Last time I saw one of these was at Cleveland in 90 and 91. Dosen't mean that they don't now exist, or people don't still use them. In a situation like Cleveland, there is a real reason to use one (maybe), the fact that you have to leave your hotel room which could be on the sixth floor and your batteries are 'flattening' as you make the long trek to the track, through tech, and have to wait till the race previous to yours is complete. So boosting the voltage would be a big advantage. Do you need one on the club level? I think not, just strategically charge your batteries so that they peak close enough to when your scheduled race is.
Will cliping leads directly to a pack from a 12vdc source harm them? It is possible. It is also possible to harm a battery pack with a standard charger that is supposed to stop charging your batteries when they peak, especially if the settings are incorrect. When they clip a 12v source to the battery pack in the car to 'bump' them back up to peak voltage, they are actually monitoring the batteries, either by temperature (actually touching them), or watching a volt meter to watch the pack peak again. I am sure that there are a vast many other ways of bumping a battery.
Speed Merchant Rev7, Tekin, TQ Racing (wire), Team Tamale
RC Excitement - Buy where you race, support your local tracks.