The result of bi(fullempty) doesn't simply return the time until fully charged but also the additional tect " from now". This makes it very inflexible to use without unnecessary string manipulation. The function should really just return a duration that is then turned into text using another function.
My mistake. It does return a timespan. Doh! I missed the tf() nefore it. Ignore.
I still would like to get rid of the " from now" portion, and this includes with ai() or without.