Spoka Night Light

Just bought 2 Spoka night lights. They have adaptors that charge a battery inside the light so that when charged the kids can remove the adaptor and carry the light with then. They were both working fine for the past few months, but now neither will hold a charge, they only work when plugged in. Kind of defeats the purpose of the light, my kids were using them on trips to the potty at night. I feel it's the battery, but it doesn't appear that it can be replaced. I really don't feel like shelling out another 30 bucks, and alas, I no longer have the receipt, so an exchange at IKEA is out of the question without it.  Any thoughts?