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:
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:
- Constant Voltage.
- Constant Current.
- 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:
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:
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.
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.