big batteries and step up DC converters

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Post by HeadsUp »


just thinking outside the box for a moment

instead of using 200 x 40 AH cells = 300 kg pack plus $ 3900 worth of BMS.

what about using 20 x 400 AH cells = 270 kg plus $ 940 worth of BMS

plus of course a step up DC-DC converter to increase your 64 volt pack up to say 480 - 600 volt before sticking it into an inverter VFD drive controller.

assuming there was a DC-DC converter that large , assuming its efficiency was at least 95 % or better

whats the pros and cons ?

if a cell died it would be more tears than if you had to replace 2 or 3 40 AH cells

what else ?

would regen potential be thrown out the window unless the DC converter was purpose built with that intention ?

or would you use a switching circuit under regen that puts the motor AC back through a big 3ph bridge rectifier and limits the max voltage supplied to the batteries ?

is it possible to make a VFD controller from scratch that would take 64 volts and invert to a higher frequency and convert to AC at 0 - 200 Hz / 400 VAC ? ? ?


showing my ignorance here but just throwing it to the wind

Edited

200 x 40 AH = 0.202768 cubic metres
20 x 400 AH = 0.17726 cubic metres = 12.5 % less space required

although some vehicles wouldnt like the height required ( 282 mm instead of 190 mm high ) but these things can be dealt with through good engineering

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Last edited by HeadsUp on Sun, 22 Nov 2009, 08:07, edited 1 time in total.
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Post by coulomb »

HeadsUp wrote:
just thinking outside the box for a moment

instead of using 200 x 40 AH cells = 300 kg pack plus $ 3900 worth of BMS.

what about using 20 x 400 AH cells = 270 kg plus $ 940 worth of BMS
Or 40 x 200 Ah buddy pairs = 240 kg plus the same $940 worth of BMS. I have no idea why that works out lighter.
plus of course a step up DC-DC converter to increase your 64 volt pack up to say 480 - 600 volt before sticking it into an inverter VFD drive controller.
Well, if you were going DC, I'd say make it a buck/boost controller. It has to be cheaper and more efficient than a DC-DC (boost converter) followed by a buck controller.

As for AC, I don't know if it is even possible to make a buck/boost 3-phase controller. I'll toy with that later.
Assuming there was a DC-DC converter that large , assuming its efficiency was at least 95 % or better
Every Prius from about 2003 onwards has one, boosting from 202 to about 500-600 VDC.
What's the pros and cons ?
Pro: BMS cost, safety of battery wiring
Pro/con depending: one size might lend itself to mass production, e.g. Toyota seem to love the Panasonic 6.5 Ah NiMH cells
Con: as you say yourself: "if a cell died it would be more tears than if you had to replace 2 or 3 40 AH cells"
Con: It is sometimes harder to fit larger cells than more smaller ones
Con: Cost, complexity, weight, efficiency of the extra converter
would regen potential be thrown out the window unless the DC converter was purpose built with that intention ?
Yes, but it's "just" one extra transistor. The Prius DC/DC is bidirectional.
or would you use a switching circuit under regen that puts the motor AC back through a big 3ph bridge rectifier and limits the max voltage supplied to the batteries ?
Huh? I'm not sure what your aiming at there. The Prius does three phase 500+ p-p VAC to 240 VDC to charge the battery; it has a standard VFD and a two transistor (hence bidirectional) DC-DC.
is it possible to make a VFD controller from scratch that would take 64 volts and invert to a higher frequency and convert to AC at 0 - 200 Hz / 400 VAC ? ? ?
Hey, my idea Image    It seems difficult at first glance. Any AC frequency is higher than DC   Image
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Post by HeadsUp »

Or 40 x 200 Ah buddy pairs = 240 kg plus the same $940 worth of BMS. I have no idea why that works out lighter.
but it does work out lighter and still saves well on BMS costs and cabling weight / joint resistance , plus the possibility that it helps to balance out issues with a weak cell if its buddied with a stronger one.
plus of course a step up DC-DC converter to increase your 64 volt pack up to say 480 - 600 volt before sticking it into an inverter VFD drive controller.....
...Well, if you were going DC, I'd say make it a buck/boost controller. It has to be cheaper and more efficient than a DC-DC (boost converter) followed by a buck controller.......
...As for AC, I don't know if it is even possible to make a buck/boost 3-phase controller. I'll toy with that later.
...is it possible to make a VFD controller from scratch that would take 64 volts and invert to a higher frequency and convert to AC at 0 - 200 Hz / 400 VAC ? ? ? [/quote]

instead of a buck/boost converter to increase DC voltage could you make one incorporated into a controller by having 3 sets of small 6 pack modules of 150 amp IGBT ,each getting paralleled 64 VDC and combine the AC outputs in series to get higher voltage ?, or could the AC outputs be connected only in parrallel so you still get the same voltage at higher current anyway ?
(i can see alot of magic smoke inside that picnic basket busting to escape)
any way to series the AC output or is the DC buck/boost simpler and cheaper ? , pardon my ignorance about electrical engineering theory.
Assuming there was a DC-DC converter that large , assuming its efficiency was at least 95 % or better
Every Prius from about 2003 onwards has one, boosting from 202 to about 500-600 VDC.
thanks for letting me know that Prius has a bi-directional step up converter -didnt know that
cutoff limit at minimum voltage.. ? if it still runs off 48 volt ( 20 x 2.4 cell volts ) then Mister-prius-wrecker could have some beer money.. however its probably only max 25 - 30 kW ( still looking for a spec sheet) .... whereas in free dreams world we would all like 100 kW

then again , it may lend itself to being dolly-the-sheeped and reverse engineered too....but i would never do that Mister-patent-attorney
or would you use a switching circuit under regen that puts the motor AC back through a big 3ph bridge rectifier and limits the max voltage supplied to the batteries ?
Huh? I'm not sure what your aiming at there. The Prius does three phase 500+ p-p VAC to 240 VDC to charge the battery; it has a standard VFD and a two transistor (hence bidirectional) DC-DC.


okay , an extra transister in a circuit is not much , i was thinking a 3ph bridge rec is smaller than a pack of cigarettes and cheaper to replace than a blown up controller


edited , the quote function got momentary menapause
Last edited by HeadsUp on Sun, 22 Nov 2009, 10:35, edited 1 time in total.
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Post by Electrocycle »

a bidirectional DC-DC converter just uses a transistor instead of a diode, so that part is easy.

Obviously running a normal VFD from the DC-DC converter is the easiest method, but you could just do a custom low voltage controller and use a low voltage motor (or rewind).

Last edited by Electrocycle on Sun, 22 Nov 2009, 11:58, edited 1 time in total.
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Post by HeadsUp »

Electrocycle wrote: a bidirectional DC-DC converter just uses a transistor instead of a diode, so that part is easy.

Obviously running a normal VFD from the DC-DC converter is the easiest method, but you could just do a custom low voltage controller and use a low voltage motor (or rewind).


a step-up DC-DC converter is a tad more complexified' isnt it ?

if you did use a low voltage battery pack and low voltage controller you are going to need double the frequency to run an overvoltaged motor wooden tyu ?

and double sized cables too

increased system frequency might work , but dont have a chart of voltage / current / frequency to assess where the sweet spot is Imagefor efficiency


the pluses i am looking for is reduced battery weight , reduced BMS costs , reduced risk from electrocution , reduced cabling and cable joint resistance .

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Post by Electrocycle »

a normal boost (step up) converter has a transistor, an inductor, and a freewheel diode - and dumps charge into an output capacitor.

If you replace the freewheel diode with another transistor, switched on when the other one is off, you get a bi directional converter.

It's a boost converter in one direction and a buck converter in the other.


You don't have to worry about frequency, it's all DC. The VFD will pump DC back out under regen, and the buck / boost converter will put it back in the battery pack.
The switching frequency of the converter will depend on the size of inductor and type of transistors used.


If you run a permanent magnet DC motor from a controller with a transistor instead of the diode you get automatic regen like an AC system (the controller works as a buck converter when driving the motor, and a boost converter in regen - using the motor windings as the inductor)
The only hassle is that your throttle ends up being a speed control rather than a torque control, so you'd have full regen when you back off the throttle unless you have some extra circuitry there.
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Post by coulomb »

HeadsUp wrote: Is it possible to make a VFD controller from scratch that would take 64 volts and invert to 0 - 200 Hz / 400 VAC ? ? ?
I think it needs an extra six inductors, an extra 6 diodes, and possibly an extra 6 smallish capacitors. Plus, the bit switches aren't commoned in the middle, so you can't use standard IGBT modules with 2 transistors already connected for you. Here is my attempt:

Image

This is one third of the back end, you need three copies of this.

To make OUT1 go positive, you turn on switch S2; when S2 is on (for part of a cycle), S1 is completely off (on for no part of the cycle). Current flow into L2 per the straight green arrow. When it goes off, the current in L2 continues via D2 and notionally through C2, as per the curved green arrow. Over a <= 200 Hz cycle, most of the current would come from another output that has its S1 turned on. The supply is in antiseries with L2, so the output can go less than or greater than the supply, depending on the duty cycle.

I have no idea how practical this circuit is, especially with the inductance at the output instead of at the input. Obviously, 6 large battery current rated inductors pretty much dashes the practicality right away. The switches carry battery current, and the diodes carry output current (less than battery current).

So I'd say that this idea is dead. It's probably better, as Toyota decided, to go with a bidirectional boost converter optimised for efficiency than to attempt this topology.

Edit: L1 -> L2
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Post by HeadsUp »

coulomb wrote:
HeadsUp wrote: Is it possible to make a VFD controller from scratch that would take 64 volts and invert to 0 - 200 Hz / 400 VAC ? ? ?
So I'd say that this idea is dead.It's probably better, as Toyota decided, to go with a bidirectional boost converter optimised for efficiency than to attempt this topology.



thanks coulomb .. anyone else is welcome to kick its corpse a couple of times just to make sure its not playin possum though Image


if the rating of the Prius DC-DC converter is too small , could i put two in parallel ( assuming they are compact enough ).
starts getting problematic then doesnt it   Image
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Post by Electrocycle »

making a bidirectional converter is probably easier than messing with prius ones anyway.
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Post by Johny »

Someone did all that - it was a ute type vehicle. They used a air-wound inductor in a buck-boost convertor. It took up a LOT of space in the vehicle. Does anyone remember where I saw this (hmmm).... Image

Edit: tooK
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Post by Mr. Mik »

How about going to the extreme and put all cells in parallel?



A 3.5V to 600V DC/DC converter!

Cells screwed directly to two thick bus plates that provide structural support as well as low loss conduction of say 10,000A?

This would get rid of any cell imbalance, ever.

No problem with cell reversals, ever.

Very easy charging, this could start with enormous power (if available), then asymptotically approach full SOC with no need for a BMS as such. Not even need to monitor cells, a few dead cells in the pack would be inconsequential! They just sit in there and the current flows around them (unless they fail as a short, then a single cell would kill the pack, but just one fuse at each cell could prevent this).

Would such a converter be too big or too expensive?

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Post by Electrocycle »

we have talked about that before (I think it was me who jokingly suggested it :P) but the efficiency would be far too low for it to be practical.

Even charging would be very inefficient with that much voltage step down.
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Post by HeadsUp »

2 inch diameter battery cables would be a beech to bend on them tight corners gungadin.

i think anything below 64 volt system would be creating dumbness>squared

above 64 volt and the sweet spot could be narrowed down to optimise weight , overall cost , power density , power density/price with efficiency
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Post by Richo »

HeadsUp wrote:
Or 40 x 200 Ah buddy pairs = 240 kg plus the same $940 worth of BMS. I have no idea why that works out lighter.
but it does work out lighter and still saves well on BMS costs and cabling weight / joint resistance.


If you are referring to prismatic lithium cells then it will be the plastic case for each cell.
Less cells means less percentage of case.
ie Higher Ah has less % of case in weight.
So the short answer is NO but the long answer is YES.
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Post by Richo »

Also the cost to build such a DC-DC would be more than the cost of the BMS to monitor lots of cell to equal the high voltage.
So the short answer is NO but the long answer is YES.
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Post by Electrocycle »

and smaller cells are easier to fit in tight places / irregularly shaped boxes, etc
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Post by HeadsUp »

Richo wrote: Also the cost to build such a DC-DC would be more than the cost of the BMS to monitor lots of cell to equal the high voltage.


at a guess a Prius Dc-DC converter would be around $ 200 at a wreckers woodentit ?

I have not got a reply on prices yet , but i think the Prius battery packs would be in high demand and be priced accordingly , but i doubt they would sell many of the DC-DC converters and fingers crossed they will be cheapish.

i am not yet dissuaded from this line of thought

when you hear it go 'clunk' in the rubbish bin that will be an indication that i have moved on.


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Post by bga »

I remember thinking about this a while back...

The idea does have merit, but as Trituim James pointed out, there is an additional complexity and efficiency hit on the up-coverter. The Prius one uses an inductor that looks like about a 100mm cube of ferrite and a dual IGBT and a capacitorr or two. It boosts the battery voltage from 200V to about 500V. It is bi-directional. I would expect the efficiency to be about 90%, meaning that an EV with one of these would need another 11% of battery capacity.

This is the killer for EV applications. Image

Why I think that the coverter works in a Prius is that most of the energy is coming from the petrol motor and going to the wheels, with the electric motors operating as an adjustable motor-generator pair to achieve the gearing needed at various speeds. Only energy to and from the 1 kWh battery is passed through the converter.

The converter allows the small Prius battery to be considerably simplified, particularly with Ni-MH cells ar 1.2V each.

Most for the time, the Prius energy flow bypasses the converter and so doesn't suffer from it's efficiency. I would guess that about 80% of the energy bypasses the converter, so its efficiency isn't so much of an issue.
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Post by HeadsUp »


dang

looks like the 2004 Prius DC-DC converter is only 20 kW

the Lexus LS600 has a 37.5 kW DC-DC step up converter . problem is they are as rare as rocking horse ships


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Post by coulomb »

HeadsUp wrote:
dang

looks like the 2004 Prius DC-DC converter is only 20 kW

Oh, right. The MG2 motor is 50 kW (60 kW in the 2010), but a lot of the time, most of that power comes from MG1 (driven by the ICE). In fact, I think you need almost the full 20 kW from the battery, plus 35 kW (maybe plus a little) from MG1, to get 50 kW to MG2.
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Post by HeadsUp »

coulomb wrote:
HeadsUp wrote:
dang

looks like the 2004 Prius DC-DC converter is only 20 kW

Oh, right. The MG2 motor is 50 kW (60 kW in the 2010), but a lot of the time, most of that power comes from MG1 (driven by the ICE). In fact, I think you need almost the full 20 kW from the battery, plus 35 kW (maybe plus a little) from MG1, to get 50 kW to MG2.


from enquiries on a few Prius forums around the web , it appears there is no rating plate on the DC-DC converter and Toyota dont publish a spec that we can find , however the link i posted above shows a comparison chart between the lexus and prius hybrids which states 20 kW is the rating of the Prius DC-DC converter

I however am going to speculate that Toyota would have designed and built that thing to have " future redundancy " when they uprate the battery pack or drive motor plus a safety or service factor in its rating so its rating could be double or more of the suggested 20 kW

if they are cheap enough i will pull one apart and try to get some maths on rating and input voltage ranges , or even stick it on a load and start pumping DC volts from 40 V and up into it to see its operating range . or until it gives smoke signals at the upper end.

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Post by weber »

coulomb wrote:
HeadsUp wrote: Is it possible to make a VFD controller from scratch that would take 64 volts and invert to 0 - 200 Hz / 400 VAC ? ? ?
I think it needs an extra six inductors, an extra 6 diodes, and possibly an extra 6 smallish capacitors. Plus, the bit switches aren't commoned in the middle, so you can't use standard IGBT modules with 2 transistors already connected for you. Here is my attempt:
...
Here's my attempt:

Image
So I'd say that this idea is dead. It's probably better, as Toyota decided, to go with a bidirectional boost converter optimised for efficiency than to attempt this topology.
I agree. High powered DC-DC converters typically use a high frequency transformer unless their voltage ratio is close to one, and isolation doesn't matter. With the non-isolated single-stage inductor/capacitor type of buck-boost converter, you need switching devices that can handle the maximum of the input and output currents and the sum of the input and output voltages.

[Edit: Added the integral diodes and the word "theoretical" to the schematic]
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Post by HeadsUp »

thanks weber

what kind of values would the components need to be ?

at least i can get a rough idea of parts costs for something like that

will let you know when i find a prius one to strip down
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Post by acmotor »

I guess the 600VDC bus would best be shunted with enough ultra caps for up to 10 seconds or more of 80kWh with a 20kW boost buck jiggy feeding it as you only need to supply the average power not the 100kW peak power to the VFD/motor ?

Hmmm that's still a reasonable cap bank. But the 20kW prius device may not be too small if you could hold up the DC bus.

A lot of the time the DC bus can be a lot less than 600V, maybe 300V or so.

Prius Gen3 is 650V so they must consider they are on to something with this boost system.

A 100V VFD has 6 times the silicon and higher switching loss (if IGBT) than a 600V unit. All the wiring is 6 times the CSA etc. There might be 20kg or so extra copper in the system. I guess these are factors that Toyota must have considered. Maybe it makes up for the booster efficiency ? Image
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