LM338 basic adjustable power supply circuit diagram - Designing a Customized Battery Charger Circuit: Part 1

Designing a Customized Battery Charger Circuit: Part 1

I have designed and published a variety of battery charger circuits in this website, however the readers often get confused while selecting the right battery charger circuit for their individual applications. And I have to explicitly explain each of the readers regarding how to customize the given battery charger circuit for their specific needs.

The entire compilation can be witnessed here:

Battery Charger Circuits

This becomes quite time consuming, since it's the same thing that I have to explain to each of the readers from time to time.

This compelled me to publish this post where I have tried to explain a standard battery charger design and how to customize it in several ways to suit individual preferences in terms of voltage, current, auto-cut-off or semi-automatic operations.

Requirement for Correct Battery Charging

The three fundamental parameters that all batteries require in order to get charged optimally and safely are:

  1. Constant Voltage.
  2. Constant Current.
  3. Auto-cutoff .

So basically, these are the three fundamental things one needs to apply to successfully charge a battery and also make sure that the life of the battery is not affected in the process.

A few enhanced and optional conditions are:

Thermal management. 

and Step charging.

The above two criteria are especially recommended for Li-ion batteries, while these may not be so crucial for lead acid batteries (although there's' no harm in implementing it for the same)

Let's figure out the above conditions step wise and see how one may be able to customize the requirements as per the following instructions:

Constant Voltage:

All batteries are recommended to be charged at a voltage that may be approximately 17 to 18% higher than the printed battery voltage, and this level must not be increased or fluctuated by much.

Therefore for a 12V battery, the value comes to around 14.3V which should not be increased by much.

This requirement is referred to as the constant voltage requirement.

With the availability of a number voltage regulator ICs today, making a constant voltage charger is a matter of minutes.

The most popular among these ICs are the LM317 (1.5 amps), LM338 (5amps), LM396 (10 amps). All these are variable voltage regulator ICs, and allow the user to set any desired constant voltage anywhere from 1.25 to 32V (not for LM396).

You can use the IC LM338 which is suitable for most of the batteries for achieving a constant voltage.

Here's an example circuit which can be used for charging any battery between 1.25 and 32V with a constant voltage.

LM338 basic adjustable power supply circuit diagram 1 - Designing a Customized Battery Charger Circuit: Part 1

Varying the 5k pot enables setting of any desired constant voltage across the C2 capacitor (Vout) which can be used for charging a connected battery across these points.

Although a constant voltage may be necessary, in places where the voltage from an input AC mains does not vary too much (a 5% up/down is quite acceptable) one may entirely eliminate the above circuit and forget about the constant voltage factor.

This implies that we can simply use a correctly rated transformer for charging a battery without considering a constant voltage condition, provided the mains input is fairly dependable in terms of its fluctuations.

Today with the advent of SMPS devices, the above issue completely becomes immaterial since SMPS are all constant voltage power supplies and are highly reliable with their specs, so if an SMPS is available, the above LM338 circuit can be definitely eliminated.

But commonly an SMPS comes with a fixed voltage, so in that case customizing it for a particular battery might become an issue and you may have to opt for the versatile LM338 circuit as explained above.... or if you still want to avoid this, you may simply modify the SMPS circuit itself for acquiring the desired charging voltage.

The next post will explain the designing of a customized current control circuit for a specific, selected battery charger unit.

4 thoughts on “Designing a Customized Battery Charger Circuit: Part 1

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. Hi sir,
    Your blog is very much interesting.
    Sir I need some help to build CONSTANT VOLTAGE OUTPUT,
    As I have 1kw alternator, It gives 0-50vdc output, but it's fluctuating & will be varied from 0 to 50vdc….I need constant voltage for battery charger, so that I can connect FAST BATTERY CHARGER CIRCUIT to that output…..Can I use LM338?………Please suggest me. Your help will be very HELPFUL.

  3. Dear Sir,
    I have recently purchased 2pcs of 6V 4.5Ah SLA and 1pc of 12V 12Ah battery but currently i am not using them.

    I will be using them in the near future as and when i need them.

    I don't want the newly purchased batteries to self discharge, so i wish to keep them on continuous charging system i.e., 24Hrs non-stop.

    What i wanted to know was how much current and at what voltage should i need to apply continuously on the batteries?

    Also is a continuous charging system dangerous to the battery OR one can implement it freely without any issues?

    • Hi Sherwin,

      to keep the battery healthy you can keep it trickle charged using an automatic battery charger.

      you can charge it at a constant current rate which may be at 1/10th of its AH rating and then float charge it with a 1/50th AH rate…. this figure is not too critical but lower current is preferred for the float charge.

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