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I have this circuit built and tested. I think this should work and charge the battery with 0.4A to about 13.5V.

The Problem: it doesn't charge and between U1 and U3 (Battery) is a current of about 0.03A, but why?

The LM2596 is a Buck converter Breakout board and that works flawlessly. R1 is a 4W resistor so that won't be the problem

Why doesn't it charge?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Why do you put the Buck converter in between? Just to get rid of that one volt? \$\endgroup\$
    – jusaca
    Aug 7 '19 at 11:23
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Read the datasheet of the LM317, on page 9 it states:

enter image description here

So when you feed the LM317 14 V it can regulate to 11 V and lower, not 13.5 V.

Also there will be 1.25 V across R1 so for 13.5 V you will need to put at least 13.5 + 1.25 + 3 = 17.75 V into the LM317.

The ~15 V you're feeding the LM2596 board isn't even enough, there's no need to have that LM2596 converter in place so remove it.

You will need a power source with a higher voltage than ~ 15 V.

As the LM317 will drop 3 V or more at a significant current, it will get hot so use a heatsink! If the LM317 gets too hot it lowers the current to lower its power dissipation (and allow it to cool down).

Note that your circuit does not have a well defined "stop charging" voltage, current will keep flowing and your battery might over charge!

I have built an LM317 based battery charger for my 12 V car battery. I use a 19 V laptop power supply I had lying around to power it. In that design I do not use the LM317 as a current source, instead I use it as a voltage regulator set to 13.5 V. Then when the battery has a lower voltage, the LM317 will hit its build-in current limit (< 2.2 A). For a car battery 2.2 A or less is fine. As the battery charges an the voltage reaches 13.5 V, the current gets smaller and smaller until only a leakage current is left.

If that 2.2 A is too much for your battery, use this circuit instead:

enter image description here

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You need more than 14V at input if you want to charge a battery to 13.5V. About 18V should be enough. LM317 needs about 3V between Vin and Vout to work. And there will be 1.25V between Vout and Adj. And then the battery will have 13.5V. 13.5+1.25V+3V is 17.75V.

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The problem you have is caused by the LM317 and the voltage you have available to it.

The LM317 requires a difference of 3V between the input and output voltage. You have 15V from your powersupply (which you drop to 14 with the LM2596.) That's only 1.5V above the output voltage (13.5V) that you need for the battery.

You really only have two choices:

  1. Use a higher input voltage (at least 17V)
  2. Use a different current regulator that can work with the available voltage.

In any case, the LM2596 is only making things worse.

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Also, if the power supply is switched off and the 12V battery is still attached, there's a negative voltage difference (Vin-Vout) on the LM317, a situation not mentioned in the data sheet as far as I can see. If that causes a leak current, it would drain the battery.

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