Normal PP3 batteries are nominally 9v although almosy no manufacturere publishes battery capacity. The Duracell Bunny test is the closest. It would be greta to use rechargeables but most are nominally 8.4V. There are  now 9.6V recharcheables and I am thinking of trying some. Has anyone tried them?   https://www.componentshop.co.uk/9-6v-220mah-nimh-pp3-battery.html   or   https://www.amazon.co.uk/Self-Discharge-Precharged-Rechargeable-Batteries-MHR9VP/dp/B01FV0U5WG/r