Technical notes from the cell factory floor

When a buyer specifies a 18650 cell for a cordless drill, the first question is almost always capacity. It is also the least informative one. A cell rated at 2,000 mAh that cannot sustain the current a tool draws will trip the tool's protection circuit faster than a 1,500 mAh cell that can.

The parameter that decides whether a tool works is continuous discharge current, measured against the cell's internal resistance. A cell with 25 mΩ internal resistance will sag far less under load than one at 60 mΩ, and it will dissipate less heat while doing so. Heat is what kills cells in tools: a cell that runs hot ages quickly, and its capacity fades long before the tool is retired.

The second decision is chemistry and case format together. Manganese-based cells such as the IMR and INR families we manufacture trade peak energy density for cost, safety and stability under load, which is exactly the trade a tool maker wants. In a 18650 format, a manganese-based INR18650-2000P5.0 delivers 2,000 mAh at a 5.0C continuous discharge rate with 20 mΩ internal resistance — a combination that suits high-torque tools where weight and cost are already under pressure.

The third decision is the temperature window. A tool used in a workshop at 5 °C or a garden at 35 °C will not see the same performance as one on a bench at 25 °C. Ask the supplier for capacity and discharge performance at the temperature extremes of your market, not at 25 °C, and check whether the cell's published figure is measured with a rest period between cycles.

Finally, confirm that the cell you are buying is actually the cell in the pack. Sorting by capacity, voltage and internal resistance before assembly is what makes multi-cell packs behave consistently. A pack built from unmatched cells will always have its weakest cell define the runtime, and in a high-drain tool that weak cell is also the hot one.

At Minmetals East we grade every cell on OCV, capacity and internal resistance before it leaves the line, so the cells arriving in your assembly are matched by the criteria that actually matter in service.

A datasheet tells you what a cell can do, but only if you read the footnotes. The three numbers that drive most purchasing decisions — nominal capacity in mAh, energy in Wh and rated cycle life — are each quoted under conditions that are often left off the summary sheet.

Nominal capacity is measured by discharging at a stated rate, usually 0.2C or 0.5C, to a stated cut-off voltage. The same cell can show materially different capacity at 0.2C and at 1C, because internal resistance converts part of the load into heat. This is why comparing two cells quoted at different rates is misleading: the lower-rate cell is not 'better', it is simply being measured under easier conditions.

Energy in Wh is the more useful figure for device designers because it is directly comparable across sizes. A 2,600 mAh INR18650 at 3.7 V nominal stores 9.62 Wh; a 1,500 mAh 18650 stores 5.55 Wh. In a device that must run for eight hours on two cells, Wh — not mAh — is the number that determines whether the requirement is met.

Cycle life is the figure most often quoted with the least context. A cell rated at 500 cycles at 0.2C, 25 °C and 80% depth of discharge may deliver far fewer in a real product, particularly if the device runs hot or is rarely fully discharged. Ask specifically for cycle life at the temperature and depth of discharge of your application, and ask whether the test used a rest period between cycles.

Finally, note the cut-off voltage. A cell quoted at 2.75 V cut-off and one quoted at 3.0 V are not interchangeable, and pairing them in the same pack means the 3.0 V cell is the one determining runtime — and the one taking the abuse. Matching cut-off voltage within a pack is part of what our grading process is designed to guarantee.

If you would like a datasheet reviewed against your application, send us the load profile and the ambient range. We will tell you which of our models fits and which does not.

Lithium cell exports fail for one reason more than any other: the documentation is treated as a single package when it is actually three independent systems, each with its own scope.

The first is transport. UN38.3 testing covers the ability of a cell to survive the rigours of transport, and the test summary is what a carrier will ask for before a container is accepted. This applies to every lithium cell leaving China, regardless of the destination market or the end use. It is a transport requirement, not a product requirement.

The second is the destination market's own product approval. In the European Union, a battery placed on the market must satisfy the EU Battery Directive, and the applicable legislation has tightened considerably over recent years, with new requirements for due diligence, carbon footprint declaration and digital battery passport arriving in phases. In Japan, PSE applies; in India, BIS; in the United States, UL 1642 under the recognised voluntary certification programme, often mandated by the retailer or the end customer rather than by law.

The third is the environmental and chemical framework of the destination: RoHS for restricted substances, and REACH for substances of very high concern in the European Union. These apply to the cell and, depending on the configuration, to the pack as well.

There is a fourth, practical category that catches exporters out: what the retailer, the OEM customer or the insurance underwriter requires, even when the law does not. Many European and North American buyers will not accept a supplier who cannot provide a complete test report for the exact model being shipped.

Minmetals East currently holds UL 1642, BIS, PSE, an IEC 62133 CB certificate, REACH, RoHS, EU Battery Directive and UN38.3 transport certification, on top of ISO 9001 and ISO 14001 management system certification. The documentation package for a given model and destination is prepared before production, not after a customs query.

If you are planning a new destination market, tell us the country and the application early. We will confirm which approvals the model needs and what additional testing is required.

A pack of four identical cells behaves nothing like a cell four times bigger. The reason is that in series, the pack voltage is the sum of the cell voltages, and the pack runtime ends when the first cell reaches its cut-off voltage — not when the average cell does.

Consider two cells of identical 2,000 mAh capacity, one with 20 mΩ internal resistance and one with 60 mΩ. Under a high current draw, the higher-resistance cell has a larger voltage sag at the same state of charge. It therefore reaches the cut-off voltage while still holding a substantial amount of its own capacity, and the usable energy of the entire pack collapses to what that cell can still deliver.

This is why cell matching is not a cosmetic production step. Grading by OCV and capacity alone will not find the cell that sags hardest under load; the internal resistance distribution has to be measured and used for matching. Our sorting stage measures capacity, voltage and internal resistance for every cell precisely so that matched packs can be assembled.

There is a second-order effect worth considering: heat. The energy dissipated in a cell during a high-current discharge is proportional to I²R. Double the resistance and you roughly quadruple the heat generated in that cell, while the rest of the pack stays cool. The cell that is already the weakest becomes the hottest, and heat accelerates degradation. This is a common failure pattern in cordless tools and in high-power lighting.

The practical guidance is straightforward. Specify a maximum internal resistance, not just a capacity, and require the supplier to state the measurement conditions. Ask how cells are matched before packing. And if you are building packs in the field rather than in a controlled factory, consider whether the cells should be sorted at pack assembly as well as at cell production.

We publish internal resistance for every model in our technical parameter tables, measured at first grade. If your pack design has a target resistance budget, we can tell you whether our current catalogue meets it.

Lithium manganese oxide — usually referred to as LMO, and identified in our product codes by the M in IMR — is frequently assumed to be a legacy chemistry overtaken by nickel-manganese-cobalt. For certain product categories that assumption is wrong.

The first consideration is safety margin. LMO has a higher thermal stability than the nickel-rich chemistries and, in the manganese-based IMR family, we work with an electrode system that resists the runaway behaviour associated with high-nickel cathodes. In a consumer product where the cell sits in a plastic housing next to a user's hand, in a bag, or in a children's toy, that margin is worth paying for.

The second is cost, and it is not marginal. LMO contains no cobalt and no nickel in the cathode. For products that do not need the energy density of a high-nickel cell — and many do not — a manganese-based cell removes two of the most volatile and most heavily scrutinised input materials from the bill of materials. That matters more each year for European and North American brands working against supply-chain due-diligence requirements.

The third is cycle behaviour. LMO cells generally offer good cycle life at moderate depths of discharge and hold their capacity well in applications with a shallow, frequent duty cycle — think lighting, audio, personal care and small household devices.

What manganese-based chemistry does not do is maximise Wh/kg in the smallest possible package. If a device has a hard volumetric constraint and a hard runtime requirement, a high-nickel cell may be the only way to fit it. That is a legitimate reason to choose NMC, and we are not going to pretend otherwise.

The honest position is that most consumer devices do not need the energy density that NMC offers, while they very much do need the cost, safety and supply-chain advantages that manganese-based chemistry provides. For those devices, LMO remains the more sensible engineering choice.

We manufacture both manganese-based cell families — IMR for standard manganese and INR for high-capacity manganese — and we supply our own LMO cathode material, so the quality of the active material is inside our control.

No consumer-grade lithium-ion cell can be made completely free of thermal runaway risk. A credible supplier does not pretend otherwise; a credible supplier controls what it can control, documents it, and tells the pack designer plainly what remains.

What the cell supplier controls begins with the active material. Control of cathode chemistry, particle morphology and the electrode coating consistency has a direct effect on the temperature at which a cell begins to vent. This is one of the reasons we produce our own LMO cathode material rather than buying it: the specification of the powder and the specification of the electrode are one continuous process, and a break in that chain becomes visible as a change in cell behaviour.

Manufacturing control follows. Electrode thickness uniformity, particle removal from the electrode surface during cleaning, the absence of weld debris and metal contamination, and correct electrolyte filling volume each affect the likelihood of an internal short. Our process applies automated inline checks at the coating, winding, filling and sorting stages precisely so that a deviation is caught at the stage where it originates.

Then there is the matter of sorting and grading. Internal defects that a cell would not reveal at 0.2C discharge can be detected by capacity, voltage and resistance grading under test, and cells that fall outside the specification are removed rather than shipped.

What remains is the pack designer's responsibility: protection circuitry, cell spacing, venting design, the charge protocol, and thermal management of the finished product. No cell can compensate for a charger that charges below freezing or a pack that traps heat.

We test thermal abuse, extrusion and needle penetration, low-voltage and high-altitude simulation, salt spray and temperature-humidity cycling on our own equipment, and we will share the relevant test summaries with your engineering team on request. That is the honest position: we can make the cell as safe as a consumer cell can be, and we can document it. The final safety case belongs to the product.