I'm designing a small solar charger with these characteristics:
Solar panel with 19% monocrystalline cells
- Voc = 7,50V
- Vmp = 6,00V (Voc -20%)
- Isc = 1,07A
- Wp = 6,42W
Microchip MCP73871 linear 1A lipoly/li-on charger with VPPC capabilities (input voltage regulation loop)
Li-ion 3,7V 6600mAh 24,42Wh
This configuration works fine, but I decided to move to SunPower cells because the solar panels fabrication, when it's needed to cut the cells, in China is made better. Mainly, due to the fact that Sunpower cells have the "back-contact" technology. So, given that the Sunpower cells are more efficient (23%), the solar panel will output more total watts, that an IC such as MCP73871 will actually waste because of its 1A current limit.
SunPower solar panel
- Voc = 7,125V
- Vp = 6,10V
- Vmp = 5,7V
- Isc = 1,35A ◁◁◁
- Wp = ~8,25W
So, I moved my attention to another IC, this time LT3652 (rated 2A). At a first glance it looked fine but then I discovered that it didn't work with a 6V solar panel. It has a VIN_START value of at least 7,50V to start the IC, after that the actual normal nominal operation voltage can be 5,95V.
A solution to this problem could be to cut the SunPower cells smaller to get an higher voltage in the same surface, given that this IC is actually a buck-converter. But someone told me I could really lose efficiency because of the big voltage difference with the input and the battery.
Or perhaps the solution could be to use a different approach with another IC. For example, the ST SPV1040 is a very cool chip with a MPPT perturb and observe embedded algorithm and it looks simple to design. It's designed as a boost converter, so almost any low voltage solar input source is desirable. My idea could be to use two uncut Sunpower cells to reach 1,22V. To build a panel with 2 full cells of 125mm x 125mm should be much easier to design and to fabricate, because it has no waste and there's no need to cut the cells. I will boost this voltage to charge the li-ion battery pack. However I can't understand yet how much current this IC can handle and if it's capable alone of a li-ion charging profile.
Within SPV1040's datasheet I found ILx, a value for the current limit.
Is ILx referred to the also called "switch current" of the inductor?
This project has a limited space available for the solar panel surface, so the whole design could be more complicated. I'm talking about 257mm x 175mm.
The very ultimate goal for this charger is to harvest the most possible amount of watts/time, but any solar-specialized technology such as an active MPPT should be taken into account without to fall in an excessive electric circuit complexity, because I'm not an engineer.
Any effort with this question will be very appreciated. Thank you.