Gel cell battery charger circuit

Last updated on May 18th, 2022 at 09:16 am

I love a gel cell battery because it is maintenance-free and cheap. Sure, we need a suitable battery charger circuit, for longer service life.

Gel cell battery charger circuit

Let’s look at this, Gel Cell battery charger circuit using LM317.
Why do I recommend it to you? Because it is a lower voltage charging, 13.4 volts only. Thus, no heating.

Moreover, it is easy to build. Even my daughter 10 years is still able to build. So, you can, too.

And, we can charge any Gel cell batteries with a charging current, 300mA, 650mA, and 1.3A.

While the circuit is running, the LED indicates charging. And the battery voltage rises to the set level (13.4V). But the current flow reduces almost zero milliamps. At the same time, the LED will go out.

What is a Gel Cell battery?

Some of you may still not know them. Let’s get to know it a little bit.

Cr: Photo from Amazon Mighty Max Battery

It is also an acid-lead battery type. But is the maintenance Free Battery group. Inside there are Positive Element Plate and Negative Element Plate with Lead Calcium or Lead Silver (expensive).

We do not have to see inside. Just use it enough.

This type of battery has acid inside the battery in gel form. To reduce the problem of leakage of acid from the battery.

Cheaper gel battery Ideas

I love the gel battery. And I use them in many projects. Because it is cheaper, we can buy it easily at any store near us. Most I will buy it from a motorcycle repair shop, 0.6$ only.

In most modern motorcycles these batteries are used. Most sizes: 12V 2.5Ah and 5Ah. They are usually replaced after 1 year of use. Of course, it can continue in electronic work, which uses less electricity and low temperature.

When it really expires. I can sell it to an antique shop (recycling) as well. Better than other types of batteries. Used and then discarded only.

Gel cell battery charger circuit working

In the circuit below, We use the LM317 as a 1.5A Adjustable Positive Voltage Regulator. It converts the input DC voltage into the stable voltage to charge the battery.

We adjust the 5K potentiometer-VR1 for 13.4V output.

We set the output current by the current sensing resistor( R3) in the ground or ( – ) terminal.

The R3 is current limiting. You can choose it to set the charging current.

  • 300mA = 2.2 ohms, 1 watts
  • 500mA = 1 ohms, 1 watts
  • 1,300mA = 0.47 ohms, 1 watts

At this point, some people (including my daughter) may not understand a battery charging system.

I always believe, explained with pictures to make it easier to understand. Let’s take a look at the block diagram below. First operating: High charging current!

We compare the charging circuit and the battery to a water tank. It has different water levels(voltage levels). The charger circuit is set at 13.4V but the battery is 12.8V.

Then we connect a water pipe (resistor R3) to pass water (electricity) from the charging circuit to the battery.

Naturally, water always flows from high to low. Electricity is the same. It will flow from the charging circuit to the battery.

And, when the water level is very different. The current will flow with full strength according to the size of the pipe. In this case, we set 1 ohm for the charging current 600mA.

charging current compare water block diagram

Then, the water continued to flow through the pipes, until the water level in both tanks is the same.

As the block diagram above. It looks like the battery voltage began to increase closer to 13.4V. the charging current began to decrease to almost 0A.

Let’s look at the real cycle again.

When the current flows through R3. This results in a voltage across the base and emitter of transistor-Q1. It is forward-biased. Thus, Q1 conducts current to LED1 and adj of IC1.

The red LED1 will show a charging battery. When the battery voltage reaches, the current will drop to a few milliamps. And it makes the voltage across Q1 and LED1 lower. When its current drops about 5%, the LED turns off, and the current drops to almost zero.

See: Automatic battery charger circuit

The AC adapter

We use an AC adapter as a power source for this. As the circuit above, the plug pack 500mA DC  for a charging current of 300mA. The current AC adapter should be more than 1500mA at 15V to 18V.

  • 300mA output requires the plug pack 500mA up.
  • 500mA  output requires the plug pack 650mA up.
  • 1,300mA output requires the plug pack 1,500mA up.

For 1,300mA: If possible using the 2000mA (2A) current is most suitable.

What is an AC adapter?

If you are a beginner, sometimes you may confuse the circuit inside it.
When you cannot buy the AC adapter use can build an unregulated power supply from parts that you have. It will save you money.

How to build it

My daughter wants to create this project. By she assembled all the equipment on cardboard. Then connect those devices with copper wire. According to the circuit above. In this way, it makes this project look easier.

IMPORTANT: Be careful to insert the correct polarity device. Such as LM317, LED, transistor. Otherwise, the circuit will definitely malfunction.

LED1: If the wrong polarity is inserted, it will turn off.

connect correct polarity

Note: Only a 3mm red LED should be used. Because when working, it will be clearly visible and the current flowing through it is very little.

LM317: Don’t put the wrong polarity at all. It may suddenly overheat and become damaged. Its metal body is connected to the output pin. We should be very careful. It short-circuited into other parts.

pinout of LM317T on TO-220

Note: While in use, it becomes warm. To prolong its service life. We should also install the heatsink on it.

Transistor: It should fit it correctly as well. It has three legs, be careful!

We may use equivalent transistors: BC547, BC548, BC549, BC550, etc.

different Small transistor legs

Look at the above, You may use other small NPN transistors, too. But different leg positions be careful!

But most importantly, it should be a transistor with high gain (100 hFE up) or a good quality transistor. Otherwise, it won’t be able to conduct current well enough to drive LED1 bright up.

Potentiometer: You might confuse its legs. At least my daughter is one of them.

Trimmer Trimpots Potentiometer

See above, It is 100K. But in this circuit 5K. It is an example.

Parts you will need
IC1: LM317, 1.5A Adjustable Positive Voltage Regulator
C1,C2: 0.1uF 50V, Ceramic capacitors
R1: 470 ohms, 0.25W Resistor
R2: 2.2K, 0.25W Resistor
VR1: 5K, potentiometer
R3: 1 ohm, 1-watt Resistor
LED1: Red LED 3mm
Q1: BC549, 0.45A 40V NPN transistor

How to setting and usage

When we finish the circuit and checked everything already. Then, apply the power supply to Vin, DC 17V at 1.5A. Without connecting the battery. Next, adjust VR1 until the Vout is 13.4V.

After that, connect the 12V battery to the output. The voltage level will drop. When measuring the current flowing through the battery initially it is approximately 600mA.

Notice that the LM317 gets hot. Because there is a lot of current flowing through it and the voltage drop across it is high.

When the voltage level starts to rise, close to 13.4V. The current through the battery will also be less. It takes about 10 hours. It shows that the battery is fully charged. Because the voltage level is exactly 13.4V.

Notice that the LM317 doesn’t get hot at all. Even I used to charge it for 24 hours. The circuit works normally. And the battery doesn’t get hot too, maintaining the battery voltage very well.

Not only that I like to keep old circuit ideas. It is maybe useful for you. See below:

Dry Cell Battery Charger circuit

This is the Dry Cell Battery Charger Circuit. This can use a charger battery to get that about 12 hours. When applying to the power supply 9 volts the equipment that fixes in the circuit used for size battery AA.

If using the size C or D should devalue of Resistor RX down be 68ohm and should not lead battery comes to serial while voltage in cell battery lower 1.6V.

Dry Cell Battery Charger using LM741 and IC4011

The Comparator Circuit with (IC741) controls Gate output from Pulse Oscillator. We use the integrated circuit CMOS 4011 bias current to Transistor that does in front charger battery until voltage tall 1.6V.

The comparator Circuit makes the LED Flasher alert known for protecting the Charger battery full.

The next time is if friends have Dry Cell Battery that use to be finished already, don’t abandon, try to apply new again.

Keep reading:
5 Lead-acid battery charging circuits


I always try to make Electronics Learning Easy.


  1. John al

    I am looking for a simple schematic for a 12volt 5Amp gel battery. I however, want to charge it all year around for emergency use with a possible low amp such as 0.2amp or lower is better. I will put it in my attic just for emergency alarm. It will support an alarm that it has only one SCR and one resister of 15k that I had already made.
    I need help of the 12volt low amp charger.

  2. Jose Aguilar

    I make the circuit , it charges the battery, le transistor limit the current of the circuit, so, the charger always work , but if the battery its very low limits the maximun current, it charger slowly, in general a practical circuit

  3. Ralf

    Realy, the red Wire from the DC adapter to ground????

    • Hello Ralf,
      Thanks for your visit to my site.
      You are great. I just updated this to clearly see.

      Thanks again.

  4. Nemo

    Very interesting circuit but I would like it charge a 20Amp so I need to boost the current.
    I’m thinking of putting a 3055 as shunt transistor across the 317 with maybe a driver transistor for the 3055.
    Would this work?

    • Hi Nemo,

      Do you mean a 200Ah battery? Yes, It is charged with 20A of current. You may use 2N3055 in parallel to boost the current up.

      I also want to try it. But I do not have a battery and transformer. It is a great idea to do!

      Have a fun day with electronics.

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