"Acceptable" 8S BMS for 18650s

[dequinox]

So I have quite the quantity of 18650 cells, and I'm trying to build my first 8S8P battery from them, and I'm not in it to spend over ~$20 on a bms, but it is also the simplest solution for my wife to charge her bike. Plug & Play.

I'm looking for a state-side warehoused option from ebay, amazon, etc that will work with your typical li-ion 18650's voltage, but I'm not familiar with BMS and haven't been reading up on the latest & not-greatest for some time. The point is I'm trying to get this project by on a shoe-string, and could use some input if other people have done the same.

I did find this on amazon, which I consider a reasonable price, but you'll notice it's 7S "24 volt" and not 8S. 8s because, well I'm replacing lead acid 24Vs and want it to be a little on the zestier side at 30.4V nominal.

Any help would be appreciated!

 

[fechter]

What is your maximum desired discharge current?

With the cheap ones, it's sort of a crap shoot, but I have had good luck with several. Take your desired discharge current and multiply by about x2 and shop for one with that spec.

Most of the cheap ones can be configured to run with fewer cells. If you got a 12s, for example, most likely you could make it work with 8s. The types with centralized chips are may not be easy to use with fewer cells.

I would avoid the ones called "Smart" or that have Bluetooth if you want low budget.

You may have a choice between "same port" or "separate port", which refers to whether it has a separate connection for the charger or you charge through the same connection as the discharge. I prefer the two port designs for anything that runs high current.

 

[dequinox]

Thanks for your input!

I do have a pair of 10S BMS I got from ebay that are pretty much equivalent to this item.

I'm building the battery for a 24V Hill-topper which has a controller rated for 15 amps max I think. So a 40A board would do it, but how do I jury-rig a 10S board to run the 8S configuration? It looks like this "BMS" is just a dumb balance board, so it might suit the application well if I can mod it to manage 8S instead. I may put a cell-log on it as well... I do have a few of those.

I think it's a shared charge/discharge port, but that's ok for a near-"junker" battery in my view. Will the above balance board work?

 

[john61ct]

cells new or used? link?

 

[dequinox]

cells new or used? link?

If you're asking where to get them or what kind... they vary and have been harvested from laptop packs.
I am still selling them here:
https://endless-sphere.com/forums/viewtopic.php?f=9&t=98436

Original acquisition is here: https://endless-sphere.com/forums/viewtopic.php?f=14&t=70676&hilit=1000

 

[krlenjuska]

Deligreen BMS from aliexpress if you have time to wait for delivery.
I use them and its ok so far. Also its cheap.

 

[fechter]

Thanks for your input!

I do have a pair of 10S BMS I got from ebay that are pretty much equivalent to this item.

Yes, that should work.

To use with fewer cells, wire normally starting from the B- end and tie the two unused balance wires to the top cell (B+).

 

[dequinox]

Ok that's simple enough, thank you!

 

[dequinox]

Do smart chargers like this one get along ok with these cheap balance boards?

https://www.ebay.com/itm/8S-28-8V-29-6V-Li-ion-Li-Po-Battery-Intelligent-Smart-Charger-33-6V-2A-US/222799127217?hash=item33dfdce2b1:g:YXAAAOSwKIpWBgVj

 

[fechter]

Do smart chargers like this one get along ok with these cheap balance boards?

https://www.ebay.com/itm/8S-28-8V-29-6V-Li-ion-Li-Po-Battery-Intelligent-Smart-Charger-33-6V-2A-US/222799127217?hash=item33dfdce2b1:g:YXAAAOSwKIpWBgVj

That should work fine.

 

[dequinox]

Thanks again!