How quickly are the batteries draining? You may be able to change some parameter settings to extend the battery life.
RBoy
(www.rboyapps.com - Making SmartThings Easy!)
3
Check out the Monoprice 4 in 1 sensor or the ZooZ 4 in 1 sensor. Note through that the battery life is between 2 and 6 months depending on the configuration and they run on 2 AAA batteries. If you decide to use AAA 1.5v lithium batteries instead of alkaline, it’ll roughly double your battery life.
There’s also the Aeotec Multisensor 6 that can be powered by USB if you have an outlet close by. It’s a little pricier but can be offset by no long-term battery cost
I use Xiaomi sensors in my bathrooms, too. Battery life is usually ~9 month.
I recommend you put an additional sensor in a central area of your home. This way you can trigger the fan based on the humidity difference rather than an absolute measurement. Overall humidity is usually lower in the winter than in the summer and so the fan will only run when it is needed.