# 12v lead acid charging circuit

#### Sbr73

##### New Member
hi.
I used a few circuits to charge a 12v-4.5AH lead acid battery but when the battery voltage is 12.9v the charge is no longer.
i am using lm317t for charging for now.
i am feeding lm317 with 19v adapter.
current limiting resistor 2.2R

#### Attachments

• mruwl (1).png
6.6 KB · Views: 31
• Nq2wkBh.gif
12 KB · Views: 26

#### Sbr73

##### New Member
unfortunately charging stops at 12.5v~12.9v

#### rjenkinsgb

##### Well-Known Member
The LM317 data says a minimum of 3V between input and output for it to work.
If you have two, one for current limit and one for voltage regulation, the minimum will be around 6V total.

19V - 6V = 13V, near enough the 12.9 you see.

You need to use low dropout regulators or combine current and voltage limits.

I use a single LM1086CT in a charger circuit, with current sense resistor on the input, a PNP transistor with base resistor monitoring the voltage across that and feeding (via resistors) the base of an NPN transistor connected across the lower resistor in the voltage setting divider, so it pulls down the reg adjust pin when the current limit is reached.

#### Sbr73

##### New Member
I used a low dropout regulator and used another regulator, nothing changed.

#### crutschow

##### Well-Known Member
Below is the LTspice simulation of an LM317 battery charging circuit that uses a transistor to limit the current, which reduces the input voltage requirements:
The yellow trace is the charging current.
It also has an LED to give an indication of the charging rate (red trace).

#### rjenkinsgb

##### Well-Known Member
This is an LT1086CT charging circuit with current limit, as I described above a one-off in a robot project I'm working on:

Do you have suitable size decoupling capacitors at the regulator inputs and outputs? The data sheet examples are concepts and not always functional in themselves, you need to look at all the requirements.

Without the caps, the regulators can oscillate at high frequency and give what appear to be nonsensical voltages..

(There are some 0.1uF ceramic caps underneath that board, not just the visible electrolytics).

#### rjenkinsgb

##### Well-Known Member
The limiting mechanism - pulling the adjust pin down - is the same in mine as Crutschow shows, but my circuit senses the positive input rather than negative as the ground is shared with other devices & I find this configuration more versatile.

A quick sketch without values or capacitors etc., just for the concept:

#### Sbr73

##### New Member
There are capacitors that are not visible in the circuit diagram I shared.
Wouldn't it be better to use lm338t or MIC29302wt instead of lm317t? How can I charge this battery with a simple manual?

#### Sbr73

##### New Member
This is an LT1086CT charging circuit with current limit, as I described above a one-off in a robot project I'm working on:

View attachment 132781

Do you have suitable size decoupling capacitors at the regulator inputs and outputs? The data sheet examples are concepts and not always functional in themselves, you need to look at all the requirements.

Without the caps, the regulators can oscillate at high frequency and give what appear to be nonsensical voltages..

(There are some 0.1uF ceramic caps underneath that board, not just the visible electrolytics).
yes i am using capacitors

#### Sbr73

##### New Member
The limiting mechanism - pulling the adjust pin down - is the same in mine as Crutschow shows, but my circuit senses the positive input rather than negative as the ground is shared with other devices & I find this configuration more versatile.

A quick sketch without values or capacitors etc., just for the concept:

View attachment 132783

Since I produced pcb circuit board before, I cannot use other different circuits as I want.
IC i can use: lm338t, mic29302, or tıp122, tıp142 transistor

Replies
2
Views
258
Replies
2
Views
255
Replies
3
Views
2K
Replies
3
Views
2K
Replies
6
Views
1K