EV range

mgfvvc

Herzlos said:

Resistance isn't quite fixed, it's lower at higher voltages for reasons I'm not sure; IIRC that's why power lines transmit at huge voltages.

I think you've misunderstood the physics. Power is current times voltage, so to transmit more power on a line you can increase voltage or current (or both). Higher voltages allow you to transmit more power for the same current. The resistance doesn't change, but increasing current increases power lost to heat in the lines, increasing voltage does not increase power losses and hence is more efficient.

mgfvvc

MouldyOldDough said:

Changing the subject slightly. :

Exactly how does charging an EV at different rates work ?

I am aware (from schoolboy physics)  that V=I*R, or Charge Current = Voltage / Resistance But surely R (resistance) is obviously fixed.

So - do they ramp up the voltage to shorten charging times (by up to 10 times)

if not - how do they force a charge in to a Lithium battery at a higher rate than it usually accepts ?

The same question is for mobile phone fast chargers ..

I'm not exactly sure what you are referring to here. Charging rates altering over time as the battery charge level increases or different charging rates for different cars?

The reference to shortening by up to ten times makes me think that you are talking about the difference between cars. There are a bunch of different factors. Teslas currently use a 400V system, some fast charging cars use an 800V system.

When you say the resistance is fixed, different batteries will have different resistance depending on both the specific chemistry and the number and layout of individual cells. A 100kWh battery contains twice as many cells as a 50kwH battery built from the same cells, thus the same amount of power to the battery as a whole would be half the power to each cell. Looked at another way, the 100kWh battery has half the resistance of the 50kWh battery.

Also the limiting factor is not always the resistance, it may be cooling and ensuring that the battery doesn't overheat.

ElefantEd

MouldyOldDough said:

Changing the subject slightly. :

Exactly how does charging an EV at different rates work ?

I am aware (from schoolboy physics)  that V=I*R, or Charge Current = Voltage / Resistance But surely R (resistance) is obviously fixed.

So - do they ramp up the voltage to shorten charging times (by up to 10 times)

if not - how do they force a charge in to a Lithium battery at a higher rate than it usually accepts ?

The same question is for mobile phone fast chargers ..

The limiting factor for charging an EV battery using 240AC (ie something from your home supply) is the cables its connected with; so using a standard 3 pin plug you'll only get about 10A going in (13A theoretical max but for extended periods 10A is more likely). Use a dedicated charger and you will get about 32A (same as a shower) because the cables are thicker.

Rapid chargers simply use a higher voltage as you suggest, which in turns gives a bigger current (could easily be several hundred amps).

Then in all cases Power = Volts x Amps, and Energy = Power x Time