Adjustable LiPo Charger Hookup Guide

Contributors: Alex the Giant
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The SparkFun Adjustable LiPo Charger is a single-cell lithium polymer (LiPo) and lithium ion battery charger. Because it’s adjustable, this charger will be able to safely charge all of our singe-cell batteries.

SparkFun Adjustable LiPo Charger


Required Materials

The Adjustable LiPo Charger parts required depends on your input power supply for charging and the size of the LiPo battery used. At the bare minimum, all that’s needed to charge a LiPo battery sufficiently is a micro-B USB cable to connect to either a computer USB port or a wall adapter. We also have 5V wall adapters with a USB cable attached.

USB micro-B Cable - 6 Foot

Wall Adapter Power Supply - 5.1V DC 2.5A (USB Micro-B)

Wall Adapter Power Supply - 5V DC 2A (USB Micro-B)

USB Wall Charger - 5V, 1A (Black)


And one of the LiPo batteries with nominal 3.7V from our catalog:

Lithium Ion Battery - 1Ah

Lithium Ion Battery - 400mAh

Lithium Ion Battery - 110mAh

Lithium Ion Battery - 850mAh

Coin Cell Battery - 24.5mm (Rechargeable CR2450)


For batteries smaller than 100mAh (i.e. 40mAh LiPo), you may also need to solder an assortment of resistors to set a custom charge rate.

Resistor Kit - 1/4W (500 total)

Polymer Lithium Ion Battery - 40mAh


Suggested Reading

If you aren’t familiar with the following concepts, we recommend checking out these tutorials before continuing.

Battery Technologies

The basics behind the batteries used in portable electronic devices: LiPo, NiMH, coin cells, and alkaline.

Electric Power

An overview of electric power, the rate of energy transfer. We'll talk definition of power, watts, equations, and power ratings. 1.21 gigawatts of tutorial fun!

What is a Battery?

An overview of the inner workings of a battery and how it was invented.

Hardware Overview

For a quick reference, here is an annotated diagram of the parts used on the Adjustable LiPo Charger:

Board Highlighted

1Charge Input - The input voltage for the MCP73831 charge IC is between 3.75V to 6V. To fully charge a LiPo battery, it is recommended to have a voltage of around 5V. The charge IC will regulate the voltage down to safely charge the LiPo battery. (Up to 500mA of current may be required depending on the charge rate).
2Power LED - When power is supplied, this red LED should turn on.
3Status LED - When a battery is connected, the blue status LED should turn on. When the battery is fully charged, the LED will turn off.
4Battery Input - Only one battery should be charged at a time. There are a total of 3 battery inputs: JST connector, PTH pins, 2450 coin cell holder (on the back).
5Charge Rate Select - Selects the maximum amount of current to charge the battery. For a detailed explaination, see the Battery Charging section of this guide.

Charge Status LED

The on-board blue STAT LED can be used to get an indication of the charge status of your battery. Below is a table of other status indicators depending on the state of the charge IC.

Charge StateLED status
No BatteryFloating (should be OFF, but may flicker)
ShutdownFloating (should be OFF, but may flicker)
Charge CompleteOFF

Hardware Assembly

Before connecting anything to the Adjustable LiPo Charger, it's important to make sure the board is configured properly.

The switches on the Adjustable LiPo Charger come shipped with all the switches in the ON position. Before configuring the board, all the switches should be moved to the OFF position.

DIP Switches Off

Setting the Charge Rate

Regardless of the size of the battery, it’s important to keep in mind the charge rate of the battery. Every time a battery is charged and discharged, it causes wear on the battery. As the battery starts to wear down, the capacity of the battery slowly decreases until it’s unable to hold a charge. Setting the charge rate comes with the trade off of lifespan and charge time; the faster the battery is charged, the shorter the life of the battery.

Rechargeable lithium batteries have a maximum recommended charge and discharge rate measured in “C”. All of the single cell LiPo/Li-Ion batteries we currently sell have a maximum recommended charge/discharge rate of 1C. Other battery chemistries allow for faster rates of 10C or even 40C. A 1000mAh battery with a 1C charge rate would charge the battery at 1000mA of current. The same 1000mAh battery charged at 500mA of current would be 0.5C.

To set the charge rate, move the appropriate DIP switch from the off position to the ON position by sliding it to the right as seen below:

400mA Select

As a general rule of thumb when selecting a charge rate on the DIP switches, select the charge rate that best matches the capacity of your battery (e.g. 440mAh battery should be charged at 440mA MAX).

Setting a Custom Charge Rate

If you want to charge a battery smaller than 100mAh, or have a battery thats in between the values on the switch and would like to have exactly a 1C charge rate, you’ll need to set a custom charge rate. The MCP73831 charge rate controller sets the maximum charge current with a resistor. To calculate the resistor needed, you can use the following formula from the datasheet:

R_PTH (kΩ) = 1000V / I_set (mA)

If you wanted to set the charge rate to 40mA for example, a 25kΩ resistor will need to be soldered into the unpopulated pins next to the micro-B USB connector. To use that charge rate, move all of the DIP switches to the OFF position and move the DIP switch labeled “PTH” to the ON position.

Battery Charging

Once the board is configured for the battery, you can connect power from either a micro USB cable or the pins below the USB connector labeled “GND” and “INPUT”. To fully charge a LiPo battery, it is recommended to have a voltage of around 5V when connecting to those through hole pins. The charge IC will regulate the voltage down to safely charge the LiPo battery.

Once power is connected, you can then install a single LiPo battery to one of the three battery connectors outlined in the Hardware Overview section of this guide. If you’re using the rechargeable coin cell battery, make sure the battery is installed in the correct orientation to avoid damaging the battery.

If using the coin cell battery holder, do NOT install any type of battery other than the rechargeable coin cell battery. Most other coin cell batteries are not rechargeable and will not work with this charger.

Once the battery is installed, the blue status LED should turn on as shown below:

Charging LED ON

Charging is complete when the status LED turns off.

Charging LED OFF

Resources and Going Further

Now that you’ve successfully got your Adjustable LiPo Charger up and running, it’s time to incorporate it into your own project!

For more on the Adjustable LiPo Charger, check out the links below:

Need some inspiration for your next project? Check out some of these tutorials:

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