Polymer batteries are widely used these days. Finding a big application in smart phones, laptops, e-cars etc. As all electronic devices are subject to defects, polymer lithium batteries are no different. These defects affect the performance of the batteries and can be detrimental. It is therefore necessary to detect defective polymer lithium cells early enough to maintain battery system reliability and safety.
This article will give an insight on how to detect defective polymer lithium cell packs.
Manual inspection
Spotting defective polymer lithium cells may start with a careful thorough manual inspection. These polymer lithium cells are typically contained in a soft lithium pouch, but when defective, swell, leak and even change color.
1. Swelling
These polymer lithium cells may swell. Swelling cells occurs usually due to some gas build up resulting from effect of over charge, deep discharge and some chemical reaction in the cell.
2. Leaking
Leaking electrolyte may also occur. This occurs where there is some chemical reaction which compromise some part of the cell.
3. Discoloration
This may occur whereThen follow these steps to check voltage:
Use a multimeter to check the voltage across the cell terminals. Compare it to the voltage range quoted by the battery manufacturer and an abnormally high or low reading of sell voltage may indicate a defect.
If a cell comes in below 2.5V could mean truly deep discharge and this has consequences. Irreversible damage may occur. Cells have over voltage readings higher than 4.2V, which should be treated cautiously as it strikes of overcharging which is said to cause instability.
3. Internal Resistance
Internal resistance signifies health or not. A cell with high internal resistance will thus also have poor charge/discharge efficiency, plus produce more heat, functions by reducing operational margin to fail. This leads to the conclusion that the higher the internal resistance in the cell, the closer it is to failing.
How To Measure Internal Resistance.
Specialist battery analyser or internal resistance meter for doing the job.
Subjecting a known load to the cell and making a measurement across the terminals to ascertain voltage drop. If a big drop is evident, then the repeatability would surely show the ‘big drop’ again. Compare with specification. A high internal resistance in the cell, says that the ‘juice’ might not be passing through as it ought to and instead goes down to keep rest including the cell warm!
This method could show [in hindsight] cells that looked ok, but stood out bad by eye.
4. Capacity Test.
Refers to the process of attempting to put less than full capacity into these saving little polymer lithium cells, ie discharging them in line with what we will bring out of them. Couple lines in how to:Discharge.
First fully charge to rated voltage.Discharge the cell on constant current load and see how much energy is delivered before the voltage drops below the cutoff level. Compare this to the manufacturer’s capacity rating. If you get only a small portion of the rated’.
Capacity testing can also show problems with energy density which can affect the usefulness of the polymer lithium in applications like electric vehicles and renewable energy storage systems.
5. Cycle Life Testing
Polymer lithium cells tend to have a certain number of usable cycles. As they age, their capacity diminishes and their performance falls off. Cells which die on you after only a few cycles could be defective.
How to do cycle life testing:
Put the cell through repeated charge and discharge cycles with a battery tester. Monitor the performance over time so that you know just how much the capacity and internal resistance has changed. Cells which die on you prematurely or show too much change in performance after just a few cycles may have manufacturing defects or other problems inside.
6. Thermal Testing
This is to make sure you don’t encounter problems with overheating and thermal runaway where the cell catches fire or explodes. Polymer lithium’s shouldn’t get too hot during normal operation.
How to do thermal testing:
Monitor the temperature of the cell on charge and discharge cycles.( Normally from 0°C to 45°C) A defective cell or a cell which is having issues internally will never sit back in this range.
A cell which is getting too hot should always be taken out of service, and should never be put back to prevent the likelihood of fire and that consequent damage to other parts.
7. Battery Sorting Machines
Sorting out defective polymer lithium cells is a lot easier and faster if you are dealing with lots of cells using battery sorting machines.
Using a combination of voltage, capacity and internal resistance, battery sorting machines (also for 18650, 21700 and prismatic lithium-ion cells) are able to group cells according to their characteristics and weed out the bad apples from the bunch.
Having battery sorting machines helps to weed out defective cells from your production line before they reach their final destination.
Conclusion
These are the ways employed in identifying and detecting defective polymer lithium cells. If you do really need to sort cells real fast and with good accuracy, you can always use battery sorting machines for that.
