Why charging deserves attention
Rechargeable cells store a lot of energy in a small space. Charging pushes energy in, and if something goes wrong at that point, such as a faulty charger, a damaged cell, an incorrect setting or heat that cannot escape, the stored energy can be released as heat, smoke or fire. Most cells are charged without any incident, but the consequences of a rare failure can be serious, so it is reasonable to treat charging as the moment to be careful.
This article gives general principles only. The manufacturer’s instructions for your specific cell, pack and charger always come first, followed by local rules and any guidance from your fire service or insurer. If advice here ever conflicts with those, follow them.
Use the right charger for the chemistry
Different battery chemistries are charged differently. Lithium-based cells, nickel-based cells and lead-acid batteries each need their own charging method and voltage limits, and a charger made for one can damage or ignite another. The differences between chemistries explain why the match matters.
Practical points:
- Use the charger supplied with the product or one the manufacturer recommends for that exact pack.
- Check the chemistry, cell count and current settings before starting, especially on adjustable chargers. A wrong setting is a common cause of trouble.
- Prefer chargers from reputable sources that carry the safety markings required where you live. A certified supply and charger has been tested against recognised standards, which an unmarked bargain unit may not have.
- Do not charge a cell that has no protection circuitry in a charger that expects it, or the reverse, unless the manufacturer says it is fine.
Supervise, and know what you are looking for
Charging unattended, particularly overnight, is the habit most often criticised in safety guidance. Many manufacturers advise staying present or at least within earshot, and checking periodically. If you notice any of the following, stop charging and, if it is safe to do so, disconnect and move away from the pack:
- swelling or puffiness in a pouch cell
- unusual heat, hissing or crackling sounds
- a chemical or sweet smell
- discolouration or leakage
- a charger that becomes very hot
Do not ignore early signs. A pack that has swollen should be treated as damaged and not charged again, and the guidance on storing and disposing of old batteries describes what to do with it afterwards.
Choose the surface and the place
Where a pack sits matters. A hard, non-flammable, uncluttered surface is generally better than a bed, sofa, carpet or a pile of papers. Keep cells away from anything that could burn and away from direct sunlight or heat sources. Leave airflow around the pack and charger so heat can leave.
Some makers use dedicated fire-resistant charging bags or containers. These may reduce the consequences of a failure in some cases, but they are not a guarantee, and they do not replace supervision or proper equipment. Consider placing the charging area where a problem would be noticed quickly and where escape routes are not blocked. Smoke alarms nearby are a reasonable precaution, and your local fire service may offer specific advice for lithium batteries.
Avoid charging damaged or extreme-temperature cells
Do not charge a cell that has been dropped hard, punctured, crushed, swollen, heavily corroded or left in a very hot or very cold place. Manufacturers often give a temperature range for charging, and charging outside it can harm the cell. If a pack has just been used heavily and is warm, many guides suggest letting it cool first. If you are unsure whether a cell is safe, treat it as unsafe.
Understand what protection can and cannot do
Many packs include a protection board, and chargers often have their own safeguards. Battery management systems typically watch for over-charge, over-discharge and short circuits. They add a layer of safety but cannot prevent every failure, for example mechanical damage to a cell or a fault in the protection itself. Treat them as a backup to good habits, not a replacement.
Check the rest of the chain
The charging path includes more than the charger and the cell. Check cables, connectors and mains adapters for damage, heat discolouration or loose fits. Loose connections can heat up, and thin or damaged leads are an easy place for a fault to start. The way wire gauge relates to current is relevant when you build custom charging leads, and where a ready-made lead is available that is designed for the job, it is usually the better choice. Ready-made certified power supplies are also the safer source for the input side.
When a charger plugs into household mains, keep in mind that faults on that side can involve shock hazards as well as fire, which is why mains voltage safety is a matter for suitable products and qualified people, not improvisation.
Keep a simple routine
A routine makes good habits automatic. For example:
- Inspect the pack and leads before each charge.
- Confirm the charger setting matches the chemistry and cell count.
- Charge on a clear, hard surface away from flammable items.
- Stay nearby and check from time to time.
- Unplug once charged, and let the pack cool before storing.
When using many cells at once, such as in a larger project, treat the combined energy with proportionally more caution. Power planning for a project should include where and how it will be charged. If you have doubts about a pack, charger or setup, stop and ask the manufacturer or a qualified person, since no set of tips can remove risk entirely.