[rompik]

Hello Electricians and EV enthusiasts,
Could some one explain why traction battery in EVs needs to be High Voltage instead eg. 12V or ~4V (single cell voltage)?

I understand that power is simply V * A. So technically you could have more cells joined in parallel delivering high Amps rather than serially (high Voltage / lover Amps for a given C rate). You should be able to charge / discharge low voltage battery at the same kW rate as high voltage.

Higher voltage increases motor spin rate but you can construct a motor that will simply spin faster / slower for a given voltage (my guess it mostly depends on motor winding but i may be wrong here). Just like transformer.

So why traction batteries cannot be 12 volt?

 

[andrew*debbie]

Because of resistive losses.

Power in a resistor is the current squared times the resistance.

For 1000W at 500V current (i) = 1000W / 500V = 2A.

1000W at 12V i = 1000W / 12V = 83.3A

For this example, assume the resistance in the battery, wires, inverter and motor is 1Ω. Power = i^2R. Power loss at 500V = 2^2 * 1 = 4Watts. Power Loss at 12V = 83.2^2 * 1 = 6944W.

You loose seven times more power heating the battery, motor and wires than you get out of the motor.

 

[cah197]

How high would the current be for a 12V pack for the same power?

 

[Spiny]

You have supplied your own answer power is V * A at low voltage the Amperage is high.
Say you need 20kW when accelerating then at
400v this is 50 Amps
48V this is 416 amps
12V this is 1666 Amps

Even a 0.01ohm resistance in the wiring would lose 16V at 1600 Amps so nothing left to drive the car.... Not possible of course so the motor becomes current limited due to lack of voltage.
At 50 Amps that is 0.5V of the 400V available.

This is why Porsche has gone for 800V for their Taycan, less wiring losses under heavy acceleration.

 

[andrew*debbie]

150kW rapid charging would be interesting. 150kW / 12V = 12.5kA

I think you would need a cooling pond.


Radio station WLW from the 1930's

 

[rompik]

Thank you @andrew*debbie ! Makes sense

 

[Big277wave]

The copper cabling to take the high currents would be huge and very expensive. This is the same reason that we use 240V for our home electricity it allows thin, flexibe cables even for quite high current devices. The National grid take this to the extreme, the super grid which takes highest power runs as 400 KV. Mild hybrid cars often run at 48V. At this low voltage they can't fit a very high power motor hence the mild hybrid name.

Electric cars have a 12 V battery to supply lights and the ancillaries that are usually fitted to all cars. A 5 W filament side light bulb would need a very thin filament if it was run off 400V and would probably blow when you drove over the first bump. The 12 V version needs a thicker filament and so it lasts a lot longer, In this case it works better with lower voltage.