What is a "Thermal Runway"?

What you're seeing is a swollen (bulging) lead acid battery. It looks alarming, but it's a known and well-understood behavior of sealed lead acid chemistry, not a sign that anything is dangerously wrong with your system. The important thing to know up front: fire risk is very low. The main thing to avoid is a case cracking and leaking acid, which is corrosive. A few safe-handling steps for right now:

• Disconnect the battery from any charger and any load.
• Leave it somewhere cool and ventilated, and don't set anything on top of it.
• Don't puncture, drop, or try to open it. If you notice a strong rotten-egg smell or the case leaking, open a window for airflow and keep the area clear until it cools.

As for why it happens: a swollen battery comes from a cell heating up faster than it can shed that heat. In our experience it traces back to one of a few things: the battery getting hot (direct sun, or a warm environment), a knock or dent from being dropped or bumped in transit, or an attempt to recharge a battery that had been run all the way flat and sat deeply discharged. Any of these can stress the battery enough to trigger it. For context, we build every system with quality components set to the correct charge parameters, and we ship thousands of these each year. A swollen battery is something we see only a couple times a year. It's rare, and it's almost always tied to one of the situations above rather than the equipment itself.

A swollen case is more fragile and may have acid near the vents. Wear gloves and eye protection, disconnect terminals (negative first), don't over-torque or pry, and set it in a sturdy non-metal container or a zip bag to catch any weeping. Keep it cool and out of the sun until it goes. Lead acid is fully recyclable and in most places can't legally go in household trash; it's regulated as hazardous/universal waste. Options: any auto-parts store or battery retailer (they're required to take them, and many pay a small core credit), a municipal hazardous-waste drop-off, or a scrap/recycling center. For a damaged/swollen one, call ahead and tell them it's swollen so they bag and store it correctly, as local rules vary.