https://www.amazon.com/dp/B086PC5962?ref=ppx_pop_mob_ap_share

I bought these. they come with a battery, and stay on all the time. I keep one in the case and one in the room near the case. With the D'addario humidipaks I've only seen my case humidity drop to 47%, and that was in the middle of a cold snap here in the northern US.