18650 Battery Capacity Tester

Ok, this Lazy Old Geek (L.O.G.) made some Battery Capacity Testers for AA and 18650 batteries about three years ago. I thought I’d published these but apparently didn’t.

Well, I thought my measurements were really low until I read this Instructable:

https://www.instructables.com/id/How-to-know-a-fake-18650-battery/

Well, that was interesting. I decided to weigh some of my 18650s. (see pictures). I used a sandwich bag to hold the battery.

Most of them were cheap 18650s bought on ebay. (The purple, blue and grey ones) But I’d recently bought some more expensive Samsungs (the pink ones) just to see if they were better. And recently I took apart an old Lenovo laptop battery pack that wasn’t holding a charge. (The red ones)

Okay, this is a cheapy scale from ebay. I noticed that it would measure only in 5 gram increments (but still good for this application):

Cheap 18650s (30 to 40grams)

Samsung and red (45-50grams)

So the Instructable testing batteries by weight seemed to make sense.

But I wanted to see if that had anything to do with capacity.

Step 1: Measuring Capacity Hardware

Well, I had a capacity tester PCB already made. I actually made one for AA and one for 18650 batteries but couldn’t find the 18650 so used the AA. (the only difference is the load resistors) It had everything I wanted except I added a start button. See second picture.

Well, I had the 4ohm 5watt resistors on the PCB but they got too hot so I moved them off the PCB and attached them to a heatsink. See picture.

Yes, I know this is a kludge but it works.

The schematic is shown.

Ealge Cadsoft files included.

Step 2: Arduino Sketch

I wrote my original sketch but I based this sketch on this Instructable:

https://www.instructables.com/id/Arduino-cell-capacity-meter/

Theory of operation:

For each battery:

Start (Open) battery voltage is measured.

When the Start button is pressed, the FET is turned on and connects a 4 ohm load across the battery.

The voltage at the both sides of the resistor are measured, the difference is the voltage across the resistor.

Since the resistance, the current through the resistor I is that voltage divided by 4.

So this current is assumed to be the same for 10 seconds (delta).

So capacity for this delta is current * 10 seconds.

These capacities are added up until the battery voltage drops below cutoff voltage.

Constants that can be changed:

float ArefV = 5.1; // Supplied 5V

float CutoffV = 2.9; // Stop discharge voltage

float Res = 4.0; // Discharge resistance

#define DELTA 10000L // Length of sample period in milliseconds

Step 3: Operating Instructions and Conclusions

Operating Instructions:

Charge 18650 batteries on charger.

Turn off power.

Plug in up to two batteries at a time into tester.

Interestingly, the batteries will provide a dim display. Make sure both batteries read 5V+

Turn on power.

Press the Start button for about 2 seconds.

Wait about 10 seconds, both batteries should so Running.

Each battery status will be displayed.

The start voltage will be displayed.

The current voltage will be displayed.

The current capacity will be displayed.

When a battery gets down to the lower limit, it will go into Pause status and will not be discharging. Without the discharge load, sometimes the voltage will rise a little so it will start running/discharging again.

So, in theory, the test is never finished. But in actuality, after a while the capacity is not going to increase very much. I usually just wait until it stays in Pause a long time or doesn’t change much when it comes out.

Okay the readings I got from the cheapy batteries were from 318 to 1000mAh.

The readings I got were way lower than ‘rated’ capacity. These were actually similar to my original software but I’d assumed that the tester wasn’t calculating true capacity.

However, these were all under 42gram batteries and now I had some over 42gram batteries.

The red batteries were used batteries several years of usage but they were all reading about 1800mAhs.

And the two pink batteries were fairly new Samsung batteries were around 3000mAHs and they’re rated at 2600mAhs.

CONCLUSIONS:

First, I would say the Fake 18650 Instructable is basically correct. Any 18650 under 42 grams is probably fake. According to some comments, it’s possible that some batteries over 42 grams can still be fake.

Second, my capacity tester seems to work pretty good and calculate capacity correctly.

Third, while I have a lot of fake 18650s, I have used and will continue to use them. I will probably toss the ones under 500mAhs. And not use the others in more important applications.