The cheaper batteries used on the more entry level folding bikes like yours probably do not have the best quality cells and may lose their usable range quite quickly. On my old Oxygen Ebikes of 2011 vintage, I have when needed. replaced their silver fish batteries with Yose ones sourced of the internet. The last one I bought quite recently from Ebay. I paid £165.62 delivered, for a 36V15Ah battery that came with a charger and is working beautifully. I believe they sell batteries of many different styles and might enable you to buy a spare battery to extend your range. You should check that there is not any "protect" technology built in to your bike to prevent you using a generic third party battery. You could also check if the suppliers of your bike could sell you another spare battery. Woosh bikes often recommended on this site allow you to buy an extra battery when you buy a bike from them.