Li-Ion "Pulse" C Rating

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celectric
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Li-Ion "Pulse" C Rating

Post by celectric » Tue, 29 May 2012, 18:14

The LiFePO4 cells that EV Works sells are rated to 3 or 4 C continuous and 10 or 12 C "pulse". How long is this pulse allowed to be? Can I use it for calculating the peak power output of my battery pack over say 10 seconds of acceleration or is it much shorter than that?

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Post by evric » Tue, 29 May 2012, 18:30

Hi Tim,
Welcome to the forum...
I am using Thundersky cells in my conversion and as I understand it, the "impulse" current of 10C is just that - a small part of a second (I have not seen the part of a second specified anywhere).
The "Continuous" rating, again as I understand it, is for approx 30 seconds at a time. Don't ask me how often this can be applied. I assume at least the same time of recovery at a much lower current.
If you find out any more or if some one else knows more, please let us know.
If your constructing a car, use the largest capacity cells that you can (afford) and you won't have to be concerned with these figures.

Don't forget the battery current under acceleration (from a start) is much lower that the motor current (thanks to the magic of the controller) eg. with a motor current of approx 600Amps the motor current may be only 300Amps (130V DC system)

Come to the next AEVA Adelaide meeting and ask more questions. Many people there have had experience with these cells.
Last edited by evric on Tue, 29 May 2012, 08:36, edited 1 time in total.
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Post by jonescg » Tue, 29 May 2012, 18:37

I'd say under 5 seconds.

With most lithium chemistry, my safe estimate is take the continuous rating, halve it, and never try to exceed this number. Your real-world continuous discharge should be well under this (already halved) figure.

For example, I have some Turnigy cells which claim to be 40C continuous. At 15 Ah, that's 600 A. I now seek to never exceed 300 A discharge, and keep the continuous discharge closer to 150 A.

It's just another way of ensuring the cells stay as capable as possible for the majority of their life.
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Post by evric » Tue, 29 May 2012, 19:18

Tim,
Don't be confused by the example Chris used with cells of very high C ratings. The Turnigy cells are a different chemistry (Lithium Polymer) and are used in model aircraft, cars and boats etc. and not in car conversions.
I tend to agree with Chris in halving the continuous current rating, to extend the cell life.
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Post by celectric » Tue, 29 May 2012, 19:53

Thanks guys for the info. So if I have 60Ah, 3C cells the most I can safely draw is 90A?

When I'm calculating the maximum power my pack is capable of delivering, can I calculate based on (nominal pack voltage * max continuous current)? If true, 90A won't get me anywhere near the 35W/kg I saw recommended here. And I can't go up to 100Ah cells because of weight restrictions (I'm already reduced to 2 seats as it is). So what are my options - upgrade the GVM or find another donor car?

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Post by weber » Tue, 29 May 2012, 19:57

This datasheet linked from the 100Ah SkyEnergy (blue) cell on the EVWorks website, says 4C for 30 seconds and 12C for 5 milliseconds. Why anyone would care what the 5 ms rating of the battery would be, I have no idea. We have capacitors inside our controllers for that!

I once saw a document on the ThunderSky website that detailed all the test they did to the cells. One of them was to subject a fully charged cell to a 10C discharge for 10 seconds. But then maybe it's only allowed to do that once in its life.

In our design, Coulomb and I are hoping the Sky Energy cells can take 1C continuous, where continuous means until they are nearly empty i.e. for nearly 1 hour. That's what they show on a graph.

We are also hoping that 5 ms is nonsense (hopefully meant to be 5 seconds). We don't plan to go anywhere near 12C, but we do plan to do 6C for up to 10 seconds.

I expect the limiting factor is the hot-spot temperature inside the cell exceeding the boiling point of the elecytrolyte and forcing the plates apart. It is very important that your cells be clamped.

When a heating process is happening fast enough that it doesn't have time to conduct the heat away, one expects an approximately constant I-squared-times-t. So if it can take 4C for 30 seconds then it should take 6C for almost 30 / (6/4)^2 = 13 seconds and 12C for almost 30 / (12/4)^2 = 3 seconds. But I could be totally wrong.

[Edit: fixed typo in last calculation by changing "*" to "/"]
Last edited by weber on Tue, 29 May 2012, 10:00, edited 1 time in total.
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Post by Johny » Tue, 29 May 2012, 20:14

celectric wrote:....And I can't go up to 100Ah cells because of weight restrictions (I'm already reduced to 2 seats as it is). So what are my options - upgrade the GVM or find another donor car?
I might have missed it but what size/weight car are you considering and what voltage pack? A 144V pack with 60AH cells may be a bit light - the usual smallish sized 144V packs use 90AH cells. As weber says, you can use 4C for acceleration so you get about 35kW, but not big hills for too long - they'll be the killer.

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Post by Richo » Tue, 29 May 2012, 20:32

evric wrote: The "Continuous" rating, again as I understand it, is for approx 30 seconds at a time. Don't ask me how often this can be applied.


Seems counter intuitive that continuous is only for 30 secs.
Sounds more like "intermittant" to me.
So the short answer is NO but the long answer is YES.
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Li-Ion "Pulse" C Rating

Post by celectric » Tue, 29 May 2012, 20:33

Car is tare=1100kg and gvm=1600kg which gives me only about 300kg to play with for batteries with 2 pax (would like 4 but I don't think it's going to be possible).

If I'm trying to estimate the performance of my conversion, is it as simple as comparing (pack volts * current limit) to the ICE kW rating? evric's comment about battery current being lower than motor current makes me think the calculation is more complicated than that but I don't really understand enough about it.

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Post by Richo » Tue, 29 May 2012, 20:41

Yeah there is a bit more involed than that for an accurate assesment.

For one pack volts is not constant.
The more current the lower the volts.
For this you need to know the loss through the cells.

A motor controller will to a degree Power in = Power out
With the motor at low RPM the Pout is low but the output current will be high.
So the Power in will be similar to Power out but the current IN will be low.
So when accelerating for 10secs the peak current doesn't happen until you get towards the top RPM's so the peak current may only be for 2-3secs.

Don't forget you could use other batteries with better specs.
So the short answer is NO but the long answer is YES.
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Post by evric » Tue, 29 May 2012, 20:43

As an example my EV conversion Barina '85 has 130Volts worth of 90Ah Thunderskys (40 cells). It now weighs only 820Kg as an EV and I struggle to go up long steep hills - so much that I avoid them. You would know Shepards Hill Rd or the Belair Rd, the're OK for my car but I wouldn't go up the Southern Expressway of O'Halloran Hill.
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Post by celectric » Tue, 29 May 2012, 21:22

OK so here's the plan so far, tell me if I have got this right:

86 x SE100AHA cells
Nominal 300 V pack (float charge at 310 V)
Max continuous current 200 A (=2C)
-> 60 kW continuous
-> power-to-weight = 37 W/kg for 1600kg GVM (target: 35 W/kg)
Peak current 368 A (Tritium WS200 controller limit, < 4C)
-> 110 kW peak (target: 100 kW)
Pack capacity 30 kWh
-> ~100km range (assuming 200 Wh/km efficiency and 70% DoD) (target: 80km)
Pack cost $13k (cells only)
Pack weight 280kg
Pack volume 180L

Now accepting any and all suggestions on how to improve this plan - cheaper, lighter, simpler conversion, etc. without compromising too much on the indicated targets...

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Post by zeva » Tue, 29 May 2012, 22:12

My 2c: As a rule of thumb I usually suggest 5C as the maximum real-world discharge from ThunderSky/Winston or Sky/Calb cells (e.g for long enough accelerate 0-100), and 1C continuous if you want your cells to last a long time. (FYI the 5C is based on voltage sagging to 2.5V at normal operating temperature.)

With 86 LFP cells I think you'd be OK with the 60Ah size, though it does only give you a ~17kWh pack which would probably offer ~80-100km max range. Factoring in voltage sag, peak power would be about 65kW - but remember it would perform much better than a 65kW petrol vehicle because the pack will deliver that the whole time you're accelerating, instead of the peaky power delivery of an ICE. It'd probably feel like a 100kW petrol car.

Typical cars need about 15kW to maintain highway speed, so with a 17kWh pack it's still under 1C. It'd only be long hill climbs that you may want to slow down a bit to be kind to the batteries.

Nominal pack voltage will be more like 280V (86 x 3.3V), peak charge voltage ~310V (86 x 3.6V)

Having a bigger pack is nice for the extra range and performance, but it is more expensive, harder to fit in the car, and often makes licensing difficult due to GVM considerations.
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Post by coulomb » Tue, 29 May 2012, 22:33

celectric wrote: 86 x SE100AHA cells
Nominal 300 V pack (float charge at 310 V)
The nominal voltage of SE cells is usually taken as 3.2 V. They will measure more like 3.3 V soon after a charge and with no load. At around 200 A load, expect a sag to (wild guess) 2.9 V.
Max continuous current 200 A (=2C)
-> 60 kW continuous
That's assuming a pack voltage of 300 V, i.e. around 3.49 VPC under load. This is not going to happen. If they sag to around 2.9 VPC, you'll get 2.9 * 86 * 200 = about 50 kW. That's electrical into the controller; you'll get around 95% of that to the motor, and around 85% of that to the transmission, say 80% to the wheels (very rough figures). So that would be 40 kW of vehicle mechanical power, continuous (for almost an hour).
Peak current 368 A (Tritium WS200 controller limit, < 4C)
-> 110 kW peak (target: 100 kW)
At 3.7C, you'll get more sag, to (wild guess) 2.6 VPC, so that would be 2.6 * 86 * 368 = 82 kW electrical, perhaps 66 kW to the wheels, perhaps 70 kW to the transmission (to compare with ICE power).
Pack capacity 30 kWh
Using 3.2 VPC, it comes to 27.5 kWh.
-> ~100km range (assuming 200 Wh/km efficiency and 70% DoD) (target: 80km)
At lower speeds, you'll likely get a bit better range than that, depending on aerodynamics, tyres, hilliness, transmission efficiency, exact speed profile, and probably other factors.

27.5 kWh is a fairly large pack, so it's heavy, expensive, but long range. It always works out that way for the lower C-rate prismatic cells. We have 29 kWh in our MX-5 (at 3.2 VPC; we often use 3.3 VPC, since the numbers happen to work out rounder, e.g. 375 V, 750 V, and 30 kWh), and hope to get 100 kW mechanical (before transmission and differential) from it. Actually, to get the 100 kW mechanical, we'll probably need to go to the 750 V configuration, so that requires the 900 V Tritium controller, which doesn't exist yet. Until it does, we'll be running at 375 V (using 3.3 VPC), and will get of the order of 80 kW mechanical at the motor shaft; I can't remember the exact figure.

So to get closer to your goals, you may need to use more cells; we are using 114 in series for our 375 V configuration. You hopefully won't need that many. To save cost and weight, you might be able to go to 90 Ah cells (if they exist and you can get them), or buddy pairs of 40 Ah cells. But that will increase the C-rate load on the cells. It's a tricky balancing act, and then you have to find the space to put them in the vehicle. The weight of all the battery boxes, contactors, cable, and so on adds up quickly too, so that's another thing to juggle.
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Post by celectric » Tue, 29 May 2012, 23:59

[Hmm, I think after this I better start my own conversion thread, it's kind of gotten off topic.]

OK so I could go down to 90Ah x 86 and still meet my targets - I reduced my peak power target to 70kW in line with zeva's suggestion that it will still feel like a 100kW car, but taking into account voltage sag it comes out about the same anyway.

Going on Coulomb's suggestion to explore using more cells, I found another configuration I like too, 107 x 60Ah. In addition to the obvious differences (higher voltage, lower current but higher C) this has the following tradeoffs:
-> Cheaper, offset somewhat by higher cell count which will increase BMS and other costs.
-> 10% lighter but similar volume
-> Lower continuous power but similar peak power

Are there any significant steps up in cost or complexity going from a 275 V to a 345 V system?

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Post by jonescg » Wed, 30 May 2012, 17:55

Just my thoughts, but any EV which under-performs so badly that you actively avoid driving up hills was probably well under-specced. I know Belair Road in the Adelaide hills (lovely spot by the way) and it is one steep mother of a hill, as is the freeway. If you anticipate driving up lots of these sorts of hills, consider going for a higher C-rate cell to begin with, and have plenty of them! About the only plausible option would be something like A123 pouches from Victpower, Emissions-Free or (hahaha) Mavizen.
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