Why does it take less total energy to fast charge?

[scottf200]

There are also less losses inside the vehicle charger (the AC->DC conversion step). I don't have the exact numbers here, but the presentation given by Jason Harper from Argonne Nat'l Labs discovered the number empirically (i.e., they measured them). I've asked him to share his numbers/methodology here directly, but IIRC the L1 net losses (wall to wheels) is around 16% and the L2 net losses (again, wall to wheels) was more like 11-12%. That extra .5 KWh principally due to both line losses and conversion losses inside the on-board charger.

Page 5 here: https://docs.google.com/open?id=0B5l...2NVSzV1ZE9ERGc

[bonaire]

If you could pull 440V at 8A from the wall, versus 220V at 16A - you'd also have less loss too and faster than 1/2 the 220V rate. It would transform the 440V down to just over 360V+ to charge the pack rather than 220V up to 360V+. It may be more like 380V+ since the cells need to hit about 3.9V-4.0V each for top SOC.

What would be best is a 440V-rated electric motor using 440V HV main pack, 440V charging and AC-DC conversions done outside the vehicle in a really good power supply. I suppose one day, with infrastructure build-out, that could be the case to keep vehicle costs a little lower by not including a charger-inverter. This is also lower amps through all the wires while running the 25% higher voltage than the 360V. Higher voltage is more efficient, as this thread indicates. HV is also a bit more dangerous to people. (it also means more cells in the main pack - but that's ok, that would be higher kWh and more miles - do it with 118S3P instead of 96S3P - or perhaps 118S2P using 20Ah cells).

[emd]

don't really know why 220 is more efficient than 110. larger home air compressor motors sometimes are wired to take either voltage. other than portable compressors most i've seen are running on 220 because the owners claim lower energy use. maybe the meters record more amperage than wattage? maybe higher voltages handle inductive loads better? 220 just plain more economical? i know my bill went down a little when i went from level 1 to 2.

[silvercorvette]

For the same reason the power coming into your house stats off at 500 kV. The higher the voltage the more efficient. Those high voltage wires are stepped down at various points but the bottom line is the higher the voltage the more efficient the delivery system will be.

[supermachoman]

Energy: Joules or KWH (why the latter exists I have no idea) - SI symbol J
Power: Watts (joules per second) - SI symbol W
Coulomb: Basically a huge cluster of elementary charges - electrons or ions - SI symbol C
Volt: Potential difference (energy in joules per coulomb of elementary charges) - SI symbol V
Resistance: Ohms - self explanatory - SI symbol omega
Current: Amperes or Coulombs per second - SI symbol A, but in a lot of places you'll see current as I

From there it's just a matter of memorizing the formula, and in this case the one that matters is power = current squared times resistance. Plugging in the numbers shows you how the 240 is more efficient, as previously mentioned.


There has been talk about switching conventional gasoline vehicles from 12V to a 24V rail because of the additional accessories and all the bells and whistles that are showing up on modern cars. It also wasn't too long ago that you used to see power-users buy huge power supplies for their computers, but since most components have been moved to the higher voltage rail, the need has mostly disappeared (although it doesn't stop some newbs from recommending it unnecessarily on forum after forum)

Remember, with great power comes current squared times resistance
http://xkcd.com/643/

[Slapshot28]

Energy losses almost always can be "sensed" as heat loss--things get warm (and lose Btus or watts or whatever you like).

The more TIME a system has to generate heat loss, the less efficient it will be.