Danzzigger wrote: ↑
Apr 14, 2018 9:25 am
I've charged 3 cells so far, and I've got a bit of variation, but I wasn't sure whether it was cause of my charging process. my results were 1785, 1884 and 1940. All from my 20R cells (2000mah)
If you test them at higher currents you may see more variation, or just less capacity in total.
One thing you should keep in mind aobut used cells is that you cant know anything about how long or how hard theyve been used, so even the ones that are still good in packs might have 100 charge cycles left or they might have a dozen, before they start to wear out more quickly.
The bike itself is not going to be overly powerfull if I end up going for the qs205 I have a odd 11kw of potential discharge at my disposal, if it only needs say 2000watts at peak (burst) thats still only 20% of the batteries potential.
I think there might be a couple of issues with some of your math
unless I ran across wrong numbers in a search for the Samsung 20R cell data. In your OP you mention a configuration of
20s 6p 74v-84v 9ah
which is a bit confusing in itself. 20s would nominally be 74v, for 3.7v nominal cells, and 84v would be full voltage. But 6p for 2Ah cells is 12Ah, not 9Ah. If its 6p, then if you used the max continuous current (whcih will probably cause a lot of voltage sag and heat) then assumign its around the nominal voltage at that point, and not sagging below that, 74v * 6p * 22A = around 9700W max. If its not 6p / 12Ah then those numbers change too.
You might get more watts if the sag is less, but given these are used cells of completely unknown condition (at this point) Id personally expect at best to get half of what theyre rated for. If you use the higher currents, Id expect more sag, and Id expect much less capacity while used at the higher currents.
Also, since teh pack (for 12Ah) is nominally 888Wh, if you actually used that max wattage a lot the pack might only last 5-10 minutes to empty.
And thats all assuming brand new cells, rather than used ones of unknown lifespan/capabilities.
Remember that the motor doesnt limit the watts--the controller does. So the motor being a 2kw motor doesnt mean anything if the contorller is different. Some are programmable, some are not, so you may be able to change the power or current limits to adapt to your motor and battery power handling capabilities.
Also, many motors can take several times their rated wattage for short periods. See threads like the Definitive Testing of Heating / Cooling hubmotors by Justin_LE, for some data on that sort of thing.
I dont know which brand would be better for you; theres a lot of them out there and sometimes generic stuff does the job just fine, and sometimes you need the features in a pricier brand.
Im using generic stuff right now, but have been collecting parts to build my own pair of Lebowski controllers for the past year and a half or so, as these are programmable and have some nice features I want (especially the way braking works), and are tunable to work with specific motor properties fro better efficiency and performance. (the phaserunner from Grin is, too, but I cant build those myself, and theyre way out fo my price range).
Should I be somehow stress testing my cells before assembling them into a pack?
You should test them under the same conditions you expect to use them in. If youre ever going to run them at higher currents, you should test them at those same currents so you know which ones can still handle it and which cant, so you know which ones to not put in your pack.
For testing methods, your best bet is to look at the other recycled-cell build and testing threads. DrkAngel has at least one thread wiht various testing info in it, and there are others around too. Probably 18650 and either used or recycled or other similar search terms would find them, looking in just title of threads, display by topic, or maybe even just in the first post.
looks like I need to find another provider for my online images.
Just use the Attachments tab below the text entry box in your posts. ES hosts teh images, so anyone that can see the post can see your pics, unlike right now when probably no one can see them (I cant see any pics at all your posts in this thread, for instance). Also, the images will always be here in the thread (which wont be true if whatever external site you hostimages on goes away or does shitty things like PB did).
So what I was assuming was right in that the meanwells are simply a PSU much like a server psu with a CV and CC, there are no 'smarts' behind it. The bms is what does the discharging, and the bms is wired up to switc the psu on and off at the end of the battery charge cycle.
Well, the BMS doesnt switch the PSU on or off. It just disconnects the incoming voltage to the BMS (from any voltage source, regardless of what it is, charger or PSU or solar panel, or even another battery or even regen on a controller), if the voltage on any cell goes above the cell-level HVC.
As far as PSUs vs chargers, and discharging, neither one does any of that. At least, not for non-RC type chargers (those are the only ones commonly found that do the job of both charger and BMS).
So a designed-to-be-a-charger unit like the Cycle Satiator or a common ebike charger, etc., may have end-of-charge termination, where it shuts off its own output voltage once current drops below a certain point. (it also may not do this, as it might be designed to continue trickling current for a BMS to do balancing with). And some of these chargers taht do shut off below some current draw, will restart if they detect a low enough voltage on their output (from a BMS reallowing input), and some will never restart until they are disconnected from both AC power and the battery, and then replugged into both. Ive had some of each of these kinds.
The PSU simply never shuts off its output voltage, so current could continue to flow if the battery never reaches the PSUs max output voltage (for instance, if the batterys full charge voltage as it ages drops below the voltage you have set the PSU to). But if the PSU output voltage equals the full charge voltage of the battery assuming its well balanced, then current will still drop to essentially zero once it does reach that full charge. Also, with the PSU, and a BMS, if the BMS needs to balance then the PSU will always let the BMS have the current it needs to do taht with, where some chargers dont (if they are the no-restart type).
How often does the bms do this in order to balance the cells correctly? seems rather inefficent to turn on and off a large psu to put a few tiny mah in at the end if it does it often. Or does it only do it like a couple times?
It happens as many times as it takes to balance the cells, and that depends on the cells themselves. WIth brand new cells that are all well-matched to each other, that are not used near (or beyond) their performance limits, theyll probably stay well-balanced until they get pretty old and worn-out, and the BMS wont have to do much, if anything, until that aging happens.
But with unmatched cells of varying capacity, internal resistance, age, and other performance differences, cells may become very unbalnced on every single discharge cycle, and require a lot of balancing each charge cycle, so it could take hours (or days, though thats unlikely) to balance such a pack on every charge cycle.
RC chargers do this same thing, though you dont see it happening from the outside because its all one unit (BMS and charger in one box).
I'm hoping that I'll be able to find enough good matched cells to make a 120cell pack. He can get me more batteries in time too if I need them. I'd rather not spend $600+ dollars on a battery if I can save the money.
Remember that if you have the space, you can also just parallel a lot more cells to make up for the failings of unmatched or poorly-performing cells.