My assumption is that when charging to the battery terminals directly, all permanently connected circuitry gets 12V + the extra supplied by the charger and the charging current supplied by the charger (which varies depending on the level of charge the battery has). So the circuitry gets more 'power' than normal, possibly more than it can be guaranteed to cope with. My further assumption is that when charging via the power socket, there is some protection electronics that limits the voltage and current that makes it into the rest of the bike, so to speak. Hence, charging via the power socket is 'safer' as some level of protection from over-voltage and over-current is afforded. I should say I am not an electrical engineer, I am an environmental scientist, who is likely to be buying an Optimate 4 with BMW CANBUS support very soon...