Why isn't my stick vac holding charge?

My iVac Ultra Light Plus stick vacuum cleaner won't hold a charge. I replaced the battery & it worked fine with the new battery until the battery ran down. So I replaced the charger but it the battery isn't getting charged. I have had identical issues with a Dyson stick vacuum cleaner & a Bose Soundlink Mini. Is there an issue with some electronics between the charger & the battery, that can be fixed? Is it possible to charge the battery externally?