110 or 220 v, is either more efficient?

I'm not an expert but most homes are provided with three phase 220 and individually the phases provide 110 VAC. But from an electrical standpoint, higher voltages reduce the power loss for identically sized conductors. We call it the "I squared R" loss.

Bob Wilson

 

FWIW, on the Leaf, 220+ volt charging is more efficient than 110. More electricity will be wasted as loss at 110.

 

In the LEAF and Volt, charging on 240 is roughly about 7-10% more efficient than charging at 120V. Most of this is likely due not to the efficiency of using a higher voltage. The real difference is due to charge rate. Charging at 120V is typically limited to 12A due to the 120V sockets that are commonly available in homes and businesses in the U.S. and many other countries. So, the charge rate is 1.4 kW or less. Charging with 240V is typically done at 16A in the LEAF and Volt (somewhat less in the PiP) so around 3-3.7 kW. Charging at a higher rate means less time is spent on the fixed overhead associated with charging independent of the rate (charging electronics, battery monitoring, active battery cooling overhead, etc).

It would likely be more efficient to charge at even faster rates like 6-7 kW which will be supported in Ford's new plugins and in the 2013 LEAF. The efficiency of charging the Tesla Roadster has been shown to peak at around 40A and then remain essentially flat up to 70A.

 

One other obscure fact, three phase charging is going to be slightly more efficient that single phase. This is buried in the bowels of how three phase rectification occurs. The simple answer is single phase rectification always has 'dead' time but three phase always has current flowing through one pair of the six diodes. This also improves the 'noise' to the filter.

I'm not an expert but Jeffry is and can probably provide an exact math model.

Bob Wilson

 

Most homes in the US are wired for 240V single phase these days. There are 2 legs that come into the house, each at 120V. For 240V appliances, the two 120V legs are combined to provide 240V.

 

In years gone by I was always told that a slower charge was better for the life of a battery. Did that change for Lithium Ion batteries?

 

Homes in the US are very rarely supplied with 3 phase power. They have 240 volt single phase and 120V is obtained by taping off one side of the 240V to neutral. It's still single phase though.

 

I did a back of the envelope calculation on the energy savings due to reduced power loss in my house wiring with the 220 volt charger and it came out to about $40 saved over a 10 year period. I am expecting a greater savings just because I'm getting more charging between short errands when I'm at home, but it's hard to predict quantitatively.

I hope the benefits of 220 volt charging will not be overshadowed by a shortened battery life due to faster charging.

 

All homes use Single-Phase power. Only commercial businesses use 3-Phase power.

 

The battery life from charging at 220 is probably fine. I believe I've seen consistent reports that the Prius PHEV charges at a rate of around 2.0-2.2 kW at 240V (208V in 3 phase commercial buildings). That is presumably slower than the 3-3.7 kW rate seen on the Volt (16kWh) and LEAF (24 kWh) in order to protect the smaller 4.4 kWh pack size.

 

The 240V charging rate is lower than the battery charge/discharge rate during regenerative braking/driving.

Hence, if you aren't worried about driving your Prius Plug-in, you shouldn't be worried about charging it at 240V.