‘A new battery was flat in a few weeks and the charger wouldn’t work again’
A liveaboard boater with very frugal electricity use couldn’t keep the batteries charged and the charger wouldn’t start charging.
A new battery was flat within a few weeks and the charger wouldn’t work again. It was described as ‘smart’, but the online information implied it only had a two- or three-amp output and gave no indication of what it did when it decided the batteries were fully charged.
I suspect that when this charger showed ‘fully charged’ it either shut down or dropped to a maintenance voltage but did not go back to high voltage when the battery discharged or a load was applied. This allowed the battery to over discharge and sulphate.
Try not to use a cheaper car-type charger: make sure you get one that will track the discharge and load and react accordingly. If you have to use such a charger, turn it off and on every day to force it to restart its charge.