Pics/Info: Inside the battery pack

[Johan]

A lot of the genious here is in the design of the physicsl pack, geometric placement of the cells, cooling, fusing etc. Electrically it's only about one order of magnitude more complicated than one of these:

 

[rabar10]

Regarding the BMS and balancing techniques, there are lots of different ways to do it -- see the paper linked on this post.

Aside from the passive switched-resistor method, active circuits can use switched capacitors, switched inductors, etc. to "shuttle" charge between the different series-connected cell groups as needed. Or even use small DC-DC converters to shuttle charge back/forth with the auxiliary battery. Many many ways to get the job done.

I'm not at all surprised that there are no power-dissipating resistors to be found -- wasting energy isn't really Tesla's style

 

[qwk]

Seems like a bad design for a battery pack. The modules with less cells would also have less capacity, so would be doing deeper cycles than the rest of the modules. They'd also limit the capacity of the pack as a whole by hitting zero when the other bricks still have charge. Total range is limited by the weakest brick. We'll have to break open a 60 to find out.

I asked around, and it looks like Tesla is using the same number of cells in each module for the 60kwh pack after all. Some cell slots are left blank, while others have something else in place. Here is the only pic I could find that shows this somewhat.

 

[ElSupreme]

I asked around, and it looks like Tesla is using the same number of cells in each module for the 60kwh pack after all. Some cell slots are left blank, while others have something else in place. Here is the only pic I could find that shows this somewhat.

From the pictures it looks like they are leaving out 10 cells per set. Maybe they are putting in dummy cells for ballast (avoid duplicate crash tests, or suspension variations between versions?) in some of those modules. So that would be 64 cells, and 14 vs 16 modules. So 85 * (64/74) * (14/16) gives ~64kWh battery pack. Seems reasonable to me.

EDIT: And great picture thanks.

 

[Johan]

From the pictures it looks like they are leaving out 10 cells per set. Maybe they are putting in dummy cells for ballast (avoid duplicate crash tests, or suspension variations between versions?) in some of those modules. So that would be 64 cells, and 14 vs 16 modules. So 85 * (64/74) * (14/16) gives ~64kWh battery pack. Seems reasonable to me.

EDIT: And great picture thanks.

Not so sure about the ballast. Maybe volume ballast, for stability/integrity reasons. But I doubt they add weight ballast (re your suspension reference) - we know a 60 is lighter and gets slightly better Wh/km than an 85 and the only differnce I know of is the battery.

 

[GSP]

.......Vastly more likely that they've just done it the same way everybody else does - simple circuit to bypass each brick with a resistor and burn the excess energy.

Agreed. The resistors are most likely on the BMS circuit board and connected to each of the 96 bricks using the voltage sense wires.

GSP

 

[woof]

Got pics of the infamous contactors?

 

[wk057]

Got pics of the infamous contactors?

Top view of them in the first post near the end of the pics.

 

[scaesare]

Total pack power... something is interesting about the pack given the assumption widely held on the site here that Tesla has been using 3.1Ah cells from Panasonic.

Earlier in the thread it's been stated that the cells are charged to a max of 4.2V. We know there are 7104 total cells, so that would net a 7104 x 4.2V x 3.1Ah = 92.5kWh
pack.

That seems awfully high. If that cell voltage is wrong, and they are charged lower, that's still 90.3kWh @ 4.1V or 88.1kWh @ 4.0V.

If the voltage value of 4.2V is correct, and the assumption of the cell capacity is wrong, it would mean that the cells are only 2.85Ah cells.

I wouldn't be surprised to see Tesla leave a little wiggle room, but not that much. Given that voltage can be directly measured, whereas current capacity cannot be, I wonder if the assumed cell capacity has been wrong all along? Perhaps the Tesla cell chemistry trades off slightly less cell capacity for longer life and/or thermal stability?

 

[rlang59]

Total pack power... something is interesting about the pack given the assumption widely held on the site here that Tesla has been using 3.1Ah cells from Panasonic.

Earlier in the thread it's been stated that the cells are charged to a max of 4.2V. We know there are 7104 total cells, so that would net a 7104 x 4.2V x 3.1Ah = 92.5kWh
pack.

That seems awfully high. If that cell voltage is wrong, and they are charged lower, that's still 90.3kWh @ 4.1V or 88.1kWh @ 4.0V.

If the voltage value of 4.2V is correct, and the assumption of the cell capacity is wrong, it would mean that the cells are only 2.85Ah cells.

I wouldn't be surprised to see Tesla leave a little wiggle room, but not that much. Given that voltage can be directly measured, whereas current capacity cannot be, I wonder if the assumed cell capacity has been wrong all along? Perhaps the Tesla cell chemistry trades off slightly less cell capacity for longer life and/or thermal stability?

Isn't the nominal voltage used not the max. Also I thought Tesla was using the 3400mAh cells.

7104 x 3.7v x3.4Ah = 89.4kWh.

 

[islandbayy]

Isn't the nominal voltage used not the max. Also I thought Tesla was using the 3400mAh cells.

7104 x 3.7v x3.4Ah = 89.4kWh.

No, Max Voltage is used to measure capacity. 18650 is "FUll" officially at 4.2v. When dealing with my own lithium cells for my projects, I dont usually drain them below 3.2, though, I'm not sure off hand what most consider dead. I do know danger zone is 2v. Below that and they are "Usually" not safe to recharge due to risk of bursting.

 

[rlang59]

No, Max Voltage is used to measure capacity. 18650 is "FUll" officially at 4.2v. When dealing with my own lithium cells for my projects, I dont usually drain them below 3.2, though, I'm not sure off hand what most consider dead. I do know danger zone is 2v. Below that and they are "Usually" not safe to recharge due to risk of bursting.

Everything I have read about batteries states to use the nominal voltage for pack capacity since the voltage drops.

Here 1 just one example: http://ftp.jrc.es/EURdoc/JRC54699_TN.pdf
Table 3

 

[djp]

@rlang59 has it right, the nominal voltage is used to quote pack capacity. Tesla uses 95% of the 89.4kWh total, giving a rated capacity of 85kWh.

 

[qwk]

I'm pretty sure that the nominal voltage of Tesla's cells is 3.6V. Tesla doesn't charge them above 4.15V, and doesn't discharge them below 3.0V.

 

[stopcrazypp]

No, Max Voltage is used to measure capacity. 18650 is "FUll" officially at 4.2v. When dealing with my own lithium cells for my projects, I dont usually drain them below 3.2, though, I'm not sure off hand what most consider dead. I do know danger zone is 2v. Below that and they are "Usually" not safe to recharge due to risk of bursting.

Battery capacity is always measured with nominal voltage. It's because "max" voltage does not last the entire discharge cycle (it drops instead of being flat) so if you take the 4.2V * mAh capacity you will get the wrong Wh rating. "Nominal" voltage is intended to account for this voltage drop by being sort of an average (actually integrating the area under the curve).

For the Panasonic 3400mAh cells, they are 3.6V cells, so 7104*3.6V*3400mAh = 86.95Wh. Leaving a bit of margin off the top and bottom it's easy to see how it's rated 85kWh by Tesla.

 

[scaesare]

Well, for any of those voltages they have to be greater than 3.1Ah cells, so assuming they are 3.4, that renders:

At 3.6v: ~87kWh
At 3.7v: ~89.4kWh

It's interesting to consider djp's statement that Tesla only "uses" 95% of the pack for the rating. Estimates here have been made on how much of the pack is reserved for "brick protection"... and it's somewhere in the 4.1kWh range according to several estimates. And that's after the car exhausts whatever "below zero" buffer it has... which seems to vary some based on firmware revision.

The interesting thing about all of those tests, is that those reserves are within an 85kWh capacity... not 87 or 89.4. So either the "brick protection buffer" is even larger than surmised, or Tesla leaves that extra 5% for other reasons... perhaps to be conservative, or to counteract initial pack degradation somewhat?

 

[djp]

The interesting thing about all of those tests, is that those reserves are within an 85kWh capacity... not 87 or 89.4. So either the "brick protection buffer" is even larger than surmised, or Tesla leaves that extra 5% for other reasons... perhaps to be conservative, or to counteract initial pack degradation somewhat?

There's another reserve at the top end. Tesla doesn't fully charge the pack (even in a Range charge) to extend the life of the cells.

 

[scaesare]

There's another reserve at the top end. Tesla doesn't fully charge the pack (even in a Range charge) to extend the life of the cells.

Hence the 4.15V max?

 

[djp]

Hence the 4.15V max?

Exactly

 

[TonyWilliams]

I'm pretty sure that the nominal voltage of Tesla's cells is 3.6V. Tesla doesn't charge them above 4.15V, and doesn't discharge them below 3.0V.

The highest cell voltage during charging that I've seen is 4100mv, but it definitely goes below 3000mv. I ran them down to 2800, with voltage sags under power to 2600mv.

I suspect contactor opening is when any cell group hits 2500mv.

This is all data from the BMS in the Rav4 EV with Tesla 41.8kWh usable capacity battery. Another interesting observation is that I have not seen the cell voltages go above 4200mv during fully charged regen.