How to charge and use a battery powered with Solar Cell Panel

Hi!

Im building a MPPT Solar Charger for a LiPo battery (2 Cells, 7.4V, 910 mAh).

My design is using a Synchronous Buck Converter, that transform the 20 Voc and 0.33 A Isc from the Solar Cell Panel to charge the battery. While I didnt test the design in real situation, the simulation in PSpice is giving me some troubles when I insert a load that has to be powered from the battery.

Arduino will be also powered from the battery, because the Solar Panel will be used just for charging the battery.

Can anyone help me to understand how to power the load while the battery is charing?

PS: If you need more information just ask. :slight_smile:

This is the schematic:

Since I cannot find the schematic, maybe it did not post. I will take a SWAG and say connect the load to the battery. You need to be aware in your design of the charger that not all power is going into the battery.

Buck convertors don't work well with a solar panel, because solar panels are primarily current sources, with the current proportional to the illumination. The combination tends to oscillate or fail completely.

By far the simplest approach is to just have the solar panel directly charge the battery, with cutoffs or current limiting to control the charge as required by the battery chemistry. Then you draw current from the battery.

You need to look carefully how lipo’s are charged - there are some power management chips about for this purpose that will allow charging and supplying a load - but not easy to use .

Using a lead acid battery would make life easier

Post your schematic for a MPPT solar charge controller for LiPo's.

Your solar panels are producing 20V? What is the power rating of your solar panel as well as its real voltage rating loaded and unloaded?

Updated with schematic

Its rated 10W, but in real use is like 6.5W

The project it is planned for a Cubesat (at least the prototip), so that type of batteries arent allowed to be used.

Could you take a look at my recently upload schematic, please? :slight_smile:

Will you be winding the coil or will you purchase one for the MPTT Charge controller.

You state

A LifePo4 will work down to -40C uninsulated.

I bought one of 500 uH

The schematic is incomplete. Please post the details of the PSpice models you are using for the sources labeled Solar, PWM_H and PWM_L.

Yeah, sure!

Updated

Which topology do you recommend me for a Cubesat?

The requirement is that the battery has to supply always the power for the loads, and the solar panel has to charge the battery when the Cubesat is on the sun side.

You are using a voltage source for Solar. That won't work, because a real solar panel is mostly a current source.

Mmm...So, I should use a current source instead a Vdc + Resistance instead? I used a Thevenin equivalent.

You could look up what other people do, and avoid flailing around.

This is one guide to modeling solar cells using SPICE: https://nanohub.org/resources/12259/download/Users_Guide.pdf

Yeah, but that isnt the problem of my simulations I guess...Because with the Thevenin equivalent Im building a current source too.

Good luck with your project!

I wrote here asking for help, not for luck :sweat_smile: