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Discussion Starter · #1 ·
Hi,

Most oem packs have active cooling (except for bolt for example), but I was wondering why. And I cant seem to find a conclusive answer.

As to why Im wondering, Im going to build my own battery pack. I simply dont have the money to buy tesla packs to get the range I want and I can get recycled/tested 18650 cells that still have a capacity of 3200+ for a bit more than a euro a piece.

For example purposes keep a 60kwh pack in mind with a ~2000kg when loaded car.
Driving will only consist off normal high way acceleration up to 130km/h.

So here come a few, basic questions:

1. What is the ideal temperature for a 18650 cell and what range would be acceptable? (I live in the Netherlands which has a fairly cold climate, avg 10 with min/max -7 to 22,3)

2. When do the batteries heat up to the extent that cooling would be required.
a. Is this only during acceleration (from a standstill or just any acceleration?) or is it continuous during driving?

b. Is it only during fast charging or also if you would charge over night?

3. Does battery pack size have an impact on heat generated with a certain amount of amps? Would a bigger pack get less hot with the same amount of amperage?

4. Here we have about 3 months that the average temp is around 2 but frequently dip bellow freezing but never further than ~-7. Would I need to get heating for charging or would insulation be sufficient?

Thanks!
 

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There are so many variables, general answers will always be inaccurate. Even adding in missing information, like which battery chemistry, will not yield enough precision.

The commercial makers spend many millions on specialist engineers and computer modeling to answer these questions, and even then the development process involves a lot of trial and error.

Heat is not only a safety issue, but has a huge impact on longevity.

If all you can afford is scrapped 18650 cells, then likely the result of your project will be an interesting experiment but perhaps not convenient nor reliable enough for production use.

Each battery type will publish data sheets on their temperature range. 18650 is not a chemistry, just a size. Remember climate is changing, and ambient can greatly vary outside of average numbers.

Heat will rise with greater amps required, hills, acceleration and fast charging.

The physical design wrt cooling from unassisted ventilation, driving airflow etc has a huge effect, cell spacing, how enclosed is the bank, etc.

The higher the amps and ambient, the more likely active cooling is needed.

C rate is the critical measure, proportion of power capacity to energy flow, so of course a bigger pack will lower the C rate.

There is a time lag between ambient dropping below freezing and internal cell temperature matching, but again, lots of variables in how long that delay will be.

Best to design for all contingencies, or accept your result will not be convenient nor reliable enough for regular production use.
 

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Your prices for used 18650s of that quality (nearly new!) are really good.

However, compare the prices per Kwh compared to a recycled OEM pack.

You're getting not quite 12watt-hours per cell. That means you need ~85 of them to make 1Kwh.

At a Euro apiece, that's 85 euro/Kwh.

Now compare used OEM pack values from crashed OEM EVs.

I think you're several multiples more expensive.

18650s have the advantage of being tiny and configurable in all kinds of small spaces, versus most OEM cells are more restrained by their big clunky form factors. That's really the only reason to use them.
 

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Discussion Starter · #5 ·
and ambient can greatly vary outside of average numbers.

Heat will rise with greater amps required, hills, acceleration and fast charging.

The higher the amps and ambient, the more likely active cooling is needed.

C rate is the critical measure, proportion of power capacity to energy flow, so of course a bigger pack will lower the C rate.

There is a time lag between ambient dropping below freezing and internal cell temperature matching, but again, lots of variables in how long that delay will be.

Best to design for all contingencies, or accept your result will not be convenient nor reliable enough for regular production use.
Very insightful and answered all my questions! Superb.

I already have a cooling method in mind but I will devote another topic to that subject when I have done more research.


I may have something of interest to you ...I will send you a PM on this.
I will look into it :)


Your prices for used 18650s of that quality (nearly new!) are really good.

However, compare the prices per Kwh compared to a recycled OEM pack.

You're getting not quite 12watt-hours per cell. That means you need ~85 of them to make 1Kwh.

At a Euro apiece, that's 85 euro/Kwh.

Now compare used OEM pack values from crashed OEM EVs.

I think you're several multiples more expensive.

18650s have the advantage of being tiny and configurable in all kinds of small spaces, versus most OEM cells are more restrained by their big clunky form factors. That's really the only reason to use them.
You make a very valid point, cost. Which is exactly why I went down this DIY road.
I think I will end up paying around 100 euro/kwh and I did try to compare that to salvaged packs.
But the packs I see (leaf/model s) all go for atleast double. Take for example the cheapest model s module I could find, costs 1116 and is comprised of 5,2kwh. Which comes to about 213 euro/kwh.
Now perhaps I have been looking in the wrong places, if so please tell me where. Because a margin of 200% will make me go DIY but 130% might just make me gut my savings account ;)
 

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Discussion Starter · #6 ·
and ambient can greatly vary outside of average numbers.

Heat will rise with greater amps required, hills, acceleration and fast charging.

The higher the amps and ambient, the more likely active cooling is needed.

C rate is the critical measure, proportion of power capacity to energy flow, so of course a bigger pack will lower the C rate.

There is a time lag between ambient dropping below freezing and internal cell temperature matching, but again, lots of variables in how long that delay will be.

Best to design for all contingencies, or accept your result will not be convenient nor reliable enough for regular production use.
Very insightful and answered all my questions! Superb.

I already have a cooling method in mind but I will devote another topic to that subject when I have done more research.


I may have something of interest to you ...I will send you a PM on this.
I will look into it :)


Your prices for used 18650s of that quality (nearly new!) are really good.

However, compare the prices per Kwh compared to a recycled OEM pack.

You're getting not quite 12watt-hours per cell. That means you need ~85 of them to make 1Kwh.

At a Euro apiece, that's 85 euro/Kwh.

Now compare used OEM pack values from crashed OEM EVs.

I think you're several multiples more expensive.

18650s have the advantage of being tiny and configurable in all kinds of small spaces, versus most OEM cells are more restrained by their big clunky form factors. That's really the only reason to use them.
You make a very valid point, cost. Which is exactly why I went down this DIY road.
I think I will end up paying around 100 euro/kwh and I did try to compare that to salvaged packs.
But the packs I see (leaf/model s) all go for atleast double. Take for example the cheapest model s module I could find, costs 1116 and is comprised of 5,2kwh. Which comes to about 213 euro/kwh.
Now perhaps I have been looking in the wrong places, if so please tell me where. Because a margin of 200% will make me go DIY but 130% might just make me gut my savings account ;)
 

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[
QUOTE=Jubo;1037227]Hi,

I can get recycled/tested 18650 cells that still have a capacity of 3200+ for a bit more than a euro a piece.
[/QUOTE]

I believe he means this part
 

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Aha.

Cells are nowhere near new if any capacity has been lost, and of course whether they are good value or not is only proven by actual testing.
 

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Sorry, whose prices? Link?
Umm...

Cells are nowhere near new if any capacity has been lost, and of course whether they are good value or not is only proven by actual testing.
... I don't know if you're just not reading or what, but... literally in the first post:

"I can get recycled/tested 18650 cells that still have a capacity of 3200+ for a bit more than a euro a piece.

Both questions answered before asked.
 

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Discussion Starter · #11 ·
and ambient can greatly vary outside of average numbers.

Heat will rise with greater amps required, hills, acceleration and fast charging.

The higher the amps and ambient, the more likely active cooling is needed.

C rate is the critical measure, proportion of power capacity to energy flow, so of course a bigger pack will lower the C rate.

There is a time lag between ambient dropping below freezing and internal cell temperature matching, but again, lots of variables in how long that delay will be.

Best to design for all contingencies, or accept your result will not be convenient nor reliable enough for regular production use.
Very insightful and answered all my questions! Superb.

I already have a cooling method in mind but I will devote another topic to that subject when I have done more research.


I may have something of interest to you ...I will send you a PM on this.
I will look into it :)


Your prices for used 18650s of that quality (nearly new!) are really good.

However, compare the prices per Kwh compared to a recycled OEM pack.

You're getting not quite 12watt-hours per cell. That means you need ~85 of them to make 1Kwh.

At a Euro apiece, that's 85 euro/Kwh.

Now compare used OEM pack values from crashed OEM EVs.

I think you're several multiples more expensive.

18650s have the advantage of being tiny and configurable in all kinds of small spaces, versus most OEM cells are more restrained by their big clunky form factors. That's really the only reason to use them.
You make a very valid point, cost. Which is exactly why I went down this DIY road.
I think I will end up paying around 100 euro/kwh and I did try to compare that to salvaged packs.
But the packs I see (leaf/model s) all go for atleast double. Take for example the cheapest model s module I could find, costs 1116 and is comprised of 5,2kwh. Which comes to about 213 euro/kwh.
Now perhaps I have been looking in the wrong places, if so please tell me where. Because a margin of 200% will make me go DIY but 130% might just make me gut my savings account ;)
 

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You make a very valid point, cost. Which is exactly why I went down this DIY road.
I think I will end up paying around 100 euro/kwh and I did try to compare that to salvaged packs.
But the packs I see (leaf/model s) all go for atleast double. Take for example the cheapest model s module I could find, costs 1116 and is comprised of 5,2kwh. Which comes to about 213 euro/kwh.
Now perhaps I have been looking in the wrong places, if so please tell me where. Because a margin of 200% will make me go DIY but 130% might just make me gut my savings account ;)
Tesla and Leaf packs are the most wanted of them all, making them expensive.

when buying directly from scrapyard Tesla prices are more like €130-150 per kWh, buying seperate modules from people is more expensive, as they take the risk of buying the pack, investing in it and sometimes loosing money if modules are not good. (and making a margin of course)

There are lots of other options, a Bolt pack (which btw does have active cooling) can go for as low as €4500-€5000 (probably because they're not easy to use in the solar storage market) Kia Soul have very good 30kWh packs, etc.

Building a pack out of used 18650's:
-Don't
-Do
-It

It will be a complete disappointment, where do I start?
-different cells with different characteristics leading to unexpected results (fire)
-cells with different age / ir / capacity (fire)
-modules with different capacity (difficult to manage, result in unusable capacity)
-lots of work, lots of work, lots of work.
-fire
-lots of 18650's are not made for high C discharge current
-difficult to make vibration proof and still serviceable (you will find defects)
-cost, even if you find enough tested cells (tested with 0,3C probably so not a good test for EV packs), you still need cell-holders, nickel strips, a good spot-welder, cell-block holders, etc. etc. you'll never end up below €200,- per kWh (prove me wrong)
-need more?
 

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It will be a complete disappointment, where do I start?
-different cells with different characteristics leading to unexpected results (fire)
-cells with different age / ir / capacity (fire)
-modules with different capacity (difficult to manage, result in unusable capacity)
-fire
-cost, even if you find enough tested cells (tested with 0,3C probably so not a good test for EV packs), you still need cell-holders, nickel strips, a good spot-welder, cell-block holders, etc. etc. you'll never end up below €200,- per kWh (prove me wrong)
These reasons are generally false paranoia.

Different cells leading to "unexpected results" like what? Many cells in parallel will be just fine. Mix and match capacity all you'd like.

Different age the same.

It's not difficult to manage "different capacity". Actually very simple.

2 methods:

1 - Use the http://repackr.com/ Repackr. Paste in a list of your capacities, it builds a balanced pack for you.

2 - Guess blindly but leave the top few percent of each parallel group empty. Then do a discharge test and see how far behind the worse groups are. Add extra cells to that group with the right ballpark amount of capacity.

Fire is not going to happen if you have a BMS. And, honestly not likely to happen anyway.

Extra costs? Plastic cell grids are pennies apiece. You don't need nickle strips, plain copper wire and connector wires is all. You don't need to buy a spot welder, you can just solder, it's not the crisis that the contrarians want you to think.

-lots of work, lots of work, lots of work.
-lots of 18650's are not made for high C discharge current
-difficult to make vibration proof and still serviceable (you will find defects)
These are legitimate reasons not to. Especially the amount of work. Whoa damn is it a lot of work.

Generally laptop cells are not meant for high discharge rates, however, on a respectable-sized pack, any pack that lasts you an hour of driving is fine, since that's only 1C discharge rate. Laptop cells are fine sustaining that. Power is not an issue unless you're using it in a racing environment.

Vibration proof and such I'm not sure about. I haven't seen any evidence that 18650s are somehow worse than other form factors. The cheap plastic grids you can buy hold them very tight.

...

I probably still wouldn't suggest using them. I'm getting free 18650s from power tools, and I'm right on the edge of whether it's worth using them versus buying OEM packs. But, it's important to be fair in the reasons to use or not use them.
 

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Matt,

could you post some links to successful EV projects that use a battery built using the methods you've advocated above please.

My use of the word succesful in this context means, long term reliability, proven safe, usable range and reasonable project completion timescales.

Thanks.
 

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These reasons are generally false paranoia.

Different cells leading to "unexpected results" like what? Many cells in parallel will be just fine. Mix and match capacity all you'd like.
Like 1/4 of cells has power below 3,5V and the other 3/4 not, those 1/4 of cells will provide most of the current in that voltage range

Likewise different internal resistance, the cells with lower resistance will provide more power than the rest. Different age the same.

Generally laptop cells are not meant for high discharge rates, however, on a respectable-sized pack, any pack that lasts you an hour of driving is fine, since that's only 1C discharge rate. Laptop cells are fine sustaining that. Power is not an issue unless you're using it in a racing environment.

Vibration proof and such I'm not sure about. I haven't seen any evidence that 18650s are somehow worse than other form factors. The cheap plastic grids you can buy hold them very tight.
1C discharge may be fine, but lets say 50kWh pack, most EV's have higher peak power, combine that with different cells, and some cells will have to withstand 3C while others do little.

A stationary powerwall is already difficult to manage / maintain with used 18650's, an EV is a whole different game.

Vibration proof: the modules + complete pack needs to be vibration proof, so not to break connections, shift modules, etc. combine that with the need for maintenance and the whole idea of using used 18650's becomes even less attractive.
 

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My use of the word succesful in this context means, long term reliability, proven safe, usable range and reasonable project completion timescales.
...

Are you kidding? On a DIY project? Who's going to record and report that to a degree that would make you happy?

Your necessary criteria includes this long string of corporate tech-speak?

DIY is maybe not for you.

I don't know that 99% of any DIY EV built by any methods or materials would satisfy someone in the boardroom for "reasonable project completion timescales" and other production/marketing wankery or the rest of that.

What does "reasonable project completion timescales" have to do with DIY anyway? It's 100% a matter of how much work you put into it. Why would you reject an option because people have taken their time on it? Again, DIY and boardroom meetings about schedules don't cross over often.

I think the biggest reason there just aren't many examples of this is because scale, not suitability. Lots of people think they'll start collecting enough for free to build a pack, but it takes thousands. So, for how good of an option it is (a mediocre one), it's probably underrepresented in terms of projects (very few).

...

Some analogs:

Jehu Garcia built a recycled 18650 pack for his Samba, just to show it can be done. It's a portion of his pack. Far as I know it's held up at least a couple years now.

And I forget the guy's name but last time I asked, skeptical that any or many had ever actually finished an 18650 project, someone said they had.

In terms of Powerwall applications (much lower power requirements but reasonably similar energy requirements), I'd say there's dozens of people with recycled 18650s. The https://secondlifestorage.com/ community is entirely people doing just that. Hundreds of people, but probably only dozens with finished projects in use.

In the Powerwall community, there was some head-butting from two of the bigger personalities last year. One guy capacity tests and IR tests everything, is extra-picky about what cells he uses. Another guy is more laid back and tests for voltage and makes sure they don't heat, but that's it. The first guy tried to demonstrate how his methods were necessary by building a pack the way the other guy did, intending to embarrass him and show how it would lead to disaster. He did an update 6 months later and seemed frustrated that everything was working fine. His only nag was that, for identical cell count, that one pack was a few percent weaker (5% or so, I think). Which only means, duh, since he wasn't capacity testing them and rejecting the lower ones compared to the rest of the banks in his powerwall, lower capacity ones were part of the pack so pack energy was a bit lower than average for the same cell count. He used Batrium I think so he had live bar graphs of every pack, data logs, the whole bit.

These are people who've had them hooked up, in use, non-stop for years now (in Powerwall context).

In E-bike context, there are hundreds of people who've used recycled 18650s for bicycles that they commute on every day in the https://endless-sphere.com/forums/ community. The difference between bicycles and cars is only scale. Power requirements for bikes are less, but so is storage room so I suspect the C draw from most E-bikes is similar to shorter-ranged EVs. You'd almost never see an EV with a 30 minute runtime at average speeds, but that's fairly normal for an E-bike, 60 minutes is a pretty long-range bike, so, it's a good comparison for power.

Heck guys have commuted long-range on e-bikes for years without even a BMS, just manual seasonal recharging.

So, aside from the obvious drawbacks of "do you actually want to disassemble and test thousands of recycled cells?", I don't think there's a problem.

Boekel said:
Like 1/4 of cells has power below 3,5V and the other 3/4 not, those 1/4 of cells will provide most of the current in that voltage range
I'm not exactly sure what this hypothetical is. Any cells in parallel are going to be forced to be the same voltage. I'm not sure what you mean by "has power below 3.5v".

Do you mean that the cell capacity tests fine, but somehow the discharge curve has changed shape below 3.5v compared to other cells? Is that something that happens to older lithium cells or is this entirely hypothetical?

I've capacity tested thousands of recycled 18650 cells. I don't sit and stare at them all day, but they're right next to my desk. I don't think I've ever noticed a cell that had a peculiar discharge curve or where the voltage plummeted faster than normal at steady current draw after some point. If a cell was half capacity, the voltage would drop twice as fast, but that's exactly normal.

It has a little bit of a negative feedback loop (self-correcting) too. If some cells end up having to supply more of the current, they'll get a bit hotter. Being a bit hotter raises their resistance, so the other cells will supply more current in comparison.

I suppose it might be possible, but, even on the bad cells (cells sitting at 0v I tried to revive), I've never noticed this even once in thousands of cells. So, as to concern about 25% of the cells in a recycled pack having a suddenly lower ability to supply current below a certain voltage... I can't see how that's possible, even on purpose.

1C discharge may be fine, but lets say 50kWh pack, most EV's have higher peak power, combine that with different cells, and some cells will have to withstand 3C while others do little.
50kwh pack. Average power draw for a car at highway speed is, what, 350wh/mile? That's 142 miles of range. At highway speed, which is high power draw, that's still only 0.42 C. At slower speeds, even less, much less.

Using a Leaf motor (~100kw), flat-out, max power, is still only 2C draw from this 50kwh pack you're proposing.

So, unless you're performance-minded or using this car on a racetrack, I can't see that it could ever, ever be a problem, even for the worst 18650s out there. You might spike to 2C for a few seconds of acceleration but otherwise there's just no way to actually use 100kw on the road, you'll taper back to 10kw in the 10 seconds it takes to hit highway speed. If some cells do a little bit of extra lifting for a few seconds, it's fine.

A stationary powerwall is already difficult to manage / maintain with used 18650's, an EV is a whole different game.
A stationary powerwall is difficult to manage and maintain with used 18650s? Who is struggling with that? It's straightforward and easy enough for beginners to get right.

...

I'm still convinced a recycled 18650 pack would be just fine as an EV battery. It's just a pain in the ass to collect a few thousand cells. But for anyone who knows that up front and is okay with it, nothing wrong with it far as I can tell.
 

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...

Are you kidding? On a DIY project? Who's going to record and report that to a degree that would make you happy?

Your necessary criteria includes this long string of corporate tech-speak?

DIY is maybe not for you.
Other than the Samba, what other DIY EV traction packs have been built using the method you are advocating - and what's the Samba using for traction now ?

You spent most of your not answering my question reply talking about static storage - and you mentioned the Second Life forum, what's your user name there ?

You might be right about DIY not being for me .....
 

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Other than the Samba, what other DIY EV traction packs have been built using the method you are advocating
Oh, I thought I mentioned that.

Hardly anyone has completed a project like this at car-scale.

I actually asked for a few months every time it came if anyone had ever actually taken a project to completion, and, someone eventually popped in and said yes (I didn't take notes of who). That doesn't mean much, it's not like everyone who ever did is around to check in with us, it was a more popular thing before OEM EV batteries became so readily available in the last few years.

You spent most of your not answering my question reply talking about static storage
Correct. Since there aren't many direct comparisons, I compared to something similar in terms of energy storage. I said that right in the post.

I also talked about E-bikes which have similar C rates but are more popular because of smaller scale. They should be analogous to EV cars.

what's your user name there ?
I hardly ever post, I don't remember what I used if it was different than this one, I'd have to dig it up. Why?

You might be right about DIY not being for me .....
Even just because you're asking the questions you are, the way you are, to me says no DIY solution is going to be acceptable. Even just, a home-built picnic table or tree house, with
"long term reliability, proven safe, and reasonable project completion timescales"... probably what's best for you is to go to Home Depot and purchase a picnic table then.

If you want the things that you listed in your criteria, a good way to solve that is to just purchase a used OEM EV.
 
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