Testing and Grading ProcessPosted on: Tue Apr 05 2022
If you want to know how to test 18650 cells, you are in the right place. Testing cells is one of the most important steps in preparing salvaged 18650 cells for reuse.
A Quick Overview of The Sorting and Grading Process
If you want a quick overview on what the process looks like, these 7 steps are on how to test and grade cells. If you are looking for a more in-depth overview on what each step looks like, visit the bottom of the article.
Step 1: Salvaging - The battery pack is removed from its original casing and the cells are removed from the battery pack.
Step 2: Visual Inspection - Cells are looked over to make sure they are not physically damaged and show no signs of leaks. Any cells that show signs of damage are discarded.
NOTE: Any cells that are discarded throughout this process should be taken to be properly recycled. Please never throw Lithium battery cells in the trash or recycling bin.
Step 3: Cleaning - The glue, paper, and tape that was used to hold the battery pack together is removed from the cells.
Step 4: Charging - Cells are brought to a full charge while their temperatures are monitored. Any cells that become abnormally warm during charging or do not hold their charge should be discarded.
Step 5: Self-Discharge Test - Voltage for each cell is checked after they have been stored for one week to ensure that voltage has not dropped past a certain threshold. If any voltage drop above .1v is detected after one week, the cell should be discarded.
Step 6: Capacity Testing - Cells should be capacity tested and labeled to ensure that they can store the correct amount of energy. Any cell that falls below 90% of original manufacturer capacity should be discarded. Some people go lower depends on your needs.
Step 7: IR Testing - Cells are tested for IR and sorted based on the cells deviation from the manufacturers specifications. Any cells that fall too far outside of the manufacturer's spec should be discarded.
Detailed Steps on Salvaging 18650 Cells
The process of salvaging 18650 cells properly can be quite involved. There are several key factors to consider and processes to understand in order to properly salvage, test, and sort 18650 cells.
It’s important to make sure that the cells you are salvaging have no leaks, and don’t get hot when they are charging or discharging. Also, it’s important to test each cell for capacity and group them based on those figures.
So, here is a detailed explanation of how we salvage, test and grade cells before they are shipped to our customers:
When mass-producing lithium ion battery packs, a significant amount of adhesives and permanent fasteners are used. This can, at times, make the salvaging of cells difficult.
That’s why our first step is to remove the battery packs from whatever device or housing they were installed in. After that, we break down the battery packs, and separate the cells.
Where Can I Find 18650 Batteries?
All over the place. Our modern world is literally powered by them. You can find 18650 cells in things like discarded scooter batteries, modem battery packs, and old laptop battery packs.
How Do You Break Down A Battery Pack?
The first step is to remove the battery pack's exterior casing. After that, you may have to pry or otherwise work loose the battery pack from the casing. Then, the nickel strip needs to be carefully removed from the cells.
Once that is done, you can generally break loose any adhesives or hot glue that is used to hold the cells together. Once you have the cells freed, the final step is cleaning any remaining debris off of the cells.
After that, the next step is to inspect the cell for any visible signs of damage. A small scratch on a cell’s wrapping is not a major cause for concern, but if a cell is swollen, leaking, or discolored, it is discarded. If a cell has leaked, it can have a somewhat sweet smell to it.
Why Do Bad 18650 Cells Smell Sweet?
Any kind of hole in a cell is a very bad thing. Holes can come from physical impact as well as internal damage. For whatever reason, the electrolytic that is used to separate the negative and positive side of lithium ion batteries. So, if you smell an odd, slightly sweet smell, the cell has a leak it’s electrolyte somewhere and should be properly discarded.
Why Is A Swollen Battery Cell So Bad?
A lithium ion cell can become swollen by overcharging, over discharging, or other internal damage. A swollen battery cell is dangerous because the swelling is the result of heat and gas that have built up within the cell. If enough heat and gas build up in the cell, it will vent with flame or could even explode.
As mentioned before, lithium ion battery packs are generally put together as a permanent, non-serviceable structure. Before we begin electrically testing any 18650 cells, do your best to remove all the nickel, glue and tape off of each and every cell.
If you find that the tape, hot glue, and other adhesives used to hold cells together are too difficult to remove, then try using a heat gun. How well any adhesive works is a function of its temperature. So, applying a little heat is usually enough to loosen the bond.
Heat is bad for batteries, so make sure to not heat up the cells for too long. It does not take much surface heat to loosen adhesives and glues.
When removing the nickel strip from 18650 cells, it's best to grab the nickel by an edge and slowly roll the nickel strip off of the cells. The rolling action requires the least amount of force to remove the welded nickel, but make sure to not short out the cell when doing this!
Once we have clean, visually inspected cells, the next step is to charge test the 18650 cells. As they are charging, cells are monitored for heat buildup. If any cells exhibit unusually warm charging characteristics, they are discarded.
How Can You Tell If An 18650 Is Getting Too Hot?
There are several devices on the market that can precisely monitor 18650 cells during their charging and/or discharging phases. While these devices are useful, that can be somewhat expensive. Another option would be to use an infrared thermometer along with a standard battery charger.
You would be surprised, however, by how much insight you can gain about a cell's temperature characteristic with something as simple as the palm of your hand. So, as you are charging your battery cell, keep tabs on their heat output with the palm of your hand. If a battery cell is getting warm under very small charge currents, then the cell more than likely has an internal problem.
Why Is It Bad For 18650 Cells To Get Hot?
Unless you are putting them under tremendous charge or discharge loads, you should not feel too much heat from your battery cells.
If you do, that means the cell either has an extremely high ISR (Internal Series Resistance) or it has some amount of internal discharge. Internal discharge is when the positive and negative electrodes are physically touching inside the cell.
Self Discharge Test
If a lithium ion cell is worn out or damaged, it will have some degree of self-discharge. Self discharge happens because the positive and negative electrodes, to a very small degree, are physically touching inside the cell. As you may expect, this is a bad thing.
So, that’s why when we are testing 18650 cells, we let the cells sit for a full week once they are fully charged. If the voltage of any cell drops by more than 0.1v during that time, the cells should be discarded.
Why Do Bad Batteries Drop Voltage Over Time?
Internal discharge. When a cell is worn out, the electrodes will literally grow towards each other very slowly. While there is a mechanism inside the cell to keep them from touching, it will eventually happen. When it does, it causes the cell to discharge into itself slowly at first, but increasingly so over time.
Do Good 18650 Cells Also Experience Voltage Drop?
Yes, but to a very small degree. There are several factors at play that cause a battery's voltage to drop over time. All cells exhibit some form of self discharge, but for healthy cells this phenomenon happens rather slowly. Expect a good cell's voltage to drop somewhere between 0.5% to 2% per month.
As you can see, there are a lot of reasons to discard cells. The cells that have made it this far are then put on a capacity tester to determine how much actual capacity the cell has. If any of them read abnormally low, you guessed it, you should discard those cells..
Why Is Capacity Testing Important?
Capacity testing is important because of how Battery Management Systems work. If you don't match the amount of capacity in each cell group, one will reach a full charge or be depleted sooner than all the others. To keep things safe, the BMS will shut off the battery if any single cell group hits the over-discharge or over-charge threshold.
We designed our Packer Tool to make this part of building a lithium ion battery pack much easier. Once you enter all your cell capacities in the tool, it tells you the most optimal way of packing the cells together. This helps ensure the longest running time and life-span for your battery.
When learning how to test 18650 cells, IR (Internal Series Resistance), is one of its most important characteristics. The lower the resistance, the less voltage drop the cell will experience under load. This leads to less heat build up, which provides for a longer total cell life and, therefore, a longer battery pack life.
For these reasons, each remaining cell is tested for ISR and it’s figures are compared to the manufacturer’s spec sheet. If any cell deviates too much from what the manufacturer states, that cell should be discarded.
Why Is It Important For All The Cells To Have The Same ISR?
The cell’s ISR governs how hot the cell will get and how much of a voltage drop it exhibits under a given load. So, if one of your P groups has a cell with a much higher IR than the other cells, it will experience more voltage drop than the other cells.
This causes energy to flow from the good cells into the bad one, which only exacerbates the issue. And remember, as a cell’s temperature increases, so does its resistance. This creates a potentially hazardous positive feedback loop that will make your battery pack fail a whole lot sooner than it otherwise would have.
Also, if a P group has a cell with a very high IR, that will cause the BMS to shut the battery pack down far before the other P groups are totally depleted.
When it comes to working with 18650 cells, learning the process of inspecting cells for visual damage, testing cells for voltage drop, and performing a proper charge and discharge test for each and every cell is crucial.
Remember, if a lithium ion battery pack is built with unmatched cells that have many different capacities and internal resistances, the end result could be unsatisfactory or even dangerous.
We hope this article helped you learn how to test 18650 cells yourself.