Generally it is noticed that while charging batteries people hardly pay any special attention toward the procedures. For them charging a battery is simply connecting any DC supply with matching voltage with the battery terminals.
How to Correctly Charge a Lead Acid Battery
I have seen motor garage mechanics charge all types of batteries with the same power supply source irrespective of the AH rating associated with the particular batteries.
That's gravely wrong! That's like giving the batteries a slow "death". Lead Acid batteries to a very extent are rugged and are capable of taking on the crude charging methods, however it's always recommended to charge even the LA batteries with a lot care. This "care" will not only increase the longevity but will also enhance the efficiency of the unit.
Ideally all batteries should be charged in a step wise manner, meaning the current should be reduced in steps as the voltage nears the "full charge" value.
For a typical Lead Acid battery or an SMF/VRL battery the above approach can be considered very healthy and a reliable method. In this post we are discussing one such automatic step battery charger circuit which can be effectively used for charging most of the rechargeable types of batteries.
How the Circuit Functions
Referring to the circuit diagram below, two 741 ICs are configured as comparaters. The presets at pin#2 of each stage is adjusted such that the output goes high after specific voltage levels are identified, or in other words the outputs of the respective ICs are made to go high in sequence after predetermined charge levels are accomplished discretely over the connected battery.
The IC associated with RL1 is the one which conducts first, after say the battery voltage reaches around 13.5V, until this point the battery is charged with the maximum specified current (determined by the value of R1).
Once the charge reaches the above value, RL#1 operates, disconnect R1 and connects R2 in line with the circuit.
R2 is selected higher than R1 and is appropriately calculated to provide a reduced charging current to the battery.
Once the battery terminals reaches the maximum specified charging voltage say at 14.3V, Opamp supporting RL#2 triggers the relay.
RL#2 instantly connects R3 in series with R2 bringing down the current to a trickle charge level.
Resistors R1, R2, and R3 along with the transistor and the IC LM338 forms a current regulator stage, where the value of the resistors determines the maximum allowable current limit to the battery, or the output of the IC LM338.
At this point the battery may be left unattended for many hours, yet the charge level remains perfectly safe, intact and in a topped up condition.
The above 3 step charging process ensures a very efficient way of charging resulting in almost a 98% charge accumulation with the connected battery.
The circuit has been designed by "Swagatam"
- R1 = 0.6/ half battery AH
- R2 = 0.6/one fifth of battery AH
- R3 = 0.6/one 50th of battery AH.
A closer inspection of the above diagram reveals that during the period when the relay contacts are about to release or move from the N/C position might cause a momentary diconnection of the ground to the circuit which in turn migh result in a ringing effect on the relay operation.
The remedy is to connect the ground of the circuit directly with the bridge rectifier ground and keep the ground from the R1/R2/R3 resistors attached solely with the battery negative. The corrected diagram may be witnessed below:
How to Set up the Circuit
Remember if you are using 741 IC then you must remove the red LED from the lower opamp and connect it in series with the base of the transistor to prevent permanent triggering of the transistor due to IC leakage current.
Do the same with the upper transistor base also, connect another LED there.
However if you use an LM358 IC then you may not have to this modification and use the design exactly as given.
Now let's learn how to set it up:
Initially keep the 470K feedback resistors disconnected.
Keep the slider of the presets towards ground line.
Now let's say we want the first relay RL#1 to operate at 13.5V, therefore adjust the LM338 pot to get 13.5V across the circuit supply line. Next, adjust the upper preset slowly until the relay just toggles ON.
Similarly, suppose we want the next transition to happen at 14.3V, ...increase the voltage to 14.3V by carefully adjusting the LM338 pot.
Then tweak the lower 10K preset such that RL#2 just clicks ON.
Done! your set up procedure is complete. Seal of the presets with some kind of glue to keep them fixed in the set positions.
Now you can attach a discharged battery to see the actions happening automatically as the battery charges with a 3 step mode.
The 470K feedback resistor can be actually eliminated and removed, instead you can connect a large value capacitor in the order of 1000uF/25V across the relay coils to restrict threshold chattering of the relay contacts.
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