Electrical Questions for Beginners: Voltage & Amperage Effects

[buddhaman]

i wanted to say hi and i have a couple questions. I'm letting everyone know that i have the electrical understanding of a basic beginner. first question: what are the effects of a voltage multiplier on amperage? second question: is there an amperage multiplier? thanks.

 

[berkeman]

i wanted to say hi and i have a couple questions. I'm letting everyone know that i have the electrical understanding of a basic beginner. first question: what are the effects of a voltage multiplier on amperage? second question: is there an amperage multiplier? thanks.

Welcome to the PF!

The simplest example of what you are asking about is a transformer, for AC signals only. By adjusting the turns ratio, you can adjust the ratio of AC input voltage to output voltage, with a corresponding (but inverse) change in the current available (because a transformer is a passive device, with power out = power in, for an ideal lossless transformer)

http://en.wikipedia.org/wiki/Transformer

For "active" devices (where you put additional power in, like with active amplifier circuits), you can do whatever you want with voltage ratios and current ratios.

 

[buddhaman]

ok i pretty much get it. I'm looking to make 14.4VDC turn into 144VAC and 100 amps turn into 900.

 

[Fish4Fun]

wow, that's a lot! (129.6kVA)! To put this in perspective, a standard 200A service to a house is only ~45kVA. Or, another way of looking at it, if you had 100 * 220Ah deep cycle batteries, you could support a 100% efficient conversion at full load for about 2.44 hours. In reality I doubt you could get better than 70% efficiency in your conversion, and you would want to limit the battery discharge to about 50%, so your "real world" inverter, with 100 batteries, might have 51 minutes of run time before you damaged them.

You are talking about enough power to run your house and two of your neighbor's houses with every single appliance running wide open!

Anyway, Good Luck!

Fish

 

[buddhaman]

ok. that's the amperage i need for an electric car motor.

 

[Fish4Fun]

Yea, I figured it had to do with a car. So, really you do not need to "convert" anything, you simply need to use series-parallel batteries to achieve the requisite voltage/amperage/capacity. The Tesla Roadster uses the 18650 form factor Lithium Ion Batteries, here are the specs:

Specifications:

* Nominal Voltage: 3.7V
* Size: 18650 (Cylindrical)
* Capacity: 2800 mAh
* Chemistry: Lithium Ion (Rechargeable Li-Ion)
* Brand: LG
* Max Charging current: 2.7A
* Max Discharging current: 5A
* Included Qty: 1Dimensions:

* Length: 65 mm
* Diameter: 18 mm
* Weight: 46.5 g (1.64 Oz)

They use 100 batteries in series (nominal 375V) per "cell" and 50 "cells" in parallel to provide 53.5kWh of storage capacity from 5000 batteries. Obviously if you plan on using a motor requiring a nominal 144V, then you would want to put ~40 in series per "cell". Your storage capacity would then be 414.4Wh per cell. If you wanted to be able to supply 900A maximum, you would need a minimum of 180 "cells" (900A / 5A max draw per cell), for a total of 7,200 batteries, and a storage capacity of ~75kWh.

Current low quantity street cost of the above mentioned 18650 2.8Ah battery is ~$15.50 setting the retail value of your battery pack ~$111,600. Buying this volume of batteries in pre-assembled "cells" would likely reduce your cost by about 50%, or ~$55k.

Assuming you wanted to be able to charge the entire battery pack assembly simultaneously at the full charging rate, you would require ~150V @ 486A. Obviously this would require more than the "normal" 200A service most homes have. And boy would this thing spin your meter, hehe.

Sounds like a fun project! Good Luck!

Fish

 

[buddhaman]

what about 12- 12v deep cycle batteries?

 

[Fish4Fun]

what about 12- 12v deep cycle batteries?

220AH is a pretty standard "heavy duty" deep cycle lead/acid battery, they run ~$450 each. 12 of them in series would give you your nominal 144V with about 26kWh of storage at near full discharge, closer to 15kWh at a sustainable discharge level. They would weigh in at slightly over 1200lbs as opposed to the 738lbs for the above mentioned 75kWh of lithium ion storage, but the up-front cost is only ~$6k for your 12V deep cycles. The lithium ion service life is 3 to 9 times as long as the lead-acid batteries, so if this is a long-term project, they are the way to go:

@ 3 times service life & 3 times capacity... 9 * $6,000 = $54,000 (Equivalent cost over time)
@ 9 times service life & 5 times capacity... 45 * $6000 = $270,000 (Equivalent cost over time)

To get the same storage capacity (assuming 50% discharge):

15kWh * 5 = 75kWh

So, you would need 12 * 5 deep cycles = $30,000, but the weight would be close to 6000lbs!

Anyway, you can figure out how far you can get on 15kWh (1 gallon of gasoline = ~34kWh of energy). Do remember that electric energy is converted to motion more efficiently than an ICE converts gasoline, so 15kWh will take you further than 1/2 a gallon of gas will. How much further depends on the efficiency of your motor and speed control electronics.

The Tesla mentioned previously, with 53kWh storage capacity can travel up to 200 miles between charge cycles. This implies it uses 265Wh per mile. Assuming charging efficiency of 75%, it would take ~71kWh to fully charge the batteries. @ $0.15/kWh it would take $10.65 per 200 miles, or $0.05325/mile in fuel. Assuming an economy car gets 40mpg under similar circumstances and gas costs $3.00/gallon the cost to drive the same 200 miles would be $15.

Using the Tesla figures of 265Wh/mile, I would think the best range you could hope for (without damaging your batteries) would be 15kWh/265Wh = 56.6 miles. If you used the 75kWh battery pask outlined above, 75kWh/265 = 283 miles.

Just thought I would finish the math for you :-) I am not attempting to discourage you, only make sure you are going in eyes-wide-open.

Fish

 

[buddhaman]

not discouraging at all, i appreciate all the help. like i said, i have pretty much no knowledge of electronics and such which is why I'm asking a lot of questions.

 

[buddhaman]

this is the motor i was thinking of using. am i trying to shoot too high on the amperage?

http://www.evparts.com/products/str...to-192-volt-street-vehicle-motors/mt2120.htm"