The real danger with these rascals is after all the lead is chemically converted back, each cell is by definition fully charged. If, however, the available voltage is high enough current still flows. Amps of current times Volts across the cell is work being done. If there is no more chemical work to be done ('cuz it's fully charged) it's gonna be heat and (worst still) splitting of the water in the cell by electrolytic action into Hydrogen and Oxygen *gas* that will leave the cell for ever. This kills the part of the cell exposed, eventually the entire battery becomes useless.
Like what you're seeing. The guy that makes it is quite happy to only give you a (cheapest possible) charger and stands ready to sell you the replacement battery it kills.
Lead acid charging *demands* voltage regulation (or extremely careful 'trickle charging') when dealing with gel cell batteries. Ideally, in fact, the voltage needs to be temperature sensitive (lower at higher temperatures).
Doug Owen