My two Di2 systems are both external batteries, one dates from 2013 and the other from 2015. Both are doing fine though I probably didn't do either
any favors by charging once/month for 2-4 yrs. Now typically every 2.5-3 months when one bike with a battery readout falls to 40% charge, both are
charged. They are stored in a garage which goes to 90-100F in summer. Li battery failure is manifest by failure to hold a charge, ie a full charge only
lasts a few weeks or few hundred miles. Considering the significant price increases in Di2 batteries and chargers I would be in no hurry,
ie ext batteries were $45 in 2016, now $80. Considering price drops in Li flashlights over the same time frame......