THE most common type of battery in the U.S.A. is the AA size battery in 1.5V alkaline chemistry.
THE most common rechargeable type of consumer battery in the U.S.A. is the AA size battery in 1.2V NiMH chemistry.
It is very hard to find a decent quality but not overly expensive rear tail light that uses the AA size battery. Almost all of them use either AAA size batteries or have built in Li-Po rechargeable cells. Long story short there is an unfulfilled market nitch sitting right there just begging for someone to take full advantage of it.