I've noticed at the track lately that some cars have a tiny battery pack in their cars. Is this a type of receiver battery so it doesn't use up power from the 4 cell battery or is this so the car doesn't run away when the battery dumps?

Thanks, just curious.

The voltage in 4-cell can drop very low under heavy amp draw even when the pack is fully charged. Most electronics are made to work at 5+ volts so this low voltage can cause glitching and other bad things to happen.

That's what I figured. Who makes them and what do they cost? Will it plug right into the battery hole in my 2 channel reciver? I use Futaba 'J' plugs.

They used mostly in 4cell mod, sometimes in 19turn but hardly in stock. The come in loose or assembled in flat or hump, where you may have to install the pins to wire then into plug housing which is usually a standardize plug to fit most receivers. Most esc's require that the positive plug on esc be removed, lift plastic retaining and gently pull out plug then wrap with tape to prevent a short (LRP, Quantum , Tekin probably Novak should be done this way). Now esc switch can be left on but before turning off radio remove receiver pack plug or install a switch and turn off.
Hank sells the packs here see HobbyShopper link and on some online sites:RC4Less, Lefthander RC, Darkside Motorsports, Pro Match Batteries etc.

We have them at www.hobbyshopper.com