used cells (or used batteries) aren't worth anything unless you can personally sit there with them and prove they are good by testing every cell, *and* you know their complete history.
without that, you can't know what they've been thru, so you can't know if they will work for you for one charge-discharge cycle, or ten, or a hundred. you can't know what their voltage sag will be under what load, and you can't know if they were damaged either by their previous usage or by the disassembly process of whatever they came from.
with any used cell, you should double the number of parallel cells that you think you need for a particular application, and halve your expectations of the pack's longevity. (if you think a new pack would last you a year, estimate the used one will last you six months or less). if you're lucky, it will make you very happy and do what you want, and do it for longer than you expect. but if you're not, it won't even do that.
cheap packs from "random sellers"
should be treated the same way. some are good, some are not, but until you actually test them yourself you won't know.
also keep in mind that batteries are very tough to ship back for warranty, even if there is one, because of hazmat shipping rules, which get stricter all the time. so you will likely be stuck with whatever battery you get, even if it doesn't work. the cheaper it is, the more likely that scenario is (though there's no perfect manufacturer or seller out there, the cheaper it is the less likely it is to have been thoroughly tested and built well, of thoroughly tested well-built parts).
eligri said:
what would you buy to get the following:
48V, 21A minimum, 15Ah+
Thinking either 12 or 13s, then 5-7 p
first, 48v is 13s. 52v is 14s. those are "standards", as much as anything is in the ebike world.
if you're determined to use cheap or used or unknown cells, you should keep the current needed for each cell as low as possible. an amp or two at most.
if you need 21a, then divide that by two for 2a per cell max, and you need at least 11p. if they're not very good cells, or well-used, 21p would be safer to need only 1a per cell max. some cells aren't even capable of *that*, so you'd need even more in parallel.
if you need at least 15ah, then for the same cheap/used/unknown cells, expect no more than 2ah each, and possibly not even that. so you'd need at least 8p, if the cells could give 2ah each.
also keep in mind that as they age, which happens faster the harder they are used, and the more used they already were to start with, they'll give less capacity and they'll sag more in voltage under load. so you should also anticipate that aging loss, and add at least some percentage (20%+ could be a good minimum rule of thumb) to your needs, to make sure the pack you get will still do what you want when it gets a year or two older.
exactly how much of any of these things you need to worry about is determined by the specific cell you use, and it's manufacturer specifications. every cell has different properties, some by a little, some by a lot, so you have to know which cell you're getting and what it's specs are. then keep in mind those specs were determined under very specific laboratory conditions (stated in the spec sheet), so any usage outside those conditions will give you different results than the factory got. how different and in what way...depends on how different your conditions are. that's for brand new cells.
if you are using used cells, then the spec sheet can only be a loose guide to their original capabilites, and may have little to do with what they can do now (which will be less, possibly much less, capacity (ah or wh) and amps (a)). you'd have to test them *under your conditions* to know what they can actually do *now*. or reinterpret whatever testing someone else did on them under their set of conditions to see if they will do what you want.
last thing--if you want everyone to be able to see your images, you'll need to use the attachments tab on your post to upload them directly to the forum. externally hosted stuff can't be seen by everyone (for example, something between me and imgur has broken those for a while now; the same is true of quite a few other image hosting sites).
