Make Lithium Battery Pack From Salvage




Introduction: Make Lithium Battery Pack From Salvage


The goal of this ible is to show you how to salvage li-ion cells, sort them properly, revive them (when possible), and re-use them safely for a new use.

Giveaway: follow my instructables and YouTube channels and post an "I've made it" picture to get the free pro membership voucher I got when this instructables was featured.

Step 1: The Basics

A few things to understand. Cells (single "batteries"), are cells. A battery is a combination of cells. Cells at a given price are usually either high capacity OR high power, or a midway of the two. You can get high capacity and power if you pay a lot of money and have lots of space, but generally one comes at a cost to the other because high power needs thicker material, and high capacity needs more material (more thin layers instead of fewer thicker layers). The exception is lico cells which are cheaper, high capacity an high power, but tend to be extremely flammable, volatile, and don't last very long, so salvaging them is not a good idea. More on this as I present the main characteristics you want to know about cells. They have a few numbers attached to them.

  • V (voltage) which is pretty much the same for most kinds of lithium batteries, except lipo and liFePo4 cells (not getting into this here). Voltage is higher when the battery is fully charged, and goes down as you deplete the battery. Normally around 4.2 volts the battery is at 100%, and at 3v completely discharged. You can keep using them a bit, but past 3v per cell, you will start damaging them.
  • Ah (ampere hour, or amp hour). This is "capacity". The higher it is, the more energy it contains. A universal measure is VA (volt-amps) or watts, which is when you multiply nominal (normal) voltage by Ah. For lithium, that is usually 3.7v +-0.1v times your Ah rating.
  • C rating; this is "power". A cell capable of 2C can output 2x the Ah rating. It's how strongly you can push those volts out. In a motor, v = speed, amps = torque. C tells you how many amps you can push with at a given moment. Continuous rating is what you battery can safely output for a long time, and sometimes you also get the momentary or burst C rating.
    • C rating is directly related to the IR (internal resistance) of the cell. When it is very high in resistance, more energy is lost as heat as you push very strongly. In high capacity, low power, cells, this can lead to a fire hazard. High capacity cells are usually rated for 0.5C-1C; high power cells can be rated for as little as 3C (so be careful what you buy) to 10C or even 20C. The 10C+ cells are usually LiFePo4 or LiMn (I will be using LiMn salvages in this ible as it happens).

Types of battery arrangements: parallel (p) and series (s). In parallel, you add up the Ah. In series you add up the voltage. You can do both too. Example with 4 3v cells at 2Ah capacity.

  • 1s4p = 3v with 8Ah capacity
  • 4s1p = 12v with 2Ah capacity
  • 2s2s = 6v with 4Ah capacity

In this ible, I focus on power tool packs, because they use higher grade cells than say laptops, and as such they are less dangerous even if they have been over-discharged. Moreover, it is easier to identify that you got the same types of cells and are not mixing incompatible chemistries. Laptop packs are meant for capacity and cost savings. Power tools packs are meant for capacity and POWER, which makes them much safer. Capacity is the number of watts in the battery (VA, or Ah relative to voltage), whereas the power is the ability to charge and discharge faster. This is important because the only way to get more power is to increase the quality of the parts to lower the internal resistance. This means that as the cell may have degraded while it sat unused below a safe voltage, way more material can corrode before it becomes too thin to be safe. This is not true of high capacity-low cost batteries.

Step 2: Material

Once you have your hands on a few discarded li-ion powertool packs, you will need a few tools.

Optionally, but useful if you have one to quickly monitor if a cell is internally damaged, an infra-red thermometer. A small rotary tool with cutting discs can also be useful if you have steady hands.

Step 3: Take Apart the Pack

Start by opening the pack. You will need to remove the screws, and most likely cut the labels (the seams often run under them to prevent tampering, but not always).

Step 4: Pre-test the Cells

This step is not absolutely necessary, but you can save some time by pre-testing your cells. Measure the voltage on each of them, and mark with an x any cell below 0.5v. These are pretty much certainly dangerous to use; absolutely not worth it. You can do this after, but this way, if you are trying to salvage the nickel strips, it is easy to know where to prioritize.

Step 5: Cut the Cells Out and Grade Them

You may already have started grading them before cutting them, feel free to continue doing so. You can either snip the nickel tabs, pull them off (but then working with the cells becomes much harder), or cut them with the rotary tool.

Once the cells are out, sort the cells by their voltage. Note that if this pack recently died (in the last few weeks) the voltage you test probably won't indicate much about how good the cells are. In the first few weeks, high quality cells at 0v can often still be recovered. However, usually you don't know how long it has been, or you know that it has been years (the ones in this ible stopped being made years ago), just assume the worst and use the following rules of thumb.

Here is my grading system

A: 2.0v-2.8v : looking good, especially above 2.5v! These are almost sure to be your best cells.

B: 1.5v-2.0v : decent, form the same pack as some A grade cells, it could be as good, but likely not quite as good.

C: 1.0v-1.5v : Whatever was lost from the max capacity in the better cells, double it here. You will probably end up with most of your decent cells around this capacity from very old packs; likely around 70% of A grade cell capacity in old packs.

D: 0.5-1.0v. These are quesitonnable, and will usually be in the 0.7-0.9 range, they rarely go below 0.8 without simply completely dying and dropping to a few millivolts. If they recover, you will likely be looking at cells with as little as 1/3 the capacity as A grade cells.

Anything above 2.8v is almost certainly still holding a charge, slap it on the charger and bring it back, but don't let it go back that low again.

Again, I can't stress enough how important the quality of the cells is. Laptop cells (very low power / high capacity) that have sat around for years would probably be much much worse off.

Step 6: Revive the Cells

Before the smart charger will accept the cells, you want to revive them. Bring them up to 3.3-3.4v using the ni-cd mode of the charger. This has to be actively monitored, and this is where the IR temperature gun comes in handy.

If the temperature starts rising rapidly, stop and discard the cell.

If the cell does not go up to 3.3v within 15 minutes at 0.5c, it is self draining, or the anode is broken. Discard the cell.

Step 7: Secondary Screening

Now that your cells are up to 3.3v-3.4v, you can do a secondary screening by waiting overnight.

Getting up to that voltage will only have taken a few milliamps, so you can easily see which batteries self-drain the fastest (and as such are the most damaged). Usually initial voltage was a very good indicator, but the next morning, re-grade your cells.

Any cell that stayed above 3.3 (assuming you did not give it a much bigger charge than the rest, this is where active monitoring changes the quality of your rating, you should not have left it lingering on the charger), then this is a high quality cell. Bump it up a grade, A becomes A+.

If the voltage went down to 3.0-3.3v, it is normal. B cells of this quality are a very good find. Likely to be very useful.

If the voltage went to 2.7-3.0, downgrade it 1 grade. D becomes a D-.

If it is between 2.0 and 2.7v, downgrade it two grades (so A->C, B or less ->D).

If it is below 1.5v-2.0 volts, it is of questionable quality. You may be able to use it, but these will normally have only about 1/4 of the original capacity at most. Not meant for heavy use.

If it went below 1.5v, I would consider it dangerous. Discard the cell. In a pack, it would risk dragging all the other cells with it.

Step 8: Fully Charge Them

I fully charged these high capacity cells with 2 to 3 steps.

For cells still above 2.7v, the charger will accept it. Start charging. You can use the rated value of the cell. Keep track of the number of mAh that go in it. 1/3+ of capacity is okay. If it is much less, consider it to be a questionable cell (this is almost always the ones that were marked questionable, or the D- cells).

For cells at or below 2.7, you will have to give them a boost in the ni-cd mode again, then charge them. Keep track of the number of mAh that go in it. 1/3+ of capacity is okay. If it is much less, consider it to be a questionable cell (this is almost always the ones that were marked questionable, or the D- cells).

Let it sit over night, and check the voltage again. They should now all float around 3.7v-3.9v. Anything that went back down to the 3.3 or lower is dangerous, discard it.

The third step is to charge it again as a slow charge, so 0.2C. Not a trickle charge, that is not safe for li-ion cells. Use the smart charger and set it to a current of the Ah capacity of the cell times 0.2. So 3000 mAh = 3Ah (m is milli, or one thou), 3A*0.2 = 0.6A. That simple. Now we are doing a slow and very full charge. This is going to be necessary for the last step of sorting the cells.

You can make your life easier by charging in parallel for this (see the picture). This works especially well with high power cells as they self balance. To do this I also had to add pigtails. I don't go into this in this ible (will do when I do my pack building ible), but you can get a preview of how to do that in the video.

Step 9: Test the Capacity

Now that you have fully charged cells, you need to discharge them. It is important to do it one at a time. In parallel, high power cells will balance each other out, and you will only get the average capacity. If you put these in series, you might be in for a surprise if you don't know what your weakest cell is.

Using the discharge mode of your smart charger, let it bring your battery down to a safe discharge level, and write the capacity in mAh on your cell. This is how much power it can now hold after having revived it.

Step 10: Figure Out the Use

Now that you have the max capacity calculated, you can figure out what applications they will be good for. Cells that are far down in capacity from nominal values (sub 70%) can't really be trusted to be used in series. However, a heavily paralleled application can still make use of these.

Again, with low power, high capacity cell, even in parallel you can run into issues of the cells not being able to self balance, in which case they can become dangerous. In this case, we have high power cells, so even if one cell in a heavily paralleled setup has minor issues, the rest of the pack will be able to handle it.

For a good example, my cells started off as 3 Ah cells with a 15C discharge rate (so safe continuous discharge at 40A), and peak discharge (momentary) at 100 A. I ended up with 3 at 1.5 Ah, 7 between 0.95-1.25 Ah, and my lower grade cells ended up at around 0.5-0.8 Ah. I decided to make a spot welder with them (more on this in another ible), but even with the high self-balance ability of these cells, I won't use the 3rd group. I might use those in a flashlight or something. However, the 1.5A cells will fit just fine with the 1.2ah cells (most of them), because they will just top up the other cells as I use my massive 12p1s battery.

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    2 Discussions


    1 year ago

    Aren`t those cells in pictures with NiMh or NiCd chemistry? Still you talk about salvaging lithium-ion-cells. Correct me if wrong and missed something.


    3 years ago

    Where's the best place to get these salvage / scrap battery packs? Thanks!