I recently got my hands on a large batch of 18650 cells pulled from old laptop battery packs. The first challenge was figuring out which ones were actually worth using. For that I needed two things: the capacity of each cell and its internal resistance, because both of those tell you a lot about the health of a cell. I came across a 18650 battery charger, discharger, and tester all in one device, and in this article I am going to walk you through what it does, how it works, and how I use it to sort through salvaged cells.

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What the Device Does

The tester has four independent positions for 18650 cells. Each position has a spring-loaded slot as well as a set of external terminals with breakout connections if you want to wire in a cell rather than slot it directly. Each channel is completely independent, so you do not need to fill all four positions. One cell, two cells, or all four all work the same way.

Power comes in through two USB-C connectors at the top of the unit. Each connector is rated at 5 volts and 3 amps, and each one powers one side of the board. That means you do need two separate USB-C power sources to get all four channels running. At the bottom there are indicator LEDs for each channel. A blinking LED means the channel is actively running an operation. A solid LED means the operation has finished.

The Screen and Controls

Once powered up, the screen shows a lot of information. You can cycle through each channel using the far right button. For each channel you see the current voltage, the current flowing in or out of the cell, the milliamp-hour count, the internal resistance of the cell, and the time elapsed on the current operation.

The internal resistance reading is one of the most useful things here. When I tested my batch of salvaged cells, I got readings ranging from around 56 milliohms all the way up to 279 milliohms. That spread immediately tells you which cells are healthy and which ones have degraded to the point where they are not worth using.

There are three function buttons on the device. The first one cycles through the setting you want to adjust. The second button changes the value of that setting. The third button starts or stops the current operation on the selected channel.

Modes and Settings

Each channel can operate in one of three modes. Auto mode will decide by itself to charge or discharge the cell based on its current voltage. Charge mode always charges up to 4.2 volts, which is fixed and cannot be adjusted. Discharge mode draws a constant one amp from the cell until it reaches a cutoff voltage that you set yourself, anywhere from 2.5 to 3.5 volts.

In auto mode you can also set a loop count from 1 to 9. This tells the tester to repeat the charge and discharge cycle a set number of times automatically. That is useful for conditioning a cell or getting a more representative capacity reading across multiple cycles.

Testing Cell Capacity Properly

If you want a reliable capacity measurement, the order of operations matters. You should discharge the cell first, then charge it fully all the way up to 4.2 volts, and then run a fresh discharge cycle while the tester counts the milliamp-hours going out. The reading at the end of that discharge is your actual usable capacity.

The reason you measure during discharge rather than during charging is that charging has losses. Some energy is absorbed into heat and circuit inefficiency along the way. Measuring on the way out gives you the true deliverable capacity of the cell, which is what matters in practice.

Inside the Device

Opening up the case by removing the four screws on the sides reveals a clean PCB sitting on an injection-moulded plastic base. On the back of the board you can see the high power resistors that handle the discharge load. The fan sitting at the center of the device is there specifically to cool those resistors down during extended discharge cycles. Looking at the board layout you can clearly see four identical sections, one for each channel, each with its own control circuitry.

The two sides of the board share a common ground. On the front of the PCB there are two test points for 5 volts, one per side. In theory you could wire directly from a bench power supply into those test points and power both sides from a single source outputting more than 6 amps at 5 volts. The problem is that there is no clean mounting point for a connector, and bare wires soldered to small test points will eventually rip the pads off. For now I am sticking with two separate USB-C cables. A custom Y-splitter built around two USB-C connectors could be a cleaner solution down the road.

Testing Parallel Cell Packs

One thing I wanted to try was connecting two cells wired in parallel to a single channel. When you salvage cells from laptop batteries, you often find them in a 3S2P arrangement, three groups of two cells in parallel connected in series. When you find a pair that has held its voltage well together, it makes sense to keep them paired because the combined capacity is roughly double that of a single cell.

When you plug a parallel pair into the tester it recognizes them and works exactly as it does with a single cell. Charging takes longer because there is more total capacity to fill, but everything else behaves the same. The only thing to keep in mind when reading the capacity at the end of a discharge cycle is that you need to divide the milliamp-hour reading by two to get the per-cell capacity, since the tester is seeing both cells as one load.

Conclusion

This is a genuinely practical tool if you work with salvaged or recycled 18650 cells. Running four independent test cycles at once makes it possible to process a large batch quickly. Knowing the capacity and internal resistance of each cell lets you sort them properly and pair cells with similar characteristics before building them into larger packs, which is exactly what you want to do when combining cells into a project battery.

If this kind of project interests you, make sure to subscribe to the Taste The Code YouTube channel so you do not miss the next build.

Links And Resources

• 2ch 18650 Battery Tester: https://s.click.aliexpress.com/e/_c4So0t85
• 4ch 18650 Battery Tester: https://s.click.aliexpress.com/e/_c4t9yLwt
• 8ch 18650 Battery Tester: https://s.click.aliexpress.com/e/_c4BHPtiD
• 18650 Storage Case: https://s.click.aliexpress.com/e/_c35y8Fbx
• Bench Power Supply: https://s.click.aliexpress.com/e/_c3Em4DeZ
• Digital Multimeter: https://s.click.aliexpress.com/e/_c4NSL4xR
• 18650 Battery UPS: https://s.click.aliexpress.com/e/_c3sG3dx7
• Battery Spot Welder: https://s.click.aliexpress.com/e/_c3SK2OVj