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EV cells to build a battery pack?

chris_m

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Nov 9, 2020
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Hi everyone,

Here in Norway there starts to be quite a few used cells out of electric cars coming out on the used market.
It's tempting to use these to build a solar battery, as prices are quite low.
But is it advisable (for a newbie)? And I understand usually these cells are 3.7v built into blocks of 2S3P or 4S3P, giving packs with 7.4 and 14.8V.

How would one build a 12V or 24V battery out of such cells? or would one use a different voltage?
 
You do not build effective 12V batteries with 3.7V nominal chemistry. They just suck. If you absolutely, positively have to, 4S is slightly more practical IF you can charge to 16V and you have an inverter capable of operating at that voltage without shutting off.

7S for 24V is the sweet spot.

Understand that this type of lithium chemistry often fails by thermal runaway and results in a fire.

I do NOT consider it advisable for a newbie.

Check out this guy's more recent videos for best practices:


Note that "best" doesn't necessarily equate to "good" when there are other similarly inexpensive ways to build a storage battery with LFP chemistry.
 
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There are many types of cells coming available from the EV Market space and there is quite a mix / variety. Many can be used for Energy Storage Systems but not all, some are better suited to larger scale systems due to the voltages & curves. Depending on pack type and how it can be broken down, the Voltages may not necessarily work with regular (average) equipment and often require Inverters / Chargers and even Solar Controllers which are highly programmable which allow for a broader voltage range.

The Chemistries Vary a lot. EV's have been and will be produced with LFP as well as several other chemistries. LFP is presently one of the safest chemistries with a profile that is "close to" Lead Acid battery profiles and therefore easier to mix in energy storage systems with little effort. Other chemistries CAN be dangerous & even volatile if not handled / managed properly. BTW: Avoid Used Nissan Cells ! lots of used & craptastic ones out there... they are generally junk. Some folks will hate that but it is true. Tesla Packs are popular but they have voltage curves that require a better grades of gear.

Pros & Cons... well there is many... but a few toppers on the list.
EV Batteries and the cells used within are ALL Matched, Batched & Binned to be identical for performance, charge & discharge, prior to assembly into packs. (main reason they cost so much more). Generally, they use Higher Energy Dense cells which are more volatile and potentially dangerous. EV batteries that have Failed Cells within a pack are decommissioned, sometimes they are repaired with defective cells being replaced but others go off to the 2nd life market if it's too costly or technically difficult (some packs cannot be repaired due to how they are assembled).

Keep in mind a very true adage: If it sounds too good to be true, it likely is false. If ANY vendor pressures you to buy something, PAUSE ! red flag !! There are always great deals to be had and they do come up often, one must be ready to act when it happens and when you "know" it is a good deal, patience is important and never ever impulse buy anything, 99% guarantee you will regret it.

There are some reputable vendors in Europe who sell new & Used cells and battery packs. Some actually use testing & evaluation centres ad class / catagorize cells for reselling or for mineral recovery. Such programs exist in France, Germany, Netherlands and I believe Belgium is putting up a faciltiy & sponsoring such as well. I also hear in the Rumour Mill (EV Side) that Norway was also working on some form of used Lithium Battery Recycling & Upcycling facility to test, evaluate and process used cells. I do not know if this is operational as yet or how soon it will be. Europe is far ahead in this area compared to North America who is just realizing this need.

In closing, I suggest you may have a look at some of our Known Good Suppliers for battery cell pricing so you have a baseline of what costs are for New LFP Cells for X capacites. That will let you compare the used EV cells costs to new LFP. As well you have to factor in equipment costs (if you have to buy higher grade configurable gear to handle the higher capacity cells) which adds up fast. At present the most popular cells are the 280AH format but due to high demand, the prices are now climbing due to supply... More is coming so not to worry.

Hope it helps, Good Luck.
Steve
 
Thanks folks for very insightful information.
Sounds maybe like I should not _start_ with this project, but save it for a bit later when I have some more understanding of chemistries etc.
But in any case, if at all, then 7S would be the "only" way to use EV cells for stock equipment?
 
If you dont know how to test the cells or dont know the km/miles from the donor car....DON'T.....SIMPLE
 
Understand that this type of lithium chemistry often fails by thermal runaway and results in a fire.
NOPE, sorry to say, but car battery are safer than other li ion chemistrys....Nope again, sorry.
Li ion strait is the most dangerous of them all(18650 cells usually), and has the fasted thermal runaway, BUT if we decide to keep the discharge low and the charge low (25% of the c rate) also and leave out the red sanyo's and do cell level fusing like tesla, you are oke.
Add cobald and aluminium you will get safe high c rate cells, add iron fosfate you will get low c rate (ess) very save cells.
Take the SOH% + IR into account and you know exactly what to buy in second hand cells and its or there value...
Secondlifestorige is a great forum btw, if your are willing to commit to 18650 and spent a lot of time with those cells, but really a very great forum to learn about everything about every li ion based chemistry....With no add ons!!!!!like ev cells!!!Just saying.
Best Igor K
 
Yes, LFP cells are supper safe and Li-Po cells can catch in fire.
But everyone need to know the fact that many EV cells are not Li-Po cells, but NMC cells. Those are way safer than Li-Po.
Personally, I use 6S battery for 24V and this work fine as max/min voltage (25v / 21v) match many 24V inverter. Of course, 7S battery can work as well (29v / 24v).

EV cells are generally such high quality cells that I think they should be consider. Especially because they are often cheap in used format.
 
Yes, LFP cells are supper safe and Li-Po cells can catch in fire.
But everyone need to know the fact that many EV cells are not Li-Po cells, but NMC cells. Those are way safer than Li-Po.

Correct, but WAY less safe than LFP


 
I have seen that second video several times. It is dramatic. It is a good example of what over charging NMC or NCA can do. The facts are that they hooked up those modules to a FLA charger without any control over voltage or current.
 
The second vid is from some electronic beginners.
The did NOT use a bms OR a overcharge/undercharge protection....
They where charging them with a lead acid charger......
Li ion CAN be very save, IF you play by the li ion RULES
I handled, and fully tested a 10.000 plus second handed of those.....

I think the only(or the most) ev's fed with lifepo cells are driving manly in China?
As for the European continent secondhand NMC cells are very tricky.
There is not even one car scrapyard owner in the Netherlands that i know of(at least not in my eara) that can (or want) to test the packs under the car.
They are very happy to sell it to you for 2000E(24kwh nissan leaf for example)....even if the donor car has less then 25000 km it could be less than 25% soh....
There is one person i know that has the right testing equipment for those battery's to test the ir, soh and cycle life at cell level.
Aaaaaaand he is going to retire next year....

Funny story:
I had some cells at 50% soh, to low for my application....i charged them full and tossed them into the fire(outside)
Some where hissing, some blowing, some did nothing....some exploded with a great force.
I was standing at 5 meters away, one casing hit me in my chest, that spot was purple blue and red colored for 2 weeks.
Over charge li ion with just 5.5v and 1 amp charger you will get a roman candle or go bang, please do this outside with a 10 meters exertion cord!
Connect a 12v bat...., please be careful or go on youtube.

Best Igor
 
As a beginner I built a couple of very small LFP batteries (7Ah, 12v & 20Ah, 12v) to familiarize myself with the process, math, terminology and equipment. It was not expensive at that scale and I ended up with a couple of very usable batteries. Not to mention is was fun. :) Might I suggest you do the same before you start building a large project?
 
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