Greetings!

I'm working on a project about Li-Po battery charging from a solar panel, using MPPT algorithm.
My specs:

PV: ​

• Vmp = 17.49 V
• Imp = 0.57 A
• Pmax = 10 W
• Isc = 0.61 A
• Voc = 21.67 V

Battery:​

• V = 3.7 V
• Capacity = 1700 mAh

I'm using a buck type DC-DC controller with P-MOS and controlling it from a microcontroller.
My problem is I don't know how to charge the battery.
I was thinking about the following solutions:

1. Using a charger IC at the output of the converter
2. Using a voltage regulator (4.2 V) at the output of the converter
3. Using directly the converter

I would like to use the CCCV charging method, but my another problem is that if the sun gives not enough power, I stop the charging but what if the charging is at the CV mode. How do I know that should I continue the charging. Or the sun gives about C/10 amps (it is typically the end of charge current).

I'm sorry if my language is not perfect.
Best regards.

 

This is a problem with such chargers. Several years ago I designed a charger for military lithium ion batteries with exactly this problem. I concluded that for that particular application I would simply limit the current to a level that was safe for the batteries and then transition to constant voltage charging. Normally the batteries would not remain on the charger for more than a day, and it isn't seriously detrimental to keeping a lithium ion battery on "float charge" for that long. The current will typically drop to a very low and safe level when the voltage is accurately controlled. In my application it was considered important to charge the batteries very fully for maximum operating life of the equipment. If I were doing it where getting maximum operating time were less critical, I would set the maximum voltage to no more than 4.1 V and maybe even down to 3.9 or 4.0 V. Lithium ion cells are not harmed by being undercharged, and in fact you will get more ampere hours over the full useful life of the battery by undercharging. It is very important to avoid going over 4.2 V for more than a very brief period. Even a few cycles of charging to excessive voltage can significantly shorten the life of the battery.

You will need to use an accurate voltage reference for controlling charging. Depending on the microcontroller you want to use, this may mean using an external voltage reference. Some have an internal reference that is quite good. Don't use the power supply voltage of the micro as a reference. It is not adequate for the task. You can probably directly regulate the voltage with the main buck converter, but you will need to be sure that the battery voltage is checked very frequently. Once you get near the end of charging where the current is dropping, the usual buck voltage regulator equation (Vout = Vin x duty cycle) will no longer apply (it only applies if the inductor current does not drop to zero each switching cycle, and at low current it will drop to zero unless you use a large amount of inductance). You could use a low dropout voltage regulator after the switcher, but since you can deliver more than 2 amperes at 4.2 V, you probably would not be able to buy a simple three-terminal regulator that will give low dropout and that much current. However, 2 amperes is probably more than your 1700 mA·H battery should be charged with, unless it is a type that is specified for high charge rate.

 

Thank you!
I was thinking on the overcurrent problem before and I've decided to control the PV on a lower powerpoint (closer to the Voc) so It would not produce the whole ~10W on a sunny day, only the less bright hours.
Is it possible to use a 1 A charger IC?
Could it work on low current charging?