This article explains a few lead acid battery charger circuits with automatic over charge, and low discharge cut off. This can be used to charge all automotive and SMF batteries up to 100 Ah, and even 500 Ah.
Lead acid batteries are normally used for heavy duty operations involving many 100s of amps. To charge these batteries we specifically need chargers rated to handle high ampere charging levels for long periods of time. Here we discuss one such automatic lead acid battery charger with high Ah charging capacity.
This 5 useful and high power lead acid battery charger circuits presented below can be used for charging large high current lead acid batteries in the order of 100 to 500 Ah, the design is perfectly automatic and switches of the power to the battery and also itself, once the battery gets fully charged.
1) Using a Single Op amp
Looking at the first high current circuit for charging large batteries, we can understand the circuit idea through the following simple points:
There are basically three stages in the shown configuration viz: the power supply stage consisting of a transformer and a bridge rectifier network.
A filter capacitor after the bridge network has been ignored for the sake of simplicity, however for better DC output to the battery one can add a 1000uF/25V capacitor across the bridge positive and negative.
The output from the power supply is directly applied to the battery which requires to be charged.
The next stage consists of an opamp 741 IC voltage comparator, which is configured to sense the battery voltage while it is being charged and switch its output at pin #6 with the relevant response.
Pin #3of the IC is rigged with the battery or the supply positive of the circuit via a 10K preset.
The preset is adjusted such that the IC reverts its output at pin #6 when the battery becomes fully charged and reaches about 14 volts which happens to be the transformer voltage at normal conditions.
Pin #2 of the IC is clamped with a fixed reference via a voltage divider network consisting of a 10K resistor and a 6 volt zener diode.
The output from the IC is fed to a relay driver stage where the transistor BC557 forms the main controlling component.
Initially, power to the circuit is initiated by pressing the "start" switch. On doing this, the switch bypasses the contacts of the relay and powers the circuit momentarily.
The IC senses the battery voltage and since it will be low during that stage, the output of the IC responds with a logic low output.
This switches ON the transistor and the relay, the relay instantly latches the power via its relevant contacts such that now even if the "start" switch is released, the circuit remains switched ON and begins charging the connected battery.
Now as the battery charge reaches about 14 volts, the IC senses this and instantly reverts its output to a high logic level.
The transistor BC557 responds to this high pulse and switches OFF the relay which in turn switches of the power to the circuit, breaking the latch.
The circuit gets completely switched OFF until the start button is pressed once again and the connected battery has a charge that's under the set 14 volt mark.
How to set up.
It's very easy.
Do not connect any battery to the circuit.
Switch ON power by pressing the start button and keep it depressed manually, simultaneously adjust the preset such that the relay just trips or switches OFF at the given rated transformer voltage which should be around 14 volts.
The setting is complete, now connect a semi discharged battery to the shown points in the circuit and press the "start" switch.
Due to the discharged battery, now the voltage to the circuit will drop under 14 volts and the circuit will instantly latch, initiating the procedure as explained in the above section.
Circuit Diagram for the proposed battery charger with high ampere capacity is shown below
2) 12V, 24V / 20 amp Charger Using two opamps:
The second alternative way of achieving battery charging for a lead acid battery with high amperage can be observed in the following diagram, using a couple of opamps:
For more auto cut-off charger ideas, you can read this article regarding opamp automatic battery charger circuits.
At low battery level, the lower opamp output remains high disabling the upper transistor relay driver, which allows the battery to initiate its charging process through the N/C contact of the relay.
As the charging voltage rises, the potential at (-) input pin of the lower opamp exceeds its (+) input level, which switches OFF the lower BC547, however the upper opamp output being still at logic zero the relay sustains its N/C relay position and carries on charging the battery.
At the full charge level when the potential at (+) input pin of the upper opamps tends to exceed its (-) input pin, the output of the upper opamp goes high, switching ON the relay and toggling its contacts to N/O.
This action switches OFF the charging current for the battery.
In this condition, a positive feed back from the 1K/10K resistive divider is applied to the (+) input pin of the upper opamp, which latches the opamp hard, and this in turn latches the relay in the N/O position.
Now, if any intended load is operated through this battery, and as it goes through a discharging process....after a period of time the battery voltage tends to drop to a point wherein the potential at (-) input of the lower opamp gets lower than its (+) input pin....this instantly causes its output to go high, switching ON the lower BC547.
The BC547 grounds and breaks the feedback latching potential and also the base triggering voltage of the relay driver transistor.
The action switches OFF the upper relay driver transistor allowing the relay contacts to revert to its N/C position, yet again initiating an automatic charging of the attached battery.
The set up of the above circuit can be visualized in the following video which shows the cut off responses of the circuit to the upper and the lower voltage thresholds, as fixed by the relevant presets of the opamps
3) Using IC 7815
The third circuit explanation below details how a battery may be charged effectively without using any IC or relay, rather simply by using BJTs, let's learn the procedures:
The idea was suggested by Mr. Raja Gilse.
Charging a Battery with a Voltage Regulator IC
I have a 2N6292 . My friend suggest me to make the simple fixed voltage high current DC power supply to charge an SMF battery. He had given the attached rough diagram. I don't know anything about the above transistor. Is it so ? My input is 18 volt 5 Amp transformer. He told me to add 2200 uF 50 Volt capacitor after rectification. Is it works ? If so , is there any heat sink necessary for transistor or/and IC 7815 ? Is it stops automatically after battery reaches 14.5 volt ?
Or any other alteration needed ? Please guide me sir
Charging with an Emitter Follower Configuration
Yes it will work and will stop charging the battery when around 14 V is reached across the battery terminals.
However I am not sure about the 1 ohm base resistor value...it needs to be calculated correctly.
The transistor and the IC both may be mounted on a common heatsink using mica separator kit. This will exploit the thermal protection feature of the IC and will help safeguard both the devices from overheating.
The shown high current battery charger circuit is a smart way of charging a battery and also achieving an auto shut off when the battery attains a full charge level.
The circuit is actually a simple common collector transistor stage using the shown 2N6292 power device.
The configuration is also referred as an emitter follower and as the name suggests the emitter follows the base voltage and allows the transistor to conduct only as long as the emitter potential is 0.7V lower that the applied base potential.
In the shown high current battery charger circuit using a voltage regulator, the base of the transistor is fed with a regulated 15 V from the IC 7815, which ensures a potential difference of about 15 - 0.7 = 14.3 V across the emitter/ground of the transistor.
The diode is not required and must be removed from the base of the transistor in order to prevent an unnecessary drop of an extra 0.7 V.
The above voltage also becomes the charging voltage for the connected battery across these terminals.
While the battery charges and its terminal voltage continues to be below the 14.3 V mark, the transistor base voltage keeps conducting and supplying the required charging voltage to the battery.
However as soon as the battery begins attaining the full and above 14.3 V charge, the base is inhibited from a 0.7 V drop across its emitter which forces the transistor to stop conducting and the charging voltage is cut off to the battery for the time being, as soon as the battery level begins going below the 14.3 V mark, the transistor is switched ON again...the cycle keeps repeating ensuring a safe charging fr the connected battery.
Base resistor = Hfe x battery internal resistance
4) 12V 100 Ah Lead Acid Battery Charger Circuit
The proposed 12V 100 ah battery charger circuit was designed by one of the dedicated members of this blog Mr. Ranjan, let's learn more regarding the circuit functioning of the charger and how it could be used as a trickle charger circuit also.
The Circuit Idea
My self Ranjan from Jamshedpur, Jharkhand. Recently while googling I came to know about your blog, and become a regular reader of your blog. I learned a lot of things from your blog. For my personal use I would like to make a battery charger.
I have a 80 AH tubular batery and a 10 Amps 9-0-9 volts transformer. So I can get 10 amps 18-0 volts if I use the two 9volts leads of transformer.(Transfomer is actually obtained from an old 800VA UPS).
I have constructed a circuit diagram based on your blog. Please have a look on it and suggest me. Please note that,.
1) I am belonging to very rural area hence there is a huge power fluctuation it varies from 50V ~ 250V. Also note that I will draw very less amount of current from the battery( Generally using LED lights during power cuts) approx 15 - 20 Watt.
2) 10amps transformer i think safely charges 80AH Tubular Battery
3) All diodes used for the circuit are 6A4 dides.
4) Two 78h12a used as parallel to get 5+5 = 10 amps output. Although I think Battery must not draw full 10 amps. as it will be in charged condition in day to day use so internal resistance of battery will be high and will draw lesser current.
5) A switch S1 is used thinking that for normal charge it will be kept in off state. and after fully charging the battery it switched to on state to maintain a trickle charge with lower voltage. NOW question is that is this safe for the battery to kept in charge unattended for long time.
Please reply me with your valuable suggestions.
100 Ah battery charger circuit diagram designed by Mr. Ranjan
Solving the Circuit Request
To me your high current VRLA battery charger circuit design looks perfect and should work as expected. Still for guaranteed confirmation it would be advisable to check the voltage and current practically before connecting it with the battery.
Yes, the shown switch can be used in the trickle charge mode and in this mode the battery can be kept permanently connected without attending, however this should be done only after the battery has been fully charged upto around 14.3V.
Please note that the four series diodes attached with the GND terminals of the ICs could be 1N4007 diodes, while the remaining diodes should be rated well over 10amps, this could be implemented by connecting two 6A4 diodes in parallel at each of the shown positions.
Also, it is strongly recommended to put both the ICs over a single large common heatsink for better and uniform thermal sharing and dissipation.
Caution: The shown circuit does not include a full charge cut-off circuit, therefore the maximum charging voltage should be preferably restricted between 13.8 to 14V. This will ensure that the battery is never able to reach the extreme full charge threshold, and thus remain safe from over charge conditions.
However this would also mean that the lead acid battery would be able to attain only around 75% charge level, nevertheless keeping the battery undercharged will ensure longer life for the battery and allow more charge/discharge cycles.
5) IC 555 Lead Acid Battery Charger Circuit
The fifth concept below explains a simple, versatile automatic battery charger circuit. The circuit will allow you to charge all types of lead acid battery right from a 1 AH to a 1000 AH battery.
Using IC 555 as the Controller IC
The IC 555 is so versatile, it can be considered the single chip solution for all circuit application needs. No doubt it's been utilized here too for yet another useful application.
A single IC 555, a handful of passive component is all that's needed for making this outstanding, fully automatic battery charger circuit.
The proposed design will automatically sense and keep the attached battery up to date.
The battery which is required to be charged may be kept connected to the circuit permanently, the circuit will continuously monitor the charge level, if the charge level exceeds the upper threshold, the circuit will cut off the charging voltage to it, and in case the charge falls below the lower set threshold, the circuit will connect, and initiate the charging process.
How it Works
The circuit may be understood with the following points:
Here the IC 555 is configured as a comparator for comparing the battery low and high voltage conditions at pin#2 and pin#6 respectively.
As per the internal circuit arrangement, a 555 IC will make its output pin#3 high when the potential at pin#2 goes below 1/3 of supply voltage.
The above position sustains even if the voltage at pin#2 tends to drift a little higher. This happens due to the internal set hysteresis level of the IC.
However if the voltage continues to drift higher, pin#6 gets hold of the situation and the moment it senses a potential difference higher than 2/3rd of supply voltage, it instantly reverts the output from high to low at pin#3.
In the proposed circuit design, it simply means that, the presets R2 and R5 should be set such that the relay just deactivates when the battery voltage goes 20% lower than printed value and activates when the battery voltage reaches 20% above printed value.
Nothing can be as simple as this.
The power supply section is an ordinary bridge/capacitor network.
The diode rating will depend on the charging current rate of the battery. As a rule of thumb the diode current rating should be twice that of the battery charging rate, while the battery charging rate should be 1/10th of the battery AH rating.
It implies that TR1 should be around 1/10th of the connected battery AH rating.
The relay contact rating should be also selected as per the ampere rating of TR1.
How to set the battery cut off threshold
Initially keep the power to the circuit switched OFF.
Connect a variable power supply source across the battery points of the circuit.
Apply a voltage that may be exactly equal to the desired low voltage threshold level of the battery, then adjust R2, such that the relay just deactivates.
Next, slowly increase the voltage up to the desired higher voltage threshold of the battery, adjust R5 such that the relay just activates back.
The setting up of the circuit is now done.
Remove the external variable source, replace it with any battery which needs to be charged, connect the input of TR1 to mains, and switch ON.
Rest will be automatically taken care of, that is now the battery will start charging and will cut off when its fully charged, and also will get connected to power automatically in case its voltage falls below the set lower voltage threshold.
IC 555 Pinouts
IC 7805 Pinout
How to Set Up the Circuit.
The setting up of the voltage thresholds for the above circuit may be done as explained below:
Initially keep the transformer power supply section at the right hand side of the circuit completely disconnected from the circuit.
Connect an external variable voltage source at the (+)/(-) battery points.
Adjust the voltage to 11.4V, and adjust the preset at pin#2 such that the relay just activates.
The above procedure sets the lower threshold operation of the battery. Seal the preset with some glue.
Now increase the voltage to about 14.4V and adjust the preset at pin#6 to just deactivate the relay from its previous state.
This will set up the higher cut off threshold of the circuit.
The charger is now all set.
You may now remove the adjustable power supply from the battery points and use the charger as explained in the above article.
Do the above procedures with lot of patience and thinking
Feedback from one of the dedicated readers of this blog:
untung suharto January 1, 2017 at 7:46 AM
Hi, you have made a mistake on preset R2 and R5, they should not be 10k but 100k, I just made one and it was a success, thank you.
As per the above suggestion, the previous diagram may be modified as shown below:
Wrapping it up
In the above article we learned 5 great techniques which could be applied for making lead acid battery chargers, right from 7 Ah to 100 Ah, or even 200 Ah to 500 Ah, simply by upgrading the relevant devices or the relays.
If you have specific questions regarding this concepts, please feel free to ask them throug the comment box below.