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I wanted to get an idea of the relationship between charging power and charging time. In my head I have this idea that charging on a granny cable (2.4kW) is less efficient than charging on a type-2 (7.6kW). That is, not only is it slower, but it costs you more as well.

I had no proof of this though, so I thought I'd conduct a little experiment to see if I was right. My chargepoint has a feature where I can control the maximum current in 1A increments. So my experiment was to start charging at 10A, which is the same as a granny cable, and then increase the current by 5A each hour. I'd then be able to look at the percentage gain each hour and see if (for example) it charged twice as fast at 20A than 10A. If it was more than twice as fast, then it's more efficient to charge faster.

This is the graph I got:

The green line is the charging current. The steps up are when I changed it. I did the last change after 30 mins because it was late and I wanted to go to bed.

We start off at 10A, and get a charge rate of ~6% per hour. Then we kick it up to 15A and.... no change . The car keeps climbing at 6% per hour even though it's getting 50% more power. You'd expect about 9% per hour. We'll come back to that.

Then we kick it up to 20A (I was 15 minutes late), and to start with there no change. The car then seems to make a correction to it's reported figure and jumps up 8%, and then continues to rise at ~13% per hour. 25A, no change again. 30A, and after a while we get another correction and it finishes off the charge at 11% an hour.

It looks like the car measures the power it's getting at the start of the charge cycle (after a little bit of stabilisation). It then predicts how long until full, including it's ramp down, says "I need 10 hours to charge this 56% remaining, so I'll report 1% increase every (600 minutes / 56% = ~11 minutes)". I then come along and increase the power, but it doesn't recalculate its "progress bar". Not for a while anyway. I think this shows that during charging the % numbers are more about giving the user an idea about how far charging has progressed, not what the state of the battery is. They're kind of "made up" numbers.

I drew a curve on top of the graph, based on what I would expect if the 10A - 6% an hour had scaled, and become 15A - 9%, 20A - 12%, 25A - 15%, 30A - 18%.

I'm not sure you can read much into it because the % numbers aren't reliable. The only thing is that each time the car corrects itself, it's over the expected curve, which suggests higher currents are a bit more efficient, but it's flimsy.

Anyway... I though some people might be interested. I'll probably conduct a few more experiments, so if anyone has anything they want me to try, let me know. Also, if anyone has managed to get OBD data out of the car I'd be interested. Maybe there's a better way of monitoring things.