by Lorn Douglas – Contributor – October 26, 2019

Have you ever wondered about how much actual energy is needed to charge your battery? 

In driving electric vehicles (EV) for 8 years I have often wondered how much electricity it actually takes to fill the battery knowing there is some loss in the power conversion. It would take complicated expensive equipment, so I had no analytical way of figuring that out.

Recently, I got a new ‘smart’ AC charger (courtesy of the Smart Charge Hawaii Program). The charger includes a mobile app that gives me a lot of options and data. Every time I plug my EV in, it keeps track of the actual power consumed by the car’s internal charger. Finally, I have the data to see how efficient the conversion of power into the battery.

EV Charger 101

Let’s take a quick dive into some of the basics of electric vehicle charging…

All batteries store energy in Direct Current (DC) voltage. Level 1 and Level 2 electrical vehicle wall ‘chargers’, like you see in some parking lots, are not actually chargers at all. They just provide AC (alternating current) power. A Level 1 charging station delivers 110v AC (like your standard wall outlet) and charges the car at a very slow rate. Level 2 stations delivers 240v AC (like your electric dryer outlet) and quickly provides power to charge the battery. 

All EV’s have an actual charger built in; this device transforms the AC power provided into DC voltage which energizes the battery that propels the car. The amount of distance one can drive on a fully charged electric car can be anywhere up to 400 miles, depending on the car. My Kia Niro has a battery that stores 64kWh (kilowatt hours) of power and gets over 4 miles of range for each kWh, and that translates roughly into 300 miles of range.

Tracking energy used to charge an EV

In newer EVs, there are more data metrics available, along with related charging features, e.g., maximum charge and charging timers. In our Kia Niro, we can specifically set the percentage of charge. As a strategy to conserve battery life, I usually only fill the battery to 80% and try not to completely discharge it either (the optimal is to stay between 20% and 80%). The capacity of the Niro battery is 64kWh, therefore each 10% represents 6.4kWh. Even with this, however, I had no way of measuring the actual amount of power consumed by the internal charger during a fill-up, until now. With my new enelX Smart Charger, I’m able to track the amount of energy that goes into the car. This has allowed me to better understand the efficiency (or inefficiency) associated with charging. Here’s how I figured it out.

When I am going to take a long ride to the other side of the island I fully charge the battery and I’ve noticed the last bit of charge seems to take longer and have guessed it’s due to a reduced efficiency (the ratio of the power used to charge the car compared to the power stored).

Efficiency =   (Power used to charge/Power stored in battery) x 100 (%)

I charged the car from 18% power to 80% and this translates to 39.68kWh stored (62% x 64kW). However, the wall charger reported using 44.6kWh. Efficiency using the above equation it came out to 88.9% (44.6/39.68). Afterwards I charged from 80 to 100% and the efficiency dropped to 80% – it is more cost effective to charge to 80%!

The efficiency of charging is well documented. EV manufacturers post the efficiency figures for their cars. Additionally, there are environmental considerations – ambient temperature, concurrent car functions like A/C, even the level of charge (Level 1 vs Level 2) – impact efficiency. Knowing how these factors impact charging efficiency can help you maximize your energy utilization.

What it means to the pocketbook…

If I were to fully charge my 64kHr Niro at HELCO rates, at the above calculated efficiency, it would use 71kWh and cost $24.14. My range is 300 miles. This works out to $0.08/mi. A gas car getting 33mpg and paying $4/gal costs $0.12/mi or 50% more than an EV. Figuring in close to zero maintenance costs for an EV power train and comparing that to oil changes, filters, tune ups, timing belts, filters etc., one can easily realize how much cheaper it is enjoying an EV. 

For me the above costs are hypothetical because I power my cars with my solar PV system and the incremental cost per mile is basically zero. My wife and I have accumulated over 100,000 gas free miles with negligible maintenance costs.

Related information:

• https://www.veic.org/docs/Transportation/20130320-EVT-NRA-Final-Report.pdf
• https://www.sciencedirect.com/science/article/pii/S0360544217303730

Lorn Douglas is BIEVA advisor for Puna. He is an early adaptor and strong advocate for electric vehicles. In 2011, while on the waiting list for a Tesla, he purchased a Nissan Leaf. Many people in East Hawaii has followed his lead. Currently he owns a new Kia Niro and an original Tesla Model S.