Prius Plug-in Hybrid - Part Four - Home Charging

Tuesday 23rd August, 2011
Whilst driving home for the evening, I'd been thinking about where to plug the Toyota Prius Plug-in Hybrid in. I had numerous cables to choose from but the long, straight, black one was the natural choice - at 7 metres long I hoped it would reach from the car to the plug socket at the back of the garage.

This scenario isn't entirely typical of EV ownership, more like visiting somebody else whilst you're driving oned. Long term owners often choose to have a permanent outdoor socket fitted for the purpose in a handy location. With a cost of around £1000 for a future proof fast charger, however, you may need to drive over 10,000 miles on electric power before you get a true return on that investment.
Opening the garage door, first I found my old faithful tool - a plug in 13 Amp power meter. This measures the instantaneous voltage and current going through a power socket and also totals the power consumption, in kWh, over time. This will be perfect to see how much charge we get into the batteries of the Prius Plug-in Hybrid.

I unwrapped the cable and plugged the charging "gun" into the socket in the front left of the car and then walked across the garage towards the mains socket in the far corner. At this point you realise that 7 metres isn't as far as you think and, with the instructions explicitly stating not to use an extension cable, it was time for another plan.

I looked around on the off-chance I had a closer power socket I had forgotten about and I realised there was actually one on the ceiling about half-way along the garage. But something was already plugged into it and, quite annoyingly, it was the electric garage door opener. If I pulled it out, I wouldn't be able to close the garage door. Let's try again.
Looking back from the socket towards the car on the driveway, I realised I was actually several feet short of the garage door. I could pull up closer and reduce the distance between them. I pulled the charging "gun" out of the car and, this time, plugged into the mains socket first. The LED came on and a small current started to flow - around 2W before even connecting the car.
These chargers aren't just straight mains adapters but rather complex devices full of intelligent charging controllers, communications (so the car can tell the charger it is full) and safety switches - you don't really want the full current to flow in the charging connector until after you plug it in, not when it's dropped in a puddle on the floor!
At this point I realised that, with all the messing around, I could have filled up with petrol by now and it also wouldn't be raining under the petrol station canopy. Instead, the charging gun is lying in the rain as I figure out what to do; watching the bricks on the driveway change colour as they get wet. I decide the only option is to close the garage door on the cable, hoping the thick wire isn't too big for the gap under the door, then go through the side door of the garage, through the gate at the side of the house and back to the car to pull it up close to the garage door.
The car ended up this close to the garage which, in an unfamiliar car with no parking sensors is quite an effort. In fact, whilst jumping out the car and back in to check the gap, I only actually hit the garage door once - you can really hear the creaking noise when there is no engine sound. But would the plug fit?
Sadly not, I was just inches short but I simply couldn't move any closer to the garage. At this point I realised I had to employ the old theory of Pythagoras - I was simply wasting cable by running it corner to corner across the garage. By moving the car a few feet to the right, it would go a more direct route and give me the extra slack I needed. I manoeuvred the car into the planned position and then tried again but...the cable was trapped under the door and I couldn't pull it across. Which meant opening the door...which required I pull the car away again...grrr.
You can imagine my relief when finally the plug went in and, in the dry refuge of the garage, I see the current beginning to flow. After a few seconds and a couple of relay-switch clicks in the charging unit, the power draw shot up to the reading you see above. That's about 2.4kW, the same power that a modern kettle might consume when boiling water for a cup of tea - except this won't just be on for a few minutes during the adverts of Coronation Street, it will be a little while yet.
A quick glance at the wireless smart meter on the mantelpiece confirms that we're burning a fair amount of electricity. Our usual 5-6pence per minute has shot up to 23 pence. You'll see our normal daily average of £1.46 in the screen-shot which, assuming we're charging for 2 hours at this rate, could add around 30% to our electricity bills.

However, it doesn't use quite as much electricity as you might predict. Whilst charging from empty starts at around 2.4kW and it sits there for a while at that rate, after a while the charging stops and re-starts after a couple of minutes. Then stops and restarts again after a few mins. This cycle repeats and, in each phase, the power draw is a little less. Towards the end of the charging cycle, just 600W of electricity is trickling in. So, in the 2 hours in which we could technically supply almost 5kWh of electricity, the Prius actually draws just 3.3kWh. This varies slightly, depending how flat the batteries are, but 3.3kWh appears to be the amount required to fully charge from empty to full.

That means, using a standard unit cost of 10 pence per kWh that we've used elsewhere on the blog, a cost of 33 pence to charge from empty - 2.75p/mile based on an EV range of 12 miles. I'll analyse these numbers, and the resulting grid CO2 emissions, in a later write-up when I've had the chance to get more of a feel for the car and how it performs in every day usage using both EV and petrol hybrid modes.

Prius Plug-in Hybrid - Part Five - How Does it Drive?

  1. 1) James B-T Said: (15/09/2011 13:53:48 GMT) Gravatar Image
    Prius Plug-in Hybrid - Part Four - Home Charging

    Ben, your drive is a disgrace. Sort it out!

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