Lithium Battery Repair, Building, Re-use and Safety

Safe Lithium Battery Building & Repair Manual

1. Introduction

Purpose of This Manual
This guide is intended to provide hobbyists, technicians, and professionals with foundational knowledge and best practices for building and repairing lithium-ion and lithium iron phosphate (LiFePO₄) batteries safely.

Scope

Applicable to cylindrical, pouch, and prismatic lithium cells
Focus on safety, proper tooling, and responsible disposal
Intended for use by individuals with basic knowledge in DC electronics theory and practice.

2. Understanding Lithium Battery Types

2.1 Lithium-Ion (Li-ion)

Higher energy density than LFP (Lithium Iron Phosphate) batteries, mentioned in the next section.
3.7V nominal voltage, 4.2V charge voltage. Discharge voltage as low as 2.8V.
Common in laptops, power tools, EVs.
Various sub-categories of chemical composition, including:

-Lithium Cobalt Oxide(LiCoO2) — LCO: Very high specific energy, limited specific power.

-Lithium Manganese Oxide (LiMn2O4) — LMO: High power but less capacity; safer than Li-cobalt; commonly mixed with NMC to improve performance.

-Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2) — NMC: Most common chemistry in lightweight devices. Provides high capacity and high power. Serves as Hybrid Cell.

2.2 Lithium Iron Phosphate (LiFePO₄)

Often defined as a separate family to the lithium-ion chemistries mentioned above, LFP is distinct in that it has a lower nominal and charge voltage of 3.2V and 3.6V, respectively. It is regarded as one of the safest lithium chemistries due to its robustness against overcharging and lower intensity and likelihood of thermal runaways.
Safer thermal and chemical stability, with thermal runaway at 270°C as opposed to around 210°C for Li-ion.
Longer life cycle a the expense of lower energy density
Nominal Voltage: ~3.2V per cell
More stable charge-voltage curve.
Most commonly used in 12V applications in a 4S configuration (four cells in series) and for stationary storage, and many brands of EV, including BYD.
Lithium-manganese-iron-phosphate (LMFP) is a manganese-enhanced form of LFP that is relatively uncommon. It is said to increase the capacity by up to 15% with a specific energy of 135Wh/kg. The average working voltage is 4.0V, and a cycle life is said to be 5,000. Economic cost and safety are other purported benefits.

3. Safety Precautions

3.1 Personal Protective Equipment (PPE)

Before beginning any lithium battery works, having the equipment below is a minimium:

Disposal/Safety bin to quickly place a battery experiencing thermal runaway. Either a bucket full of moderately salty water (to enhance conductivity for rapidly discharging cells) or 2 buckets of sand: one to place the battery into and another to cover the battery up. Water is the ideal medium as it can absorb large amounts of heat and help reduce the intensity of the fire by discharging the cells rapidly. Sand cannot reduce the intensity or time of the fire, but it will smother flames to a small degree and reduce the release of gases. If neither is available, a thick-walled steel container with a secured non-airtight lid will help contain fire but will do nothing to reduce the large amounts of toxic gas released or the intensity of the thermal runaway.
Cut-resistant gloves and Arc flash safety goggles. Any battery assembled with a welded nickel strip or other current collector metals will be full of sharp edges during disassembly. When working on batteries higher than around 30V, there is a risk of uncomfortable but not necessarily dangerous electric shock, which can be mitigated by wearing gloves. Voltages higher than around 60V (depending on the individual) present a very real risk for injury from electrocution. Even at lower voltages, gloves and safety glasses may help protect you from arc flash and burns from high-temperature projectiles that can be made during accidental short circuits.
Fire extinguisher (Class D or ABC) on hand. These will not extinguish a battery experiencing a thermal runaway, but are to help contain any other fires that may be made around the battery.

3.2 Environment

When working on lithium batteries, you will need to be in a ventilated area free from flammable materials.
No metal jewellery to be worn (including keychains and lanyards) or conductive tools placed loosely in the work area.
During a thermal runaway event, massive amounts of smoke will be released even from the smallest of battery cells, enough to displace all breathable air in a room or a warehouse in a matter of seconds. It has been an underappreciated risk, with the focus on the intensity of the fire being prominent in media coverage. The fumes released from lithium batteries during a fire are extremely lethal and carcinogenic. Therefore, it is not advised to never work in a closed room and always have a clear accessway to an outdoor area.

3.3 General Handling Rules

Never puncture or crush cells. Any simple deformations or punctures to lithium cells can create an internal short circuit between the very closely spaced electrodes in the cell, producing a short circuit that may sometimes cause a delayed onset of thermal runaway.
Avoid short circuits. Although simple in theory, there are many less obvious current paths that can be created to cause short circuits when working on batteries. Nearly all the wires you will see in a battery will be at a unique voltage potential, and contact with those wires together or in any part of the battery can create a short circuit. Some common, often cheap, batteries will have poorly secured or no insulation between cells. Small, apparently insignificant manipulations of the cells can displace this insulation or air gap and allow the electrically charged cells in different series strings to come in contact with each other. Particular attention is needed for configurations where cylindrical batteries are placed end-to-end with each other. COMMONLY ON THIN E-SCOOTER AND BIKE BATTERIES. Sometimes this insulation is only a small piece of paper-like material that can easily be crushed to create short circuits, often on the positive terminal of 18650 and similar cylindrical cells.
DO YOU KNOW WHERE THE NEGATIVE AND POSITIVE TERMINALS EXTEND TO? Nearly all cylindrical cells will have a casing that will be electrically connected to the negative terminal. This casing will wrap all the way around to the positive terminal and enclose it, leaving a very thin gap (often less than a few millimetres) between the positive and negative terminals. Simply touching the end of a cell with a damaged insulation ring or sleeve can cause short circuits. Become familiar with the anatomy of the cells you are working with, including testing the voltage potential of the casing regardless of the type of cell.
Use insulated tools. This may seem obvious again, but accidentally dropping long conductive tools onto the battery will quickly bring bad fortune. For low-voltage batteries, specialist insulated tools are not necessary, and covering the shafts and handles of tools with electrical tape may suffice.
Tape/cover all exposed electrically charged components when access to them is not needed.

4. Required Tools and Equipment

Before considering building or repairing a battery, the following tools would be the minimum requirements:

Spot welder (preferred for cylindrical cells). Get in touch at econtact@slbatteries.com.au to get our suggestions on brands to purchase or avoid.
A soldering station with temperature control. Having a soldering iron rated to at least 50W will be necessary for most current-carrying connections and allow for fast joining of BMS leads to avoid heat soaking to your cells.
Multimeter
Insulating tape and heat shrink
DC power supply

5. Cell Inspection and Sorting

Before building or repairing a battery, each cell needs to be individually inspected and tested. This applies to both new and used cells. Some tests cannot be performed directly on all cells if already configured in a parallel configuration, such as IR tests. However, tests can be performed on groups of connected cells to determine any irregularities, without enough resolution to identify single faulty cells.

5.1 Visual Inspection

Check for swelling, rust, corrosion, or leaks
Discard damaged cells safely. For small batteries and cells, they can be disposed of at B-cycle collection points in Australia. Be sure to cover the terminals with clear tape.

5.2 Voltage and IR Matching

Match cells within ±0.05V before assembling them into a pack. If cells are connected in parallel with different voltages, then the higher voltage cells will charge the lower ones with unrestricted current at a rate that can be close to the short-circuit current for the cell. In the worst-case scenarios, this can lead to thermal runaway; in the best-case scenario, there will be degradation of the cells; in both cases, the mistake may not be immediately obvious (or noticed at all) until it is too late. For cells connected in series, having different voltages amongst the series groups (referred to as 'unbalanced') will lead to reduced usable capacity and power, or BMS shut off. Most common BMSs have inadequate balancing ability, so this may not resolve itself in any reasonable time. If a BMS failure occurs and an unbalanced pack can still be charged, this will very likely lead to a thermal runaway event even if it is connected to the correct charger.
Internal resistance should be within 10–15% tolerance. Within parallel cells, uneven internal resistances will lead to uneven current distribution through the different cells. If a pack is designed to be close to the limit of the rated charge or output current of the cells, then a wide IR difference within a parallel group will lead to cells exceeding their rated current, as current will divert towards the low IR cells. Even under less extreme conditions, the wear on the cells will be different as the battery is cycled. A difference between the IR of different series groups will result in uneven voltage sag under load, resulting in BMS imbalance errors, it will also likely result in uneven degradation and depletion of the cells under normal cycling.

6. Battery Pack Assembly

6.1 Configuration Planning

Determine the required series (S) and parallel (P) layout. Just as important as working out your desired capacity and voltage, doing the correct calculations here will require dividing the max and continuous current of the pack amongst the number of parallel cells to determine that they are individually rated for the expected charge and discharge rates with a reasonable margin of error.
ALWAYS use cell holders for insulation and spacing when using cylindrical cells. Although there are plenty of instructions and examples online and commercially of cylindrical cell packs constructed without cell holders/spacers. It should never be done, the benefits are minimal, and it is usually only a cost-saving exercise for mass-produced, cheap products... If they have tried to save $10 by avoiding using cell holders... where else have they cut corners? There are only a few examples where it would be appropriate (such as some very small designs immune to flexing and vibration).
We won't go into this too deeply, but the main benefits of using cell holders are: massively reduced risk of series group short circuits, ease of construction and welding, structural rigidity, impact resistance, and better cooling.

6.2 Connecting Cells

Spot welding nickel strips for series/parallel connection, there is a lot to cover here, but these are the main points:

Minimise heat exposure to cells by doing a few good welds instead of many bad ones. Take time between welds on a cell by moving to the next one in the group and coming back for the second welds.
Use a high-quality spot welder (get in touch for our recommendations)
Test your nickel strip is not nickel plated steel by filing or grinding the surfaces and leaving exposed to salty water and air, this will quickly produce rust on any steel exposed within a day. Other methods exist but are not as definitive or foolproof.
CALCULATE YOUR CURRENT LOAD THROUGH SERIES CONNECTIONS. This can be more deceptive than it seems. This will need to be done between every unique series connection, and for all batteries at the B0- and B+ and from the battery to the load. A safe conservative maximum of 10A per 10mmx0.2mm nickel strip will work for nearly all batteries designs.
Parallel connections, if done with a separate piece to the series connections, are best done after/above the series connections so that they will not increase resistance in the main current path.

6.3 Adding a BMS

Get in touch to receive our recommendations for BMS for various applications. For large, high-value systems, we recommend Prohelion.
Each BMS will often have its own setup procedure, however, it is normally best practice to connect the B- port before the balance leads to prevent damage to the battery.
The BMS chosen MUST match the series count and chemistry of the battery; this is a no-brainer. There is no scenario, even if it appears to work, that a battery with the wrong type of BMS will have any kind of useful protection.
When choosing a BMS, the second most important safety consideration is that the peak output power does not exceed the rating of the cells in the battery and that there is adequate temperature sensing available. Most BMSs will have a temperature sensor built into the BMS board, but the ability for this to react in time to a single faulty cell getting too hot at the other end of the battery pack is negligible. Find a BMS with temperature sensor leads you can place near the centre of the pack, or ideally, a BMS with multiple temperature sensors you can evenly distribute around the back.
Most BMSs are supplied from China, with questionable QC. It is always advised to treat any battery you are dealing with as having a potentially faulty BMS or one that may fail unexpectedly at any time, even due to random environmental interferences. This means understanding that there may always be risks present with charging or discharging a lithium battery, and fire containment considerations should never be ignored. For most reputable brands (such as trade-quality power tools, Australian-delivered EVs, and Australian-made BESS), the risk profile will be adequately in line with what is reported by the supplier. This does not translate at all to any E-mobility device on the market at the time of writing this. Our experiences have shown that the build quality in the majority of overseas-built e-scooters is wholly unsatisfactory, and they should always be treated as if they may spontaneously combust at any time. This means never storing or charging them inside, only outside in an undercover area.

7. Testing and CQ

The tests we recommend for lithium cells and assembled packs, other than visual inspections and items discussed in section 5, are: Voltage, internal resistance AC, internal resistance DC, self-discharge, self-heating, capacity verification, and charge profile testing.

We will cover these in more detail below. The results of these tests should be verified against the factory-specified data for the cells to determine which do not underperform:

-Voltage Test: Lithium-ion cells will have a maximum charge voltage of 4.2V and a rated discharge cutoff voltage of between 2.5-3V. Some manufacturers will specify a more precise charge voltage of 4.25V, but it is important to note that the amount of energy stored in the last 0.05V of lithium-ions charge cycle is insignificant, and using this value as a threshold for testing or charging will increase the likelihood of accidentally exceeding safe charge voltages when also taking into account the margin of error in the BMS or testing equipment's accuracy. Degradation of lithium cells will increase dramatically when charged to or stored close to 4.2V. Using 4.1V as a max charge voltage (when using a configurable BMS) will drastically improve longevity and safety at the expense of a minimal decrease in usable capacity (approx. 5% decrease).
See the Graph below for the charge profile of an 8AH Li-ion battery.

If a cell has a voltage above 4.25V, it is best to assume it has been irreparably damaged internally and may present a real fire risk. It should be slowly discharged to 0V (less than 0.1C until 4.2V) and then disposed of. See this page if based in Australia for the best recycling options. Similarly, but to a lesser severity, if the cell has been discharged below the recommended discharge voltage it should also be disposed of.

-Internal resistance AC and DC: The internal resistance of an individual cell, not connected to a pack, can be determined using an IR tester, which will send a high-frequency, low-voltage AC current into the cell and instantly calculate the Impedance, referred to generally as IR. Contact us for our recommendation on the best brands available for this. An IR test can be performed on a pack with a known SP configuration and cells, and an estimate calculation for the IR of the cells can be performed, which gives a basic estimate using the inverse formula for parallel resistance. However, this will neglect IR in the series connectors (which should ideally be negligible anyway) and only works on the assumption that all the IR values of the cells are identical. This test can be performed to positively identify if AT LEAST one cell is above the rated IR for the pack or group by substituting the value of the highest acceptable IR value from the cell spec sheet into the formula for all cells to determine if the maximum theoretical IR under this scenario is exceeded. This should be done with extreme caution as it is highly susceptible to experimental and calculation errors and will not identify which cell in a parallel group may be suspect.

The second type of IR test, being a DC current test, will give more real-world applicable results but will also be subject to notable experimental errors unless performed by an experienced technician under lab conditions. This is performed by running a known discharge current from the cell and back calculating the voltage sag using:

V(sag)=I(discharge) * R(discharge internal resistance)

Although not very accurate, this can be done easily on a complete battery when using a smart BMS with live voltage and current readouts.

The IR of a cell will also be increased at the upper and lower levels of charge, therefore, performing this test in these two scenarios will give the most safest/conservative results.

- Self-discharge: Lithium cells will self-discharge, although at a much lower rate than most other chemistries. If a single cell or pack is left to sit at close to full state of charge (SOC) then the decrease in voltage can be verified against the specifications for the cell for what should be expected for that time frame. After one day, this difference may be negligible, so the longer it can be performed, the more accurate the results will be. Performing this at close to full SOC will give the highest self-discharge rate and result in the most notable change in voltage per the V vs SOC graph shown above. Note that most BMSs will have a standby current draw and will discharge a pack slowly even when not being used.

- Self Heating: By operating the battery under the maximum charge and discharge conditions it is designed for whilst monitoring temperature at all points on the pack (ideally using an Infrared Camera or Temperature Probe) hotspots can be identified. These hotspots may be from high IR cells or weak connections, etc. Make sure this test is done at various intervals, including when the pack has reached thermal equilibrium with its surroundings.

- Capacity Verification: Inline with the above, doing a full charge and discharge of the cell under the test conditions specified by the cell manufacturer for capacity verification will expose issues of damaged or worn out cells. After two full charge cycles, there should be a negligible difference in the cell voltages in a multiple series pack, and the pack should remain balanced. Making the final measurement twice, once the pack is fully charged and once when fully discharged, will give the most exaggerated voltage differences to better identify issues.

- Charge profile testing: The charge and discharge profiles of the battery should resemble the example graph above. For instance, it may be seen that the charge current reduces early at constant voltage charging (CV), and the cells are unable to reach full charge voltage in a timely manner.

8. Common Repairs

8.1 Replacing Dead Cells

Generally speaking, there are three common repairs to be performed on Lithium battery packs: Full cell repack, damaged cell replacement, or BMS or BMS wiring repair or replacement. If a single cell group is faulty, it is usually a sign that the others may be ready to fail soon also, so a single cell group replacement is generally not advised. Additionally, if a parallel group has had one or more cells fail, then the other cells will have experienced additional wear compared to other cells in the pack and will also need to be replaced.
Occasionally, the standby current draw of the BMS may allow the cells to drop below the cutoff voltage of the BMS. If this is the case, AND the cells are above their specified minimum cutoff voltage, then the cells can be slowly recharged to a higher voltage that the BMS accepts (normally 2.8-3V).
Unbalanced pack: if the balance current is insufficient in the BMS, then the cells may slowly become out of balance over a long period of cycling. However, caution is needed to be sure there are no other underlying issues. Unbalanced groups will normally indicate some cells are dramatically underperforming, and if the pack has had a relatively short service life, this will likely be the case. Rebalancing the cells will, at best, give you close to the original lifespan it previously had before failing. At worst, it will hide underlying issues, such as some dead cells in a group, causing other cells in the parallel group to be exposed to excessive charge and discharge conditions that can result in thermal runaway.
The flowchart below is a summary of the testing procedure that should be done on a suspect battery:

8.2 BMS Replacements

Document existing wire layout by taking many photos and labelling terminals and wires if the existing plug cannot be reused.
Replace with equal or higher-rated BMS: A higher-rated BMS does not mean one that has a higher maximum charge or discharge current, as this could lead to damage to the cells or the load. A higher-rated BMS is one with a larger balancing current, additional temperature sensors, a better brand and/or additional features. It is best to replace with an identical BMS, or one with lower charge and discharge ratings that will not be exceeded by the intended use case.

9. Fire and Thermal Runaway Response During Testing or Repair

Per section 3, we will re-establish some basic safety procedures:

Isolate the battery if smoking or swelling occurs. Place in fire containment box, and if possible (no leaking fumes or active fire), move the box into an open area. Evacuate the area and allow thermal runaway to complete if fire begins. Notify relevant authorities. Do not re-enter space until there are no more toxic fumes present. Australian firefighting services carry equipment that can test for some of these chemicals.
Do not perform any tests or charging/discharging without constantly supervising the battery.
Fire is always a possibility, there is no room for complacency
NEVER place batteries in general trash. Contact us at econtact@slbatteries.com.au if you are unsure of how to dispose of and recycle any kind of lithium battery.

10. Where To Buy Battery Building Supplies

11. Disclaimer

Important Notice – Limitation of Liability

The information provided in this manual is for general educational and informational purposes only. While every effort has been made to ensure the accuracy and reliability of the information presented, Sustainable Lithium Cells Australia PTY LTD (trading as Second Life Battery Sales) makes no guarantees, representations, or warranties, express or implied, regarding the completeness, safety, suitability, or accuracy of the content.

Building or repairing lithium batteries carries inherent risks, including but not limited to fire, explosion, serious injury, and damage to property. By using the information in this manual, you acknowledge and accept that you do so entirely at your own risk. You are solely responsible for ensuring that any work you undertake complies with applicable safety standards, regulations, and manufacturer guidelines.

Sustainable Lithium Cells Australia PTY LTD, its authors, contributors, and affiliates shall not be held liable for any loss, damage, injury, or claim, whether direct or indirect, arising from the use of, or reliance upon, any information or practices described in this manual.

This manual is not a substitute for formal training or professional advice. It is your responsibility to seek appropriate certification or guidance where required.

This disclaimer is governed by the laws of the Commonwealth of Australia. If any part of this disclaimer is found to be unenforceable under applicable law, the remainder shall remain in full force and effect.