# Li-Po/Li-Ion solar charger and 3.3V stabilized output

I'm working on a project with ESP8266 and I want to power it with solar panel and a Li-Ion/Li-Po backup battery. I searched on google and I found a buck/boost regulator is required to stabilize battery voltage to 3.3V, because it can be higher or lower than that. While searching online circuit I found usually are chargers only, buck/bost regulators only or both but with 5V out.

I need 3.3V and ~300/400mA max.

So my question is: does anyone have a simple and well tested schematic to achieve that?

I'm planning to embed it into the final PCB, so if possible I prefer pure schematics instead of ready-to-use PCB (like Adafruit, SparkFun, etc).

• You probably don't want to discharge a Lithium battery too much below 3.3V, so you might only need a buck regulator. The solar panel adds a wrinkle though. I assume it would not be able to power the device on it's own, so it is used to charge the battery, in which case you may have a boost/buck converter or MPPT for charging. – AngeloQ Feb 7 '17 at 21:10

I suggest you to first consider:

1. Estimate of how much power your application may draw from the battery.
2. Space constraints, if you have some, in order to better select the solar panel.
3. A DC-DC converter is the best choice when converting energy, because the switched components can achieve a very high efficiency. The most applications in solar are build with a buck converter, because the solar panels has usually more voltage than batteries voltage.
A buck-boost converter instead, is a device that can adapt a both higher or lower input voltage to the output fixed value.

I suggest you to consider open-hardware projects first to make a prototype and select the components (battery, solar panel).

Then you can look for the free schematic files and study the circuit for the design of the embedded PCB. Remember: even if you have a schematic you need skills to design a PCB layout, especially for switched circuits.

A good charger is SunnyBuddy from Sparkfun, sporting a Linear Instruments LT3652 switched buck charger with MPPT (passive) capabilities. It's a well documented project. But it needs a solar panel capable of at least $7\text{ Voc}$ to start properly.

If you want to have your life easier you can select an easy-to-find $6\text{ V}$ solar panel and use it with a linear charger with good efficiency, specifically hacked for solar. Here you can find the project based of Texas MCP73871.

Now you have a charger. You will need also a device to lower the battery's voltage to feed the ESP8266. You may consider an LDO component to simply lower the voltage from $3.3\text{ V}-4.2\text{ V}$ to $3.3\text{ V}$. This is the most simple choice but not the more efficient.

Or you can opt for another DC-DC buck converter. This Adafruit project has a nifty $3.3\text{ V}$ output with a 90-95% efficiency.

You can ask Adafruit support to provide you the schematic as it is not published yet.

• Thanks! I'm thinking to mix up the MCP73871 (to charge) and a regulator for the ESP8266. I have two questions: why do you mention a "buck" only regulator? (it will not work with voltage lower 3.3). The other one: the Adafruit circuit seems very cool, but is quite expensive, so may be ok to prototype, but does exist any similar circuit at lower price? – Noisemaker Feb 8 '17 at 12:56
• A Li-Ion battery will go down only to 3.2 V. You can consider to waste that 0.1 V or to move to a buck-boost regulator that has a more complicated circuitry. – Nic1337 Feb 8 '17 at 15:49
• Adafruit is not low price. Try to take a look to Seeed Studio projects, but don't expect half prices ;) – Nic1337 Feb 8 '17 at 16:07

As for your 3.3V you might even want a buck-boost converter, as your input voltage range includes the desired output. Eg Pololu S7V8F3