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In The Beginning, emergency backup power for industrial purposes was done with Flooded Lead-Acid batteries (FLA) and inverters. It worked well at the time, but there are drawbacks:
AGM addressed all these issues and became the standard for backup-power banks.
There are use cases where AGM makes a lot of sense, but otherwise AGM is not a natural fit for vandwelling. It's advantages are difficult to realize in a van and the disadvantages are front and center (2x cost per kWh, charging issues discussed see below).
So why do people default to AGM?
The dark side of AGM's ability to discharge and bulk charge at higher rates is