Solar Charging

Solar charging is easy, don't forget to prepare your solar panel and solder in the electrolytic capacitor beforehand!

Once you've done that, you can simply plug in the solar panel into the DC jack - look for the  PWR GOOD LED to indicate that the solar panel is providing power and then plug the battery into the BATT slot in the left. Use only 3.7V/4.2V lithium ion/polymer batteries.

We have a tutorial about the batteries in case you have some questions about how to use them.

When the CHRG charging light is lit, the battery is being charged. Make sure to have the panel facing direct sunlight not shaded and not behind any glass or plastic! when the battery is full, you'll see the green DONE LED light up.

USB & DC Charging

Of course, sometimes is just really dark out and you can't solar charger, so there's a USB port on the board as well. Use any mini-B cable to plug in and charge.

If there is something connected to the DC jack, it will mechanically disconnect the mini USB connector so be sure to unplug the solar panel when USB charging
You can also connect a DC wall plug adapter directly into the jack, just make sure its a 5.5mm/2.1mm inner diameter connector, which is very common!
If you need the DC power for something else, you can also connect to the DC input via the 0.1" breakout. While you can feed power into the breakout pins as well, it's not polarity protected so make sure you use a schottky diode or just be really careful.

Indicator LEDs

There are three status LEDs on the charger, which you'll find very handy!

The red PWR LED indicates that there is good power connected to the charger. If this LED is not lit, something is wrong with the power supply

The orange CHRG LED indicates current charging status. When this LED is lit, the charger is working to charge up a battery! It also acts as a low battery indicator (fixed at 3.1V) when no power is connected. So, if you don't have USB/Solar wired up, when the battery voltage drops below 3.1V, the orange LED will come on.

The green DONE LED is pretty easy to understand as well - when it's lit the battery is charged up! Very handy for when you want to know that everything is done.

If you need to connect larger LEDs or a microcontroller up to these status pins, you can us the STATUS 0.1" breakout on the bottom edge of the PCB. The pins are open drain, so they will short to ground when 'active' and float when 'inactive' - you'll want to use a pullup resistor if you need a digital signal or connect LEDs just like shown below on the schematic, if you want bigger lights.

Load Sharing

The MCP73871 chip in the usb/solar charger has a very nifty feature called 'load sharing.' Say you have an every day lipoly charger and you want to use the battery while its charging. To do this, you might connect the project directly to the battery output. This means, however, that the charger is both charging a battery and driving your project at the same time. The charger is working extra hard and the battery is being charged and discharged constantly.
As an improvement, this design has a pass transistor inside the chip, connected to the output load from the input voltage, so that you dont lose efficiency from charging/discharging the battery. When the USB/Solar charger is powered from a USB port or panel, the load current goes directly from the input voltage to the output. If the current required is higher than what the panel or USB port can provide, the current is supplemented by the lipo battery, up to 1.8A
The smart load sharing means that the LOAD output can be as high as 6VDC if in direct sun because it will draw current directly from the 6V panel instead of from the battery. If using this with an electronic project, make sure it is OK for up to 6VDC input or use a low-dropout-regulator (LDO) to regulate the voltage down.
This is all managed at the same time as the solar current maximization. Simply plug in the lipoly battery on the left into the BATT port and connect your project up thru the LOAD port on the right.

Temperature Monitoring

If you plan to have your project outside or unattended, we suggest adding temperature sensing to keep the charger from overheating the battery or attempting to charge when the battery is too hot or cold.
Simply remove the 10K surface mount resistor from the THERM pads (or cut the trace going to it), and solder in a 10K NTC thermistor. Test out the system by trying to charge while you place the thermistor in a freezer or against some ice, as well as in a cup of > 50°C hot water. The charger should stop charging the battery. Once you are sure it is working, attach the sensing element (the epoxy bulb in this case) so it is resting against the battery.

Adjusting the Max Charge Current

The USB/Solar charger comes with a preset rate of 500mA which will work great for USB ports, USB wall adapters and solar panels up to 3 Watts. If you have a project that uses a larger panel, or perhaps some other sort of setup, you can easily adjust the current by soldering a resistor into the PROG pads.
The current is set by 1000/RPROG Amps, where RPROG is the resistance. So for 2 Kohms, that would make it 1000/2000 = 0.5 A = 500 mA. If you want 1A, you would use a 1K ohm resistor. If you want to increase the current, you need to decrease the resistance, so you can just solder over the existing 2K. So for example, soldering another 2K resistor into RPROG will give you 1K total resistance and 1000 mA current draw. See above for a 2.2K resistor soldered for about 950mA of max current draw. If you want to set the max current draw lower, you'll need to remove the 2K resistor.

Adding External LEDs

If you are placing the charger in a box, you may want to have external LEDs for indicating charge state. This is easy! Simply use any 3mm, 5mm or 10mm LEDs you wish and 1K resistors and wire them like so:

This guide was first published on Jul 29, 2012. It was last updated on Mar 27, 2024.

This page (Using the Charger) was last updated on Aug 02, 2012.

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