# can I buy a 72V - 288V DC-DC converter?



## jasonjungreis (May 2, 2011)

I am looking for but cannot find an off-the-shelf DC-DC converter to go from 72V to 288V: does this exist?

I have average skills. I have built a 48V electric motorcycle, with no BMS, 16 100AH lifepo4 cells, each cell with its own charger (about 5 years of trouble-free operation). I'd like to do the same concept for a car, but with better performance.

High voltage is the key to performance, and fewer cells is the key to an easier build. My thought would be to have 24 cells (say, 200AH) producing 72V and convert that to 288V to get better performance from the motor. This way I could again avoid using BMS by having each cell use an individual charger.

I have looked around online but cannot locate any such DC-DC converter product. Obviously, it might not exist. Or, I just can't find it.

I'd appreciate assistance. Thanks.


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## Moltenmetal (Mar 20, 2014)

My suggestion: give up. A high current "boost" type DC/DC converter is going to be more costly than just putting more cells in series, especially if you (unlike me) are comfortable without a BMS.


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## jasonjungreis (May 2, 2011)

@ Moltenmetal -- Are you saying that such a DC-DC converter does or does not exist? This is my question. Can you point me in the direction of such a thing?

And as to BMS, my point is that I would simply prefer not to have it. Those several people I know with BMS have to a person reported everything from repeated small problems to really severe problems -- I have never met anyone who has had nothing but positives to report about DIY BMS. But, if I had to employ more than, say, 24 cells, I see BMS as an inevitability. 

Lastly, to get to 288V, there is the cost and complexity and weight and space issues of so many cells. If there is a cheap, small, light, easy way to get to 288V, please let me know. And I do not have the expertise to, say, figure out how to use a Leaf pack.


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## dcb (Dec 5, 2009)

with a boost converter you will be borrowing from peter to pay paul (with some taxes thrown in). Have a look at page 70 here: https://info.ornl.gov/sites/publications/files/Pub26762.pdf there is a lot of heat being generated in the prius boost converter. It isn't as big an issue for a prius since it has a gas engine to get you home though. But assuming you could run it continuously @200v input and 25kw, the best you could hope for would be 9kw at 72v input.

No EVs use a boost converter that I know of, it is better to have more batteries to get the required voltage as the controller and motor itself is basically a buck converter. Best to come to terms with dealing with more cells.

But yes, boost converters do exist, they are not well suited for 72 volts, or non-hybrid applications.


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## jasonjungreis (May 2, 2011)

@ dcb -- thanks for the information (albeit I admit I did not follow the figures you provided re input and output on the Prius converter). But you've introduced to me the new term "boost converter." Yet, again, with responders asserting that they exist, I cannot find one.


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## dcb (Dec 5, 2009)

because they are inside the inverter of every 2004+ prius in every junkyard (plus probably a lot more models of hybrids). It is basically an inductor and a large igbt and some control logic. If you mean a pre-packaged, select-a-size, high power inductor that does exactly what you want, for cheap, good luck w/that  But they aren't terribly complicated either, but I don't know anyone who has thought it through actually recommending such an approach.


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## WolfTronix (Feb 8, 2016)

You might just have to make one:
http://www.wolftronix.com/buckBoost/index.htm

Mine is designed to take in 48V nominal and boost to 48-550V output.
And it can buck high voltage back down to 48V to charge the battery.
It uses a Prius inductor and 600V IGBTs from some damaged motor controllers.

It also has 1.2KW of programmable resistance load.

It is designed as a battery pack simulator, and load for repairing motor controllers and battery chargers. Where I can set the output voltage to what ever range the motor controller / battery charger expects.

Bi-directional buck-boost converter playlist: 
http://www.youtube.com/playlist?list=PLQdu_G7xyFIT2YItJQL1JZujxFJl6LOWL


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## jasonjungreis (May 2, 2011)

@ dcb 

Yup, an off-the-shelf DC-DC converter for 72V to 288V, running up to perhaps 600A or so, is exactly what I'm looking for. I recognize that such a thing would not be free, or ideally efficient, or tiny, or cool to the touch. But I'd just like to know where I can locate such a thing -- if it does indeed exist -- so that I can judge for myself the cost, efficiency, weight, size, heat, etc.

Thanks.


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## WolfTronix (Feb 8, 2016)

jasonjungreis said:


> @ dcb
> 
> Yup, an off-the-shelf DC-DC converter for 72V to 288V, running up to perhaps 600A or so, is exactly what I'm looking for. I recognize that such a thing would not be free, or ideally efficient, or tiny, or cool to the touch. But I'd just like to know where I can locate such a thing -- if it does indeed exist -- so that I can judge for myself the cost, efficiency, weight, size, heat, etc.
> 
> Thanks.


600A @ 288V?! That would be more than 2400A @ 72V!!!


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## jasonjungreis (May 2, 2011)

@ Wolftronix

1. Wow, that is crazy -- good for you! Very impressive. Truly professional.

2. I have no inclination to try to do what you did. At some point, to quote the movie, a man's gotta know his limitations. Building such a device is mine.

3. I just wanted to get better motor performance without having to deal with 100 cells and complex BMS. If it can't readily be done, I will shrug my shoulders and move on. But if I could have paid a few hundred dollars and had a plug-and-play device delivered to my doorstep, I would have considered it.

Thanks.


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## jasonjungreis (May 2, 2011)

WolfTronix said:


> 600A @ 288V?! That would be more than 2400A @ 72V!!!


See? Goes to show what little I know. I figured a DC-DC converter would just change the voltage, and leave the amperage the same. Apparently not. (I had been thinking that my battery pack would be a peak of perhaps 600A at 72V, and that it would therefore convert into 600A at 288V -- wrong again.) I think I'll stick to my simple ways and leave the heavy work to people who are capable.


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## dcb (Dec 5, 2009)

did a little digging,


nice bike btw:
http://www.evalbum.com/3918


realize you are looking at a new controller anyway, and there is a LOT of unused potential with your ac-15 @[email protected] as it is. hpevs shows it running with a 96v, 650a controller. You really don't need to go all the way to 288v to get massive performance improvements. 

Not many batteries will be happy with 650A discharge, but you can run 650a through the motor at lower rpm, plus you will have a lot more power at speed.

re: bms, I'm sympathetic, that is why I kept it as a simple first order balance monitor (but you have to assume responsibility for monitoring and acting, like a car that you can NEVER let run out of gas or overfill). http://www.diyelectriccar.com/forum...-pack-balance-monitor-batt-bridge-165953.html


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## WolfTronix (Feb 8, 2016)

jasonjungreis said:


> See? Goes to show what little I know. I figured a DC-DC converter would just change the voltage, and leave the amperage the same. Apparently not. (I had been thinking that my battery pack would be a peak of perhaps 600A at 72V, and that it would therefore convert into 600A at 288V -- wrong again.) I think I'll stick to my simple ways and leave the heavy work to people who are capable.


Assuming Ideal, with no losses:
600A * 72V = 43,200W
43,200W / 288V = 150A

You would need about 3 Prius inductors to implement that boost converter.


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## jasonjungreis (May 2, 2011)

@ dcb

Hey thanks for finding my bike. Yes, it works well. I'm happy with the bike.

But my 72V-288V quest is not for my bike, it is for a prospective project car I have my eye on. Nice car, nice Warp11 motor, and a ruined battery pack -- because, of course, its BMS crapped out and wrecked the cells. 

I had wanted to get 288V performance with 72V simplicity. But, based on all the above, I give up.


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## PStechPaul (May 1, 2012)

As explained above, it is impractical at the power level you want. But it may be quite reasonable for much lower power applications, such as the 2 or 3 kW electric tractor I am working on. I have designed and built several prototype versions of a nominal 1 kW DC-DC converter for 12, 24, or 48 VDC input, with output of nominally 240 or 480 VDC. I have also used automotive inverters rated 12 VDC to 220 VAC at 1 kW, and 24 VDC to 1500 watt, to obtain nominal 250 VDC that was able to power a 240V 2 HP VFD to spin a 2 HP motor.

For such a high voltage boost ratio, and high power, it is probably best to use a high frequency transformer. It is theoretically possible to get up to 5000 watts using an E55 N78 ferrite core at 100 kHz or higher. That's only a little more than 3" cube. Parts should be about $50, and you could connect these together in series or parallel to get higher power. Still, 40 kW would cost at least $400 as a major DIY project. And there is still the problem of high input current.

But you could use one or two for a limited power concept vehicle. 5000 watts is enough for a small car and low speed up to maybe 40 MPH.


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## Noq (Dec 11, 2016)

WolfTronix said:


> Assuming Ideal, with no losses:
> 600A * 72V = 43,200W
> 43,200W / 288V = 150A
> 
> You would need about 3 Prius inductors to implement that boost converter.


I've done that power on my bike in a much simpler manner, no inductors required. 10 Leaf modules in series for 74V nominal, and a Kelly KLS controller pulling 620A through my 8kW QS hub motor. I was around 78V at the time, so 48kW give or take. Just wanted to say that it's very possible to get high power and speed without overly complicating things. You might reconsider the Leaf packs because they're quite simple and safe. Nice bike BTW.


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## dcb (Dec 5, 2009)

Noq said:


> I've done that power on my bike


I think something is amiss there,

was it 650 motor amps or battery amps?

was it 72 motor volts or battery volts?

there is a voltage/amperage conversion that happens in the controller (basically a buck converter) so for battery hp you have to monitor the product of battery voltage * battery current, otherwise you will wind up in the weeds with your analysis.

edit: I took a peek at your ES thread, so you have gobs of startup torque, which means you are getting lots of motor amps at startup, but at startup the motor voltage is quite low (power is volts*amps), and battery current is low. And as the speed picks up the torque falls off I assume, so you probably are NOT anywhere near 48kw. If you had startup torque (600 motor amps worth of torque) at 120kph, THEN you would be at ~48kw.


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## Noq (Dec 11, 2016)

Hi, sorry for the delay here. To clarify, I only measure voltage and current at the battery using the cycle analyst, so those power levels I mentioned would seem to be accurate. I'm at 74V nominal (84V full), and I've pulled over 600 battery amps here with sag from say 84V to 73V (voltage climbs back up as speed evens out). This is WOT from stand-still. It's temporary power, as once I hit 100KMPH or so (takes about 3 point something seconds), the induced motor voltage equalises with my battery voltage and I reach my voltage-limited top speed. At this point, current tapers off because it doesn't require much current to maintain speed (as opposed to reach that speed). Thus, top speed on flat road draws 120-180A at 75-80V (depends on state of charge), = peak 14kW battery power once top speed is attained. I'm pretty confident I'm pulling 48kW battery power during maximum acceleration though. Anyway, it's pretty ridiculous and I've had to replace melted phase wires before. Will update my ES thread soon.


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