The lipo experts on RC Groups maintain storage between 3.7 to 4v/cell is fine, 3.8v gets tossed around a lot only because its in the middle of the range.


Your charger manual doesn't speak of a storge mode per se, but what is called TCS -

http://media.hyperion.hk/dn/eos/EOS0610iDUO-MAN-EN.pdf


It is adjustable, but you'll have to experiment with it until you find what TCS %'age corresponds to 3.8v.