If I had a big car battery and wanted to charge a smaller battery with it (say a laptop), how would I do that without breaking the laptop? First of all, a battery has a certain voltage and DC current which is the same thing a laptop battery uses, but you'd have to ditch the AC adapter because that has a transformer on it for converting AC to DC. I know that I=V/R but how can I charge one battery with the other battery (assuming they are rechargeable in the first place). If I connect the leads directly and one is a much bigger battery won't that damage the smaller battery? Is there a way to reduce BOTH the incomming voltage and current so that it isn't harmful to the battery? How would you do this? A capacitor?