What's the safest way to use power from my cigarette lighter to?
Your cigarette lighter socket can supply around 10amps at 12v. This is plenty to charge a second battery with. Buy a 12v trickle charger plug it into the cigarette lighter socket and connect it to battery no. 2. The advantage of this approach is the cigarette lighter socket should only turn on when the key is inserted and turned, so usually when the engine is running. This means you don't run down your starter battery, so you can always start your vehicle. It's a slow gentle charge just to keep the second battery topped up. A disadvantage is also that it is a slow charge. If you want to be charging faster you need a split charger this is either a relay, or diode system that splits the current from the alternator to the two batteries when the engine is on, but which won't allow devices attached to the second battery to drain the first one, so you can start your car. A battery to battery charging system is a more sophisticated version of the 12v trickle charger, it intelligently draws power from battery 1 and the alternator when it senses there is sufficient surplus and maximally charges battery 2.