In our previous tutorials we learned how to customize a constant voltage while building a battery charger, while in the next section we understood how to add a customized constant current to the design for assuring a safe charging for the connected battery.
How to Add an Auto-Cut 0ff in Battery Charger
In this section we'll discover how an auto cut-off may be added to a battery charger which is one of the most crucial aspects in such circuits.
A simple auto cut-off stage can be included and customized in a selected battery charger circuit by incorporating an opamp comparator.
An opamp may be positioned to detect a rising battery voltage while it's being charged and cut off the charging voltage as soon as the voltage reaches the full charge level of the battery.
You might have already seen this implementation in most of the automatic battery charger circuits so far published in this blog.
The concept may be thoroughly understood with the help of the following explanation and the shown circuit GIF simulation:
In the above simulation effect we can see that an opamp is been configured as a battery voltage sensor for detecting the over charge threshold, and cutting off the supply to the battery as soon as this is detected.
The preset at pin (+) of the IC is adjusted such that at full battery voltage (14.3V here), the pin#3 acquires a shade higher potential than the pin (-) of the IC which is fixed with a reference voltage of 4.7V with a zener diode.
The previously explained "constant voltage" and "constant current" supply is connected to the circuit, and the battery via the N/C contact of the relay.
Initially the supply voltage and the battery both are switched off from the circuit.
First, the discharged battery is allowed to be connected to the circuit, as soon as this is done, the opamp detects a potential that's lower (10.5V as assumed here) than the full charge level, and due to this the RED LED comes ON, indicating that the battery is below the full charge level.
Next, the 14.3V input charging supply is switched ON.
As soon as this is done, the input instantly sinks down to the battery voltage, and attains the 10.5V level.
The charging procedure now gets initiated and the battery begins getting charged.
As the battery terminal voltage increases in the course of the charging, the pin (+) voltage also correspondingly increases.
And the moment the battery voltage reaches the full input level that is the 14.3V level, the pin (+) also proportionately attains a 4.8V which is just higher than the pin (-) voltage.
This instantly forces the opamp output to go high.
The RED LED now switches OFF, and the green LED illuminates, indicating the changeover action and also that the battery is fully charged.
However what may happen after this is not shown in the above simulation. We'll learn it through the following explanation:
As soon as the relay trips the battery terminal voltage will quickly tend to drop and restore to some lower level since a 12V battery will never hold a 14V level consistently and will try to attain a 12.8V mark approximately.
Now, due to this condition, the pin (+) voltage will again experience a drop below the reference level set by pin (-), which will yet again prompt the relay to switch OFF, and the charging process will be again initiated.
This ON/OFF toggling of the relay will keep on cycling making an undesirable "clicking" sound from the relay.
To avoid this it becomes imperative to add a hysteresis to the circuit.
This is done by introducing a high value resistor across the output and the (+) pin of the IC as shown below:
The addition of the above indicated hysteresis resistor prevents the relay oscillating ON/OFF at the threshold levels and latches the relay up to a certain period of time (until the battery voltage drops below the sustainable limit of this resistor value).
Higher value resistors provide lower latching periods while lower resistor provide higher hysteresis or higher latching period.
Thus from the above discussion we can understand how a correctly configured automatic battery cut-off circuit may be designed and customized by any hobbyist for his preferred battery charging specs.
Now lets see how the entire battery charger design may look including the constant voltage/current set up along with the above cut-off configuration:
So here's the completed customized battery charger circuit which can be used for charging any desired battery after setting it up as explained in our entire tutorial:
- The opamp can be a IC 741
- The preset = 10k preset
- both zener diodes can be = 4.7V, 1/2 watt
- zener resistor = 10k
- LED and transistor resistors can be also = 10k
- Transistor = BC547
- relay diode = 1N4007
- relay = select match the battery voltage.
If you further doubts, you may feel free to express them through your comments.