[SAJ]

Before I fitted voltage regulator cut outs to my 2 cars, I could smell the batteries out-gassing as they over-charged and electrolysed the acid. I do have a very acute sense of smell, as the chemists who work in my factory will attest!
Could this be a factor in what you are smelling?
My charging rate sits at 3 amps, lights on or off. It peaks at 10 amps just after using the starter, and falls to 3 amps or less in a very short time as the battery regains lost charge from starting. System is standard 6 Volt. The 10 amp recharging phase is now too short to make a detectable odour in the cabin.
The smell was unpleasantly acid and irritating to my throat.
I use John Reagan's Fun Projects regulator/cut outs by the way.
SAJ in NZ