Mobile Device Battery Safety
The following information is mainly directed toward our hotspots, but applies to any battery in a mobile device (phone, laptop, Chromebook, iPad, smartwatch, etc.). Recommendations from the U.S. Consumer Protection Safety Commission regarding mobile phones, batteries and chargers follow the pictures, and are applicable to almost any rechargeable battery.
If you ever notice the hotspot getting extremely hot to the touch while charging or indications that the battery has swelled as in the following photos, please stop using the device immediately and let us know.
While it is normal for them to become warm, it is not normal if it gets hot enough that you cannot hold it in your hand for more than a few seconds. The swelling of a battery is not normal in any case. If either of the previous conditions occur, the battery has the potential to explode 
or catch fire 
. Obviously, we would all prefer that not happening.
The following info is borrowed from an article at
http://www.amta.org.au/pages/Mobile.Phone.Battery.and.Charger.Safety.
"To promote the safe use of mobile phones, batteries and chargers, the U.S. Consumer Protection Safety Commission recommends the following:
- Do not use incompatible cell phone batteries and chargers. Some Web sites and second-hand dealers, not associated with reputable manufacturers and carriers, might be selling incompatible or even counterfeit batteries and chargers. Consumers should purchase manufacturer or carrier recommended products and accessories. If unsure about whether a replacement battery or charger is compatible, contact the manufacturer of the battery or charger.
- Do not permit a battery out of the phone to come in contact with metal objects, such as coins, keys or jewelry.
- Do not crush, puncture or put a high degree of pressure on the battery as this can cause an internal short-circuit, resulting in overheating.
- Avoid dropping the cell phone. Dropping it, especially on a hard surface, can potentially cause damage to the phone and battery. If you suspect damage to the phone or battery, take it to a service center for inspection.
- Do not place the phone in areas that may get very hot, such as on or near a cooking surface, cooking appliance, iron, or radiator.
- Do not get your phone or battery wet. Even though they will dry and appear to operate normally, the circuitry could slowly corrode and pose a safety hazard.
- Follow battery usage, storage and charging guidelines found in the user's guide."
User guides for most devices can be found on the manufacturer's website.