# Current limiting of battery chargers

#### phoenixyx

Joined Feb 8, 2017
18
Hi,

I am designing a circuit where the output current will be from 500mA and up

I would like to have a constant current of 1A at max

What would be the best way to attack this problem? is there any IC I can use to achieve this limiting of the current with a constant voltage?

Best regards

#### dl324

Joined Mar 30, 2015
14,493
The first thing we need to know is what type of battery you'll be charging.

#### Delta Prime

Joined Nov 15, 2019
1,326
I am designing a circuit where the output current will be from 500mA and up
Is very precise , so much so that you .don't even have to worry about any voltage level requirements.Pick a circuit any circuit it's okay
I'll work with you!

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#### AlbertHall

Joined Jun 4, 2014
12,028
is there any IC I can use to achieve this limiting of the current with a constant voltage?
You can't control both voltage and current simultaneously.
If the voltage is constant then the current will be whatever the load (battery?) draws at that voltage.
If you limit the current then the voltage will consequentially be less than it would have been without the limiting.

#### crutschow

Joined Mar 14, 2008
29,844
is there any IC I can use to achieve this limiting of the current
A simple current limiter can be made with a LM317 IC regulator (LTspice simulation below).
It can be installed between the input supply voltage and your constant-voltage circuit.

You can see the current is limited to about 1A (≈1.25V / R2).

Rload (horizontal axis resistance value) simulates the increase in voltage as the battery charges.
In your design, the point where the current starts to drop is the constant-voltage value from your regulator.

Note the minimum voltage drop across the limiter is about 2.5V.

The LM317 needs to be on a heat-sink that can dissipate (V1-Vout) * 1A, (for whatever V1 is in your design).

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#### phoenixyx

Joined Feb 8, 2017
18
A simple current limiter can be made with a LM317 IC regulator (LTspice simulation below).
It can be installed between the input supply voltage and your constant-voltage circuit.

You can see the current is limited to about 1A (≈1.25V / R2).

Rload (horizontal axis resistance value) simulates the increase in voltage as the battery charges.
In your design, the point where the current starts to drop is the constant-voltage value from your regulator.

Note the minimum voltage drop across the limiter is about 2.5V.

The LM317 needs to be on a heat-sink that can dissipate (V1-Vout) * 1A, (for whatever V1 is in your design).

View attachment 210477
Great, thank you very much

#### Irving

Joined Jan 30, 2016
2,676
An alternative, if you want to go higher amps is a TL431 regulator and pass transistor. Though a linear solution is getting inefficient (i.e. hot) at an amp or so and a switching solution would be preferred.

The TL431 is more than just a regulator, its a current limiter and a voltage clamp. The circuit below will charge a 3.7v Li-Ion at 1A for example, clamping at 4.1v once fully charged and reducing charge current to near zero. U1 regulates the current via sense resistor R1, when battery volts gets to 4.1v U2 takes over and controls the pass transistor Q1. For a fully discharged cell at 2.5v Q1 will dissipate some 4W and needs to be on a heat-sink that's around than 15degC/W, and R1 will dissipate around 2.5W so needs to be a 5W power type.

Advisory: I've not tested this at 1A. There's no reason it won't work but I'd not charge anything smaller than 5Ah at that rate as there is no monitoring of cell temperature. Don't charge anything in your house unsupervised. Always use a fire-retarding charging pouch or substantial metal enclosure.

To be honest, I always use a Revolectrix PL8 charger with my Li-Ion and LiFe packs! The Li-Ion cell Spice charge/discharge model is my own based on one of my packs.

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