# Charging board for 3 battery 3.6V used for supply an Arduino board

I would like to build an autonomous system that contains: - An Arduino board (later, it will be just the chip) - A LED strip

I decided to use 3x3.6V lithium batteries in series. So, the idea is to have the 3 batteries in series that supply the Ardiuno board and the LED strip in parallel.

Now, my problem is the charging the batteries.

After research online, I see that the best solution would be to charge them in parallel. There are many dedicated charging circuits on eBay or aliexpress for charging at 4.2V, is that good ? To be able to have a good charging time, I need a output current of 2 or 3A, right ?

If this solution is good, when a will plug the 5V input and charge batteries, there is no risk for LED strip and ardiuno board ?

Or I will have to open the circuit when I plug my supply ?

When I will plug my supply, it will apply 4.2V on each battery. So, in series, I will have 3 x 4.2 = 12.6V. So, I need to check if LED strip and arduino if ok with that voltage, right ?

For arudino, it should be ok, for LED strip, I'm not sure, the normal voltage is 12V.

Note that you cannot just put lithium cells in parallel without balancing them first (to make sure they are equally charged).

Also, you cannot just charge Li-cells by applying a voltage, if you do that, don't be surprised if the cells get too hot or even explode ! You need a dedicated charging circuit or module.

But let me propose a much simpeler solution: Just balance your cells, then connect them in parallel. Charge them using a charging module, feed that module form a 5 V power supply. These modules can easily be found on ebay.

To make the 12 V for the LED strips, use a DCDC boost converter. On ebay you can find these and most have a trimmer to set the voltage to the 12V you need for the LEDs.

You can also feed the arduino from this 12 V, make sure you connect the 12 V to the power adapter socket (Vraw) so that the voltage goed through a regulator which will supply 5 V or 3.3 V (depends on your Arduino module) to the microController.

You can make it extra fancy by monitoring the battery voltage through a voltage divider and feed that to the Arduino. When the battery voltage gets too low then the Arduino can detect that and switch off the LEDs so that you know its time to charge the batteries. :-)

• Thanks for your response. Yes, I was thinking to use dedicated charging circuit (found on aliexpress). I need to inform myself on the balancing, I don't known anything about it. Maybe a solution could be to buy one charging circuit for each battery ? About your much simpler solution, you mean I can't use batteries in series for LED and Arduino, and in parallel for charging them ? So I'm obliged to connect them in parallel for charging them and supply Arduino and LED (through a DCDC boost converter) ? Commented Oct 18, 2015 at 9:16
• I''m not saying you CAN'T put the batteries in series, it is just more complex when you place the batteries in series. To charge batteries in series properly you need a more complex charging circuit especially if you want to charge quickly. That charger has to monitor the voltages across each cell individually. It's a lot of hassle. The solution I propose is so much easier to implement and also has the advantage that you can use as many batteries as you like. But you have to balance them first, this can be easily done by connecting them in parallel WITH A RESISTOR in between. Commented Oct 18, 2015 at 10:11
• The resistor is needed to limit the current, a 10 ohm 1 Watt resistor should do the job. Leave that connected for a couple of hours so the cells with a higher voltage will charge the more empty ones. In the end all cells must have the same voltage then you can place them in parallel without the resistor. Commented Oct 18, 2015 at 10:13
• Note that you CAN'T use the batteries in series, light the LEDs, and then when empty charge them in parallel ! Because when in series the cells are not balanced. Before connecting in parallel, you MUST balance !!! Or you can charge the cells individually but that's just too much work. Don't discharge in series and charge in parallel, that's too complex and error-prone. Commented Oct 18, 2015 at 10:18
• Thanks for yours comments. So, if I understand correctly, the solution your propose is to use batteries in parallel and a DCDC boost module (3.6V to 12V). But, which I don't understand is the balancing. In parallel, I will have to balance them, but just the first time ? If one battery use itself more quickly than the others, it will not create problems ? Commented Oct 18, 2015 at 17:54