Giffgaff is pretty much the same sort of deal as Aldi Pay As You Go mobile - no contractual obligation to pay a monthly fee continuously. Costs about £10 per 180 days to keep the number active. There's no way to turn off notification by text upon low battery, so I charge it every couple of days when the bike is used a lot at night, and every four days when the bike is mostly idle. Overall, it's a good product.