What is it?

This is a 18650 Li-ion battery discharger board that discharges the cell down to 2.4V and then automatically removes the load. The battery will quickly recover voltage to about 2.85V so it wont be outside the safe 2.5 to 4.2V zone for long.

Why did you make it?

When I was discharging a battery to a certain voltage I needed to check the voltage from time to time. There was a risk I would forget to pull out the battery at the oppurtune moment. Therefore I designed this device that would automatically handle the process.

Why you need an empty battery?

Sometimes you need an empty battery. For example when you need to test how fast your battery charge circuit charges an empty battery. Another reason to drain a battery is for EMC testing in charging mode. These tests could take up to an hour and you want to start with an empty battery so it is charging the whole test.

What makes it special?

It is super simple, just four power resistors, a battery holder, a rocker switch, a LED and a TP4056 charging module. The TP4056 is used for it's onboard battery protection circuit with the DW01 and FS8205 ICs. The TP4056 module will break the connection when the voltage drops below 2.4V, and ow yeah, it can charge the battery back up if you connect up a USB cable. This tool double functions as a recharbeable hand warmer, but I that is not the intended use.

Specifications

• four 10W 10 ohms power resistors in parallel (total is 2.5 ohms)
• a rocker switch to enable the discharging
• a red LED to indicate it is discharging
• the discharge current is about 1.5A, this is a safe 0.5C for 3000 mAh cells
• below 2.4V the load is automatically disconnected
• above 2.9V the load is automatically reconnected
• quiescent current is 3uA above 2.4V and 0.9uA below 2.4V battery voltage
• the TP4056 module safely charges the battery in constant current constant voltage mode
• the TP4056 module has micro USB connector for connecting a USB cable
• the TP4056 module charges at a safe 1A current
• the TP4056 module has a battery protection circuit with overvoltage protection at 4.3V, undervoltage protection at 2.4V and overcurrent protection at 3A

What you get

• a soldered 18650 discharge tool as shown in the pictures
• no USB cable
• no battery

Legal warnings

• use this only for discharging of 3.7V Lithium Ion cells
• use this device at your own risk
• use this device only in an electronics lab
• this device may only be used by electronically trained people
• the load resistors are HOT HOT HOT!
• do not short circuit because unprotected Li-ion cells may get hot, smoke, catch fire, explode, etc.
• do not place the device on metal opbjects because of risk of creating a short circuit
• this is not a childrens toy
• after use please remove the battery and store it safely