Short answer: a battery that is warm while charging or during continuous shooting is normal; one that has swollen is not. With a W126S-class battery charged over USB-C, the charging circuit sits inside the battery itself, so heat is generated internally and the shell feels a little warmer than one that came off a dock charger. That is a consequence of the design, not a fault. Only two signals mean stop and deal with it: too hot to hold, and a visibly bulging shell. The first says the cooling conditions are wrong; the second says the battery has reached the end of its service life.
The risk in summer shooting is not heat on its own but heat stacking: high ambient temperature, a body pulling heavy current for a long stretch, and a battery on charge. When all three arrive together, that is when trouble starts.
1. Three heat sources with very different causes
| Situation | Source of heat | Normal? | Response |
|---|---|---|---|
| During USB-C charging | Conversion losses in the battery’s built-in charging circuit | Normal; warm is expected | Charge in open air, not in a sealed pocket |
| Burst shooting or 4K recording | Cell internal resistance under high current | Normal; the body warms too | Shoot in bursts and give it breaks; do not hold the body against your palm for long |
| In a car or direct sun in summer | Radiant heat from the surroundings | Not normal; this is a hazardous condition | Never leave a battery in the car |
The first two happen in the course of normal use, and a battery that feels warm but is comfortable to hold is within its design envelope. The third has nothing to do with the battery: the environment has pushed it into a temperature range it should never be in. Most lithium batteries have a charging ceiling around 45°C and a somewhat wider discharge range, with the exact figures set by the battery or body label. A closed car in summer, particularly near the dashboard, passes that number easily.
2. How to judge swelling, and why feel is not enough
Many people assume swelling is obvious at a glance. In the early stage it is a few tenths of a millimeter, the battery still fits and works, and the eye cannot see it. There is a more reliable method:
- Take the battery out of the body and wipe the base clean.
- Lay it flat on a glass table or tile and press one corner lightly to see whether it rocks like a spinning top.
- If it rotates noticeably, or the base flexes when pressed, the bottom has begun to bow outward.
- Then hold it edge-on to a light source. A healthy battery has a straight long edge; a swollen one shows a slight curve.
No tools are needed and once a month is enough. A second signal is fitting: if a battery feels tighter than before and needs force to click into place, that is usually the swelling pressing against the compartment wall.
3. What to do once swelling is confirmed
Swelling is irreversible and there is no repair. Any suggestion of freezing it or piercing it to release gas is extremely dangerous — the latter can start a short circuit and a fire.
The correct order is: stop using it immediately, do not charge it again, do not try to press it flat, and do not keep it in the body to finish the day’s shoot. Set it aside away from anything flammable and dispose of it through your local waste-lithium battery channel as soon as you can, or contact the seller for after-sales support. A swollen battery is also a restricted item for shipping, so do not drop it into ordinary mail.
One easily missed point: swelling rarely appears out of nowhere, and it usually arrives alongside a clear drop in runtime. If a battery you have used for most of a year now delivers less than half its old frame count, it is time for the desk-spin test above.
4. A few boundaries for summer shooting
Do not charge in the sun. It is common to leave a power bank and a battery on an outdoor table and top up while sunbathing. The battery then absorbs ambient heat and charging heat at once, which is the worst combination available. Find shade, a doorway or a corner indoors.
Do not charge inside a closed bag. Backpacks do not dissipate heat, and it accumulates faster than you would expect. If you need to charge, take the battery out, hang it on the outside of the strap, or put it on a table.
Do not press an ice pack against it. A sudden drop in temperature causes condensation on the shell, and moisture in the connector is worse than the heat. Coming in from a hot day to an air-conditioned room, let it sit in the bag for half an hour and come back to temperature on its own.
Do not store spares at full charge for long periods. For extended disuse, four to six tenths of charge is the better level; full-charge storage accelerates capacity loss. This has no direct link to swelling, but it does affect service life.
For behavior at the cold end, the notes on internal resistance and voltage sag in the cold-weather battery test are relevant — lithium cells follow the same rules at both temperature extremes. To see more models of this specification, browse the NP-W126S category page. The YO version follows the same boundaries structurally; the direct-charge port adds convenience, but it also means the battery needs better ventilation while charging than a dock-charger setup does.
FAQ
It feels hot while charging. Should I stop?
If you can hold it comfortably, that is normally within range and you can let it finish. If it is too hot to hold for any length of time, unplug it, let it cool in the shade, and try again with a lower-output charger. If it still runs abnormally hot, stop using it.
The camera body feels very hot. Is that the battery?
Usually it is the processor and sensor, and the battery is simply being heated along with them. The test is easy: fit a battery you know is good. If the body still runs as hot, the battery is not the cause.
Can I finish today’s shoot with a swollen battery?
Better not. Swelling means gas has already formed inside, and continued high-current discharge worsens the expansion. Switch to another battery for the day and set this one aside to deal with later.
Does a new battery need to be “activated” or charged and discharged three times?
No. Lithium batteries are formed at the factory and work normally from the first charge. Deliberately deep-discharging them shortens life; charging as you go suits them better.