The proposed 48 V automatic battery charger circuit will charge any 48 V battery up to an optimal 56 V full charge level, utilizing very ordinary components. The circuit is highly accurate with its over charge cut off features.
As shown in the circuit diagram, the main element in the circuit is the opamp IC 741, which has been arranged as a comparator.
Pin#3 which is the inverting input of the IC is referenced with a fixed voltage of 4.7V through the respective zener/resistor network.
The other input is applied with the sensing voltage which is actually the voltage merged from the supply and the from the battery, in other words the charging voltage which is applied to the battery for charging.
The resistor network at pin#2 along with the preset forms a voltage divider network which is initially adjusted such that the voltage at this pin stays below the voltage level at pin3, which is the reference voltage set at 4.7v by the zener diode.
The preset is set in such a way that the voltage at pin#2 rises above the 4.7 mark as soon as the battery voltage rises above 50V or the fill charge threshold level of the battery.
The moment this happens, the output of the opamp goes low switching OFF the mosfet, and cutting off the voltage to the battery.
Initially as ling as the battery voltage and the over all voltage from the 48V supply remains below the full charge threshold level of the battery, the output of the opamp stays high and the mosfet us kept switched ON.
This allows the voltage to the battery for charging, until the above explained threshold is reached which automatically inhibits the battery from further charging.
The mosfet can be selected as per the AH rating of the battery.
UPDATE: For converting this into a Solar version you can read this article
1) Using Mosfet Cut Of
2) Current Controlled Version of the above Design
NOTE: The above diagrams mistakenly shows 48V as the input, the correct value is 56V. Because the full charge level of a 48 V battery is around 56/57 V.
NOTE: You will have to connect the battery first and then switch ON the input supply, otherwise the mosfet will fail to initiate for the charging process. Make sure the green LED remains illuminated after power switch ON, this will confirm the charging status of the battery.
The above design can be also built using a TIP142 and a red led charging indicator.
Simple 48 V 100 Ah charger Circuit using OP Amp and TIP142
3) Making a Fully Automatic Version
The above circuit can be upgraded into an over charge cut off, as well as low charge restoring battery charger system, for charging 48V batteries.
The modifications enables the circuit to switch OFF the battery charging process at the set over charge threshold and restore back the process when the battery voltage falls below the low threshold value.
The 10k preset must be adjusted to set the full charge level while the 22k preset for detecting the lower threshold of the battery.
NOTE: In the above two circuits, please connect the RED LED in series with the BC546 base. This will prevent the op amp offset voltage from reaching the BC546 base and false triggering.
The above design can be further simplified as shown in the following image. Notice that the input pins of the op amp are swapped in this design, which allowed the elimination of the extra PNP BJT from the circuit.
How to Set up the above Circuit:
For setting up procedure, the sample power supply should be connected across the points where the battery is connected, the mosfet does not require any attention initially. DO NOT connect the battery while carrying out this procedure.
Also keep the 22k preset link disconnected initially.
Apply the higher threshold level across the above mentioned points and adjust the 10K preset such that the RED LED just switches ON. Seal the adjusted preset with some glue.
Now reconnect the 22k preset link back into position.
Next, reduce the sample voltage to the lower threshold value and adjust the 22k preset such that now the green LED just lights up, while switching OFF the RED LED.
If you find no response from the circuit try using a 100K preset instead of the 22k preset.
Seal the adjusted preset as above.
The setting up of the circuit is over and done.
Please note that during actual operations, the above circuit will remain functional only as long as a battery stays connected at the shown points, without a battery the circuit will not detect or respond.
Feedback from Mr. Rohit
I have a 50-52v solar panel setup which is charging a 48v 78ah battery. What I want is when my battery is fully charged that is it reaches to 54v the battery charging stops and the supply which is coming from the solar panels is directed to another port from which we can charge any other device connected to the port. This charging should only continue till the battery is above 48v. Once it reaches 48v the battery again starts charging on solar panels and the supply to the other port is stopped.
Hoping you will reply soon.
My Response to the above Circuit Request
You can try the last circuit from the following artcilehttps://www.homemade-circuits.