# Solar Panel and Battery Management - 3.3V output

I'm developing a power source based on a solar panel and a battery.

Basically, during night or rainy days the power comes from the battery. At sun light the power comes from the solar panel (extra power recharge the battery).

During my search for a buck-boost converter (example TPS63024x) and a linear charger for the battery, I found bq2512x.

Looks like bq2512x has all I need (linear charger, regulated output, load switch and battery voltage monitor), but I'm a quite confusing about the circuit.

• Solar panel is this one -- 5.5V - 90mA
• Battery is this -- 3.7V - 1200mAh
• My device is a Sensor with a Wifi transmitter. Consumes 3.3V - 15mA (200mA peak during 2 seconds, every 5 minutes)

My question is: If my understanding is correct, can someone explain to me the minimum requirements for bq2512x work as I need? Or maybe post a sample circuit with a buck-boost converter and a battery charger in a answer, so we can discuss about it.

• You need $3.3V\cdot 200mA\cdot 2 s = 1.32 J$ every 5 minutes (or 288 times a day) for $\approx 380 J$ per day. That's $4.4mW$. It's very likely that such a solar panel will be able to supply that much even on poor days (though $5.5V\cdot 90mA \ne 0.6W$ as stated on the web page.) The big question is what happens when almost nothing comes from the solar cell because of a very long stretch of bad weather. But the battery holds almost $16,000 J$ so $500 J$ per day (discounting for efficiency) will still last a lot of days. – jonk Aug 27 '16 at 0:04
• @jonk I agree with you. My primary point is the circuit to put it all together. Do you have any sample? I was thinking about use bq2512x, but it looks like very 'advanced'. – BrunoAraujo Aug 27 '16 at 0:09
• I really don't have a specific recommendation or circuit here. Usually, TI will have a demonstration board of some kind that uses the part. You might look for one of those. But I'm not prepared to attempt a design for you. Given the solar panel, the battery, and the load, I think you are in a good position and don't have to carefully balance trade-offs. Even a relatively inefficient conversion to the battery is likely to do okay. So you have options, I think. – jonk Aug 27 '16 at 0:13
• Your solar source has power to waste; so, just protect the battery against overcharging. Most of the bq2512 functions are irrelevant, which is why they are confusing. – Whit3rd Aug 27 '16 at 6:20