1. Why Does the Same Battery Have Two Numbers, mAh and Wh?
mAh (milliamp-hours) is a unit that represents the amount of electric charge that flows. When you multiply this by voltage (V), you get Wh (watt-hours), which measures the total energy stored in the battery. The formula is "Wh = mAh × V ÷ 1000" [1]. For a battery with a nominal voltage of 3.7 V and a capacity of 20,000 mAh, the calculation is 3.7 × 20,000 ÷ 1,000 = 74 Wh. A 10,000 mAh battery would be 37 Wh, exactly half the energy. Airlines decide whether you can bring a battery on board based on this Wh figure, not the mAh [1].
To understand this with water: mAh is like the amount of water, while voltage is like the height from which the water falls. The power to turn a water wheel depends on both the amount and the height. However, electricity is not water itself, so this is just an analogy to help grasp the concept. Since some products do not have a 3.7 V voltage, if the device lists Wh directly, prioritize that figure [1]. How are these numbers used in airplane rules?
2. How Many Mobile Batteries Can You Bring on a Plane Under the Rules Starting April 24, 2026?
The Civil Aviation Bureau of the Ministry of Land, Infrastructure, Transport and Tourism announced new rules on April 14, which took effect on April 24. Passengers may carry up to two mobile batteries in the cabin, with each individual battery limited to 160 Wh or less. Spare batteries between 100 Wh and 160 Wh are included in this count of two. You cannot put them in checked luggage, as before. Store them at your feet or under the seat in front of you; avoid overhead storage bins. Charging the battery itself using cabin power, or charging other devices from the battery, is prohibited [2].
This change aligns domestic rules with international standards after ICAO (International Civil Aviation Organization) approved a revision on March 27. The background is that fires involving lithium batteries on planes have increased globally [3]. Let’s look at the numbers again. Dividing 160 Wh by 3.7 V gives about 43,000 mAh (160 × 1,000 ÷ 3.7 ≈ 43,243). 100 Wh is about 27,000 mAh. Even large commercial models with 20,000 mAh (74 Wh) fall within this range. Batteries over 160 Wh cannot be brought on board [2].
However, some explanations distinguish between batteries under 100 Wh and those between 100 Wh and 160 Wh, so the number of allowed items and conditions vary slightly by airline. JAL has stated that due to IATA rule changes, the limit might become 100 Wh after January 2027 [2]. Since this is not finalized, checking the airline's latest guidance before flying is the safest approach. What should you watch out for in daily life on the ground?
3. Why Are Heat, Shock, and Dust Bad for Mobile Batteries?
According to points highlighted by local governments and news outlets citing the Consumer Affairs Agency, mobile batteries are weak against "shock, pressure, and heat." Avoid leaving them in cars under direct sunlight or pinning them between car seats [6]. Other advice includes not placing flammable objects nearby while charging and checking if your model is subject to a recall [6].
In a reproduction test by the National Consumer Affairs Center of Japan (March 2021), charging with conductive foreign objects inside the terminals caused the terminals to heat up, smoke, and melt the plastic. When watching videos continuously while charging a smartphone, the temperature rises; if heat dissipation is blocked, it rises even more [5]. "Foreign objects" might sound special, but they are often small metal pieces or dust in your bag. Simply checking around the terminals occasionally can prevent accidents.
The same report lists signs of trouble: if the terminals feel hot or there is a strange smell, stop using it immediately. If the battery is swelling, refrain from using it and replace it or dispose of it properly [5]. There are also markers when buying. Since February 2018, mobile batteries have been regulated under the Electrical Appliance and Material Safety Act; products that do not meet standards cannot be manufactured, imported, or sold [4]. However, having the PSE mark does not guarantee absolute safety. So, what should you do when a battery reaches its end of life or swells?
4. Why Can't Used Mobile Batteries Go in Regular Trash?
You cannot put mobile batteries in regular trash. Bring them to cooperating stores or cooperating municipalities of the JBRC (Japan Portable Rechargeable Battery Recycling Center). Do not take them apart; bring the whole unit [8].
Before bringing it in, insulate the metal terminal parts. JBRC recommends using transparent vinyl tape. Cellophane tape cannot be used [8]. This preparation prevents electricity from flowing if the terminals touch each other.
Batteries with abnormalities such as swelling, damage, or water exposure, or batteries where the manufacturer is unknown, are not accepted by JBRC [8]. In these cases, consult your local municipality or the manufacturer. Because advice varies by location and company, first look for guidance on their phone lines or websites.
5. How Can You Calculate Wh for Your Battery and Check Where You Keep It?
Look at the back of your home mobile battery for "mAh" and "V" labels. If found, use a calculator to compute mAh × V ÷ 1000 to find the Wh. If the device lists Wh directly, prioritize that. This tells you how far you are from the 160 Wh or 100 Wh limits.
Think about your usual storage spot. Check if it is near a window with direct sunlight or inside a car in summer. Look visually for foreign objects, dust, swelling, or heat around the terminals. Do not touch the terminals; if you see something odd or think it looks wrong, do not use it and tell an adult.
You can read official information in Japanese from the Ministry of Land, Infrastructure, Transport and Tourism's new rules, the National Consumer Affairs Center of Japan report [5], and JBRC guidance [8]. Checking your airline's latest notice before flying is certain to keep you safe.