In this Instructable I will review how to process and identify usable 18650 cells from salvaged dead laptop batteries, cordless power tools or any other source.


Step 1: You Have a Recycled 18650, With Most Laptop Circuitry and Metal Sheeting and Adhesive Removed, What Next?

Replaced the damaged shrink wrap cover and if needed the top ring.

  • The entire outside of the battery is a negative pole including everything on the top except the center it must be put back to its original condition before safely proceeding.

What you will need:

Shrink Tube Battery Wrap for 18650 $10.99

Battery Insulators $7.99

Heat Gun $18.50

To apply the shrink wrap use a blow drier (high heat setting) or heat gun (low setting). Get the heat gun, nothing ends a relationship quicker than "borrowing" your significant others blow dryer to wrap 200 batteries.

Tip: Take a photo of the cell ID prior to removing the damaged original cover. You can write this information on the new wrap prior to applying it (unless you like writing on curved surfaces) to look up the batteries specs.

Step 2: 18650 Cell Voltage Basics

A fully charged 18650 is at 4.1-4.2 volts.

You rarely want it to stay there as it will degrade the battery over time.

Fully charge to 4.2 volts just before starting your e-bike for a ride or place any other load on it. Or better yet charge it only to 4.0v to extend the battery life.

You never want to go above 4.2 volts, or you may set up a reaction that both damages the cell and can start a fire.

You prevent charging over 4.2 volts by charging using a quality charger where the upper voltage limit is placed in the settings, or use a charge control board matching your battery specs placed between your DC source and the battery. These boards can be purchased for $1-2 apiece.

Charge Control Board:

An example for a high-quality charger/discharger:

Your recycled cells will mostly be at 0-3.5 volts You should not discharge 18650 below 3 volts once you identify usable cells but these are recycled from DEAD products, so you get what you get.

You store usable cells not in demand for immediate use at 3.4-3.6 volts to extend their life.

That is one reason your charger is also a discharger

A recycled cell below say 2.3 volts on its first voltage check may not charge. It probably is suspect and should be recycled but may be useable. I will show you how in a separate section below.

Step 3: Sort by Voltage

You want to evaluate each cell to know its capacity, internal resistance, and its ability to charge and hold a charge over time. Here is how to proceed.

To test a cell you charge it then partially discharge it to learn the capacity, maximal voltage attained and internal resistance. Using math, a resistor, and Ohm's law you can evaluate the cell manually yet 99.9% of all makers will simply use an automated charger/discharger to test the cell and mark down the digital results.

Sort by voltage:

Bin One: 3.0-4.1 volts

Bin Two: 2.5-2.9 volts

Bin Three: <2.5 volts down to zero

To be efficient you can charge up to 16 cells at once in parallel using a charger with a 5-amp capacity. This will charge them at 5000/16 or about 312 milliamps (1000 milliamps in an amp), which is generally a safe charging current for each cell. If you charge less, say four cells in parallel, cut the charge amperage to 1.2 amps (1200 milliamps) to keep the current at 300 milliamps/cell.

Why sort?

To charge many cells quickly the cells are placed in parallel, (all the positive terminals are lined up say north and the negative terminals are all lined up south, then all the positive terminals are connected with a thick wire, nickel strip or premade battery holder and then all the negative terminals are connected).

If the high voltage cells in this parallel set-up are mixed with low voltage or shorted defective cells they will immediately dump current into the bad or low voltage cells. This may overheat the high voltage or the low voltage cells and in the worst case start a fire.

The risk is reduced by pre-checking the recycled cells initial voltage and sorting them as above. Only the bin one cells are placed together in parallel. Then the bin two are kept together. This is a quick and dirty pre-balancing of the cells, and a way to keep very low or no voltage cells out of the parallel charge groups. Bin three is handled below. Even with this pre-sort a recycled cell or two may heat up quickly, safety tips next.

Step 4: Safety

Don't be intimidated by horror stories of exploding cells and house fires. Cars, gas stoves and heaters do that as well, and yet we learn how to operate them safely.

Safety is simple:

Do not charge above 4.2 volts

Do not discharge rapidly use 1/10 the battery capacity or around 300mA-500mA

Check the cell temperatures a minute after starting a charge cycle on an unknown cell and 10-20 minutes later. Remove any that get above 50C, 122 F while charging and recycle them.

Charge and store the cells in a fire resistant ballistic bag. Even if it all goes wrong if the cells are in a closed fire resistant, ballistic container you will have significantly improved your safety.

Check the cell voltages prior to charging. Remove dead and very low voltage cells and balance the remaining cells by voltages before parallel charging.

Examine cells for singed, discolored areas, physical damage, or leakage and recycle rather than try to reuse.

These are single cell safety tips. Cells grouped into a battery have their own balance requirements not covered here.

You will need:

1) A temperature gun $15.00

2) Two Lipo Cell Safe Fire containers, one to store cells, one to charge. $13.00 each

Step 5: Reviving Low Voltage Cells

Take a 2 slot 18650 battery holder and solder a wire across the positive terminals and the negative terminals.

Place a known fully charged cell in one well the low voltage or dead cell in the other, keep the positive terminals and negative terminals aligned in parallel.

Get out your new temperature gun and monitor both the cells temperatures for one minute.

If either cell goes above 50C stop the process by removing a cell.

After a minute re-check the low cell voltage. Three things will happen.

1) The low voltage cell is now 2.6 volts or above, and can be further evaluated.

2) The low voltage cell improved but is less than 2.6 volts, toss it in the battery recycle pile (located in your fire proof, ballistic cell storage container). These cells may be internally shorted, damaged, or simply dead cells with higher risk, get them in the bag.

3) The cell stayed dead, recycle as above.

The cells that revive to a decent voltage are still suspect as you never want a 18650 to go below 3.0 volts to preserve capacity, yet as we have said these are recycled and by definition imperfect cells, that my find some additional use, so if they revive to a decent voltage they can be further evaluated with a charge/discharge cycle to determine their capacity and internal resistance.

In the examples above a dead cell went from essentially zero to 3.06 volts in 1-minute, the low volt cell went from 1.84 volts to 3.38 volts. I know many would toss all low volt recycled cells, and that is their right. I parallel charge, watch temps of both batteries during the process, and do the full test charge/discharge in a fire proof bag and save many cells for low current demand items like powering an Arduino.

Step 6: Charge and Discharge Testing, the Bottleneck!

You can charge 16 cells at a time with a good charger such as Tenergy. Charging successfully does not tell you enough about the cell health.

To charge multiple cells at once use 4-cell holders and join the top and bottom terminals with a soldered wire (as pictured above). Then keeping these holders in parallel join the 4-holders to get to a 16 slot charge configuration. Use a wire gauge that can safely handle 5 amps, 14-12 gauge.

Join them to the charger with a XT-60 connector.

You need to use your chargers charge/discharge cycle to get the current capacity and internal resistance. When a cell is charged then discharged in a measured manner you get the cells current capacity.

Current capacity is rated in milliamps it is dependent on temperature and the rate that the load is discharging the cell. A 3,000 mAh capacity means that in one hour, the cell can roughly discharge 3,000 milliamps or 3 amps.

In the above picture a Zanflare C2 is sued to test four cells, one of them on the left is finished.

Tip: Write down the cell capacity and any other data on the cell you save prior to removing the cell from the charger as all data is lost once it is removed.

At 300 mA this "Fast Test" mode can still take hours to slowly charge then discharge each cell to find their capacity and internal resistance. To test a large batch takes days or weeks. There is no way to speed this up other than buying multiple chargers or a very large capacity, very expensive charger/discharger.

A Tenergy type charger can charge 16 cells at once but can only test one cell at a time!

Resistance (IR): IR is both simple and complex. How it is measured should
consider the state of charge, temperature and other factors if you are a manufacturer developing an accurate cell spec sheet. IR varies with the purpose of the cell. Is it a rapid discharge cell? It will have lower IR when new. Is it designed to discharge slowly but last longer? The IR will be higher. That makes it hard to compare IR on two different cells you are recycling, they may have been made for very different purposes. If they are all from dead laptop batteries they will at least be similar cell types.

As the cell is charged there is buildup on the anode that increases IR with each charge and discharge cycle. This process goes faster if you charge right to 4.2v. The anode buildup progresses slower extending the life of the cell if you charge to only 4.0 or 4.1V.

You could compare the battery spec sheet IR to the one you measure to get a rough idea of the cell health, but the conditions in which you measured it will be different than the manufacturer, so it may not be valid data. In general, over time the IR goes up as the cell is cycled.

The simple part is this. Using the same charger to compare the internal resistance of two identical cells (say from the same laptop battery pack), the cell with higher internal resistance:

1. Will heat up more in use

2. Will have a reduced capacity

3. Is in poorer shape, closer to the end of its useful life, with likely small internal short circuits

4. If the IR difference between cells from the same battery pack is large with cell one IR = 150, and cell two IR = 400, the IR 400 cell would be more likely to overheat enough to cause a fire recycle it!

5. In grossly general terms an IR <150 milliohms usually indicates a cell in good shape.

Step 7: Final Steps

Once the cells are all re-wrapped, the insulator ring replaced, the cell tested and found to be usable, the cell data- labelled, and the cells grouped by capacity for future use, you have one more step.

Label the exact voltage of the fully charged cell. It should be >4.0 volts or recycle it.

Let the cell sit for 2-4 weeks and recheck the voltage. If a cell loses 0.2 or more volts over that period it does not hold a charge well, and should be used for minor low voltage tasks, or not at all.

If you want to discharge all the cells you acquired below 4.2 volts (or fully charged) to say 3.6 volts then check for a voltage drop from that position, you would place less stress in the cell and likely increase its lifespan. Remember these cells charge to 4.2 volts but degrade if you leave them fully charged.

Trying for ideal voltage conditions when working with recycled cells that were never at an ideal charge state for months or years in a dusty box of used laptop batteries is going to play into how perfect you try to be here.

Finally, until you plan to use them in the next day or two:

Discharge the cells to 3.4-3.6 volts to preserve their lifespan, and put them in your fireproof, ballistic bag for storage.



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    Penolopy Bulnick

    15 days ago

    Thanks for all this great information about recycling battery cells :)