Hi Liz, thanks for getting in touch.
I totally understand your want to be eco, and having a form of rechargeable power pack is something we would love to have in the future.
There are two main reasons for not using rechargeable batteries. Rechargeable batteries are 1.2V as opposed to a normal AA battery being 1.5V. This means that when using rechargeables, you aren’t supplying enough voltage to power everything properly. Sparkles are especially prone to not functioning correctly. Secondly, we believe CLEAPPS recommend not using rechargeable batteries, especially in primary schools, as these pose a much greater fire/explosion risk when they are short-circuited.
I hope that this helps!