Published Thursday November 6, 2014: Updated 5/29/2015

Reducing Electric Vehicle Inefficiencies During Charging

I am always on the lookout for energy wasters around the house, in appliances and electric vehicles. Even though I am an expert of sorts in areas of energy efficiency, occasionally, I still run across things in my house that waste power when they don't need to. 

I tend to obsess over monitoring the energy usage of things. But in order to find and make improvements, it is necessary. After all, if you can't measure it, you can't improve it. Right? 

I recently found a significant power waster in the area of trickle charging an electric vehicle. It comes from the electrical wiring ITSELF! 

Conductors Are Also Resistors

One day while charging up the Geo EV, I noticed after plugging it in, the AC voltage on the Kill-A-Watt meter dropped by nearly 8 volts.

At zero amps, it read 118.4 volts but at 11.9 Amps, it only read 110.5 volts. All running electrical appliances will cause the voltage to drop to some extent, so I never really gave much thought to this, but after some quick Ohm’s law number crunching, (7.9 volts x 11.9 amps = WOW! I was shocked and amazed that 94 watts was being wasted as heat in the AC wiring ALONE!

Although trivial, maybe using a relatively long extension cord isn't the most energy efficient choice for daily electric vehicle trickle charging. I wonder where else am I unknowingly wasting energy? 

The On-board charger power cable

The wasted power in the AC wiring also extends into the car, up through the wiring and up to the battery charger itself. Although I can't very easily measure it directly, I estimate the resistive heating in the on-board power cable itself consumes about 33 watts whenever the charger is charging at maximum power (~1300 watts).  This brings the total, resistive AC power losses to 127 watts.  

Original Charging Configuration

 Wire Segment Charging (11.9 A) No Load  Power Loss
 Just AC house 
 wiring (12 AWG)
 115.8 v 118.5 v 34.7 watts
 Extension Cord
 (16 AWG)
 110.5 v 118.4 v 59.3 watts
 Charging Cable
 n/a (8 feet long) n/a ~33 watts
 Total Losses n/a n/a 127 watts

I noticed that the 8’ long power cord supplying the on-board, Elcon battery charger gets kind of warm whenever it is plugged in and charging. It doesn't need to be that long (but that is what was supplied by default with the charger), so I replaced it with a short 3’ one that I picked up on Amazon for about $6. Now its resistive losses only consume about 10 watts. 

I also swapped out the relatively thin extension cord I was using with an old, heavy gauge one I had laying around. This instantly reduced the losses by an additional 34 watts.

12 AWG Extension Cord From the Wall

 Wire Segment Charging (11.08 A)  No Load  Power Loss 
 AC house
 wiring (12 AWG)
 115.8 v 118.5 v 34.7 watts
 Extension Cord
 (12 AWG)
 114.1 v  116.6 v  32.4 watts 
 Charging Cable
 n/a (3 feet long) n/a ~10 watts
 Total Losses n/a n/a 77.1 watts

That's a little better but I figured going from a 16 AWG extension cord to a super heavy duty 12 AWG one would have gained more improvement than that. Maybe the 12 AWG one is too old.  

AC wiring from breaker to outlet

The original 12 AWG, AC electrical wiring that I use for charging up the Geo EV starts at the breaker box in the basement before routing through a plug socket in the basement, then up a wall, out to the garage, up the garage wall and across the garage ceiling to a plug socket. It's probably a 60 foot length of wiring. 

I decided to go all in and run a new, dedicated, 15 Amp circuit but using 10 AWG wiring strait out to the garage. It’s only about a 20 foot run from the breaker box to the garage outlet near where my car is parked. 

Bingo! The resistive power loss of the old circuit used to be about 35 watts, now it is only 17.5 watts. 

I also ran a new 10 AWG extension cord on a boom. 

The 10 AWG boom charging cable with energy meter and switches.

New 10 AWG Boom Wire

 Wire SegmentCharging (10.98A)No LoadPower Loss
 AC House
 Wiring (10 AWG)
 118.8 v 120.4 v 17.6 watts
 Extension Cord
 (10 AWG)
 118.3 v 120.4 v   5.5 watts
 Charging Cable
 n/a (3 feet long) n/a ~10 watts
 Total Losses n/a n/a 33.1 watts 

After making all of these changes, I am truly surprised at how much power is being saved now.  Notice that the current draw also went down by almost a full Amp (from 11.9 to 10.98)?  

On average, the charger runs solid for about 5.5 hours per day, 4 days a week, 50 weeks a year.   Going from 127 watts of loss down to 33.1 watts works out to about (127-33.1 = 93.9 watts x 5.5 x 4 x 50) = 103.3 kWh or $10 in wasted electricity that is now prevented each year (assuming $0.10 per kWh). 

Total Energy Savings 93.9 watts 516.5 Wh/day 103.3 kWh/year

Wow! Who knew that these trivial losses could add up to so much energy lost? And these are just the losses while the batteries are charging. What about phantom power on the car itself, where power is being wasted 24-7? 

Energy Wasters on the 12 Volt System

I have noticed that after charging up the car on a Friday evening and having it sit idle all weekend, the battery pack is about 1.25 kWh low by Tuesday. Working backwards, there seems to be a 17.5 watt, power leak somewhere.
The BMS circuit in the Geo EV draws 15 watts all the time. That might not sound like a lot but that 15 watts is being consumed, every hour of every day. That works out to 131.4 kWh per year, or about $13 in electricity, (at $0.10 per kWh).

It may not seem like much but that is pretty substantial. That's like 8% of my total driving energy needs for a year. 

That’s the funny thing about super fuel efficient vehicles. Even trivial power draws can waste a significant percentage of your total energy needs. 

In comparison, a large pickup truck idling its engine in the driveway will blow through that much energy in 3.5 hours.  

To remedy this 15 watt energy loss, I installed a main breaker switch on the input of the DC-DC converter for all of the 12 volt circuits that can easily be switched off when the car is not being driven. 

The only down side is when this main breaker is turned off, the BMS doesn't keep track of energy usage and I have to manually re-set it after each charge. It takes about 13 seconds to do. For my daily commute I already know roughly how much energy I use so I end up only needing to reset the BMS about once a week. 

There is still a 2-3 watt power leak somewhere else in the car. I’m suspecting it is in the motor speed controller or maybe even in the battery chemistry itself. That’s pretty low so I’m not too worried about it for now.

Total Geo EV energy savings:

In total, reducing these seemingly insignificant power losses resulted in a 234.7 kWh/year energy savings.

Driving about 8500 miles each year in the Geo EV required about 1620 kWh in energy, so that’s a 14.5% energy reduction. WOW!

That $23 in electrical energy savings is enough to drive the Geo EV an extra 1466 miles every year, or heat the house in the winter time for 2 weeks. That is 2.5% of what the PV solar panels on the house produce. 

Caution:  Learning Opportunity Ahead

Here is a great little chart I found online that shows the formula relationship between voltage, current, resistance and power. 

Here is another little graphic explaining the relationship between volts, amps and resistance (ohms).  I don't know who drew this but it's a great analogy. 

Hardwired Meter

At the end of the work day, I come home and plug in, but I make sure a Kill-A-Watt meter is in line, so when the car charges up, I can measure its energy use for that day. Sometimes I need to run an errand after I have plugged in. Not wanting to lose data collected on the Kill-A-Watt meter thus far, with a tiny zap, I unplug the energized charging cable. Doing this dozens of times wears out the connector and eventually the Kill-A-Watt meter needs to be repaired or replaced. 

As with any appliance that has large filtering capacitors in it, plugging in the car also results in a tiny zap, which results in eventual connector damage.

In order to re-set the data on the Kill-A-Watt meter, it needs to be un-plugged and plugged back in again. This action eventually wears out the plug connection on the back of the meter. 

Commercial electric cars like the Chevy Volt or Nissan Leaf use a J1772 connector that is designed to last for 10,000 cycles of being plugged in and unplugged.  At that rate it would take 27 years before the connector would need to be replaced (assuming it is used once a day).  The J1772 charging standard also has smart handshaking and safety protocols that remove power before the connector is mated or un-mated. The battery charger onboard my Geo EV isn’t that smart though. 

To minimize excessive wear and tear, I initially used a couple of power strips on either side of the Kill-A-Watt meter, but after all the lessons learned about power loss (above), I decided to go with a more robust, hard-wired design. 

Using a couple of common, 120 volt, 15 Amp switches on either side of a modified, hardwired Kill-A-Watt meter solves all of these problems.

The upper switch safely de-energizes the car’s charger without losing data on the Kill-A-Watt meter and allows the charging cable to be un-plugged or plugged in without any corresponding zap. This will greatly extend the life of the plug and socket connectors.

The lower switch completely turns off the Kill-A-Watt meter, resetting the daily energy usage data back to zero.

The 10 AWG wire routes up, over and down the boom so there are no wires to trip over, inside the garage. It is high enough that I can easily move large items underneath it. Either power switch will safely de-energize the boom cable so there is no live voltage while the cable is hanging out in the open. 

It also acts as a guide for precision parking every time. 


What About Energy Losses In High-Powered Appliances?

High Power draw devices like vacuum cleaners and toaster ovens will also cause large resistive losses in the house wiring. But since you typically only use them for only minutes a week, (instead of hours per day), those losses don't really add up to much energy lost each year. The energy loss will costs you on the order of fractions of a penny to pennies each year. 

The following chart shows the resistive power loss in the house wiring when running a toaster or a vacuum cleaner. Costs assume $0.10 per kWh. 

 Appliance Power
 Wire AWG
 of Circuit
 of loss
 Toaster 1366 w 12 gauge 2.5 v 29.2 15 min 0.38 3.7¢
 Vacuum 986 w 14 gauge 5.1 v 45.7 30 min 1.19 12 ¢

The length and wire gauge of the house wiring itself also factors in. Notice the vacuum cleaner draws less power than the toaster but because it was plugged into a circuit with a thinner gauge of wire, higher voltage drop and more resistive power loss results.  In both cases, because of the low-frequency use of each appliance, the energy loss is trivial.  If you made toast or vacuumed for hours a day, these losses would quickly add up. 

Next Article: L2 Charging Inefficiencies