He says: "It is certainly possible to charge an 8 volt battery with a 12 volt charger, and certainly not recommended.
Because the charger is build to read for a specific voltage cutoff point and charge curve. A 12v charger is build to know the float voltage is ~12v, but the peak will actually be higher (13.5v? 14v?). So a 8v charger and a 12v charger with the same Amp charge rate will peak at different voltages; a 12v charger will cook an 8v battery if left unattended, and it's not covered by the warranty."