You can't tell much about the battery from the website. To start you need to find out which cells they are using so you can decide if the price is even fair, and then if the capacity rating is in the ball park. LUNA sells a 52V 21Ah battery they are claiming is 1100Wh, but you can go to the manufacture's data sheet and many independent tests and find out it's capacity is really only a at most 840Wh, and since most ebike manufactures don't let you run the battery to empty you can expect it is less in real-word use. However; at least they list enough information about the battery that you can find the correct information. This is the real problem; because if you want to compare it to another battery you have to get the true specifications for the other battery, and most of the time that is very hard.
That said; for the price, I'm sure the deal is fine. It would be good to find out if the use one of the big 3 name brand cells though or something else.
Is that battery 840Wh, no, but then all the other manufactures are inflating their specs also; so knowing the cells you can at least say if it is better or worse than other 17.5Ah rated batteries.
