Where Did my Power Go? 18650 Batteries and Buck Converter

Projects General Guidance
1

Hey everyone!

I'm doing a project where I want to use 2x18650 batteries in series to power 5V LEDs.

Here's my basic strategy:

  • Charge batteries and limit discharge using a TP4056 module.

  • Output from TP4056 goes to input of an XM1584 buck converter, which steps down voltage to 5V.

  • Output from the buck converter goes to my LEDs.

Here's what happens with the meter, though:

  • Output from battery terminals - 7.4V (as expected- 2x3.7V batteries in series)

  • Output from TP4056 terminals - 7V (a little voltage drop)

  • Output from buck converter with nothing drawing power in the circuit - 4.3V (no matter how I move the pot)

  • Output from the buck converter if I add an Arduino Uno to the circuit - .7V (and no power to the Arduino no matter how I move the pot)

What's going on? :slight_smile:

Thanks in advance!

Battery module and buck converter images attached.

tp4.jpg
tp4.jpg1024×639 152 KB

buck.jpg
buck.jpg800×800 85.5 KB

2

Is the battery voltage still 7.4 Volts when the buck converter is putting out 0.7 Volts?
Do you get the same results without the TP4056, in other words, connect batteries direct to buck converter?
What happens if you apply power from a 12 Volt wall wart?
You may have a bad buck converter...

3

Due_unto:
Is the battery voltage still 7.4 Volts when the buck converter is putting out 0.7 Volts?
Do you get the same results without the TP4056, in other words, connect batteries direct to buck converter?
What happens if you apply power from a 12 Volt wall wart?
You may have a bad buck converter...

  1. Yes

  2. I'll try.

  3. I can try that, too.

Thanks

4

Looking at the picture of your buck converter shows very little labelling, only a + & - on one side. On the back of the board there is usually an arrow which indicates the flow of power. The arrow points to the output side of the converter. Just make sure you don't have it wired backwards.

5

Due_unto:
Looking at the picture of your buck converter shows very little labelling, only a + & - on one side. On the back of the board there is usually an arrow which indicates the flow of power. The arrow points to the output side of the converter. Just make sure you don't have it wired backwards.

Yes, you're right. There is a flow marker on the back. I do have the flow going in the right direction, but I just noticed something else now:

My input and output solder points are parallel with each other, but In/Out (+) are on the inner track, and In/Out (-) are on the outer track (there are two "tracks" for each group as you can see in the photo).

Would this matter?

6

I believe the pairs of pads in each corner are the same, just two connection points for each terminal. Also, the IN - and Out - are connected together.
With the batteries connected to the buck input, are you able to get your desired output?

7

A lithium cell only gives 3.7 v when it nearly completely discharged. It should be at more than 4v when charged.

perhaps you should charge the cells.

Allan