# Boost Converters



## Twilly (Jan 22, 2008)

Watt = Watts = watts... Basiclly, you will use the same amount of energy boosted or not...

IE. 12000 watts could be 120 volts at 100 amps or 240 volts at 50 amps... The same amount of energy is used in both situations


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## rednellycat (Sep 2, 2008)

Twilly, thank you for your response...could you elaborate in further detail why a 72v, 500A total capacity parallel-series battery set up not give you more power than 120v 100AH hooked in series? Does power (watts) not equal volts times amps? So if you boost the voltage, with some current loss, would power not be increased? And if so could extend range or reduce weight with less batteries? Is this not what Toyota engineers were accomplishing by using the converter to reduce battery weight?


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## Twilly (Jan 22, 2008)

There are a lot of variables.... First, a motor would be happier with 200vdc at 100a then with 100vdc at 200a because current causes heat. If it was possible to get 800 amps from a 72 volt string, you would have about the same performance as a car with a 144 volt string that max's out at 400 amps ( until the motor overheated )

Secondly, a 200 a/hr battery string will last twice as long as a 100 a/hr string at the same current level, But the 200 a/hr string might be able to supply 500 amps to the motor, where a 100 a/hr string might only be able to supply 250 amps.

In a car, power = amps. 

In the Toyota example, I can only guess that they are using a higher voltage to lower the power cable size, which would reduce weight.


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## frodus (Apr 12, 2008)

rednellycat said:


> Twilly, thank you for your response...could you elaborate in further detail why a 72v, 500A total capacity parallel-series battery set up not give you more power than 120v 100AH hooked in series? Does power (watts) not equal volts times amps? So if you boost the voltage, with some current loss, would power not be increased? And if so could extend range or reduce weight with less batteries? Is this not what Toyota engineers were accomplishing by using the converter to reduce battery weight?


72V x 500A is 36,000W
120V x 100A is 12000W

Not even CLOSE to the same power level.

The reason the engineers boost the voltage, is because at higher voltages, since current is lower, the losses are lower and efficiency can be a LOT higher. Power is relative to weight too... to get 10,000W you still need the same total power to get the performance and distance you want regardless.... whether its 100V at 100A or 50V at 200A... its still essentially the same weight in batteries, just a different configuration.

You do NOT increase power by boosting.... you just boost voltage and decrease current... Power can never be more than the input.... EVER... there are ALWAYS losses.

72V at 100A is the same power as 144V and 50A. If You want the same power level, and want to increase voltage, the current must DECREASE. 50A through a 0.1ohm load is 5V drop and 250W lost in heat, with 100A, its a 10V drop and 1000W lost in heat. When the FET's inside are switching lower currents, there are less switching losses, less voltage drop and lower heat..... less heat, the more continuous power you can switch and the higher the efficiency. Also, if they can use smaller cables, its much easier to build, cheaper (less copper), lighter and less lossy with higher voltages.


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## rednellycat (Sep 2, 2008)

The point of my post was to invoke debate as to the merit of utilizing a boost converter. My lack of electrical knowledge lead me to imply the creation of power rather to use boost converters for a more efficient system. 

I threw the question out to the forum in hopes for insight regarding these devices instead a negative shut down reaction from an electrical engineer. Have you ever wounder why junior members have not posted more often?

However I will attempt to present my case in more clear and descriptive manner as possible lest to confuse the issue.Keep in mind this is NOT meant to create power but to utilize power in a more efficient way.

The goal is to have 144volts and 100ah for the traction motor.
Configuration #1 is to have 12-12v @ 100ah batteries hooked in SERIES.

Power for configuration #1 P = 144 x 100 = 14.4Kw

Configuration #2 is to have 8-12v @ 300ah batteries hooked in PARALLEL/SERIES, with a boost converter

Power for configuration #2 P = 48 x 300 = 14.4Kw 

Both configurations have the same power, however when voltage is boosted to 144v in the second configuration the amperage will be reduced to 100a 

In both cases the end result is the same, but the later set up eliminates 4 batteries, with less capacity.

Has this not reduced weight?

Has this not reduced battery cost?

Has this not utilized components more efficiently?

Is there no advantage here? If I am wrong I'll accept defeat and hope others have learned thur my error. But I get frustrated with negative statements that eventually cease creative thinking.


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## Twilly (Jan 22, 2008)

I see your point, But a few factors need to be considered. 

First, Would 8-300a/hr batteries be cheaper than 12-100a/hr? 

Second, in any type of electronic conversion, there is going to be losses in heat, so be sure to figure about 10% to 25% loss there.

And third, Charging a battery string in a series-parallel config will lead to under/over charged batteries which will lead to the entire pack going bad earlier than the series pack.

Dont let the electrical engineers here get you down. They tend to talk over everyones heads, but their knowledge is invaluable... There is more to this than meets the eye sometimes


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## rednellycat (Sep 2, 2008)

Actually the parallel-series set up would be cheaper and considerable less weight. Here is an example using Deka sealed battery:

Configuration #1 ... 12 - Deka 8931 12v 108ah 70lbs $241.70
Configuration #2 ... 8 - Daka 8924 12v 80ah 54lbs $186.54

The series set up costs $2900.40 and a total weight of 840lbs were as the parallel-series set up costs $1492.32 with a total weight of 432lbs. You can see that a savings of $1408.08 and a total weight loss of 408lbs. That means a nearly 50% savings and weight loss.

All systems no matter how the configuration is designed I would guess a varied amounts of lost power due to heat. And concerns over under/over charging a prudent converter/builder may want to incorporate a battery management system.


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## rfengineers (Jun 2, 2008)

rednellycat said:


> Configuration #1 ... 12 - Deka 8931 12v 108ah 70lbs $241.70
> Configuration #2 ... 8 - Daka 8924 12v 80ah 54lbs $186.54


Config-1: 12 * 12V * 108ah = 15.5kWhr
Config-2: 8 * 12V * 80ah = 7.7kWhr

I'm confused, where did the 300ah battery go?


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## Twilly (Jan 22, 2008)

rednellycat said:


> Actually the parallel-series set up would be cheaper and considerable less weight. Here is an example using Deka sealed battery:
> 
> Configuration #1 ... 12 - Deka 8931 12v 108ah 70lbs $241.70
> Configuration #2 ... 8 - Daka 8924 12v 80ah 54lbs $186.54
> ...


I see the cost savings, But I dont see the comparable A/hrs... How are you going to connect those 80a/hr batteries to create a 108 a/hr string?

2- 48v 80ahr strings in parallel would = 48v at 160a/hr = 7680 w/hr
1- 144v string = 144v at 108a/hr = 15552 w/hr

Sure, you saved cost and weight, but you also lost over half your power

I was told early on in my EV life, "Lead = Power" ... I dont see a way to reduce lead and add power... with the exception of switching to nickel or lithium


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## Frank (Dec 6, 2008)

Wait a minute fellas. Power is volts X amps, not amp-hours. Volts X amp-hours is watt-hours, a measure of energy.

I don't know why Toyota scales the voltage up (I always thought it was to minimze losses somehow) but will ask the question at a Prius forum.


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## rednellycat (Sep 2, 2008)

You all are forgetting configuration #2 is parallel-series. Which I understand the battery set up or hook up would be 4 ( in series)12v/80ah set of batteries parallel with another 4 (in series)12v/80ah set and should total 48v and 320ah. Therefor total power would be 48 x 320 = 15360w. And configuration #1 are 12 in series 12v/108ah so total power will be 144 x 108 = 15552w


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## rfengineers (Jun 2, 2008)

rednellycat said:


> You all are forgetting configuration #2 is parallel-series. Which I understand the battery set up or hook up would be 4 ( in series)12v/80ah set of batteries parallel with another 4 (in series)12v/80ah set and should total 48v and 320ah. Therefor total power would be 48 x 320 = 15360w. And configuration #1 are 12 in series 12v/108ah so total power will be 144 x 108 = 15552w


That would be 48Volts * 160ah = 7680 Watt-Hours


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## Bottomfeeder (Jun 13, 2008)

4 in parallel and 4 in series would require 16 batteries. The amount of energy that lead acid batteries can hold is fundamentally limited by the mass of lead. There really is no way to arrange the batteries so that less lead can hold more energy. A rule of thumb I heard somewhere is that 700lbs of lead will store about as much energy as 1 gallon of gasoline.

Don't take it too hard. Everyone goes through a phase when they try to out-think the conventional wisdom. There are always places for improvement, but the 150 year old lead batteries are probably not the place to start.


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## Frank (Dec 6, 2008)

I made the query about the Prius on priusonline.com and was told: 

"The higher voltage motors have more power, and the gain in efficiency outweighs the conversion losses."


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## Evan (Feb 20, 2008)

Frank - Higher output HP motors have higher efficiency but voltage is just one part of the input power. You could make a high HP motor with high efficiency and use a low voltage with a very high current. The catch is that we would rather deal with higher voltage than higher current. This is however theory I don't know of anyone who has ever built a very high current low voltage motor.


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## Frank (Dec 6, 2008)

I wonder if it's also an AC/DC thing... most OEM-style EV/hybrids (including the Prius) use an AC synchronous motor. As for low-voltage/high-current motors, the only time DC-series motors see pack voltage is full-throttle: at part throttle/low-PWM motor voltage is relatively low with current high. I guess it depends on what we consider "high."


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## TheSGC (Nov 15, 2007)

Frank said:


> I wonder if it's also an AC/DC thing... most OEM-style EV/hybrids (including the Prius) use an AC synchronous motor. As for low-voltage/high-current motors, the only time DC-series motors see pack voltage is full-throttle: at part throttle/low-PWM motor voltage is relatively low with current high. I guess it depends on what we consider "high."


They use AC motors because regen braking is much easier to do than with DC. AC also runs at a higher voltage so the actual wiring can be smaller and the battery AH ratings are lower. That's the reason why the Honda Insight can use D size NiMH batteries for its pack.


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## Evan (Feb 20, 2008)

Well if you go up to 500HP nominal in AC motors you can get motors around 95%.

I though Honda went with D size cells due to patent restrictions.


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## Johny (Jul 21, 2008)

Higher voltage motor have lower current losses. For an AC controller this is important because there are more active devices (transistors) involved in controlling the motor.
A plausible reason why Toyota went with a voltage up-converter is because they used Nickel Metal Hydride batteries. These batteries have cell terminal voltage of 1.2VDC so to produce a high voltage pack would require a huge number of cells. You would want to keep the cell count down to minimise costs in the Battery Management System.


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