Is it possible to use an 8 battery pack holder with 4 batteries? I bought an 8 pack holder that has a DC output so i can use it with the arduino. When i put 8 batteries into the holder, the aruino works perfectly on 12 volts. However, when i try to use 4 or 5 or 6 or 7 batteries, the arduino does not turn on. Do i really have to buy a 4 battery holder now? Or can i use the 8 pack one
Those batteries are all put in series. That means they are all stacked. The red wire is to one side, the black to the other side. If you want to use less batteries, you need to make sure that the last battery connects to the last contacts (and the first to the first). You could consider to use some metal bars instead of the last batteries. I would recommend to use 5 or 6 batteries by the way. That will give 7 to 9 volts so the voltage regulator can do its job comfortably.
I've used a little piece of metal or aluminum foil between the end of the last battery loaded and the holder side to provide a connection point.