Op amp Battery Charger Circuit with Auto Cut Off

The post discusses a two opamp IC 741 and LM358 based auto cut off battery charger circuits which are not only accurate with its features but also allows a hassle free and quick setting up of its high/low cut-off threshold limits.

The idea was requested by Mr. Mamdouh.

Circuit Objectives and Requirements

1. As soon as I connect the external power automatically it will disconnect the battery and supply the system, in the meanwhile charging the battery.
2. Overcharging protection ( which included in the above design).
3. Battery low and full charging indications (which included in the above design).
4. Also i don't know what is the formula to help how to determine the voltage required across my battery to charge it with( battery will be extracted of old laptops.total will be 22V with 6 apms at no load)
5. Furthermore, I don't know the formula to indicate how long my battery will last, and how to calculate the time if i want a battery to last me two hours.
6. Also, the cpu fan will supplied by the system too. It would be great too to add the option of a dimmer, my original plan was to vary between 26-30 v not need much more than that.

Circuit Diagram

The Design

In all of my previous battery charger controller circuits I have used a single opamp for executing the full charge auto cut-off, and have employed a hysteresis resistor for enabling the low level charging switch ON for the connected battery.

However calculating this hysteresis resistor correctly for achieving the precise low level restoration becomes slightly difficult and requires some trial and error effort which can be time consuming.

In the above proposed opamp low high battery charger controller circuit two opamp comparator are incorporated instead of one which simplifies the set up procedures and relieves the user from the long procedures.

Referring to the figure we can see two opamps configured as comparators for sensing the battery voltage and for the required cut-off operations.

Assuming the battery is s 12V battery, the lower A2 opamp's 10K preset is set such that its output pin#7 becomes high logic when the battery voltage just crosses the 11V mark (lower discharge threshold), while the upper A1 opamp's preset is adjusted such that its output goes high when the battery voltage touches the higher cut off threshold, say at 14.3V.

Therefore at 11V, the A1 output gets positive but due to the presence of the 1N4148 diode this positive stays ineffective and blocked from moving further to the base of the transistor.

The battery continues to charge, until it reaches 14.3V when the upper opamp activates the relay, and stops the charging supply to the battery.

The situation is instantly latched due to the inclusion of the feedback resistors across pin#1 and pin#3 of A1. The relay becomes locked in this position with the supply completely cut off for the battery.

The battery now begins slowly discharging via the connected load until it reaches its lower discharge threshold level at 11V when the A2 output is forced to go negative or zero. Now the diode at its output becomes forward biased and quickly breaks the latch by grounding the latching feedback signal between the indicated pins of A1.

With this action the relay is instantly deactivated and restored to its initial N/C position and the charging current yet again begins flowing towards the battery.

This opamp low high battery charger circuit can be used as a DC UPS circuit also for ensuring a continuous supply for the load regardless of the mains presence or absence and for getting an uninterrupted supply through out its usage.

The input charging supply could be acquired from a regulated power supply such as an LM338 constant current variable constant voltage circuit externally.

How to Set the Presets

• Initially keep the 1k/1N4148 feedback disconnected from the A1 op amp.
• Move the A1 preset slider to ground level, and move the A2 preset slider to the positive level.
• Through a variable power supply, apply 14.2 V which is the full charge level for a 12 V battery across the "Battery" points.
• You will find the relay activating.
• Now slowly move the A1 preset towards the positive side until the relay just deactivates.
• This sets the full charge cut off.
• Now, connect the 1k/1N4148 back so that the A1 latches the relay in that position.
• Now slowly adjust the variable supply towards the lower discharge limit of the battery, you will find the relay continues to remain switched OFF due to the above mentioned feedback response.
• Adjust the power supply down to the lower battery discharge threshold level.
• After this, begin moving the A2 preset towards the ground side, until this turns A2 output to zero which breaks the A1 latch, and switches ON the relay back to the charging mode.
• That's all, the circuit is fully set now, seal the presets in this position.

Answers for other additional questions in the request are as given under:

Formula for calculating full charge cut off limit is:

Battery voltage rating + 20%, for example 20% of 12V is 2.4, so 12 + 2.4 = 14.4V is the full charge cut off voltage for a 12V battery

To know the battery back up time the following formula can be used, which gives you the approximate battery back up time.

Backup = 0.7 (Ah / Load Current)

Another alternative design for making an automatic over/under charge cut-off battery charger circuit using two op amps, can be seen below:

How it Works

Assuming there's no battery connected, the relay contact is at N/C position. Therefore when power is switched ON, the op amp circuit is unable to get powered and stays inactive.

Now, suppose a discharged battery is connected across the indicated point, the op amp circuit gets powered through the battery. Since the battery is at a discharged level, it creates a low potential at (-) input of the upper op amp, which may be less than the (+) pin.

Due to this, the upper op amp output goes high. The transistor and the relay activate, and the relay contacts moves from N/C to N/O. This now connects the battery with the input power supply, and it begins charging.

Once the battery is fully charged, the potential at (-) pin of the upper op amp becomes higher than its (+) input, causing the output pin of the upper op amp to go low. This instantly switches OFF the transistor and the relay.

The battery is now disconnected from the charging supply.

The 1N4148 diode across the (+) and the output of the upper op amp latches so that even if the battery begins dropping it has no effect on the relay condition.

However, suppose the battery is not removed from the charger terminals, and a load is connected to it so that it begins discharging.

When the battery discharges below the desired lower level, the potential at pin (-) of the lower op amp goes lower than its (+) input pin. This instantly causes the output of the lower op amp to go high, which hits the pin3 of the upper op amp. This instantly breaks the latch, and switches ON the transistor and the relay to initiate the charging process yet again.

PCB Design

Adding a Current Control Stage

The above two designs can be upgraded with a current control by adding a MOSFET based current control module, as shown below:

R2 = 0.6 / charging current

Adding a Reverse Polarity Protector

A reverse polarity protection can be included to the above designs by adding a diode in series with the positive terminal of the battery. Cathode will go the battery positive terminal, and anode to the op amp positive line.

Please make sure connect a 100 Ohm resistor across this diode, otherwise the circuit will not initiate the charging process.

Removing the Relay

In the first opamp based battery charger design, it may be possible to eliminate the relay and operate the charging process through solid state transistors, as shown in the following diagram:

How the Circuit Works

• Let's assume A2 preset is adjusted at 10 V threshold, and A1 preset is adjusted at 14 V threshold.
• Suppose we connect a battery that is discharged at an intermediate stage of 11 V.
• At this voltage pin2 of A1 will be below its pin3 reference potential, as per the setting of the pin5 preset.
• This will cause the output pin1 of A1 to be high, turning ON the transistor BC547 and the TIP32.
• The battery will now start charging via TIP32, until is terminal voltage reaches 14 V.
• At 14 V, as per the setting of the upper preset, pin2 of A1 will go higher than its pin3, causing the output to turn low.
• This will instantly switch OFF the transistors, and stop the charging process.
• The above action will also latch the A1 op amp through the 1k/1N4148 so that even if the battery voltage drops to the SoC level of 13 V, the A1 will continue hold the pin1 output low.
• Next, as the battery begins discharging via an output load, its terminal voltage begins dropping, until it has dropped to 9.9 V.
• At this level, as per the setting of the lower preset, pin5 of A2 will drop below its pin6, causing its output pin7 to turn low.
• This low at pin7 of A2 will pull pin2 of A1 to almost 0 V, such that now pin3 of A1 becomes higher than its pin2.
• This will immediately break the A1 latch, and the output of A1 will once again turn high, enabling the transistor to switch ON and initiate the charging process.
• When the battery reaches 14 V, the process will repeat the cycle yet again

Single Op amp Automatic Battery Charger Circuit

Automatic battery chargers just aren't economical, but the protection they provide from overcharging and potential battery degradation is extremely appealing. The circuit illustrated here is meant to be a low-cost replacement to commercially available fully automated chargers. The concept is to pick a basic battery charger and install an add-on module that will automatically check the condition of the battery and turn off the charge current as soon as the battery gets fully charged.

How it Works

The circuit is simply made up of a comparator that checks the battery voltage in relation to a preset reference value. When  the battery voltage surpasses a certain peak value, a relay is turned OFF, causing the charge current to be terminated. When  the battery voltage declines below a certain specified lower limit, the relay activates, allowing the charge current to flow again. A 741 op-amp serves as the comparator. The op-amp's supply voltage is stabilized by R3 and D1, thus it is immune to fluctuations in battery voltage.

The reference voltage, that is supplied to the op amp's non-inverting input through R4 and D2, is generated through this stabilized supply. The reference voltage is compared with the battery charge voltage, via the resistive divider. As the battery charges, the voltage at the inverting input of the op-amp finally becomes higher than that on the non-inverting input, causing the output of the op-amp to go low, switching off T1 and T2. This causes the normally closed contact of the relay to open, cutting off the input charge current to the battery.

The battery full level will then illuminate LED D3 to show that it is completely charged. A part of the op-amp output voltage is sent back to the inverting input through P2 and R5 to discourage the battery from reverting to the charging mode at the smallest reduction in the battery voltage. The op-amp therefore works in the same way as a Schmitt trigger, with P2 determining the level of hysteresis, or the battery potential where the op-amp output can turn low again.

How to Setup

The easiest way to setup the circuit is to use a adjust stabilized voltage to simulate the battery voltage.

An input voltage of around 14.5 V is determined, and P1 is tuned so that the relay simply clicks off (opens). The voltage of the 'battery' is then lowered to 12.4 V, and P2 is tweaked until the relay reconnects and switches ON. Because P1 and P2 will have an effect on each other, the operation should be done numerous times.