To alter or readjust; tinker with: sought to raise output this year by rejiggering its agricultural policies. [first form 1940s+, second 1960+; fr mid-1800s jigger, ''shake or jerk rapidly,'' related to jig as a rapid movement, dance, etc, hence ''rearrange or readjust by shaking,'' semantically similar to shake up]
AGM Batteries. Absorbent glass mat (AGM) is a class of lead-acid deep cycle battery in which the electrolyte is absorbed into a fiberglass mat. The plates in an AGM battery may be flat like wet cell lead-acid battery, or they may be wound in a tight spiral.
An AGM battery is a lead-acid battery. You test an AGM battery just like you would test any other lead acid battery. You can measure its state of charge by measuring its voltage. Most AGM batteries are good for a service life of three years. That is probably why the seller is selling them.