Workbench Li-ion Battery Charger – ElectroSchematics.com

A few weeks ago, I started collecting defunct laptop batteries, particularly the modern smart battery packs. These battery packs cannot be repaired by simply replacing the faulty 18650 lithium-ion cells with new cells. You can take the batteries apart and re-use them if you are lucky. Often, you’ll get half dozen 18650 single-cell Li-ion batteries. Throw away the dead batteries and re purpose the rest.

If you know what you’re doing, this is a great way to stock some ‘low-grade’ Li-ion batteries for your little electronics hobby projects. If you are going to attempt this, you must note that there is a very real possibility of a fire hazard if you hastily pry the lid of a laptop battery pack out. Never let your metal tool to make an accidental contact between any solder points inside. Be very careful, in a sleepy midnight moment one of my laptop battery packs went up in smoke instantly!

The Workbench Li-ion Battery Charger

Obviously, you need a standalone charger to restore your Li-ion cells/batteries lifted from the old/defunct laptop battery packs. You can find a number of such charger circuits here, but most of them are based on dedicated Li-ion battery charger ICs in MCP,LTC,TP… series (that’s good). This time I’m giving you a crude circuit with the help of the good old adjustable linear voltage regulator IC – LM317 (perhaps you may have a dozen in your junk box). Next one used in the design is the TL431 adjustable precision shunt regulator IC, again a very cheap and easy to use component available everywhere.

First, note that this quick design does not attempt to replace a dedicated Li-ion battery charger circuit. This workbench charger is very flexible, and lets you charge both 1S (3.7V) and 2S (7.4V) Li-ion batteries, if you’re ready to make a little design modification. Basically, the circuit is configured as a current-limited universal workbench charger for 1S Li-ion.

Now take a look at the schematic:

As stated above, this basic design is for 1S Li-ion, therefore a 9V/1A external power source is needed to run the circuit practicably. This is the recommended input voltage (VIN) as IC1 itself requires up to 3V headroom to operate in regulation. You may conditionally feed a dc voltage close to 14V if you want to charge two cells in series (2S). The LM317T (IC1) is a simple current-limited power source set to 4.2VDC output. The TL431A (VR1) is used as the voltage reference to set that output. The maximum available output current is 1.5A for IC1, here it’s estimated to 570mA by the 2.2Ω/2W resistor (R1). Likewise, the resistor network R3-R4 sets the output voltage to 4.2V – this is the target voltage of a 3.7V (nominal) Li-ion battery. You should choose a ‘power resistor’ with minimum 2W rating for R1. Also, you must use 1% tolerance resistors for R3 and R4.

This is what my quick breadboard assembly looks like. Do not follow what I did. Never attempt to build this circuit on a breadboard. Always use a good quality perfboard for your construction. Remember to mount IC1 on a beefy heatsink, as well. Naturally, IC1 and R1 will get hot while running. Be equipped for that!

Furthermore, the 100nF capacitor (C1) is essential in this circuit because the LM317 datasheet says that we must include the capacitor if LM317 is not in close proximity to the power supply filter capacitors. A 100nF or 1μF ceramic or tantalum capacitor will provide sufficient bypassing for this application. The 1N4007 diode (D1) provides a low-impedance discharge path to prevent the battery from discharging into the output of the LM317.

Another Weekend Craft

This is an add-on/optional circuit to indicate the battery charging status. The little circuit monitors the voltage across the battery while keeping the LED lit, and as soon as it reaches the pre-set threshold (4.2V), the LED goes out to indicate a fully-charged battery – that means, a mere indication of the charging termination. The threshold is set by the resistor network R5-R6. With a minor tweak, you can modify this simple circuit as a battery-low state indicator/alarm.

This is my breadboard version:

I will not go into the fine details of TL431, everything is well described in its datasheet. You can find a few TL431-related do it yourself projects in this space as well.

Nothing terrible so far, right?

From an old laptop battery, you can pull out many completely working 18650 batteries. And you can use this crude workbench charger to recharge them effectively. From the practice of many weeks of use, I can say that my workbench charger will work for you too. It is an unorthodox design. The merged schematic is given below for your quick reference. Give it a try!

Laptop Battery Firmware Hack?

Once again, note that a laptop battery pack is more than just a series-parallel combination of lithium-ion batteries. There’s an intelligent controller board inside that keeps track of everything related with the power management system. The firmware can be read and programmed by using I2C protocol.

Next, I will share a couple of simple tips and tricks you can use to hack a smart laptop battery pack without seeing the magic smoke leaving from it. So maybe stay tuned for a primer on hacking of today’s laptop battery packs.

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