# Simple battery charger

#### tnn

Joined Jul 21, 2017
49
Hi everyone,

to charge the 12V - 1,2Ah lead acid battery I found this scheme very simple and unpretentious ... what do you think?

with the V1 trimmer I adjusted the charge voltage to 13.7V as indicated on the battery ....

3 questions:

- does the battery have to be disconnected when I adjust the charge voltage?

- the L2 led lights up when the battery is charged right? above 11Volt ...?

- is a 15V power supply for LM317 okay?

thank you.

Last edited by a moderator:

#### bertus

Joined Apr 5, 2008
22,304
Hello,

You already had a reply on Electronics point in this thread by Harald Kapp:
Simple battery charger

Bertus

#### crutschow

Joined Mar 14, 2008
34,834
The normal maximum charge rate for a Ld-Acid battery is about 0.3 times the battery Ah rating, so would be 360mA for your battery.
To start with that charge rate from a low battery (about 12V), the value of R3 would be (14.6-12) / 0.36 = 7Ω (for a nominal 14.6V charge voltage setting of the LM317).

#### dl324

Joined Mar 30, 2015
17,110
I found this scheme very simple and unpretentious ... what do you think?
It's better than nothing, but that's about it. You don't need R3 or D2 (increase R5 appropriately). I'd probably also get rid of D1 and just remember to not have a battery connected when the charger isn't powered.

For lead acid batteries, you need to charge harder (2.35-2.45V per cell) for a period of time to prevent sulfation. Then switch to float charging. Battery University will have the information.

Charging Information For Lead Acid Batteries – Battery University

#### crutschow

Joined Mar 14, 2008
34,834
Here's the LTspice simulation of another simple charger circuit you might want to consider:
It adds the common TL431 programmable reference, which allows the circuit to provide a constant-current charge of around 300ma (green trace) until the final charge voltage of 14.6V is reached (yellow trace).
That will charge faster than the design you posted with the series resistor (R3), where the current tapers off as the final voltage is reached.

LED D1 turns on when the final charge voltage (14.6v) is reached (red trace) to indicate the charge is essentially complete.
Leaving the charger on for a few hours after the light comes on will help top-off the charge, but don't leave it connected indefinitely as that can overcharge the battery.

To maintain the battery on trickle-charge (13.5V) when not being used open switch S1 to ground.
If you don't need the trickle-charge setting, then you can remove S1 and R3 (connect R6 to ground).

#### Wendy

Joined Mar 24, 2008
23,475

#### LowQCab

Joined Nov 6, 2012
4,290

#### tnn

Joined Jul 21, 2017
49
thanks for the answers ...
but returning to my wiring diagram, I have a doubt that the zenner diode D2 must not be connected after the diode D1?

#### dl324

Joined Mar 30, 2015
17,110
but returning to my wiring diagram, I have a doubt that the zenner diode D2 must not be connected after the diode D1?
The zener diode isn't required. You can eliminate it by increasing the LED current limiting resistor value. Before or after D1 is a nit. D1 isn't required if you don't connect a battery when the regulator isn't powered.

#### tnn

Joined Jul 21, 2017
49
well...
Diode D1 must remain because the battery remains connected when I unplug the charger ...

if I adjust the charge voltage to 13.5V with the battery disconnected,
what value of the resistance for the led do you recommend?

#### BobaMosfet

Joined Jul 1, 2009
2,123
BZX85C11:

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#### dl324

Joined Mar 30, 2015
17,110
Diode D1 must remain because the battery remains connected when I unplug the charger ...
Why does the charger have to remain connected to the battery?
if I adjust the charge voltage to 13.5V with the battery disconnected,
what value of the resistance for the led do you recommend?
$$R=\frac{V}{I}=\frac{13.5V-2V}{10mA}=1.15k\Omega$$
Use 1.1k.

#### crutschow

Joined Mar 14, 2008
34,834
The zener diode isn't required.
It's there so the LED doesn't come on until the battery is near full charge.

#### dl324

Joined Mar 30, 2015
17,110
It's there so the LED doesn't come on until the battery is near full charge.
Did NOT notice that. Also don't understand why such a crude circuit is being considered...

#### BobaMosfet

Joined Jul 1, 2009
2,123
Did NOT notice that. Also don't understand why such a crude circuit is being considered...
Well, I think the TP isn't all that knowledgable about electronics- that's why they want help. They are also unfamiliar with the significant complexities involved in safe battery charging.

#### LowQCab

Joined Nov 6, 2012
4,290