Roadster battery pack specs?

[malcolm]

Thanks Embassy.* That's a useful insight.* Regen braking is set to mimic conventional engine braking, but if it is under software control then it could be varied.* One possible scenario would be the car leaving a house on a full charge and having to go straight down a steep hill.* Unlike brushless motors, induction motors do not have to act as generators.* (It's one of those phase/timing things)* * Regen braking would be off so it would just feel like freewheeling with the clutch in.* Mechanical braking would have to be used.

Does this put us back to a Tesla battery pack containing mixed battery types?* Astonishing achievement if true.

[iisjsmith]

So Embassy, are you suggesting that the two packs are separate? That in the ESS there is one sub-pack with a low capacity but high discharge rate, and one sub-pack with a high capacity and low discharge rate, and there are no connections between the two? And the inverter is switched back and forth between the two depending on the driving load?

Wow, if they did that then I agree they deserve all the kudos they are getting. I don't know why they are so hush-hush about it...that is one hell of an accomplishment.

[Embassy]

Iisjsmith, I think it might be a little simpler than two separate packs and a switching inverter, but still clever all the same. And if I were designing such a thing I would want to scatter my high current lanes in and around my low current lanes so that the heat disappation would be distributed. A hot spot inside the pack would be bad.

Imagine a pack with 99 series by 47 parallel 2C cells and 99 series by 22 parallel 10C cells. Each cell has a current limiter so that the 2C cells never pass more than 4.6 amps each (assuming 2300 mah capacity) and the 10C cells never pass more than 15 amps each (assuming 1500 mah capacity). You probably want to throw another current limiter on each series stack as a whole, too.

Now when the inverter wants amps, it'll get them from the battery pack. And each "lane" if you like will discharge at the correct rate. But as soon as you as for more than 2C, you'll hit the limiters on the 2C cells leaving the 10C cells to pick up the current demand. I'd have to look at the individual cell specs but I expect that the current limiter would also protect the cells against overcharge during regen braking, too.. and the 10C cells would regenerate quicker than the 2C cells, most likely.

Of course, as soon as you do this, you'll have your two types of lane at slightly different voltages, which would be problematic. Maybe there's some trickle recharge of the 10C lanes. Or maybe both types of lane are stepped down to a voltage they can both always meet before they go to the inverter.

[iisjsmith]

That was my concern. If the two packs are discharged at different rates, their voltages and capacity will eventually be different. And if the two groups can "see" each other, then the one will start to charge the other, resulting in a loss of power and therefore range.

Keeping them isolated stops this issue. It would be like having an ICE vehicle with two gas tanks...one with regular fuel for normal driving and one with jet fuel that gives you a power boost for high accelerations.

But that brings up a host of other concerns, though. If you do a lot of high power driving you could "drain" the jet fuel tank sooner, leaving you with no power for high accelerations and only normal fuel for normal driving. Not good if you suddenly find yourself needing to accelerate up an freeway entrance ramp.

I would love it if the first people who got their hands on their Roadsters just rip the thing apart to see how it works! What's $100k to these multi-millionaires?

[Embassy]

Well I'm not taking apart mine! Not that I'll see it this side of next September

But does this make sense? Suppose the inverter wants, say, 360V. Though physically in the same container, the high-rate and low-rate strings could probably always manage 360V. And I think you're right.. they'd have to be electrically isolated to prevent them from cross-charging. Maybe each individual series stack is isolated - why would you want inevitable battery variation to permit cross charging even in the 2C stacks?

Something like this with a mixed-cell pack seems to be the cheapest explanation that fits all the facts we know so far. Unless they're using a honking big capacitor, a NiMH boost pack, or some other trick. I don't get the impression that they are.

[malcolm]

New information on the Tesla battery pack in this week's blog (30th November):-

http://www.teslamotors.com/blog1/

Seems max charge available is between 4.15 and 3V.* Maybe this is the clue to Tony's recent post about 1/3 of the battery pack being used.

[tonybelding]

I think that comment about using 1/3 of the capacity must have been garbled in translation somewhere along the way. The new blog entry really clears up the subject for me. It's good stuff.

[iisjsmith]

:-\ Eh, I wasn't too impressed with the recent blog entry. Some new stuff, like the user-programmable charging levels and the fact that they limit the discharge rate, but nothing really new. Nothing about how the batteries are wired, what the specs of each cell are, etc.

In fact, this brings up even more questions. We have already determined that the cells need to be high discharge capable...on the order of 4C. But if they suggest that discharge should be limited to C/2, and that they have designed the pack "such that even at maximum discharge rate, the current required from each cell is not excessive", then how do they get enough power out of the battery?

Every time these guys talk, it just brings up more questions. Don't get me wrong...I think the fact that they regularly talk to the community is fantastic. I just wish they'd get a little more technical than 'our battery is good!'

[malcolm]

Another piece to the puzzle:-

http://powerelectronics.com/mag/602PET06.pdf

18650 form factor Li Ions rated at 10C for the power tool market, still with 1500 mAh capacity.

I agree however that the C/2 rate mentioned in the article doesn't seem to make sense.

[iisjsmith]

Yea, I saw those powerelectronics cells. My post on page 1 has a link to similar cells on the BipowerUSA site.

The problem is 1500mAh and 3.7V discharge isn't enough. If we continue with the assumption that the 6831 cells are 99 series by 69 parallel, then 1.5Ah x 69 = 103.5Ah total capacity. With a total voltage of 3.7V x 99 = 366.3V. So 366.3V x 103.5Ah = 37912Wh or 37.9kWh....far less than the 50kWh number cited by Tesla Motors.

Now if you look at the table on page 2 of that PDF, there is the Sony 26650VT which definately fits the bill. It has an average capacity of 2400mAh at 10C discharge, with a discharge voltage of 3.75V. This is pretty sweet, considering it has an average capacity of 2500mAh at less than 1C discharge. This constant capacity at a wide range of discharge levels is freakin sweet...and is exactly what the Roadster could use.

2.5Ah x 69 = 172.5Ah total capacity. 3.75V x 99 = 371.25V total voltage. 371.25V x 172.5Ah = 64041Wh or 64kWh. A bit higher than the Tesla's number...but I'm sure that number will come down in reality. Looks like these cells fit the bill.

Now we just need confirmation of the 99S69P wiring.