regulator to 6.6-6.9v / 13.5-13.8v to prevent overcharging and keeping batteries in a perfect condition. I hope this will help you understanding the whole process and you will choose the best method.
Note: A 3rd stage ( Temperature controlled ) is also used in more complicated chargers but it will make it more complex so I haven't described it here.
After reading many books and articles and experiments, I found that the best method of charging is constant current charging in the 1st stage and constant voltage in 2nd stage when battery is fully charged. Manufacturers and experts recommend that battery must be charged at the rate of 1/10 (1/8 in some cases) of total battery capacity. This means that if we have a 12v battery of 40AH, it should be charged with a constant current of 4-5 amperes and when battery is fully charged, charging voltage must be reduced to 13.5-13.8 volts in order to prevent overcharging and keeping battery in perfect condition. Constant current charging means that charging current through battery remains constant from start to end. Unforunately, it is difficult to control current at high amperes in charging by simple method as current varies with the charging of battery e.g, if a discharge battery is taking 7-8 amperes at start, it will reduce with the increase in battery voltage and in the end current will be about 2 amperes so the battery will take a long time to reach its full charging point. It's very simple to regulate voltage by using a voltage regulator but current regulation is complicated. Many people recommend LM317 I.C for constant current charging but its max output current is 1.25 ampere which means 32 hours for charging a 40 ampere battery. At high amperes, switch mode technique is used for current regulation. A switching transistor is used with a VCO ( Voltage Controlled Oscillator), an error detector (basically an Op-Amp) and a series resistance with the battery. When a discharge battery is connected to the charger, more of the current flows through the series resistance which means more voltage across the resistance. This voltage is monitored by error detector which output is connected to VCO which is connected to switching transistor. As a result, transistor turns on lately and current is set to a pre-set level. Now when battery gains voltage, voltage decreases across series resistor. As a result, transistor turns on early until current reaches to its pre-set level. All this process results in constant flow of current through battery from start to end. Constant current charging is complicated and expensive but if you want your batteries at their best, you must use it
From all this, we conclude that lead-acid batteries should be charged at constant 1/10 or 1/8 of total battery current and when battery reaches to 7.1-7.2v (6v battery) / 14.2-14.4v (12v battery), charging voltage must be reduced by a voltage regulator to 6.6-6.9v / 13.5-13.8v to prevent overcharging and keeping batteries in a perfect condition. I hope this will help you understanding the whole process and you will choose the best method.
Note: A 3rd stage ( Temperature controlled ) is also used in more complicated chargers but it will make it more complex so I haven't described it here.