For daily battery life, most last 20-30 hours, but this can be reduced by how much Bluetooth streaming you do. Consider it a 1:2 ratio, where 1 hour of streaming equals two hours of battery life.
Most of the manufacturers are now including a battery pack with the charger, which is good for 5-7 days battery life in case you go camping or have a power outage.
And yes, the battery pack is also rechargeable.
Another consideration is the overall lifespan of the battery. Z-power batteries should be completely replaced every six months and can be replaced by your local provider. Lithium ion batteries last 4-6 years, but have to sent to the factory for replacement. In addition, if the hearing aid is out of warranty, replacing a lithium ion is considered a repair, and can be quite expensive($200+ each!). So right before your warranty expires, have the aids sent in to the factory to get the batteries replaced. This way, because of the replacement, you’ll get 8-10 years out of the. Instead of just 4-6.
The battery life on a hearing aid can vary depending on how it's programmed, whether it has been paired to a mobile phone, how much time you are streaming a live audio signal from a phone, etc. Fortunately, all of the rechargeable solutions I'm aware of, including those mentioned, use a charger that runs on 5 volts DC using standard USB connectors. So if you can carry a small portable power pack (can be a cheap generic one), and the HA charger with you, then you could charge them while camping or backpacking, for example. Having said that, some of the chargers are definitely bulkier than others. The Oticon charger is probably the bulkiest, the Signia charger is probably the least bulky.
I also agree with the previous poster recommending Z-power. Although Z-power has had its issues, it really excels in its flexibility. You can charge it most of the time, but when you travel or go on a camping trip, you can leave the charger at home, and just bring a few disposable 312 batteries to use until you get back.
The Phonak Marvel hearing aids will go for a good 36 hours without needing to be recharged. If you are planning on being away for a long time (maybe a week or more) then I would go with something like the Starkey Livio AI which has 2 power packs that can hold 7-10 full charges without needing an outlet.
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