I have not seen any battery problems with my S50, so it might be a problem with your unit or the charger you are using. Mine charged to 100% the first time and since then I always keep it powered trough a portable battery unit, it always stays at 100% even with defrost turned on (I live in Canada). This might or might not be a good idea, depending on the type of battery ZWO is using, but at least it is a replaceable one, so you will not have to throw the S50 away the moment the battery dies, which it will, this is as certain as death and taxes.
The S50 is not built to last forever, but neither are our much more expensive smartphones for example, and we keep buying a new one every two to three years. If I get 3 years of observations out of my S50 I would happily buy another one. If ZWO listens to user requests and adds the features we really need, of course.