Hi and welcome Martin
I dint know the device you ask about but am nervous about trying to use a cigar lighter to cigar lighter for much more than a trickle charge, if they have tested this and let's hope they have then it will eventually charge up the flattest battery. Then again so would a pair of jump leads? The advantages are it's cleaner and less fuss to connect so if it's been tested then it's a simple solution.
Where is the car stored? Is it in an underground car park or a surface car park? If the latter there are a range of solar charger panels available that may be of interest, connected with long leads to the cigar socket is fine to keep a battery topped up, they provide a steady trickle charge and quality ones will not fry the battery and can be left connected for long periods, check with a caravan spares shop. Of course the ultimate is a float charger/conditioner but without mains electricity that's no help to you.
Good luck finding a solution and safe miles to you.
