Introduction: 3 X 18650 Battery Capacity Tester

There are many instructions how to build arduino based capacity testers over the internet. The thing is, that it's rather a long process to test battery capacity. Let's say you want to discharge 2000mAh battery with ~0.5A current. It will take forever (precisely: 4 hours). I've tried to find much faster way to indicate many cells capacity. Increasing discharge current isn't a safe thing, especially when your load is simple resistor. Lower resistance = higher load = more power (heat) to be dissipated.

Basically we are discharging cells in order to achieve two different goals:

  • capacity indication
  • discharging to ~40% of total capacity, to provide safe torage for cells being unused for rather long time

To fulfill above mentioned, I decided to create multiple cells discharge station. There are two modes and simple menu, able to be handled by just one button. Additional feature is internal resistance (Rw) calculation.

I'm not an expert in this matter, so you are doing everything AT YOUR OWN RISK. Suggestions and feedback are welcomed.

Inspiration and basics comes from two projects that I've found:

Step 1: BOM

We gonna need:

  • 1x Arduino Nano
  • 3x IRLZ44N Mosfet
  • 1x 3 battery holder
  • 3x Cement resistor - e.g. 10R 10W - read about this in the next section
  • 3x 5mm red LED
  • Push button
  • LCD - in this project I utilized 16x2 i2c LCD
  • 1x 10k resistor
  • 9x 4k7 resistor
  • 3x 1k resistor
  • 1x 100R resistor
  • 1x Screw terminal for power supply connection (7-12V) - optional if you want to power up the device with arduino mini USB
  • 1x 4 goldpin female header, 2.54
  • 1x 15 Goldpin female header, 2.54mm (optional - if you want to go modular)
  • 1x Buzzer (optional)

Step 2: Schematic and the Principle of Operation

The brain of my project is arduino nano. Arduino controlls 3 mosfet, which are used to open / close 3 battery circuits with corresponding loads. We are measuring (using 3 voltage dividors) voltage of those circuts to determine the current flowiong through the power resistors - using an Ohm law.

I = V / R

Voltage drop across the power resistors is almost equal the voltage measured at the battery terminals (assuming quality solder joints and good wires), therefore there is no need to measure voltage before and after the resistors. Voltage dividors are used to prevent tested cells from powering up our device.

Knowing the voltage and current over the discharging time, we are able to calculate cell capacity.

Step 3: Power Resistors Selection

Resistor value depends on discharge current that we want to achieve. Assuming max 0.5A current, resistor value should be:

R = V (max cell voltage) / I (discharge current) = 4.2V / 0.5 = 8.4 Ohm

Using 10R resistor, you will get:

I = V / R = 4.2V / 10 ohm = 0.42A

The lover resistor value, the higher current.

IMPORTANT!! There are a lot of power to be dissipated, therefore resistor will get hot. We can determine minimum resistor power accordingly:

Min Power = I^2 * R = 0.42^2 * 10 = 1.76W

I'm using 3R3 17W resistors, however my advise is to use 10R (10W or so) - it will handle the power flowlessly and its temperature will remain safe.

Step 4: Arduino Code

You need to adjust following parameters according to your measured values:

R1, R2, R3 - power resistors values [ohm]

RB1, RB2, RB3 - B1-B3 circuit resistance. R1+0.1 is close enough [Ohm]

X1, X2, X3 - voltage dividers ratio. If you don't wanna measure it really precisely, you can enter just 2

interval - measurement Interwal (ms) - default 5000 ms

voltRef - Reference voltage measured between arduino pin 5V and GND - default 5.03

Step 5: PCB

Ready for ordering / etching :)

Step 6: Menu

Short press (with ~1s interval between next click) - change value

Long press - confirm

First level of the menu: mode selection (capacity test or simple discharge to preset voltage)

Second level of the menu: minimum voltage selection, where the end of measurement occurs.

When the measurement of any particular cell is done, final screen is displayed, where you can find the battery capacity and internal resistance (Rw).

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