# Car battery charger with LM741

#### danmih83

Joined Nov 30, 2020
15
Hello,
I have a car battery charger that uses a schematic similar to this one. It is basically a voltage stabiliser set at 14.4v. The design works very well: the charge current decreases constantly and when the battery reaches 14.4v it is charging at about 100mA. The battery does not boil.

The original schematic did not have the led in series from the output of 741 to the base of the first transistor. When the led turns off, the charging is finished.

Instead of this, is there any posibility to do the opposite? Turn on a led when charging is completed? I have tried to make a match from a zener diode and a resistor, but the led stays on at a certain level of luminosity and you really can't tell when charging has finished.

#### MrChips

Joined Oct 2, 2009
26,112
The LM741 is serving as a voltage comparator.
Remove both LEDs.
Add transistors to drive the LEDs.

#### Ian0

Joined Aug 7, 2020
5,147
With that circuit, charging is never completed. The battery remains on float charge indefinitely at 14.4V (which is rather too high). Charging will only terminate when all the water is electrolysed.

#### crutschow

Joined Mar 14, 2008
29,800
Remove both LEDs.
The LED at the transistor base is needed so that the minimum output voltage of the 741 (about 1.5V above ground can turn off transistor.

#### MrChips

Joined Oct 2, 2009
26,112
The LED at the transistor base is needed so that the minimum output voltage of the 741 (about 1.5V above ground can turn off transistor.
In that case, replace the LM741 with a proper single supply voltage comparator.

As AG would say, stop using the lousy 741 which is a 50-year old design.

#### crutschow

Joined Mar 14, 2008
29,800
I simulated that circuit and it does not work.
There is no feedback from the battery voltage so the charge current varies very little with the battery voltage as it charges.

So post your exact battery charger circuit, not one that is "similar".
How do you know it's similar?