# Battery Charger with Auto Cut-off Project

#### tphuogg

Joined Apr 24, 2021
19
Hi,
I need help on my battery charger project.

The requirement of this project is that the battery charging current has to be between 400 mA - 500 mA. Once the battery is fully charged (13V - 14V), the Iout has to be reduced around 150 mA and below.

So far, I figured out R4 = 12 ohm as I am able to charge the battery around 450 mA constantly. However, I still can't figure out how to reduce output current when the battery is full.

What I am doing right now is to place potentiometers at R1, R2 and R3 and a 13V Zener Diode at Z1. The best Iout I got until now is 190mA which is still very high.

My question is what is the best approach I can use to analyze this circuit to get the resistor values. I don't want to connect the battery and twist the potentiometers anymore since I already hear the boiling sound in my Lead-Acid 12V battery. I used Simulink to simulate this circuit, but the practical values are so different.

Thank you.

#### crutschow

Joined Mar 14, 2008
32,926
Note that circuit does not reduce the charge current when battery is charged, it only limits the maximum charge voltage, but that will have the effect of limiting the current also.
You need to remove the battery from such a charger when the battery is charged, otherwise the battery will tend to overcharge.

#### Ian0

Joined Aug 7, 2020
8,407
Note that circuit does not reduce the charge current when battery is charged, it only limits the maximum charge voltage, but that will have the effect of limiting the current also.
You need to remove the battery from such a charger when the battery is charged, otherwise the battery will tend to overcharge.
If it is set for 2.27V per cell and temperature compensated, and it's a sealed battery then it could be left on float-charge indefinitely.

#### crutschow

Joined Mar 14, 2008
32,926
If it is set for 2.27V per cell and temperature compensated, and it's a sealed battery then it could be left on float-charge indefinitely
True.
But the typical charging voltage is about 2.4V per cell, and the proposed circuit does not reduce the voltage from that voltage to the float voltage (which is not sufficient to fully charge the battery).

#### tphuogg

Joined Apr 24, 2021
19
Note that circuit does not reduce the charge current when battery is charged, it only limits the maximum charge voltage, but that will have the effect of limiting the current also.
Yes, that is exactly what my professor was saying. However, he also said if we tune the circuit correctly, we can leave the battery on float charge. It does require a lot of tuning. Can you please suggest me a simple circuit that can limit leave the battery on float charge when it is full to fulfill my project?

Last edited:

#### crutschow

Joined Mar 14, 2008
32,926
he also said if we tune the circuit correctly, we can leave the battery on float charge.
You can't "tune" the circuit you have for that.
You would need an added circuit that will lower the charge voltage when the battery reaches full charge.

One way would be a comparator circuit, that triggers a latch to change the voltage resistor value in the LM317. and lower the voltage when the upper charge voltage is reached (typically from about 14.4V to 13.6V).

You would also need a power-on reset to the latch to initially put it in the high-voltage charge state.
A simple RC pulse circuit can typically do that.

#### Ian0

Joined Aug 7, 2020
8,407
Generally the switchover occurs when after a period of absorption charge, when the current has fallen to 0.05C or some other low level with the voltage at boost charge level 2.45V/cell.
However, there's a snag - as batteries age, the current ceases to fall that low, with a result that the charger stays in boost until the battery is ruined.
I use an algorithm which changes over when the current has not fallen any lower for 4 minutes, but it needs a MCU to implement it.

#### tphuogg

Joined Apr 24, 2021
19
Hi,
I need help with my 12V battery charger project.

The requirement of this project is that the battery charging current has to be between 400 mA - 500 mA. Once the battery is fully charged (13V - 14.6V), the Iout has to be reduced around 150 mA and below. V no load ( before plugging the battery) = 18-20V.

Please suggest me a circuit that can satisfy those requirement. I was trying a the circuit below, but the result was not as expected.
Thank you so much.

#### crutschow

Joined Mar 14, 2008
32,926
Why did you repeat your original post?

#### ericgibbs

Joined Jan 29, 2010
18,051
hi C,
The user started a new Thread, so I merged it into the earlier Thread.
E

#### tphuogg

Joined Apr 24, 2021
19
Why did you repeat your original post?
I am so sorry, I didn't mean to duplicate the post. I was trying to ask for a visual circuit that I can follow. I am not good enough to understand literal instruction yet.

#### crutschow

Joined Mar 14, 2008
32,926
Can you use an IC such as an LM317 regulator in your design?

#### tphuogg

Joined Apr 24, 2021
19
Can you use an IC such as an LM317 regulator in your design?
Yes, I can use LM317. My project is to use a 120V-24V CT transformer to connect to a bridge rectifier and then filter it with 1000 uF. The output = 34VDC.
Then I use LM317 to step down the voltage to 20V for the battery charger.

#### crutschow

Joined Mar 14, 2008
32,926
Yes, I can use LM317. My project is to use a 120V-24V CT transformer
Use the CT transformer output with a two Schottky-diode full-wave rectifier, which will give over a little over 16Vdc, to minimize power dissipation.
Then use the LM317 to directly charge the battery with the appropriate voltage and current-limit.
Below is an example from the TI LM317 data sheet:

And here are some from a Google search.

#### tphuogg

Joined Apr 24, 2021
19
Use the CT transformer output with a two Schottky-diode full-wave rectifier, which will give over a little over 16Vdc, to minimize power dissipation.
Then use the LM317 to directly charge the battery with the appropriate voltage and current-limit.
Below is an example from the TI LM317 data sheet:

And here are some from a Google search.

View attachment 237318
My professor requires to have a V no load = 18-20V for 12V, so 17V is not enough.

Do you think the 6V and 12V battery charging characteristics are the same.

I guess I will have to switch to 6V lead-acid battery. I plan to use my dead 12V battery to observe the charging current. Then I will buy a new 6V battery to observe the current when the battery is full.

Thank you.

#### crutschow

Joined Mar 14, 2008
32,926
My professor requires to have a V no load = 18-20V for 12V, so 17V is not enough.
Okay, the go with your original plan.
Do you think the 6V and 12V battery charging characteristics are the same.
If they are both lead-acid batteries, yes.
The only difference is the charging voltage.
I guess I will have to switch to 6V lead-acid battery.
Why?
You just adjust the charging voltage to what you need.

#### tphuogg

Joined Apr 24, 2021
19
Why?
You just adjust the charging voltage to what you need.
So far, the Lm317 is still very hot due the huge voltage regulation even though I stacked 2 heatsinks together. It should fine since it is touchable.

The Vno load is measured to be 20.2V which is ok. The charging current (when the battery is not full) is 0.460 A.

The problem I am having for weeks is I am unable to turn on the NPN when the battery is full. As a result, the battery voltage get ramped up to 16V.

I know it took you so much time, but can you show me the way to calculate the bjt.
Thank you.

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#### crutschow

Joined Mar 14, 2008
32,926
I know it took you so much time, but can you show me the way to calculate the bjt.
Your BJT is trying to regulate by shunting the excess current to ground which will cause high current and power dissipation.
You need to put the BJT in series with the output with the emitter towards the battery and the Zener connected from the base to ground.
The emitter output voltage to the battery would then be about 0.7V greater than the Zener voltage.
You would also need a base current bias resistor between the collector and emitter to provide the necessary base and Zener current.

Sample circuit below:
You need to calculate the value of R1 to give a nominal Zener current of perhaps 5mA and a base current which is the maximum charging current divided by the minimum current gain (beta or hFE) of the transistor you are using.
You can do that using Ohm's Law, Using the Pwr and D1 voltages to give the voltage across R1.