Is a Big Cell of ANY use?

[Dauntless]

With all the NiMH paranoia continuing all these years after Chevron gave up their share of Cobasys, I'm wondering the relevance of the bone of contention to begin with.

For those not familiar, Cobasys wouldn't sell "Large" NiMH cells until someone ordered 100,000 of them. Never mind Chevron owning a share, anyone who didn't want to order 100,000 large cells a year for 3 years had to settle for the smaller ones, which Chevy and Toyota did. This was the socalled stranglehold. 2009 saw the end of Chevron owning a share, but not the end of the paranoia. Many complicated workarounds to increase blame on Chevron continue to this day.

But why do people care about larger cells? Tesla thought it a good idea to stitch together such small lithium cells instead of putting less work into larger cells. Why doesn't anyone feel the same way about NiMH? If it's good enough for Tesla, why isn't it good enough for anyone else?

 

[gogo]

Tesla is leveraging current technology and manufacturing base. If Tesla develops larger cells in conjunction with their new battery factories, waht will this tell you?

 

[Dauntless]

Tesla is leveraging current technology and manufacturing base. If Tesla develops larger cells in conjunction with their new battery factories, waht will this tell you?

It will tell me it's then no longer working with the older technology being discussed here or even with anything that exists today. It'll also further make me question the obsession with the bigger cells that weren't available while Tesla was saying they weren't a good idea anyway, since Tesla had to develop something new to be bigger.

 

[major]

But why do people care about larger cells? Tesla thought it a good idea to stitch together such small lithium cells instead of putting less work into larger cells. Why doesn't anyone feel the same way about NiMH? If it's good enough for Tesla, why isn't it good enough for anyone else?

From my experience there is an inherent problem charging parallel connected NiMH cells. Thermal runaway often results. I suspect some smart people could have figured out how to do it, but maybe it proved to costly, unreliable or unsafe.

 

[Hillhater]

But why do people care about larger cells? Tesla thought it a good idea to stitch together such small lithium cells instead of putting less work into larger cells. Why doesn't anyone feel the same way about NiMH? If it's good enough for Tesla, why isn't it good enough for anyone else?

Im not clear what exactly your question is ?
Q:..Why dont more EV makers use small capacity Li cells to make large packs ?
A... Because its too time consuming , complex and consequently costly ,..compared to the alternatives.

Q: Why does Toyota use larger format/capacity HiMH cells rather than multiple small format cells ?...
A: ... as Major said above. Charging gets tricky

Q: why do some EV's use NiMH instead of Tesla type Li pack chemistry ?
A : ...it seemed a good idea at the time ! ...OR... Most likely cycle life and abuse resistance considerations

 

[Dauntless]

Im not clear what exactly your question is ?

Well, since you're ready to hit to all fields, maybe you can come up with an even better answer.

Basically your first answer as in the ballpark. At the same time as all the screaming that Chevron DESTROYED the NiMH by being a coowner, Tesla seemed to disprove the only objection, that being that noone could have the large cells that were so much better, without explanation of how they were better. Tesla decided it was better to use very small cells. Although of course they were using lithium.

I suppose thermal runaway is concern. Couldn't the same charging process as the lithium prevent it?

 

[mvly]

First off this is 2014. No one gives a shit about NiMH unless you are Panasonic and manufacturing them in AA or AAA package. Please excuse my language, but I just find it funny and stupid how people are bringing up old tech not fit for ebike or car usage.

As for large format vs small, I think Tesla just worked with what they had at the time.

Small format have their pros and cons. At the time, the best batteries were manufactured in 18650 format because it is the least risky format. Essential Panasonic can we them to anyone if Tesla did not use their cells. If they manufactured large format at the time, it would be a huge risk because if Tesla did not take off, who would buy their large format cells?

IMO, large format cells have the advantage of reduce complexity. But it also puts more risk on the pack as a whole. If you only have one large format cell Ina series go bad, your whole pack is pretty much dead. However if you have many in parallel, then you can make your cell BMS to bypass the bad cell and effectively reduce your capacity.

So it all comes down to thea age old problem. More reduancy with increase complexity or simplier configuration, but sacrifice redundancy. I think Tesla went with the 18650 format because at the time, cells were shotty. Even if the cells were manufactured by Panasonic, there are bound to have a few bad cells. Having many cells in a pack increases the probability of the pack lasting longer, which mean more reliable car in the long run.

However, I think as the technology and manufacturing get better, large format cells will ultimately dominate small to large EV, relegating the small format cells to flashlights and hand tools.

 

[dnmun]

i wonder if that is the real reason they tended to use these small can formats. if they had concerns about the reliability there may have been an engineering decision to keep the size of the individual units small so that defective cells could be removed in a later production test or maybe even for vehicles on the road.

i expect that they actually are data logging each of the individual cans for case temperature and charging profile and using the data to estimate failures in advance to ward off failures of the pack on the road. be able to know in advance if one of the individual cans is malfunctioning by overheating during charging. so they pull that section and send it back to the factory for rebuild or rebuild in a local facility.

i think that would be a reasonable management decision based on what they could expect in terms of reliability of the pack with no prior large fleet vehicle maintenance information.

 

[Lebowski]

How about the argument that you can give a pack built with small cells any shape you want, but a pack from large square blocks will always be, well, a large square block

 

[dogman dan]

I thought they just got better performance in terms of c rate from a little can, vs a big pouch.

Plus, the little can factory like, exists.

 

[major]

I suppose thermal runaway is concern. Couldn't the same charging process as the lithium prevent it?

As I remember, the charge characteristic for NiMH has a negative delta V region near the top. When in parallel, if the cells are not perfectly matched, circulating current develops when a cell reaches that point before its buddy. The cells heat which causes increased current and temperature. This will continue even after charge current is stopped. I've seen it happen with D cells. On the other hand I used a NiMH battery with 10S2P AA cells and never had a problem. But I think it was always undercharged floating on a DC/DC in a vehicle. Or maybe I was just lucky.

I think that negative delta V is unique to NiMH. I've never heard of it with Li based batteries. For the NiMH small cell parallel connected battery, I'm guessing the BMS would have to manage each cell as opposed to each parallel group as often done with Li. However Tesla does manage each Li cell in their batteries, don't they? It could be that the mass produced small Li cell was less costly than an equivalent NiMH cell.

 

[Dauntless]

First off this is 2014. No one gives a shit about NiMH unless you are Panasonic and manufacturing them in AA or AAA package.

I wish I could agree with you on that. But not only are there still people saying it's the way to go, but they still complain on and on about Chevron 5 years after they sold off.

As for large format vs small, I think Tesla just worked with what they had at the time.

Small format have their pros and cons. At the time, the best batteries were manufactured in 18650 format because it is the least risky format. Essential Panasonic can we them to anyone if Tesla did not use their cells. If they manufactured large format at the time, it would be a huge risk because if Tesla did not take off, who would buy their large format cells?

And that's what was going on with the NiMH's.

if they had concerns about the reliability there may have been an engineering decision to keep the size of the individual units small so that defective cells could be removed in a later production test or maybe even for vehicles on the road.

Planning ahead for possible warranty work.

How about the argument that you can give a pack built with small cells any shape you want, but a pack from large square blocks will always be, well, a large square block

That would be fitting, just to use a double entendre. You see the kitcar electrics lining the square SLA's along the chassis tunnel, because that's close to the shape. But could probabaly tuck additional small cells in.

I thought they just got better performance in terms of c rate from a little can, vs a big pouch.

They suggested it at the time, probably didn't say it flat out because the public at large wouldn't understand anyway. It would be the same with SLA's, right?

As I remember, the charge characteristic for NiMH has a negative delta V region near the top. When in parallel, if the cells are not perfectly matched, circulating current develops when a cell reaches that point before its buddy. The cells heat which causes increased current and temperature. This will continue even after charge current is stopped.

For the NiMH small cell parallel connected battery, I'm guessing the BMS would have to manage each cell as opposed to each parallel group as often done with Li.

It could be that the mass produced small Li cell was less costly than an equivalent NiMH cell.

That last sentence would be a bolt from the blue. But they used LAPTOP batteries, so it would make some sense.

Please excuse my language, but I just find it funny and stupid how people are bringing up old tech not fit for ebike or car usage.

It was the fact that I wanted to think that way that inspired the question. I understand in converting an old car just using what's cheap, but check around, you find people still talking that SLA's are getting better. And it would be a good thing if they did and kept a simple, cheap format useful to people.

But if Korthof was still around, he'd probably still be touting SLA's in general and Cobasys in particular. And expecting everyone in lockstep with him.

http://adoptacharger.org/what-we-do/doug-korthof-memorial-fund

 

[Hillhater]

The reasons why Tesla chose Li 18650's over all other available cells are well documented.
There is a "white paper" on Tesla's site that goes into some detail, but in summary it was based on,..
1) safety, (small capacity units , with individual protection)
2) performance,..power and energy density.
3) consistent , reliable quality, with a proven history
4) thermal control,..ability to integrate a heating/cooling system throughout the pack
5) availability ,.. the need to have a reliable, high volume, quality supplier.
6) cost,..or rather i suppose "value" ..for a cell that met the necessary criteria.
Li Laptop cells were one of the few cell formats that were actually available in the volumes needed at the time.
I dont believe NiMH was even a starter on several of the above points.

but check around, you find people still talking that SLA's are getting better. And it would be a good thing if they did and kept a simple, cheap format useful to people.

Yea, and there are people who still talking the world is flat !
Really, does anyone still think any form of lead cell is a good idea for an EV ? ..
...Weight, cycle life, energy density, peukert effect, etc etc.
They would have to be almost free to even be considered by any thinking EV engineer !

 

[spinningmagnets]

By making a long-term commitment to the 18650, Tesla is also leveraging the high-current cordless tool industry. Tool cells will continue their development and evolution without Tesla's input or purchase contracts. And by avoiding a proprietary cell, they also avoid any maneuvering that could bottleneck their suppliers.

 

[Hillhater]

SM,..You may find that it is the other way around, with the cordless tools leveraging off the huge volumes created by Tesla's decision back in 2005/6.
And really the whole 18650 market was initiated by Toshiba, IBM, Sony, etc...for their laptops back in the '90's
It is far from certain that Tesla will continue with the 18650 cell for the new car packs being produced at the "Mega Plant" !

 

[zener]

The 18650 size standart makes the cells from different vendors comparable.
And the competition on the market together with the easy comparability makes them evolve the fastest. (if you ignore the new (and old) bettery breaktruth's which never come to marked.)

 

[liveforphysics]

NiMH is kinda unique in it's charge curve with cell voltage dropping as it finishes charging. Hence, if one cell starts to finish charging before the one it's in parallel with, it starts rapidly sucking current out of the lower SOC cell, and this is positive feedback loop as the higher SOC cell's voltage keeps dropping as it's finishing it's charge, so it often enters thermal runaway. Because of this, it's quite a design challenge to make parallel strings of NiMH work well in practice.

Hence why if you want decent EV range from a NiMH pack, you must have single cells that each have the full capacity you wish the pack to have. That is the entire reason why large format NiMH was critical, as it was the required enabling technology to have NiMH EV packs work (and hence why Texaco bought and buried it).

For most all Lithium ion based cells (which includes every type of rechargeable lithium battery we use on ebikes and EVs), directly paralleling cells is safe and easy. There was a time when the world could have benefited from the use of large format NiMH, but that ship sailed many years ago, and Lithium simply does a much better job in most every way now.