Round Trip Charging Efficiency

I have a REX that has now over 30,000 miles of which 26,000 are electric.

Recently I compared the kwh to return the car to 100% charge as against the kwh used derived from the trip computer - trip miles divided by miles per kwh. On the this comparison I obtained just 74% efficiency , i.e significantly more kwh to return the car to full charge than the kwh derived from the reported miles per kwh.

I am using the standard charger runiing off the UK three pin system.

The concern I have is that I am seeing a "hidden" battery degradation. I would welcome views from other high mileage I3 drivers.

It would be interesting to compare your 74% to what it was when the car was new. Or better yet, a chart of efficiency vs odometer miles. Do you have that data ?
I realize you're looking for comparison from other high mileage i3's....sorry, can't help there but it might be interesting to see the chart if you happen to have that data. Thanks.

A U.S. national testing lab measured the i3's on-board charger's efficiency at 240V to be in the low 90% range. The EVSE's efficiency should be in the high 90% range because it's just a smart switch. If you fully charged your battery pack, the charger's efficiency would drop a bit as the battery pack neared full charge. If the battery pack's cells needed balancing, some of the battery pack's energy would have been dissipated as heat during the balancing procedure. Nevertheless, there seems to be a significant discrepancy. Are you confident that you were able to measure the charging energy accurately? I assume that no battery pack or cabin conditioning were occurring.

I doubt that this discrepancy would be an indication of battery pack degradation after so few miles driven.

Hi,

Let me suggest doing an experiment: use different charge rates and calculate the round-trip, efficiency.

My expectation is lower charging rates will increase the round-trip efficiency. The waste heat is generated in part from (I**2) * R (current, I, squared times resistance.) By reducing the current, the wasted heating loss goes down much faster at the cost of longer charging time.

Our batteries may require cooling to get rid of the waste heat. This cooling load is also energy lost.

In a perfect world, plugging in the car would let us enter 'hours to full charge.' So at home, we might choose the hours to the next day. At work, four hours for lunch or eight hours to leave for home. At a shopping center, take the max. Regardless, the car would then tell the EVSE what rate to use and tweak as needed to meet the target. Call it adaptive charge control.

Bob Wilson

FWIW, the car DOES just pull the amount of energy it wants (up to the limit of the EVSE as determined by the pilot signal it sends out)...so, the car can decide how much and how fast it wants to recharge itself. UP to a point, the car might NEED to warm the batteries during charging to be able to achieve maximum capacity. Cooling often isn't needed in the cooler seasons, but certainly would during a hot summer or after a long, high-speed run. FWIW, while not an issue in the UK, in the USA, at 110vac input, the car is not as efficient as it is at 240vac inputs when charging.

bwilson4web said:

My expectation is lower charging rates will increase the round-trip efficiency. The waste heat is generated in part from (I**2) * R (current, I, squared times resistance.) By reducing the current, the wasted heating loss goes down much faster at the cost of longer charging time.

Click to expand...

The Idaho National Laboratory's charging efficiency tests showed just the opposite: 240V charging at 5.7A was 2.4% less efficient than at 30.1A. Possibly the considerably higher total harmonic distortion at 5.7A overwhelmed the greater resistive loss at 30.1A.

It would be useful for other i3 drivers to post the results of a similar test together with electric miles recorded and the type of charger used to see if the round trip charging efficiency does reduce and also the variation.

I looked at this issue as the overall kwh consumption for the place where the car is charged showed an increasing trend and the I3 is the prime suspect.

alohart said:

bwilson4web said:

My expectation is lower charging rates will increase the round-trip efficiency. The waste heat is generated in part from (I**2) * R (current, I, squared times resistance.) By reducing the current, the wasted heating loss goes down much faster at the cost of longer charging time.

Click to expand...

The Idaho National Laboratory's charging efficiency tests showed just the opposite: 240V charging at 5.7A was 2.4% less efficient than at 30.1A. Possibly the considerably higher total harmonic distortion at 5.7A overwhelmed the greater resistive loss at 30.1A.

Click to expand...

Excellent!

That make sense. I remember reading that switching power supplies are most efficient near their design limit and it falls off under partial loads. I just checked the datasheets for switching regulators, Linear and Maxim.

I stand happily corrected.

Bob Wilson

Other than a small internal power supply to enable the pilot signal, interlocks, and idiot lights, all the power the EVSE utilizes is determined by the car it is attached to...the car decides how much of the available power to take, and when. There should be nearly no difference in the EVSE used, for those that enable the same output level (say 32A).

The power supply in the car needs to make fairly high DC voltages to recharge the batteries...that's more efficient when you start with a higher ACV input. A lower amperage unit will need to run longer, which means potentially a bigger heat buildup, or at least cooling requirement for longer periods of time. The starting state of the battery pack, ambient temperature, SOC, size of the EVSE, all will play into the ultimate energy required to recharge the car. If you've turned on preconditioning of the cabin, that will have a big effect on how much energy is used, depending on the current ambient conditions.