this is my first question in the forum because i can't find a solution on my own... i hope you can help me. Also english is not my first language so excuse my writing errors.

i design an arduino system to be installed in a foresty truck, it has a series of sensors and microcontrollers and it has to be power on even if the truck is power off. For that reason i know that a need a sort of UPS (uninterruptible power supply) so i get one of this UPS for 18650 li-ion batteries.

18650 UPS Link

Previously, i think that i can use a powerbank to work as ups, but i realise that i cant, the powerbanks can be charged or charge something, but not at the same time. For that reason i take the 4.2 lipo batteries from that powerbank and i connected to the 18650 ups like in this picture.

enter image description here

so, my system is the next one,

enter image description here

The idea is that when the 24V battery is connected, the buck converter converts 24V to 5V and powers the UPS,powers the arduino circuit and charges the lipo battery.

I test this system and it works, but the problem is that the arduino system consumes more current (around 350mA )that the given by the buck converter from the 24V truck battery (150mA), so the system works but the lipo battery is been discharged, so i need a way to take more current from the 24V battery...

I study the 18650 ups module and it uses a tp4056 as lipo charge IC and it should charge up to a current to 600mA... so i dont know why it charges with so low current.

For the buck converter i am using a L2596 module, it can get a maximum input voltage of 42V and deliver up to 3A, i test it with a 12V input and a 10 Ohm load and it gives a current of 500mA, so it can deliver more than 150mA...

I dont know what to do now, for now, i'm simulating the 24V battery with a 42V ebike charger with a buck converter but i dont think that is the problem...

I will be attentive to answer any questions. I hope you can help me, thanks in advance.


  • \$\begingroup\$ I don't understand why you need the LiPo battery if you can connect to the truck's 24V battery. Can you explain that, please? Also, we will need a link to the manufacturer's datasheet or users manual for the UPS, a link to aliexpress (or ebay or amazon) is not very helpful. \$\endgroup\$ – Elliot Alderson Feb 9 at 13:38
  • \$\begingroup\$ @ElliotAlderson , thanks for your reply, the Lipo battery is used to power the arduino system when the truck is off and the 24V battery of the truck is disconnected. this is because the arduino system must work uninterrupted. i can't find a datasheet for the module, it is chinese and very cheap, but i found posts about their internal IC like electroschematics.com/battery-shield \$\endgroup\$ – matzelwow Feb 9 at 14:18

First, TP4056s are limited in their current draw by a resistor on the PROG pin.

PROG Pin 2

What is this resistor value on your UPS?

Rprog Current Setting

Second, where are you measuring current from the truck battery, on the 24V side? P=VI so at higher voltage current will be less, 150mA at 24V is approx 720mA at 5V. That would be enought to supply the Arduino system and charge the battery.

If this doesent solve your problem then, you have already started breaking down your problem, I would encourage you to continue the process.

  1. Can the car battery provide enough power? The battery will likely have no problem with this but as you mention it is disconnected when the car is off there is some circuitry inbetween that may limit this.

  2. While the car is on can it power the Arduino system with the selected buck converter? This is a combination of Q1 and can the buck converter provide enough power at the supplied voltage.

  3. Looks like you are using an external battery pack in place of your 18650. Is there any circuitry there that could be interfering with the logic on the UPS?

  4. What point in the battery charging cycle is it? If the battery is at full capacity or close to it then the UPS may accept less current from the buck converter and draw from the battery as well. Depending on design.

  5. Is there power path management (or similar)? Meaning does all the power for the Arduino system come from the battery and it is charged at the same time or does the UPS allow for power to flow "around" the battery, directly from the buck converter to the Arduino system? Depends on UPS design.

  6. The UPS includes a FP6298 boost converter, are there limits there similar to the TP4056?


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