I have not bought disposable AA or AAA batteries for over a decade, but I recall being at work with low batteries for my taillight, I accidently left them on. There was a Walgreens a block away, bought some AAA batteries for my light for the ride home. You can't do that with internal battery.

I use two different Planet Bike Superflash taillights, both take a pair of AAA batteries. For straight roads, the plain Superflash, it has a tightly focused beam that when carefully aimed straight back is very bright to following traffic. But if I will be on a winding road with some ups and downs, the Superflash 65 has a much wider beam so vehicles will see a bright flash if they are not straight behind.

I use both for bike touring, typically one at a time depending on road conditions, but if overcast or foggy will run both on flash mode.

I charge up the batteries once a week, even if they are still bright because I want them to stay bright.

The batteries that I prefer are no longer sold but Eneloops are a very good battery that I would recommend.

Make sure you get a smart charger that knows how to charge them up to full and then quit charging, the cheaper chargers (dumb chargers) just charge at a slow rate and keep charging in perpetuity, avoid those.

The charger I use at home is made by Accupower. For touring the charger I use is no longer sold, for touring I want a AAA/AA charger that operates on USB power so my dynohub powered USB charger can charge them.

For bike touring, the GPS, taillight flashers and headlamp for my head all use AA or AAA batteries so that I can swap batteries if batteries in a device are low so I can keep using it.