The battery rule, which is the only actual rule

Lithium cells have real restrictions and they are not about embarrassment, they are about fire.

• Installed in a device: allowed in both cabin and checked baggage. Put it in the cabin if you have the choice, because a fire in the hold cannot be reached.
• Spare or removable cells, and power banks: cabin only. These are not permitted in checked baggage at all.
• Terminals must be protected on spares — tape over them or keep them in the original packaging.

Almost everything in this category has a sealed, non-removable cell, so it falls into the first line. If yours has a removable one, it goes in your hand luggage.

The bottle is the real risk

Cabin pressure is held at the equivalent of roughly 6,000 to 8,000 feet. That is not a dramatic change, but it is enough to push air out of a bottle that was sealed at sea level, and it is enough to make a thin liquid creep past a cap that was not fully tight.

Silicone lubricant is the worst offender because it is thin, it wets surfaces readily and it does not evaporate. A small amount escaping a cap will spread through a wash bag and then through whatever the wash bag is touching.

The fix is not complicated:

• Tighten the cap, then put the bottle in its own zip bag. Not the wash bag — its own bag, inside it.
• Do not fill bottles to the top. Leave air space, so the pressure change compresses air rather than pushing liquid.
• Do not rely on a pump dispenser. Pumps leak in luggage far more reliably than screw caps.
• Put the zip bag somewhere the leak will not spread. A hard case is better than fabric.

For hand luggage, the liquid rules apply: containers of 100ml or less in a single clear bag, unless your departure airport has the newer scanners installed. That rollout has been uneven across UK airports, so check the specific airport's current rules rather than assuming, because the difference between "100ml in a bag" and "two litres, no bag" is the difference between a smooth queue and abandoning a bottle at the tray.

The cable you will lose

If your device uses a proprietary magnetic charger, this is the highest-risk item in the whole exercise. There is no shop at the destination that sells it, the brand may not sell replacements at all, and without it the device is dead weight for the rest of the trip.

Take two. They are small and light, and one of them will be left in a hotel socket.

Cleaning away from home

You cannot boil anything, and you probably should not trust a hotel sink for something that goes inside you.

For silicone, warm water and a mild unscented soap is enough, then dry it properly rather than putting it away damp. For anything porous, a wipe is not a substitute for washing — it cleans the surface and leaves the interior as it was.

A small bottle of dedicated cleaner is worth the space if the trip is longer than a few days. If it is a weekend, water and soap will do.

The hotel room detail nobody mentions

Silicone lubricant will mark a wooden bedside table permanently. It penetrates the finish and swells the surface, and no amount of wiping gets it out. The same is true of leather and of some painted surfaces.

This is not a hypothetical — it is one of the more common ways people leave a deposit-sized mark on a hotel room. Put the bottle on a tissue, a glass, or back in its bag.

Discretion

Hotel housekeeping will move your things. If that matters to you, a small lockable pouch is more useful than trying to hide something in a suitcase, because a suitcase is exactly what gets searched.

In a shared room or a hostel, the same logic as storage applies: something that lives somewhere you would not want opened tends not to get used, and the whole point of bringing it was to use it.

Check the destination

A small number of countries treat the import of these items as a criminal matter rather than a customs annoyance. The list changes, and it is not something worth guessing about.

Check your own government's travel advice for the destination before you pack, and if the answer is unclear, leave it at home. No device is worth a customs interview.