I believe that Thais use "battery" to refer to batteries within cars, computers and smartphones.
If seeking batteries at 7-11, I suspect you would see better results if you use the Thai word for "charcoal" (It would be helpful to write Thai script here, but I believe its usage is prohibited outside the language forum.).
After saying the word for "charcoal," then specify by using the Thai word for "size" (a two-syllable word beginning with a "k" sound), and then add "A" or "AA" or "AAA."