3 X 18650 Battery Capacity Tester

by zygfryd1991

12K2935

Intro: 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:

https://www.instructables.com/id/DIY-Arduino-Batte...

https://arduinowpraktyce.blogspot.com/2018/02/test...

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).

23 Comments

RobertM158 1 year ago

Can't make this work. It's still running overnight voltage is still reading 4.06 volts and saying the capacity is now at almost 4700 mAh. Only thing different I'm using is 10k resistors but have made the appropriate changes in the code. Not sure what's going on. The FET is passing current through it to ground so not sure what's up with it. I know this is an old post but hoping someone has an insight.

Mohammad Mhm 2 years ago

I manufactured the device and added minor modifications to the design. The device works and detects the battery voltage, but when checking after 10 seconds, it turns off automatically and restarts itself. Is there a solution?

JakubP41 5 years ago

Witam
Na Twiom schemacie noga D2 jest podlaczona do GND i wylacznik zwiera ta noge przez opornik 10K tez do masy to troche nielogiczne.
Nie jestem pewien ale wydaje mi sie ze opornik 10k powinien byc polaczczony D2 do masy a przelacznik powinien podawac chwilowo VCC lub odwrotnie.
Mozesz to wyjasnic ?

BartoszD13 2 years ago

super projekt. Czy masz może jeszcze płytki ?? Chętnię odkupie. Pozdrawiam

JakubP41 5 years ago

Dzieki za odpowiec dam zna c czy wszystko dziala.

techno6666 3 years ago

Czy kolega będzie rozwijał projekt? Przykładowo na tester 10 ogniw?

JonC163 3 years ago

These capacity testers work great!!! Look no further ... I have 20 of them running 24/7

thanks @zygfryd1991

robertkazmierski40 4 years ago

Witam
Wykonałem tester i zastanawia mnie jaki powinien być prąd rozładowania u mnie jest na poziomie 0,16-0,25 mA .Mam wrażenie że jest to za mało użyłem innych rezystorów po prostu takie miałem zmieniłem kod jak poniżej przy takich parametrach test trwał ponad 20 h i jeszcze się nie zakończył .Proszę powiedz czy to normalne ? Robert
float R1 = 20; // Resistor R1 value [Ohm]
float RB1 = 20.1; // Measured B1 circuit resistance [Ohm]
float R2 = 15; // Resistor R1 value [Ohm]
float RB2 = 15.1; // Measured B2 circuit resistance [Ohm]
float R3 = 10; // Resistor R1 value [Ohm]
float RB3 = 10.1; // Measured B3 circuit resistance [Ohm]

zygfryd1991 4 years ago

Cześć. W przypadku rezystora 20 ohm i ogniwa 18650 prąd powinien być na poziomie 200 mA, więc zakładając rozładowanie porządnego ogniwa 3000mAh zajmie to 15 godzin. Osobiście rozładowuję prądem ok. 1A i cała operacja zamyka się w 2-3 godzinach.

AvonWolf 4 years ago

Cool project. Would this work with 8.5V battery packs? Or do I need to change some resistors and so on?

zygfryd1991 4 years ago

Sure, it would, but you need to modify the code and recalculate resistors

AvonWolf 4 years ago

Thanks for your reply, code is no problem. :-)
Can you tell me which resistors I need to change?

KrystJan 4 years ago

Witaj :-)
Super projekt!
Może masz na zbyciu taką piękną płytkę jak na zdjęciu. Jestem zainteresowany zakupem.
Pozdrawiam.

zygfryd1991 4 years ago

Mógłbym Ci jedną podarować, jeśli Ci po drodze do Krakowa :)

JakubP41 5 years ago

Witam
chcialem powielic Twoj projekt ale mam problem z podpieciem klawisza wyboru mozesz mi powiedzieć jak polaczyc.

zygfryd1991 5 years ago

Cześć! Jaki konkretnie problem masz z klawiszem wyboru? Podłączasz uklad na płytce stykowej, czy PCB?

nikdo_nic 5 years ago

Hi
how accurate are final measurements ?
do you have something to compare measured values
also i want to know what you have on display when you measuring all batteries
and if is you can measure only one battery
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