Add a "memo" field to contain the battery type (Es. AAA, CR2032, AA, Lipo)

[robertolamm]

I am using the secondary_info to store the last charge/substitution timestamp, but there are tons of batteries and I would like to know what battery type (AA, AAA, CR2032 ...) is needed when it's time to change it

I think is good enough to add a "battery_type" field as a string

Thanks