Recovering 18650 Lithium Cells from Damaged Battery Packs

Introduction

Disassembling old battery packs from devices such as garden equipment, laptops, or power banks often uncovers 18650 lithium polymer cells that may be salvaged for reuse under proper precautions. These cells can be repurposed for building new battery packs for robotics projects or reassembling “zombie” battery packs. However, using cells of varying states or ages in the same pack is unsafe and poses a significant fire hazard. Such configurations should never be used in environments where safety is critical. Additionally, salvaged battery packs will void warranties and may not be covered by insurance policies.

In most cases, only one or two cells in a series chain are damaged, while the others remain functional. Parallel configurations often have more damaged cells, yet functional ones can still be salvaged. By carefully disassembling the packs and testing each cell individually, you can identify usable ones and attempt to recharge them.

However, cells that fall below a certain voltage threshold are deemed unsafe due to the risk of copper dendrites forming inside the battery, potentially causing internal short circuits. Such cells present a considerable safety hazard, as they can ignite or even explode.

Safety cannot be overstated when working with 18650 cells. Mishandling them may result in fires that cannot be extinguished with water, as lithium reacts violently upon contact with water. If you lack the necessary expertise, it is strongly recommended to avoid attempting to recover these cells. This process is inherently dangerous and should only be undertaken by individuals with the proper knowledge and equipment.

This article outlines my personal experience in salvaging and recovering 18650 cells, including how I test and charge them. It is intended as a project log rather than a comprehensive guide or recommendation.

• A word on economics of this project
• Used tools
• Disassembly of Battery Packs
• Selecting Candidates
• Recovering the cells
  • Phase 1: Very low current, 3.3V target voltage
  • Phase 2: Using a Standard Charge Controller up to 1A
  • Phase 3: Monitoring Voltage Over Time
• Building Battery Packs
• Cell Log Example
• Proper Disposal of Unsafe Cells
• Conclusion

A word on economics of this project

If you try to rescue cells out of economic reasons - don’t do it. Your time is worth more than what you are going to save if you are not doing this on a very large scale. And the induced safety risk is usually not worth it. In case you just need a working battery pack just buy a cheap one (note: this is an Amazon affiliate link, this pages author profits from qualified purchases), they are available in all different configurations for hobby applications, replacement for devices, etc. Building your own battery pack can be fun and challenging - and also interesting from a resource saving perspective - but not from a economic or safety point of view.

Used tools

Note: All supplied links are Amazon affiliate links, this pages author profits from qualified purchases

• Hand held small diameter grinder (Drehmel or Proxxon) to cut open packages and the interconnection between cells
• Various screwdrivers to open up packages
• A multimeter to monitor voltage
• A KA3005D benchtop lab power supply for current limited and voltage limited operation as well as PC readout of voltages during the recovery process
• 1S charge controllers including a BMS for supplying small scale microcontroller appliances
• A cheap battery powered spot welder
• Some permanent marker to uniquely label the cells

Disassembly of Battery Packs

The first step in recovering 18650 cells is to disassemble the existing battery packs. The construction of these packs varies significantly. Some are straightforward to open with screws, while others are sealed with glue or secured with one-time-use locking hooks. When opening these packs, it is crucial to avoid using cutting tools that could penetrate the cells, as this could lead to explosions or intense fires.

Once the pack is opened, you will often encounter arrays of batteries spot-welded together and housed in plastic spacers. To remove the nickel strips welded to the batteries, cutting discs are highly effective. However, extreme caution must be taken to avoid damaging the cells. Additionally, it is essential to prevent short-circuiting any cells or arrays. Series connections within the pack can result in significant voltages, posing a risk of electrical shock or burns caused by arcs. Short circuits can also lead to welded connections that are difficult to separate, and the resulting high discharge currents may cause explosions or fires.

When storing the recovered cells, ensure they cannot form a loop, where the positive terminal of one cell contacts the negative terminal of another. Such loops can result in high-current discharges, leading to fires or explosions. Proper storage and handling are critical to maintaining safety during the recovery process.

It’s a good idea to uniquely label the cells with your own markings though cells already come with a unique serial numbers. It’s just easier to work with ones own markings usually.

Selecting Candidates

As a first step, begin by measuring the voltage of each cell to assess its condition. This initial check is crucial for determining whether a cell is safe and worth recovering. The following voltage thresholds can help guide the selection process:

• 0V: Cells at this voltage typically indicate blown internal fuses. These fuses are an integral safety feature designed to protect against catastrophic failures, and bypassing them is strongly discouraged. While some cells may have resettable fuses (CID/PTC). CID stands for Current Interrupt Device, which acts as a pressure-activated fuse to disconnect the internal cell components if internal pressure builds up dangerously, usually due to overcharging or overheating. PTC stands for Positive Temperature Coefficient, a resettable fuse that increases resistance as the temperature rises, reducing current flow and preventing overheating. These fuses can sometimes be reactivated, allowing you to re-measure the battery. However, it is important to understand that fuses usually blow for a valid reason, such as internal damage or overcurrent. Reactivating a fuse without addressing the underlying issue may lead to significant safety risks, including fire or explosion.
• Below 2.0V: Cells in this range are likely damaged and considered unsafe due to potential copper dendrite formation and internal short circuits. Copper dendrites form when a cell is undercharged for prolonged periods, as the low voltage conditions lead to the electroplating of copper from the current collector. These dendrites can pierce the separator within the cell, causing a direct short circuit between the anode and cathode. Attempting to recharge such cells can trigger thermal runaway or explosions due to the internal short and resulting heat buildup. These cells often exhibit increased heating during charging, which serves as a potential warning sign. Additionally, even after the charging current is removed, they may remain warm or continue to discharge internally due to the self-sustaining current path created by the dendrites, until they are fully discharged. This behavior further underscores their instability and the associated risks. When attempting to reuse such cells, always monitor their temperature closely. Only conduct this process in a controlled environment equipped to manage potential explosions or intense fires, as these cells can fail catastrophically.
• 2.0V to 3.0V: These cells are heavily discharged but may still be recoverable. However, they are often below specification and may have reduced capacity due to the degradation of the electrode material and loss of active lithium ions, which chemically occurs when a cell remains in a deep discharge state for extended periods. Prolonged exposure to such conditions increases internal resistance and reduces the cell’s ability to hold charge. Additionally, most standard chargers are designed not to recharge such cells for safety reasons. While battery management systems typically prevent cells from discharging to dangerously low levels, damages to the battery pack or prolonged storage can still lead to such low voltages.
• 3.0V to 3.7V: This range represents a normal discharged state. Cells within this range are usually in good condition and can be safely recharged. However, some standard chargers may automatically attempt to charge such cells without verifying their condition, potentially masking underlying issues. Always test these cells thoroughly before use.
• 3.7V to 4.2V: Cells in this range are at a typical storage level (around 3.7V) or fully charged (4.2V). These are generally safe and in good working order.

Always handle cells with care during testing. Use a multimeter or dedicated battery analyzer to accurately measure voltage. Never attempt to charge or use cells that fall into the unsafe categories, except if you fully understand the risks and are prepared to manage catastrophic cell failure (not only during charging but also during subsequent operation). Properly recycle cells that you do not plan to reuse to minimize environmental impact and prevent hazards.

Recovering the cells

Phase 1: Very low current, 3.3V target voltage

To begin recovering cells, start by charging them very slowly to a safe baseline voltage of approximately 3.3V. This process must be performed using a constant current power supply, such as a benchtop lab power supply, to ensure precision and safety. For this purpose, set the voltage to 3.3V and the current limit to 18mA. This configuration allows for a controlled constant current charging process until the cell reaches the desired voltage.

Throughout this process, monitor the cell’s temperature carefully. Use a multimeter to periodically measure the voltage and confirm the power supply’s settings. If the cell begins to heat up significantly during this phase, it is likely unsafe due to internal discharge paths, as described earlier. In such cases, immediately discontinue charging and allow the cell to cool down before disposing of it properly.

Precautions: Always perform this recovery process in a controlled and safe environment, away from flammable materials. Ensure the cell is placed on a non-conductive and heat-resistant surface like a stone slab, and securely fixate the cell to prevent it from being ejected in case of an explosion. Maintain a clear zone free of flammable materials, and keep a fire extinguisher rated for lithium fires (Class D orspecialized lithium battery extinguisher) nearby for added safety but expect to just let an burning cell burn out fully. Regular monitoring with a multimeter helps ensure that the charging voltage and current remain within safe parameters. If you notice any unusual behavior, such as excessive heating or voltage fluctuations, the cell should be considered unsafe and discarded appropriately. Proper disposal ensures environmental safety and reduces the risk of accidental hazards.

Phase 2: Using a Standard Charge Controller up to 1A

In the second phase, connect the cells to a standard charge controller for charging. It is crucial to use a charger that limits the charging current to 1A or less for safety reasons. Set the target voltage to 4.15V and continuously monitor both voltage and temperature throughout the process.

If the voltage stagnates significantly below 4.15V, it may indicate internal discharge paths within the cell. In this case, the charger’s energy is dissipated internally, rendering the cell unsafe for further use. Additionally, while it is normal for cells to warm slightly during this phase, a rapid or excessive temperature increase is another clear sign of internal shorts. Cells that fail to reach the target voltage or become excessively hot during charging should be disposed of immediately.

If a cell successfully reaches 4.15V but remains hot even after disconnecting from the charger, it is also likely unsafe due to internal short circuits. Such cells should not be used and must be disposed of properly.

Phase 3: Monitoring Voltage Over Time

Once the cells have been charged to 4.15V, allow them to rest for at least two weeks before conducting further evaluations. After this period, measure their voltage again. A significant drop in voltage indicates internal shorts or self-discharge, making the cell unsafe for reuse. These cells should be properly recycled to avoid potential hazards.

For reference, the typical self-discharge rate of 18650 lithium cells in good condition is minimal, with a drop of only a few millivolts over several weeks. Any noticeable deviation from this behavior should be treated as a warning sign. Additionally, cells that were previously below 2.0V at the start of recovery remain at a higher risk of instability, even if they appear to function normally. When building battery packs, only use cells in non-critical applications where the risk of failure does not pose a significant hazard, such as in fire- or explosion-prone environments.

Building Battery Packs

Now that you have a collection of (hopefully working and recovered) 18650 cells, you can begin building battery packs. This process requires careful consideration of safety and proper design principles to ensure the pack functions reliably.

Compatibility and Safety: Never use recovered cells in circuits not specifically designed to handle lithium cells. Lithium-ion cells have very high discharge currents, which is one of their key advantages, but this also makes them dangerous in unsuitable circuits. Always ensure that charge controllers are not only designed for 18650 cells but also match the specific lithium chemistry (e.g., lithium cobalt oxide or lithium iron phosphate). Chemistry mismatches can result in improper charging profiles, which can lead to overheating or even catastrophic failure. If you are using single-cell packs for microcontroller projects, such as with ESP8266 or ESP32 boards, low-cost charge controllers and voltage regulator boards are readily available.

Connecting Cells: When constructing larger battery packs or designing for higher currents, it is essential to use proper connectors. Avoid using spring-loaded battery holders for high-current applications, as these can introduce large contact resistances, leading to overheating and potential fire hazards. Instead, connect cells using nickel strips that are spot-welded to each other. The thickness of the nickel strips must be appropriate for the target current; if needed, multiple layers can be spot-welded together to increase current capacity. Never solder directly onto the cells, as this introduces extreme thermal stress, creating both immediate and long-term risks of fire or explosion.

In a series configuration, the total pack voltage is the sum of the individual cell voltages, but the current capacity remains that of a single cell. Any imbalance in cell performance or aging will disproportionately affect the weakest cell, potentially leading to overcharging or over-discharging, both of which pose serious risks. Conversely, in parallel configurations, the voltage remains constant across all cells, while the current capacity is the sum of all the cells. Here, differences in internal resistance or aging between cells can lead to uneven current distribution, with weaker cells contributing less and possibly overheating. Understanding these differences is critical for designing packs that are both safe and efficient.

Balancing Leads: When building multi-cell battery packs, always include balancing leads for the charge controller. Balancing ensures that all cells in the pack maintain equal voltage levels during charging and discharging, preventing individual cells from overcharging or undercharging. The charge controller monitors the voltage of each cell individually, detecting imbalances and correcting them. This is achieved either by discharging cells with higher voltage into ballast resistors or by transferring excess energy into an inductor (tank element) that redistributes it to less charged cells.

In serial configurations, balancing is particularly critical as all cells must maintain near-identical voltage levels to avoid overcharging or over-discharging of individual cells, which could lead to failure or fire. Parallel configurations are less sensitive to such imbalances since the cells naturally share the load and voltage, but differences in cell quality can still result in uneven energy distribution. Proper balancing prevents these issues and ensures the longevity and safety of the battery pack.

High-Current Connectors: For high-current applications, use connectors designed to handle the required load. Examples include XT30 connectors for currents up to 30A and XT60 connectors for currents up to 60A. Proper connectors minimize resistance and ensure safe operation under load.

Temperature Monitoring: If you are designing an appliance with an integrated battery pack, consider adding temperature sensors to monitor the pack during operation. For small packs, a single sensor may be sufficient, while larger packs may require multiple sensors placed at critical locations. Monitoring temperature can provide early warnings of overheating, allowing you to shut down the system or reduce the load before a failure occurs.

Cell Log Example

The following log has been taken while recovering a few cells from old garden equipment to illustrate how a typical flow could look like:

Date	Cell	Start voltage	Charge current phase 1	Target voltage phase 1	Charger phase final voltage	Conclusion and comments	Useable	Checkup Date	Checkup Voltage	Result
01-20	1	2.4V	37 mA	3.3V	4.15V	Interrupted charger phase once at 3.7V	Most likely	04-14	4.13V	Ok
01-23	10	1.9V	18 mA	3.3V	4.15V		Most likely	04-14	4.08V	Ok
01-23	9	2.25V	18 mA	3.2V	4.09V	got pretty hot at charger phase, take a second look	Cautious	04-14	3.77V	Ok (stay cautious )
01-23	8	1.58V	18 mA	3.3V	4.12V		Most likely	04-14	3.94V	Ok
01-24	6	1.88V	18 mA	3.3V	3.9V	got pretty hot at charger phase, never reached 4V	Cautious	04-14	3.67V	Ok (stay cautious)
01-24	3	1.48V	18 mA	3.3	4.14V		Most likely	04-14	4.05	Ok
01-25	4	0.939V	18 mA	3.3V	4.16V		Most likely	04-14	4.12V	Ok
01-25	5	0V					CID blown?	04-14	0V	CID blown most likely, dispose
01-25	7	0.095V	18 mA	3.3V	4.04V	got very hot at the end	Cautious	04-14	0.813V	Dead, dispose
01-26	2	2.5V	18 mA	3.3V	4.16V		Most likely	04-14	4.12V	Ok

As one can see from the above log most cells that looked good during recovery kept their charges very well. The cells that got very hot showed very high discharge - a sign for formation of copper dentrites that lead to higher self discharge rates and thus rapidly increasing degradation of the battery. Those batteries impose a potential safety hazard - they are prone to exploding spontanously so they should be disposed as soon as possible or at least stored at a safe location. In addition I personally would not mix cells marked cautious (i.e. who got warm during the recovery charging phase) with potentially good ones in the same battery pack. It goes without saying that any pack built with such batteries should never be operated without supervision or in an area that could not tolerate cells exploding.

Proper Disposal of Unsafe Cells

Cells that are deemed unsafe—such as those showing signs of internal shorts, excessive heating, or deep discharge (especially below 2.0V)—should never be reused or stored with other batteries. Before disposing of such cells, ensure that the terminals cannot accidentally short-circuit: covering both ends with non-conductive tape (e.g., electrical or Kapton tape) is strongly recommended during collection and transport. Store these cells in a fireproof container away from flammable materials until they can be taken to a certified hazardous waste collection center or battery recycling facility. Never throw lithium cells into general household trash, as they pose a fire risk even when fully discharged (actually they tend to explode inside waste collection trucks and threatment facilities from time to time inflicting serious damage). Always treat potentially unstable cells with caution—lithium batteries can fail violently days or even weeks after initial damage or mishandling.

Conclusion

Recovering 18650 lithium cells from damaged battery packs can be a rewarding process for those with the necessary experience, tools and respect for the risks involved. It allows for the reuse of valuable resources and can support hobby electronics projects in a cost-effective and environmentally conscious way. However, safety must always be the top priority—working with lithium cells carries serious hazards, from fire to explosion, especially when dealing with unknown or degraded components. Always test and monitor recovered cells rigorously, never mix questionable cells with good ones, and avoid using them in critical or unattended applications. When in doubt, prioritize safe disposal over potential reuse. This process is not a shortcut to cheap power, but a methodical and cautious exercise in extending the life of otherwise discarded components.

This article is tagged:

• DIY
• Electronics