Can I lechatchilla buy hearing aids that have rechargeable batteries, and on Shabbos place them in and out of the charger when it is turned off with a timer. The charge is supposed to last close to 24 hours, and probably will not last through Shabbos. Or is it preferable to get the type that require replacing the batteries, each one lasting around a week, which would avoid dealing with them on Shabbos. I have not yet purchased them, so I could go either way. Thank you.
Buy the type that has the regular batteries.
Recharging the batteries in the way you are describing has a number of issues. First of all, the batteries and the charger are both muktza, and can not be moved on Shabbos. The batteries after they are now dead are muktza machmas gufo, and removing them from the hearing aid is moving them. Additionally, placing them inside the charger is moving that part of the charger, which is also muktza. Aside from that, placing them inside the charger to charge is hachana- preparing on Shabbos for weekday use. This is even without getting into the issue of causing melacha to be done by placing something on a Shabbos clock to do melacha when it turns on. But even without all of this, you can’t put new batteries into the aid on Shabbos, (because it is an issue of metaken kli to put in new batteries, and turning the hearing aid on afterwards), so you will be without the aid until the end of Shabbos.
Therefore, buy the kind that has regular batteries. Even if it cost you more for the aid and constantly buying new batteries, it is hotzoas Shabbos, because this type is better for Shabbos. Therefore the expense of the batteries, etc. will be paid for by HKB”H.