Prismatic cells: the easy way to lithium?

Hi Guys,

I currently have a 72V 35Ah lead acid battery on my motorbike. It works nicely, but I find it quite a bit heavy and It only have about 35km autonomy at full speed. And now that winter is coming, the autonomy starts to drop.
So, I would like to upgrade to lithium.

I've spent a great amount of time looking through the forum, but didn't really find what I was looking for so far. There are a lot of threads explaining how to build lithium batteries from 18650 cells, pouch cells or whatever, but not so much using prismatic cells.
When I talk about prismatic cells, I mean stuff like that:
https://item.taobao.com/item.htm?spm=a230r.1.14.49.4214db2gC6476&id=526379816361&ns=1&abbucket=12#detail

My plan is to go for Lithium iron, just for safety and lifespan reasons, even if it's not the best solution regarding performance.
I think those would be pretty easy and convenient to use for me, I'd just have to connect them in series to reach 72V and BOOM. Well, more or less.

But I'm wondering about cells protection. Does anyone has a clue about that?
My controller draws a continuous 200Amps at 72V and about 400Apeak, so I would need something beefy enough to support that kind of current draw.

Let me know if you guys need any more information. I'm not a complete noob so don't worry about my ability to follow your advices and understand your recommendations, I've built many electric bikes and scooters, but so far I haven't worked on BMS systems, so this part is a bit hazy for me.

Thanks in advance!

If you haven't already, you can look up posts about Winston, Thundersag, Thundersky, CALB, etc.; they are the same types of cells.

They are susceptible to swelling (and damage from this) ifyou don't build them into compression-boxes, so they can't just be packed in with foam or something into existing areas. (see my music studio thread; I think I show/discuss the failure of four Thundersky(sag) cells from that problem, even though they were not used at any high rate charge or discharge, not overcharged or overdischarged, etc. The swelling happened anyway, because I didn't have them compressed though I had seen this happen to other people's cells and had planned to build such a box for them. )


For a high-current BMS, you can look at the various Vectrix threads; I think at least one of them discusses a method of using a regular BMS to monitor but use its' FETs (or at least the gate controls) to control a contactor. (it might not be in a vectrix...but I think it was).


BTW, regardless of what vendors might say, these types of cells have been generally shown to be best used at 1C. So if you have 400A current demands, you'd want to use 400Ah cells, which will be very large and heavy.

They also are slow-charging cells; probably 0.25c to 0.5c.

I don't know those specific linked cells, but of the stuff I've read the projects of back on DIY Electric Car forums, and here on ES, and my personal limited experiences with the TS60Ah cells, the general type is not all that great.

There is the rub. Though you might be able to pack 70 ah of prismatic into the space your 35 ah of lead took, its been pretty much proven that that type of lifepo4 cell is not a good choice for a high c rate use.

They may call them 2c or even 3c cells, but in fact they don't like real world amps spikes, like a vehicle does on starts. Lots of voltage sag under load. So your vehicle would best be tuned down to a mere 130 amps spike, 70 continuous, if you wanted to use the prismatic lifepo4, and could fit 70 ah in there. ( and expect the cells to last a good long time)

This is why the smaller, lighter 18650 assemblies are chosen when space is limited, such as bicycles, motorcycles, and even cars that need more range. The c rate that they can put out is more like a true 2 or 3c, and the same space you have might fit 100 ah of them. Now you can at least do 200 amps continuous, and tune down to 300 peak.

The other option, lipo pouch cells such as drones and RC toys use, can have true c rates that would allow a smaller pack to put out the 400 amps, and this is why the hot rodders, racers, etc will choose that type. rated 30-60 c, but in reality perhaps 5-10c, you can still get your 400 amps peak easily with just a 40 ah pack. There are downsides to the RC cells, particularly fire hazard, but if you want the big amps in a smaller size battery, this is the choice.

Well, that's a bummer

But I'm not sure about the figures you guys mention. For instance, I have a 72V 50Ah liFePo4 battery on my electric scooter. Controller delivers 150A continuous and 380A peak and the battery still works just fine after two years of daily use, riding full speed all the time.

It is not made of prismatic cells, probably 18650, but they are not particularly high quality. Yet, they managed to survive so far.
So, I thought I would be just fine with double capacity and only one third more power consumed.

You might. Depends on the actual cell quality and specifications.

Personally, I think it's a crapshoot, based on what I've read and seen.

I'd recommend looking around the forum at other users of this type of cell and see what they got for results, and also over at DIY Electric Car forums, where they've also been used.

Of course there's a chance they aren't bad, but I would not trust any chinese battery cell without a large number of knowledgeably people have used/tested and recommended the exact cell from the same seller. I was just watching videos yesterday about all the complaints/problems people have when trying to get a good deal on cells, especially when salvaging from non reputable sources. TBH I was almost laughing... some people were successful, some were complaining when they absolutely shouldn't have been... many wasted their money on complete garbage. I'm just glad that I've been getting my cells out of brand new Bosch battery packs... these things are so incredibly well built, and are a relatively new product, so little to no concern about improper storage, or even worse, fakes/counterfeits. They aren't exactly cheap, though really not a bad price (of course, no where near these $1 a cell deals)... and the fact that I'm getting well known, very high quality cells... on top of utilizing other parts from these battery packs (the battery holders themselves are really nice, plus the batteries already have copper spot welded to them, and are in the configuration I need)... so I'm getting more than just cells for my money, but if I am only counting the cells, they are still only $5 each, including shipping... for new Sanyo 20700A cells, that can produce 30A each continuous. Watching all those videos made me feel even better about my choice... and even more hesitant in trying to find a really cheap deal for a future battery pack (still on the lookout for a solid price on quality cells here in the US.. on that's not a complete gamble... eventually I'd like to build a "cheap" high range pack, but not until I find something viable to build it with).

Anyway, good luck if you go with those, I will be very interested to hear your results... especially whether or not the cells meet their advertised specs, or anywhere close (Honestly with most Chinese cells, if they were 75% of their advertised spec or better, I'd probably consider it a win... I've had totally horrific experiences with anything other than the major name brands... but luckily all on very small deals, not more than a couple of batteries... tho most of the ones I tried were literally not even 10% what they were supposed to be, hopefully that's not too common).

Dude aliexpress have a seller called YM lipo battery store they are selling 5amp pouches at 50c con 100c burst have a look, I'm planing to get these for my next ebike project, dont know if its for real though most of the reviews are positive so high chance it could be.

Remember that "reviewers" often dont' really know anything about how their devices are performing, only that whatever they got does basically what they want when they first tried it. They dont' usually do lifespan testing, nor do they usually do any kind of specification-performance testing to be sure the stuff actually does what the seller says it does.

Another problem is there can be fake reviews both positive (by the company itself) and negative (by spammers of competing companies), and you can't really know how many of them there are.


I don't mean to sound always negative about stuff like this, but there's so much crap out there that "looks" good, that I hate seeing people burned because they didn't know to check it out.

those lifepo4 you first posted look like the old 0.3c calb 60ah cells I have in my scooter. emax 90s. I think that maybe people are half right about the sag, they do sag but they are also capable of 10c burst so 600amps (apparently) but only 18am continuous so to me
in my scooter I have 16s but eventually to upgrade to 26s. That is at 3.4v charge

26s
88.4 volts hot off charge x 600 = 53000 watts burst
88.4 volts hot off charge x 18 = 1591 watts steady

or

16s
54.4 volts hot off charge x 600 = 32640 watts burst
54.4 volts hot off charge x 18 = 979.9 watts steady

so my scooter hub is rated at 4kw. but I think that once up to speed I'm not consuming 4kw. I suspect they are ok but not as good as if you had a hybrid lipo pack to help buffer the burst.

edit
here is the 100ah and some specs 10secs at 8c
http://store.evtv.me/proddetail.php?prod=SE100AHA
I bought mine as a second hand lot from Lithuania.

Anyway keep up the good work dui, here and on mpcnc.