AEVA Forums

Looking at big packs of 18650's for a possible bike or even car project, due to their good energy and power density. There are loads of them on eBay, marketed as 1000-1500W ebike batteries. Such as this one:
https://www.ebay.com.au/itm/48v20ah-Ele ... 0005.m1851

It appears by the numbers to have much better energy and power densities than prismatic LFP or NCM.
Has anyone pulled this sort of thing apart to see how wee they are made?
Does it have per-cell or per-string BMS? (it claims to have balanced charging and discharge protection)
If not per-cell, would there be reliability issues/cooling issues/fire risks?

18650 cells are usually NCM, but also LiCo. They have great energy density, but typically rather low power - that is, they sag heavily under load.
All cells will benefit from a life spent at 20'C - so the closer you can keep them to 20'C the better.

Oddly the cheapest that I have found are the balance / hover board packs which are 36V 4.4Ah.
They are made from 20 x 18650's and have BMS built in.
1 x 48V20Ah=960Wh for $595 vs 6 x 36V4.4Ah=950Wh for ~$390.

112844990880
https://www.ebay.com.au/itm/4-4Ah-36V-L ... Sw4YxamLFv

I am planning to make a little PCB that allows me to plug them all in to make a large pack.
If one pack fails I can either unplug and bypass it or swap it out with a spare pack.

I'd only use them for toy stuff not for motorbikes or cars.

edit: fixed link

aegidius wrote: ↑Mon, 14 May 2018, 10:28 Has anyone pulled this sort of thing apart to see how wee they are made?

I doubt they are made any different to any other packs.
Tabbed cells spot welded.
Wires per cell going to an all-in-one BMS.
Kapton tape all around that.
Sometimes foam and thin film such as polyester or pet.
Then shrink film around that.

aegidius wrote: ↑Mon, 14 May 2018, 10:28 Does it have per-cell or per-string BMS? (it claims to have balanced charging and discharge protection)
If not per-cell, would there be reliability issues/cooling issues/fire risks?

Usually it's a multicell self contained BMS.
They have wires going to each cell.
Reliability issues with something made in China - never

In terms of the hoverboard cells most of the issue was with water/moisture getting into the unit.
AND assembly defects where the metal casing would cut through the battery pack from vibration.
Boom...

But the packs themselves seem fine.

Richo wrote: ↑Mon, 14 May 2018, 12:32 I am planning to make a little PCB that allows me to plug them all in to make a large pack.
If one pack fails I can either unplug and bypass it or swap it out with a spare pack.

I'd only use them for toy stuff not for motorbikes or cars.

What will you use them for? have you been seeing failures in experience?
(Just wanting to pick brains before spending $$)

I can certainly see hoverboard packs getting a rough life (close to ground, getting wet, chucked in the back of cars, etc.)
but if you keep them out of the elements, vibration isolated, well cooled (perhaps through holes in the shrink wrap), and not discharged below 80% DOD they should be OK for quite a while I would have thought.

Well I haven't seen any packs fail so far.
I use them like the Ryobi stuff - one pack does them all.
I found 36V 4.4Ah a nice size building block.
I have about 6-7 that I unplug and plug into various things around here.
Robots, kids mobility (e-scooter retrofits) and remote test equipment so far.
But I have been working on a controller for e-bike and mowers sub 1.5kW.
They are bit more convenient than messing around with tabbed 18650's or worse plain 18650's!
Plug and play

I'm not too sure what the difference would be between the hover pack and the large 48V pack in terms of cell quality or construction.
I would expect them to be similar so I don't know why the extra $200.
The hover packs are mass market so more available.

I did pull one apart to have a look inside - yeah I just can't help myself
I can post some pics if you want.

Despite the hover boards reputation the battery pack itself is not too blame.
So as long as you don't use them in hoverboards they should be fine!
I don't use them much but have had them for about 2 years now.

I guess it comes down to if multiples of 36V suits your needs.