Why does it take less total energy to fast charge?

[MrEnergyCzar]

What is the reason it takes about .5 KWH less to recharge the battery on 240v? Is it strictly because it does it faster thus the idle Volt draw is lower over shorter time? Or is there some electrical reason? Just a laymen here....

MrEnergyCzar

[voltbeaupre]

Great question, it is probably because of amps carried through 240v vs. 120v (bigger pipe) when we first bought our 240 v charger(self installed) we thought because it took half the time it would cost half as much to charge, we have learned a lot in three months.The Volt inspires us to learn... .5kwh is better than no savings..would like a real answer from an electrician though.

[bonaire]

The Google told me when searching: "higher voltage more efficient"

http://www.newton.dep.anl.gov/askasc...9/eng99174.htm
http://www.madsci.org/posts/archives...8929.Ph.r.html

[mixedpixel]

I believe (but could be wrong) that the current is the same at 120 V or 240 V, so the resistance is the same. But since the 240 V charges in half the time, it results in half the loss. If someone has a better understanding, please correct me.

[MrEnergyCzar]

I believe (but could be wrong) that the current is the same at 120 V or 240 V, so the resistance is the same. But since the 240 V charges in half the time, it results in half the loss. If someone has a better understanding, please correct me.

this statement is a foreign language to me... someone here will know...

[Neromanceres]

I believe (but could be wrong) that the current is the same at 120 V or 240 V, so the resistance is the same. But since the 240 V charges in half the time, it results in half the loss. If someone has a better understanding, please correct me.

The 240V charger pushes near 14A. The 120V charger is near 12A.

The losses on a line are proportional to the resistance of the lines and connections times the current squared. Power is the the Voltage times the Current. So when you charge with 240V vs 120V assuming same current you can push twice the power with similar wire losses in the same period of time.

The 240V charger because it charges at near 14A will have slightly higher instantaneous line losses than the 120V at 12A. But the 240V will charge the car in 4 hours vs 10 hours on the 120V so overall the 240V charger will use a little less power.

So let's assume the line resistance is 1 ohm.
Energy lost at 240V would be 1 * 14^2 * 4 hours = 784 watt*hours
Energy lost at 120V would be 1 * 12^2 * 10 hours = 1440 watt*hours

[Ron C]

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.

[Room_A113]

There's also a higher percentage of power available for actually charging the battery due to battery-temperature maintenance. The following numbers are not exact, but the logic is sound:

The Volt keeps the battery at around 70 degrees. Let's say on a really cold night that it takes 500 Watts to keep the battery 'warm'.

On a 120V charger, there is approximately 1500 watts available.
On a 240V charger, there is approximately 3000 watts available.

1500 watts - 500 watts (for warming) = 1000 watts available to charge the battery
3000 watts - 500 watts (for warming) = 2500 watts available to charge the battery

As you can see, 2500 watts is more twice as much as 1000 watts (these numbers are not exact, but the concept is valid).

[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

[Sterling Silver]

Think of it as if you were going to Sam's Club to buy orange juice... When a product comes in bulk (240 vs 110, or orange juice, socks, pencils) there are greater efficiencies to be realized and the product is less expensive to deliver, inventory and market.