How can i charge 2 3.7v li-ion batteries using 1 power source.

Other Hardware General Electronics
1

Ive never drawn any schematics but to let you better understand my question i added simple schematic, down below. So my goal is to charge 2x 3.7V li ion batteries using 1x 5V source. So i can then power the fan which can take a maximum voltage of 12V (i know it wont always be on since the input voltage is lower). I found some answers on the internet that connecting 2 batteries behind eachother is bad. But i have no idea what i should do, i get mixed answers. I have alot of different charging boards and other electronic components. What would be the best thing to do? Put the batteries behind eachother or connect a charging board to each battery. And when i merge the output pins on the boards wil i get 2x 3.7v (7.4v)? Or will it stay 3.7V but the amp goes up? I am new to electronics so have no idea what is the most efficient thing to do. Thanks for reading.

This is the charging board used in the picture:

But i also have this one that only charges a battery and doesnt give have an output:

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2

I think a better approach might be to use only one charging board and a larger capacity battery. You can obtain 3.7v LI batteries with a variety of mAh. A larger cell, over two smaller cells, might be the better way to go.

I think you should read this:

3

Thanks for the reply, so i read the article a few times. But capacity isnt really my problem i have 3.7v that are 4000mAh instead of the smaller 3.7v 1200mAh. But the fan spins slower with 3.7V instead of the 7.4V. And it isn't easy to find pre assembled 7.4V batteries. Is there an alternative way?

4

Yes there another way. Use just One charging board and One battery, then use a small DC-DC step up (boost converter) to take 3.7v input to give you a 12v output and the fan will run at full speed.

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I presume the fan you're using is a brushless pc cooling fan that consumes below 200mA

5

Hi thanks for the reply that is a good idea. However i should have mentioned i also wanted to power 1 3.3v arduino (can take an input of 12V). I could input 12V but then isnt that a waste? Also can i use 5V on 2 charging board so the arduino has its own battery supply? I prefer that and have a second larger capacity battery for the fan. Here is a modified scematic:

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6

dennis_arduinouser:
However i should have mentioned i also wanted to power 1 3.3v arduino

What else is being powered? is the arduino there just for the sake of it or is it being used to control anything?

Please post more information like. Fan current consumption, arduino current consumption, other components if any.

7

The arduino is used to send a signal to a motor driver. The motor driver is attached to the fan. I dont really know that much about consumption but the fan and arduino works when powered by 1 18650 battery or 16340 battery.

8

dennis_arduinouser:
The arduino is used to send a signal to a motor driver. The motor driver is attached to the fan. I dont really know that much about consumption but the fan and arduino works when powered by 1 18650 battery or 16340 battery.

You should have told this in the beginning.

You cannot select a boost converter module unless you know how much current the fan draws.

Post a picture of your fan, that might help.

9

Thanks for the help so far. Sorry didnt know that but i found the specs:
Color:Black
Size:90x90x25mm
Speed:2300rpm ±10%
Noise(dBA):19.0
Voltage:12V DC
Air flow(CFM):48
Life(hour):50000
Bearing System:Sleeve
Power(W):2.76
Material:Plastic
Wires/Connector:2wires/2x 4pin
Length of wire:20-24cm

10

dennis_arduinouser:
Voltage:12V DC
Power(W):2.76

Current=Power/Voltage
I=2.76/12=0.23A

So your fan needs 230mA to run at full speed. So select a boost converter with at least 500mA output capability.

As for the arduino and the motor driver. What exactly is the arduino supposed to do? control the speed of the fan or just turn it on and off or both?

What motor driver are you using? Your arduino is a 3.3V pro mini variant am I right?

11

The arduino is 3.3v and switches the motor driver on and off. I have a boost converter that can output up to 2A. But isn't inputing 11V into the arduino pro mini wastefull?

12

dennis_arduinouser:
The arduino is 3.3v and switches the motor driver on and off. I have a boost converter that can output up to 2A.

That boost converter will do just fine. Now which motor driver are you using? This IS important.

dennis_arduinouser:
But isn't inputing 11V into the arduino pro mini wastefull?

But you're not inputting 11V into the arduino. Just see you're own diagram in comment #4, you're inputting only 3.7-4.2V into the arduino.

dennis_arduinouser:

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13

A motor driver is wasted on a fan. The benefit of a motor driver is that it can drive the motor in either direction. You don't do that with a fan. You can use a simple transistor such as bc337 to drive a fan, you can even control the speed.

Note that if the fan has 3 or 4 wires, you don't need a driver at all because it will already have one built in. You can identify which wire is the speed control wire and connect that directly to an Arduino pwm pin (via 220R for safety) if you want to control speed, or any digital pin for simple on/off.