I'm an engineer, and the thought of buying a generic battery from China fills me with dread. I think I'd apply an exception to Mr. Ping, who is well regarded as supporting his product, but in general you have no protection at all if something goes wrong or the product doesn't do what you expect.
In China they produce almost identical products for different world markets, but the quality can vary from excellent to awful - not to mention dangerous. Even the same production facility can do this in parallel - rubbish to be sold cheaply in 'third world' countries being manufactured alongside top-quality kit designed to be sold at a premium in the West. I've seen it in domestic electrics - you buy Chinese made light sockets in Ghana (a country I have considerable dealings with) and they look fine but they're nasty, and the plastic soon becomes brittle and falls to bits in the heat. What looks identical but bought here - also made in China - is of high quality, and the Ghanaians have learned that if they have the contacts and can afford it it's better by far to import from the UK and pay double the price.
Getting back onto the subject of ebike batteries, it's tempting to pick up a bargain rather than pay £500 for something which you know will work and carries a decent warranty. I'm surprised we haven't heard (yet) of anyone having their house set on fire because a nasty battery has internally shorted itself. I know there are many on here who have bought batteries from China and not had any problems, but there's a significant risk, and NO WARRANTY other than the goodwill of the supplier, who may or may not send you a replacement if you have a problem.
To repeat what has been said before, ebike batteries are a design nightmare, because there are weight and size constraints which mean that what you have to use gets abused from day one, and that's reflected in what you pay and the relatively short life. You can't avoid applying excessive discharge currents in order to get decent assist when you need it - on hills - and if you have a borderline illegal bike the problem becomes much worse as a relatively small increase in top assisted speed carries a big penalty in battery discharge current.
I wouldn't be so blunt as to call someone who buys a battery direct from China 'an idiot' but I can see Frank's point. If anything, the more you know about the issues involved, the more cautious you become, and it can be naive to buy a dangerous pig in a poke with little chance of redress.
Rog.