They all have a battery in them, even the "pure solar" versions, AFAIK. They need it to store some level of energy to keep the unit working when there isn't quite enough light (like when you aren't yet welding). Some make it easy to change those batteries, others not so easy as they are permanently installed and you would have to tear the unit apart and possibly do some soldering to replace it. The solar cells provide a boost and operating power when welding, but they can't stay on in dim light conditions without a battery, so the cell helps charge that too.