How to use Solar LiPO Chargers

Updated 08 November 2018

Every day, Solar energy bombards Earth with about 173,000 terawatts of power. That's close to the yearly electricity bill for Planet Earth. Solar energy is free power from the stars. So the question stands, why aren't we using it more? Price and space are the two common arguments, understandable if you are trying to power a metropolis. We aren't doing that though, we can power an electronics project on sun juice quite easily; let us show you how.

The way we harness light for electricity is we sandwiching semiconductor materials together, and let light strike it. One material will have stacks of electrons heaped together, and other will have tonnes of space for electrons. Light energy kamikazes into the panel, driving into our stack of atoms at high energies. These collisions cause energy to transfer into our electrons, and they get all excited jump out of our stack. We connect our circuit right here, allowing these freedom-seeking electrons to get out and do some work. Our semiconductor (the other one) connects to the other side of this load pulling the free electrons through.

It's easy enough to understand, and the bright side is now we can utilise the method for solar powered projects. Instead of relying on our laptop's USB port or the power outlet at home, we will use a solar panel. Additionally, we need a circuit manages the dynamic range of voltages and currents generated by said panel. An understanding of what is going on behind the scenes might help us out in that area.

USB Solar Charger circuit board labelled with LiPO Attached

Why Battery Charging?

If you read in between the lines above, you don't directly power your projects from a Solar panel. Well, you can try, but it's not reliable enough to consistently power something. Instead, you use circuitry to regulate the panel power output into a stable voltage, then top up your battery with that. Think of it as topping up a bucket of water with a dripping tap (Charging), while pouring yourself a glass of water from a tap at the bottom of the bucket (Applying a load). To get on the same page as us when it comes to which battery is suitable for project power; take 5 minutes to scan over our previous articles on the topics:

In fact, you can probably just read the headlines to get the gist. The best batteries available in maker electronics at the moment are Lithium Polymer Batteries. It's due to their higher energy density, smaller size and standard voltages options. There are a plenty of choices for charging and regulating those batteries too, but only a select few when it comes to solar charging them.

We're going to go green with the USB/DC/Solar LiPO Charger from Adafruit. It's a Plug-n-play charger for any 3.7V battery and only requires a 6V Solar panel. A 6V Panel is a standard sized panel, made up of 12 x 0.5V cells! Let's take a closer look at what makes it the best option for charging your LiPO batteries.

MPPT and Charging batteries

USB LiPO chargers use a stable voltage and current to charge a battery, 5V @ 500mA for example. Essentially, the power system here is static, drawing X current at Y voltage; taking the battery Z time to charge fully.

If we use a Solar Panel as the power source instead of USB, the Voltage and Current will always shift. As the amount of light hitting the panel will be dynamic. Using a regular LiPO charger with a solar panel will result in our circuit flat-out attempting to take the current required; regardless of the Current available. An overdraw in current will lead to a supply voltage collapse; our charger will simply become unstable and turn off. To get around this problem, we can use Voltage Proportional Charge Control to Ensure that our circuit will only provide current when it can conserve the voltage level. Ideally, we can set that voltage to our 3.7V LiPO charge voltage (4.2V).

Then we add a hefty capacitor to the circuit, helping to stabilise any over current draw. All of a sudden, you've got a very affordable solar charger for a LiPO battery. This method also bypasses issues regarding DC-DC converters, such as losing a lot of power as heat and doubling the price of a module.

That's the reasoning behind Adafruit's USB/DC/Solar LiPO Charger. This module also has load sharing and USB charging; it's a complete charge (and use) solution for Solar Powering your project!

STOP! Before you harness the sun...

Go ahead and get out a 6V solar panel, a solar charger module and a LiPO battery. There are a few quick steps you need to take. First, solder in your capacitor. Next, think seriously about adding a 10k Thermistor to the board.

The thermistor will cut off the circuit if we hit temperatures of over 50 degrees Celcius. An important point if you want to prevent your battery from exploding. Connect them up as shown, and you will be good to go!

Setting up a solar power system for powering a electronics project is just that easy! You should have a good idea of Solar Cells, the ways we connect them to electronic circuits and the advantages and disadvantages of using them to power projects. If you decided that solar energy isn't the best way to power your project, we have got you covered with tutorials on a bunch of other project powering methods:

Alternatively, if you want to know more about Solar Power, in particular, let us know in the comments below! I'd be happy to expand on this topic some more in the future.

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