4 ms·

The OP didn't give enough information to produce a specific answer. For a higher voltage and lower current (same power), the wire could be thinner with the same

by lutusp 14y ago

The OP didn't give enough information to produce a specific answer. For a higher voltage and lower current (same power), the wire could be thinner with the same outcome. The reason electricity utilities string million-volt lines between cities is to minimize the heating losses in the conductors, not to impress the civilians. It's the same reason electric car designs favor putting a lot of cells in series -- a high-voltage electric motor gets more of the power delivered by the batteries, less of it is wasted in the connecting wires. Also, in the car's charging circuit, the high voltage choice maximizes power transfer for a given connecting wire size.

mbell 14y ago
You still need to convert this high supply voltage to a lower voltage to match the cell voltage for charging. The currents involved mean this converter is still likely to dissipate a lot of heat, even if it is extremely efficient.
- lutusp 14y ago
  > You still need to convert this high supply voltage to a lower voltage to match the cell voltage for charging. No, instead you charge the cells in series. That delivers the same power while avoiding the problem of having to produce overly high currents. > The currents involved mean this converter is still likely to dissipate a lot of heat, even if it is extremely efficient. Yes, but there are ways to minimize this loss factor. Obviously a fast-charging scheme for a car is going to involve a lot of current, but the system designers do all they can to minimize that current. The primary way they do that is by both charging and discharging the cells arranged as much in series as is practical. In the Tesla design, there are 11 modules connected in series and delivering 375 volts nominal (and requiring 375 volts to produce an optimal charging current). So there's no issue of changing individual cells at their normal cell voltage, instead the system charges modules consisting of 621 cells in an optimized series-parallel scheme: http://en.wikipedia.org/wiki/Tesla_Roadster#Battery_system http://en.wikipedia.org/wiki/Tesla_Roadster#Battery_system Quote: "Tesla Motors refers to the Roadster's battery pack as the Energy Storage System or ESS. The ESS contains 6,831 lithium ion cells arranged into 11 "sheets" connected in series; each sheet contains 9 "bricks" connected in series; each "brick" contains 69 cells connected in parallel (11S 9S 69P)"
  - mbell 14y ago
    By 'cell' I meant the overall pack, not the individual unit within the pack. From your link: "A full recharge of the battery system requires 3½ hours using the High Power Connector which supplies 70 amp, 240 volt electricity" Multiply that current by 10-100 to 'charge it faster' and you have a huge problem with thermal dissipation, not to mention using truly massive cables.
    - lutusp 14y ago
      > Multiply that current by 10-100 to 'charge it faster' ... No, that can't be done without damaging the batteries. A fast charge in this context means multiplying the default charging current by roughly three, not 10, and certainly not 100.
cmccabe 14y ago
Once you start getting into the thousands of volts, it becomes very dangerous. That stuff will arc through the air and kill you. There is a reason why we use a hundred or two volts in consumer appliances.
- userulluipeste 14y ago
  Not necessarily. If the connection would be unmanaged, as having the power available for drainage all the time, then you're right! ...but we have computers and all kind of complex systems now. The cable may be "empty" of that high voltage current until the cable is safely plugged in and ready. After plugging, before unleashing energy, the car's system and the power source's system may first communicate and verify their sensors if everything's ready to go, like isolation and other things. The high voltage risk and its management it's an engineering solvable problem.
  - cmccabe 14y ago
    You have the wrong idea of how voltage works. High voltage electricity can and will kill you before the ground fault interruptor (or whatever) can kick in. It takes time to break a high voltage circuit, (on the order of milliseconds), and during that time, you will die. If the current is across your heart it's especially lethal. If the voltage is high enough, there is no way to break the circuit anyway. The electricity will just arc across the (ionized) air, even if the computer physically disconnect it. Assuming the computer itself isn't killed by the spark, which is pretty likely. This is why linemen on high voltage lines don't even try to electrically insulate themselves when they're working with it live. Ask wikipedia: For high-voltage and extra-high-voltage transmission lines, specially trained personnel use "live line" techniques to allow hands-on contact with energized equipment. In this case the worker is electrically connected to the high-voltage line but thoroughly insulated from the earth so that he is at the same electrical potential as that of the line. Since training for such operations is lengthy, and still presents a danger to personnel, only very important transmission lines are subject to maintenance while live. Outside these properly engineered situations, insulation from earth does not guarantee that no current flows to earth—as grounding or arcing to ground can occur in unexpected ways, and high-frequency currents can burn even an ungrounded person. Touching a transmitting antenna is dangerous for this reason, and a high-frequency Tesla Coil can sustain a spark with only one endpoint).

by lutusp 14y ago

mbell 14y ago

You still need to convert this high supply voltage to a lower voltage to match the cell voltage for charging. The currents involved mean this converter is still likely to dissipate a lot of heat, even if it is extremely efficient.

lutusp 14y ago
> You still need to convert this high supply voltage to a lower voltage to match the cell voltage for charging. No, instead you charge the cells in series. That delivers the same power while avoiding the problem of having to produce overly high currents. > The currents involved mean this converter is still likely to dissipate a lot of heat, even if it is extremely efficient. Yes, but there are ways to minimize this loss factor. Obviously a fast-charging scheme for a car is going to involve a lot of current, but the system designers do all they can to minimize that current. The primary way they do that is by both charging and discharging the cells arranged as much in series as is practical. In the Tesla design, there are 11 modules connected in series and delivering 375 volts nominal (and requiring 375 volts to produce an optimal charging current). So there's no issue of changing individual cells at their normal cell voltage, instead the system charges modules consisting of 621 cells in an optimized series-parallel scheme: http://en.wikipedia.org/wiki/Tesla_Roadster#Battery_system http://en.wikipedia.org/wiki/Tesla_Roadster#Battery_system Quote: "Tesla Motors refers to the Roadster's battery pack as the Energy Storage System or ESS. The ESS contains 6,831 lithium ion cells arranged into 11 "sheets" connected in series; each sheet contains 9 "bricks" connected in series; each "brick" contains 69 cells connected in parallel (11S 9S 69P)"
- mbell 14y ago
  By 'cell' I meant the overall pack, not the individual unit within the pack. From your link: "A full recharge of the battery system requires 3½ hours using the High Power Connector which supplies 70 amp, 240 volt electricity" Multiply that current by 10-100 to 'charge it faster' and you have a huge problem with thermal dissipation, not to mention using truly massive cables.
  - lutusp 14y ago
    > Multiply that current by 10-100 to 'charge it faster' ... No, that can't be done without damaging the batteries. A fast charge in this context means multiplying the default charging current by roughly three, not 10, and certainly not 100.

cmccabe 14y ago

Once you start getting into the thousands of volts, it becomes very dangerous. That stuff will arc through the air and kill you. There is a reason why we use a hundred or two volts in consumer appliances.

userulluipeste 14y ago
Not necessarily. If the connection would be unmanaged, as having the power available for drainage all the time, then you're right! ...but we have computers and all kind of complex systems now. The cable may be "empty" of that high voltage current until the cable is safely plugged in and ready. After plugging, before unleashing energy, the car's system and the power source's system may first communicate and verify their sensors if everything's ready to go, like isolation and other things. The high voltage risk and its management it's an engineering solvable problem.
- cmccabe 14y ago
  You have the wrong idea of how voltage works. High voltage electricity can and will kill you before the ground fault interruptor (or whatever) can kick in. It takes time to break a high voltage circuit, (on the order of milliseconds), and during that time, you will die. If the current is across your heart it's especially lethal. If the voltage is high enough, there is no way to break the circuit anyway. The electricity will just arc across the (ionized) air, even if the computer physically disconnect it. Assuming the computer itself isn't killed by the spark, which is pretty likely. This is why linemen on high voltage lines don't even try to electrically insulate themselves when they're working with it live. Ask wikipedia: For high-voltage and extra-high-voltage transmission lines, specially trained personnel use "live line" techniques to allow hands-on contact with energized equipment. In this case the worker is electrically connected to the high-voltage line but thoroughly insulated from the earth so that he is at the same electrical potential as that of the line. Since training for such operations is lengthy, and still presents a danger to personnel, only very important transmission lines are subject to maintenance while live. Outside these properly engineered situations, insulation from earth does not guarantee that no current flows to earth—as grounding or arcing to ground can occur in unexpected ways, and high-frequency currents can burn even an ungrounded person. Touching a transmitting antenna is dangerous for this reason, and a high-frequency Tesla Coil can sustain a spark with only one endpoint).