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Following on from a post I made in the "i3 slow charging on home 7kW" thread I decided to run a little experiment.

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:

127411


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 :oops:. 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%.

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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.
 

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I love this as it was something I was considering. My question is it more efficient to charge over the weekend at 1.4 kw or do a fast charge in the early hours of Monday morning at 7kw? Which uses the least Kw and thus saves me money?

Does this analysis answer my question ?
 

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Interesting. Seems to me that if 30Amp charging was as efficient as 10 Amp charging, you would expect it to acquire 3 times the charge. From 22.05 to 22.35 on 10 Amps, I'd say there was an increase of 4%. From 1.50 to 2.20 on 30 Amps, charge increases by 10%, not the 12% you might expect.
 

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I'm assuming you used the app to track this? I'm afraid that won't work. You need to read it from the car and then you'll get actual results.

To save bandwidth the car uploads the charging speed at the beginning of a session. This is then tracked against an expected charge curve and reported in the app. The car only goes online and refreshes if it's estimate becomes more than 10% or so out of sync with reality. Or if something else changes like the car is unlocked. The updated time in the app means nothing when it's charging, that's the last time it updated the estimate from the server.

The dash estimate should be accurate.
 

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The 94Ah i3 has 3 internal independent 16Amp chargers. Each one can only do a few throttle down rates. I think 6A is the minimum for any single charger. There is
a 10A setting. 16A is the maxiumum.


For example, 20A is probably running two chargers at 10A each.

As a rule, chargers are most efficient at full power. 20A (2x10) should not be as efficient as 16A where a single charger is running at 100%.
 

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These experiments are misleading at best:

A proper trial would be to charge from a fixed state of charge, say 15%, to say 80%, at reduced amerage and then repeat again at full charge amps. Measure the input real energy with an Emlite meter (very cheap and readily available).
 

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How is your charger controlling the current? I'm trying to make my dumbish Rolec charger smarter. If I cut the power to the contactor the car seems to go to sleep after a period of time, and I can't get it to wake up.
 

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Discussion Starter #9
Thanks for the feedback. I've had a few days away from the site, so I'll answer a few things all in one go.

My question is it more efficient to charge over the weekend at 1.4 kw or do a fast charge in the early hours of Monday morning at 7kw? Which uses the least Kw and thus saves me money?

Does this analysis answer my question ?
That was the question I was trying to answer. I'd say no it doesn't answer that, at least not conclusively. It maybe points to it being more efficient to be faster, but it is difficult to say with the other effects clouding things.

To save bandwidth the car uploads the charging speed at the beginning of a session. This is then tracked against an expected charge curve and reported in the app. The car only goes online and refreshes if it's estimate becomes more than 10% or so out of sync with reality.
That would explain the effect I'm seeing. I'd certainly seen a delay between the car status updating and the app changing, but I thought it was just limiting the updates to 1% points. Something is obviously following a pre-calculated curve, and I can quite believe that's the server.

I'm getting the data from the ConnectedDrive server using a script, so not using the app directly, but it's the same data source. So without living in the car taking notes, do you know of a way of getting the car data? That's why I was asking about OBD snooping.

These experiments are misleading at best:

A proper trial would be to charge from a fixed state of charge, say 15%, to say 80%, at reduced amerage and then repeat again at full charge amps. Measure the input real energy with an Emlite meter (very cheap and readily available).
My chargepoint already has the voltage sensor and current clamp inside which is the equivilent to that meter. That's how I've got the current readings (They're often 14.x A rather than 15 for example.)

Doing lots of runs from a fixed state of charge would be the foolproof way of doing it, I agree. That's a lot of time though, and I was trying to take a shortcut. It revealed this other effect which I found interesting.

The 94Ah i3 has 3 internal independent 16Amp chargers. Each one can only do a few throttle down rates. I think 6A is the minimum for any single charger. There is
a 10A setting. 16A is the maxiumum.
The car seemed to follow what was being asked of it pretty well. The green line is actual measured current flow, not just the value I set. I don't think it was even more than 0.5A below the desired value until the ramp down at the end.

How is your charger controlling the current? I'm trying to make my dumbish Rolec charger smarter. If I cut the power to the contactor the car seems to go to sleep after a period of time, and I can't get it to wake up.
It controls the Control Pilot signal to say how much current is available to the car. It uses this mechanism for diverting solar power into the car, but it is also manually controllable from a web interface.
 
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