Electronic – arduino – Is it okay to connect 2 lithium ion battery in parallel to a charger that can maintenance 4.2V? (more detail in post)

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. :-)

I wouldn't connect any battery to your phone but the one designed for it. The charging circuit is designed for the specific battery.

You can technically probably connect a Li-Ion and LiPo battery in parallel with a properly designed charger, but it is not recommended. Usually only identical cells are connected in parallel or series.

One big issue is current hogging in the constant-current charge phase. Initial charging of a lithium ion battery is done at constant current, generally expressed as some rate like "1C" which means charging a 2500 mAhr battery at 2.5 amp (i.e., a one hour charge rate). If you put two cells in parallel, you might think you could increase the charge rate, but unless the cells are identical, one of them may hog most of the current. So if you are charging with 5A, it might be that 1.5 amp goes to one battery and 3.5 amp goes to the other: exceeding it's safe limit.

Another issue is with charging voltage. Lithium ion chargers switch to constant voltage mode when the battery gets to a certain pre-set voltage, nominally 4.2 volts. However, different anode types have slightly different voltages, in the 4.1-4.2 range. If you look at the data sheets for li-ion charging circuits, they usually have some mode selection to trim the voltage for slightly different cell types. If you connect two batteries with different voltages in parallel, you could overcharge one.

Finally, lithium ion chargers are designed to stop charging when the constant voltage phase drops to a certain minimum current to avoid trickle charging, which will cause overcharging and cell damage. If you put batteries in parallel, that threshold needs to be adjusted. If the batteries are not identical, that could be off.

In practice, if you set your charger to charge at the rate of the smaller cell, and you make sure that the maximum voltage of the two cells is the same, nothing bad is likely to happen. However, charging will take about twice as long as if you used identical cells, so this is not usually done. Multi-cell battery packs are usually made from brand new identical cells ideally from the same manufacturing lot, and permanently connected so that they age together.