# Buck/boost converter?



## jorhyne (Aug 20, 2008)

I had this idea in the back of my head for a long time, and somebody mentioned it on the thundersky yahoo group and it reminded me I had never looked into it. There has been tons of discussion lately on the best way to balance cells..what if you only had to have 4 to 6 cells for your entire conversion?? 

I have little practical experience but I'm hoping someone knows more about buck boost converters, which from my knowledge takes a low voltage source and steps it up to a higher voltage. Think if you had 4 of these bad boys in your conversion: http://www.evcomponents.com/ProductDetails.asp?ProductCode=TS-LFP800AHA 

Not only would you have a 2400amp discharge capability, but keeping 4 cells instead of 40 balanced would be 10x easier! I really couldn't tell you whether a reliable 12v to 144v (or any voltage) buck boost converter exists, or is cost efficient, or is energetically efficient, but with the potential to make conversions much easier I think it is an exciting idea.

Anyone have any knowledge or experience about buck boost converters? I've heard the prius uses them to get a higher motor voltage. Is it feasible that they could be used in conversions?


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## madderscience (Jun 28, 2008)

The reason this is not done is basic electrical theory and conservation of energy. You can boost the voltage but in doing so you will proportionally reduce the amperage. For example, if you bump 12V to 120V, and you need 120V at 10A, you will have to draw 100A off the batteries.

Put another way, power (as in watts, or horsepower) is volts * amps. a boost converter can't bump up the volts without reducing the amps, because you can't get more power out than you put in.

Also consider that a small 120V car accelerating even slightly briskly needs 300A or so from the battery. So to get the same acceleration from a 12V battery you would need 3000A. that amount of current is difficult to manage.


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## jorhyne (Aug 20, 2008)

madderscience said:


> The reason this is not done is basic electrical theory and conservation of energy. You can boost the voltage but in doing so you will proportionally reduce the amperage. For example, if you bump 12V to 120V, and you need 120V at 10A, you will have to draw 100A off the batteries.
> 
> Put another way, power (as in watts, or horsepower) is volts * amps. a boost converter can't bump up the volts without reducing the amps, because you can't get more power out than you put in.
> 
> Also consider that a small 120V car accelerating even slightly briskly needs 300A or so from the battery. So to get the same acceleration from a 12V battery you would need 3000A. that amount of current is difficult to manage.


Thank you for your response madderscience, do you think a buck/boost converter would perhaps make more sense on a smaller conversion like a motorcycle? I understand that one of these converters can't magically take 100amps at 12V to 100 amps at 144 volts, but if it could take 2400 amps at 12v and produce something useful at 144 it could work, correct?

Also it seems maybe using an AC motor in conjunction with one would be a better choice because AC motors tend to use a higher voltage and lower amps..or am I off the mark entirely?


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## MN Driver (Sep 29, 2009)

Running 1000+ amps through battery cables requires thick wire for fairly intermittent duty. To use 6 cells (19.2 volts nominal) and try to get 144 volts out of it is going to be pulling 2400 amps from the 19.2 volts and giving you 320 amps. At 2400 amps to climbing even a modest hill I somehow think the battery terminals of those cells can't handle it, even with 4 terminals. Even though TS rates them at 3C, I have my doubts that they can handle that properly with even heat distribution in the cell. I can't help but think very edges of the cell end up pass that current through to the terminals where near the terminals there may be quite a bit more heat in the first place, but I could be way off on this. I'm also thinking of the voltage drop and heat loss. I don't think you could get a convertor that would handle the load. I think it would be a gigantic efficiency loss to do this. I think those 800Ah cells would do a great job in a renewable energy(solar, wind, hydro, etc) type of application very nicely however.

...but I'm in the camp where I'd rather upsize the cell along with the voltage in the appropriate dimensions to provide the needed range and power. For me it's a balance of less cells for ease of management, better volumetric density due to less plastic taking up space, and easier wiring versus more cells for higher voltage and less amperage draw, although with less amperage draw means less cells which means more space per kWh and more stuff to wire up and pay attention to. There is another advantage too, with more cells you could lose a few cells and remove them entirely and it would be less noticeable than having a pack of say 24 cells(72 volts). I'm thinking of going with a relatively lower voltage pack at 112 volts getting 200Ah although I've considered 260Ah cells but I'm shying away from that because I don't really think I need range higher than 100 miles and not really looking to carry more weight than I need either, plus with 200Ah, things could be more flexible in terms of ditching my controller/motor some day and raising voltage and/or putting the pack into another car.

In short: I'm sure it could be done, but I don't think it will behave the way you would like it to, it adds weight, you lose efficiency, you have less flexibility to where those monsters go, dealing with very large amp loads for an extended period of time.

I've thought of this sort of thing for use with an electric bike but the solution isn't there, that sort of equipment is large, heavy, and generates more heat than in my opinion is acceptable in an EV environment.


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## frodus (Apr 12, 2008)

2400A and 12V is 28,800W
28,800/144V = 200A.... which seems pretty low for a car conversion.

the sheer cost of a buck boost that could deal with 2400A would likely exceed the cost of the battery pack, motor, controller....... 2400A and 12V would be somewhere around 30kw.... not going to be easy to come across if you can find it, and if you did, 30kw of power in a car won't be that great for driveability.

Essentially, its 2 controllers, one for DC-DC conversion from pack to controller and another for DC-DC conversion... from controller to motor.... all to save on a few wiring connections. The kwh stays the same whether its 20 100Ah cells or 10 200Ah cells..... you still need the same amount of energy either way.

Its better to go much higher voltage and convert once at a voltage the motor desires, than it is to use high amp low volts and convert twice. The cost of connections is MUCH less than the cost of a boost converter.

Oh, and even with converter at 90% efficiency, thats almost 3000W of loss in the form of heat. Add that to controller efficiency and motor efficiency.... and now you're really low.


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## major (Apr 4, 2008)

frodus said:


> 2400A and 12V is 28,800W
> 28,800/144V = 200A....
> Essentially, its 2 controllers, one for DC-DC conversion from pack to controller and another for DC-DC conversion... from controller to motor....


Travis,

Why would you need 2 controllers? Certainly the buck/boost converter could output a lower than maximum output voltage. So it could become the motor controller, could it not? 

Or, simply get a 12 volt 2400 amp motor. 

I know the whole idea doesn't make much sense. A while back it was discussed on the EVDL I think, but using a single cell for the battery pack. No worry about cell balancing then. But can you imagine the size of the cables? Or the charger cord? Or charger?

There is usually a happy medium. Something which isn't perfect, but manageable to a large degree. A low voltage battery pack isn't a good idea.

Regards,

major


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## frodus (Apr 12, 2008)

> Why would you need 2 controllers? Certainly the buck/boost converter could output a lower than maximum output voltage. So it could become the motor controller, could it not?


Yeah, i guess he could, but it'd have to be a custom controller that does buck-boost AND controls its output with respect to throttle..... I just don't know of any available....


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## jorhyne (Aug 20, 2008)

Yeah I wasn't really planning on doing something like this, but I had the idea in my head and wondered if there was any merit to it. So it seems like it COULD be possible but definitely would be using expensive custom hardware, and any advantage of having only a few cells to balance would be outweighed by the disadvantages of inefficient operation and high price.

maybe one day in the future..


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