Battery compatible retroid flip?

@squeepty usually there are numbers printed on the battery with which you can identify the battery. Post some pictures of your battery with your Question so we can assist you with that. iFixitでの質問に画像を追加する方法 Yours could be a 655093G 3.8v 5000mAh 3.8V battery.

If you can't find it by the numbers, measure your battery and then search for one (AliExpress etc.) for one that has the same electrical characteristics and dimensions to fit your Retroid. There can always be issues if these are custom batteries. If so you can always go smaller in physical size as well as mAh which will just lower the time your Retroid can be on battery usage.