Something that I've done in the past, quick and dirty and somewhat tricky to get it right (but works) is a PTC thermistor in parallel with a power resistor. These two devices are then in series between the cigarette lighter and the battery being charged.
The key element is the PTC thermistor. When the battery is completely discharged, it will attempt to sink lots of current. The thermistor then heats up and goes into a high impedance state, and on if left on its own, the available charging current would be too low. That is when the parallel resistor comes into play. It allows additional current to charge the battery.
As the battery starts to charge and the voltage differential drops, the thermistor cools down and goes low impedance, allowing again some extra current until it heats up and goes high impedance again.
This cycle repeats itself several times until the battery is almost charged and the thermistor always remains in the low impedance state.
The circuit is very simple, but requires significant experimenting and measurement to get the values correct. You will have to have an assortment of resistors and thermistors for your experiment. It is also better if you have a current probe to actually measure the charging current. And the battery voltage, of course.
EDIT: the PTC thermistor cannot be a polyfuse. Polyfuses have a wearout mechanism with each heating/cooling cycle which would render them useless after a few days. I used ceramic PTC thermistors.