Car Battery Auto Disconect garunteed Start?

Awhile back i was loooking at a police gear magazine,(not sure of the company) and saw a device you connect to your car battery and it prevents your battery from being drained. Example: You leave your headlights on and it will only allow your battery to drain to a certain ammount so you will still be able to start your engine. If anyone could give me the name to this device or a link to buy one that would be great and i will give you BEST ANSWER. thanks!