LiPo USB Charger Hookup Guide

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Contributors: Jimb0
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Setting the Charge Current

Before you plug a battery into the charger, you should be aware of your battery’s capacity and the charge current supplied by the charger. To be safe*, you should keep the charge current at or below 1C of your battery. That means you should charge your 850mAh battery at 850mA or less, and a 100mAh battery at 100mA or less.

The charge current controls how fast your battery will charge. If you have a 1000mAh battery, charging at 1000mA will fully charge that battery in 1 hour. Charging it at 500mA will mean a full charge takes twice as long – 2 hours. So more charge current is better…as long as it doesn’t exceed your battery’s specifications.

The featured component on the LiPo USB Charger board – an MCP73831 – has a programmable charge current feature. It can be set to deliver anywhere between 15mA and 500mA to a battery. To program that value, a resistor is connected from the PROG pin to ground. There are already two resistors on-board, which can set the charge current to either 500mA and 100mA. A small jumper is used to select between those. You can also add your own resistor, to set a custom charge current.

Jumper-Selecting

Next to the charge-status LED there are three bare pads that form a two-way jumper. The center pad connects to the MCP73831’s PROG pin, and the outer two pads connect to a pair of resistors. The labels next to those outer pads indicate the charge current that they set.

Jumper location highlighted

If you look really closely at that jumper, you may see a small trace connecting the middle pad to the 500mA-labeled outer pad. As such, this board is configured to deliver a 500mA current by default.

To change the charge current to 100mA, you’ll need to cut that small trace between the pads (a hobby knife is recommended), and apply a solder blob to connect the 100mA-labeled pin to the center pad.

As an advanced trick, you can short both pads to the center (connecting both resistors in parallel) to set the charge current to 600mA.

Custom Charge Current

If neither 100mA or 500mA will work for you, there is an unpopulated resistor footprint to allow you to set a custom charge current.

Resistor plugged into board

Before adding the resistor, disconnect both jumpers discussed in the section above. Then use this equation to select your resistor:

\I_{charge} = \frac{1000V}{R_{prog}}\;\; \mathrm{(or)}\;\; R_{prog} = \frac{1000V}{I_{charge}}

For example, if you want to charge a 400mAh battery at exactly 400mA, solder in a 2.5kΩ resistor (you may have to series a 2.2k and 330).


* Most batteries include over-current protection – implemented on the little circuit board under the yellow tape – which will keep the battery from blowing up if you supply too much current. But it’s best to not rely on that circuit: you’ll save power and your sanity.