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Basic Schematic for the circuit and measure point

I am trying to get a 5V output out of a 2S 18650 cells (getting a 7.5V from them.)

I am trying to use a LM2596 module, but as soon as the battery is connected to the LM2596, the input reading goes down to 2.5V. The LM2596 does not work with this voltage as expected. I also tried using an Arduino 5V as the input to the LM2596 and it works as expected, bringing the voltage down. This is being tested without any load at the output as I have to first regulate it.

Why does this happen with the 18650 batteries?

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    \$\begingroup\$ Could you post a schematic? Did you measure the voltage directly at the poles of the batteries? \$\endgroup\$ Commented Jun 13, 2022 at 20:32
  • \$\begingroup\$ If you are clamping down you batteries to 2.5 V, you will either have a fire very quickly or the batteries have tremendous high ESR. \$\endgroup\$ Commented Jun 13, 2022 at 20:47
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    \$\begingroup\$ @winny Or the battery protection system doesn't like what you're doing, and acts as a very high ESR. \$\endgroup\$ Commented Jun 13, 2022 at 21:01
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    \$\begingroup\$ Some batteries have 18650 type, but may have protection chip inside. They may be longer, approximately 70 mm. Protection may be activated. \$\endgroup\$ Commented Jun 13, 2022 at 21:09
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    \$\begingroup\$ Exactly which 7.4V 2S 18650 battery pack do you have, and where did you get it from? Does it have a PCM (protection circuit module) inside? Show us a photo of your setup. \$\endgroup\$ Commented Jun 14, 2022 at 0:43

2 Answers 2

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This fake 18650 battery cell is why we ask where did you get them from: fake

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  • \$\begingroup\$ i got it in a brazilian equivalent of ebay: produto.mercadolivre.com.br/… the seller has supposedly good reputation. i cannot however see wether it has protection or not. \$\endgroup\$ Commented Jun 14, 2022 at 2:29
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You have two Lithium-Ion rechargeable battery cells in series but they are not charged. They will produce 8.4V when they are properly charged.

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