Charging LI bike light batteries with my camper's 12V system

[ewarnerusa]

I like to use my camper's 12V system to recharge various batteries because the camper is solar powered and it just gives me warm fuzzies to use that solar power for something else when I'm not camping. I do a lot of night time mountain biking and use various AA/AAA battery and li-ion battery powered lights. I've got my AA/AAA/18650 battery charger that runs off of 12V plug so I don't have to use the camper inverter. But I currently only have the 120V AC charger for my 8.4V Li-Ion battery pack that my primary light uses. I found a 12V DC charger for it; however, it only charges at half the current of the AC powered one. I realize that using the 12V DC charger will take about twice as long to recharge the battery. But my question is if the net energy requirement of charging at half the current but for twice the length of time going to be less than just using the inverter and using the 120V AC charger?

[time2roll]

I would use the inverter to avoid spending money on one more cord to keep track of.

[MrWizard]

no way to know with out testing

i all depends on the efficiency of design of the voltage "bucker" in the 12v charge adapter

how much does thing cost ?
is it worth the effort and expense, how much will you use it,
will the power savings ( if any ) be worth the expense