Feedback for ebike battery build: 20s lithium polymer pouch

[owhite]

Folks,

i'm looking for feedback for on the upsides and downsides of pouch cells. The following is a summary of ratings for a number of pouch cells, and a comparison to two popular 18650 cells:

The links for each row are posted below. So what I'm taking from this information is there seem to be two classes of pouch cells floating around - one that operates around 5-8 C and others (in green) that are 25C. For the higher Cs that cost / Wh is basically double, but are also lighter. For any combination that gets me to 72v and >150A continuous they all cost around $600-$1,000, while some of the 25C jobs would presumably would mean massive peak amps.

The pouch cells compare just fine with 18650 cylinders, one thing I found interesting is the cycle life of 18650s is usually HALF of pouch cells - so in a sense I'm not sure prices compare well at all. Even if the cost of round cell are equal, you'd still be retiring those packs sooner than pouch cells. It seems like the products in links 3, 4, 5 the proposition could be: "hm, very high ampacity and weighs less than any comparably priced 18650, and would take more cycles".

But I'm also like the nice Ah range of the other pouch cells, and I'm leaning towards making my build with them instead of round cells.

I would very much welcome some feedback regarding any downsides.

o

Link1 https://www.aliexpress.com/item/329...68#1000022185#1000066055#0_668#3468#15609#252
Link2 https://www.aliexpress.com/item/329...68#1000022185#1000066055#0_668#3468#15609#252
Link3 https://www.aliexpress.com/item/328...AlpZCqxLrHzYHfCKl9saAorAEALw_wcB&gclsrc=aw.ds
Link4 https://www.aliexpress.com/item/328...AlpZCqxLrHzYHfCKl9saAorAEALw_wcB&gclsrc=aw.ds
Link5 https://www.aliexpress.com/item/328...AlpZCqxLrHzYHfCKl9saAorAEALw_wcB&gclsrc=aw.ds
Link6 https://www.aliexpress.com/item/330...earchweb0_0,searchweb201602_,searchweb201603_

 

[john61ct]

RCgroups has much more detailed and recent threads, both on performance data and lifespan testing.

Mostly focused on crazy high C-rate usage, but to me that implies top notch QC.

The best are expensive but they do recommend some mid tier units that hold up surprisingly well.

LiPo is fire prone though, do stay on top of maintenance testing and monitoring SoH

 

[john61ct]

Last I checked, top value:

Turnigy HD 5000 60C 840g (820g) $75

CNHL G+ 5000 70C 885g $80


these are 6S

 

[john61ct]

this thread

https://www.rcgroups.com/forums/showthread.php?1767093-Battery-Load-Test-Comparison

work your way from today, backwards

look out for a couple new entrant vendors, one is to performance but pricey per Ah

The other, Richard, is likely excellent value and rare excellent customer service.

You want 5S units I assume?

 

[john61ct]

Here is Richard

https://www.rcgroups.com/forums/showthread.php?3683649-Liperior-LiPo%C2%92s-%C2%96-There%C2%92s-a-new-player-in-the-LiPo-market-LOW-prices-too

 

[owhite]

Wow that performance data is incredible! Thanks a lot. That is really helpful.

The other thing that I've been think should be possible is the idea using the JST balance plug of premade plugs to connect to a BMS. So for example, suppose I was using four, 6s premade packs in series to create a 100v, 24s battery pack. Each of the wire ends going to the balance plug of each pack should serve as a node between each cell in series. I could then wire all 4 packs to a single BMS for charging and balancing across all the lipos - correct?

thanks again,

o

 

[john61ct]

Personally I would balance when necessary as 5S sub-packs using hobby chargers

Then assemble into 20S for normal usage

The BMS then only needs to be good at discharge protection, cutting that based off the first 1S group hitting my chosen LVC setpoint.

Inexpensive BMS usually suck at balancing.

 

[owhite]

Thanks for this information.

My situation will be a bit more complicated.

I am considering purchasing getting a total of something around 20 - 26 Ah, by using 10,000-13,000mAh packs in parallel.

I just contacted Richard at rcbattery.com to see if he'd sell me individual 1s pouches that are used to construct:
https://rcbattery.com/liperior-13000mah-6s-12c-22-2v-lipo-battery-with-xt90-plug.html. These are only available in 6s packs right now.

So given that I am considering a 2P build, I imagined that using a BMS would make sense. Regarding your point about inexpensive BMSs sucking do these fall into that category?

https://www.aliexpress.com/item/400...earchweb0_0,searchweb201602_,searchweb201603_

Or search https://www.google.com/search?sxsrf...hUKEwjDooGFurbrAhXog3IEHbEnAPAQ4dUDCA0&uact=5 on the googles.

I've certainly seen them in a lot of 18650 builds. What thoughts?

 

[john61ct]

Do not let your plans revolve around a single supplier.

Adapt your design to use standard off the shelf units that are readily available from many vendors, e.g. 5A 5S capable of 25+C

and build or have built the custom harness that lets you connect them together into your desired xPyS layout

example
https://endless-sphere.com/forums/viewtopic.php?t=83872#pHigh%20Power%20LiPo%20harness%20-%204x%20parralell%20XT90,%20HXT4mm,%205.5mm%20-

Then you can have multiple sets in rotation with spares standing by for when (not if) the weaker members reveal themselves and start needing replacement over time.

Do not think you know in advance exactly what rated vs actual Ah capacity you need, if you think 20 actually usable Ah is required you mayneed to build a 30Ah rated pack to get there.

The best way to approach the BMS issue is to recognize that term as a collection of functionalities, and not expect to get everything you need in just one device off the shelf.

With some issues you will want redundancy, e.g HVC is primarily served by your charger(s) and **must** be based on per-cell voltage

Balancing may only be needed every 20 cycles to start, as a scheduled maintenance routine, after packs get worn over time that need getting more frequent.

LVC while riding could maybe be done at the pack level rather than cell level voltage if you are conservative leaving plenty of juice in there for the sake of longevity, maybe just a good wattmeter display coupled with your stating aware, building up your experience over time developing your intuition, voltage based alarms as a backup rather than an actual cutoff.

OCP maybe best implemented with a fuse, etc

I have yet to find "a BMS" that I would trust to do it all, and would certainly not just wire it in as if a permanent part of the pack, but make sure it is easily swapped out as needed, used in conjunction with a variety of other instruments used for SoH benchmark testing / monitoring / balancing etc.

 

[john61ct]

Also, there are 20Ah to 40Ah range LI pouches available that might suit your use case better than the RC Hobby LiPos.

Have a look at A123 NMC cells, known to be absolutely top notch consistent quality. But not cheap.

Also, these appear to be very good value

https://endless-sphere.com/forums/viewtopic.php?t=102908

 

[owhite]

So I really appreciate input from both of your posts. The posts are interesting in their contrast. I completely agree with idea of doing something modular. Youre right it's a mistake to make one 24S2P big mega-block of batteries all soldered together, and one pack goes out and I'm screwed. i completely like the idea of swapping out a blown pouch or hobby pack without sacrificing the rest. It's basically consistent with the very nature of commodity items - never expect them to work well - just replace as needed.

On the other hand, I really do want to get away from hobby lipos, if a decent quality 20-30Ah pouch is out there, I'm really interested.

Now, I also am not ignoring your point that I might think I only need 20Ah, but should be prepared to go higher. My preference is the following, and I'm curious what you think:

To me, this seems completely affordable for a 20S1P or maybe 24S1P system:

https://a123batteries.com/a123-systems-3-7-volt-26ah-prismatic-pouch-cell-amp26-nmc/

(26Ah NMC Prismatic Pouch Cell AMP26.) If they're trustworthy, I'm in.

and, I dont think it would be hard to make a modular system that could be easily disassembled if one of the pouches blew.

So this still takes me back to the BMS issue. I take your point that they have different roles - the main role I would have is to do the load balancing on charging. The secondary role is to protect against high amperage discharge of the battery pack. But there would be other failsafes in place to reduce high discharge - a fuse or circuit breaker, hell yes i'd keep very close watch of my wattmeter/display, and setting the HVC on the controller.

So with that in mind - what's your reaction to 20-24 A123s, @26Ah in series, with this:

https://www.aliexpress.com/item/400...earchweb0_0,searchweb201602_,searchweb201603_

to help with load-balanced charging?

 

[john61ct]

Yes, just like IBM in the early days of computing

no one ever got fired recommending A123, top notch maker.

Just make sure the selling vendor is trustworthy, so many sales channels are corrupt.

Do make up your mind about 20S vs 24S or otherwise, well before you're locked in, many other components selections flow from that.

Sharpen up your google-fu exploring physical pack construction, welding vs soldering vs just bolting / clamping those type pouches together.

Again, charging is primarily the job of the charger.

HVC is only an issue with charging. If your charger is not a balancing charger, IOW only works on a per-pack basis, then a BMS watching per-cell/group voltage is essential, otherwise just for redundancy, acting as a failsafe.

"Load balancing" is not anything related to charging, sounds like maybe a discharging thing, not a BMS function afaik.

Keeping cells in balance is also (again) not something I would do with "a BMS".

Balancing chargers and dedicated balancers ("non-protective BMS") are IMO the way to go for that

but in fact something you may not even need to worry about more than say annually, if you are starting with Grade A brand-new genuine A123 cells.

So long as the balance wiring is accessible for monitoring per-cell/group voltages and you have adjustable PSU/chargers available, you could see how you go just handling balancing manually to start with.

That would not necessarily be the case if you were buying a random cheap-chinese pack, or scavenging scrapped cells.

"High amp discharge" aka shorting and OCP can be done IMO more reliably using fuses.

The real key function IMO for a BMS to be hooked up while the rig is in use (discharging) is watching the per-cell voltage as the average pack-level drops below 3.8, 3.7Vpc.

 

[john61ct]

what's your reaction

I'll leave to others to give BMS recommendations and discuss specific makers / lines / models.

If you don't get much feedback here, maybe participate in the many existing threads on that topic, or start a fresh thread

but best to know your definite voltage range / string-count beforehand.

If you agree on the wisdom of wiring your packs so the BMS can be easily removed and replaced, you don't even need to come to a "final" decision on that particular component anytime soon, experimenting with a variety of cheap ones first just to get hands-on familiar with them first.

 

[owhite]

my mistake i thought the purpose of the BMS was to monitor charging.

i'll do some research for balanced chargers.