maximum charging rate for 120V?

[jhu321]

just adding 1 more datapoint to the discussion -


i'm on an eMotorwerks juicebox and its registering 14.1amps. my understanding is the juicebox sends in a 15Amp signal so the car is opting to pull 14.1amps... its still higher than the 12amps that i was seeing with the OUC. I also see the improved charge times which would be consistent with the 14Amp current.

 

[alohart]

i'm on an eMotorwerks juicebox and its registering 14.1amps. my understanding is the juicebox sends in a 15Amp signal so the car is opting to pull 14.1amps... its still higher than the 12amps that i was seeing with the OUC. I also see the improved charge times which would be consistent with the 14Amp current.

Don't put too much faith in your JuiceBox's current measurement. I suspect that your JuiceBox is supplying 15 amps to your i3 when your JuiceBox's charging current is set to 15 amps. My JuiceBox always indicates that it is delivering 1-2 amps less than the current setting that I've specified. However, when a Tesla Model S, which displays the charging current on its LCD, was being charged by my JuiceBox, it showed a charging current that equaled the set current. So I'm pretty certain that the JuiceBox's current measurement circuit isn't very accurate or needs calibration. I have added a correction factor to the current measurement logic in the firmware of my JuiceBox so that the charging current that it displays matches the set current as long as the car is able to accept the set current.

 

[jadnashuanh]

FWIW, without a calibrated meter to check, neither could be correct! The car may be indicating the pilot signal's value, and not how much current it is pulling a the moment.

 

[jhu321]

FWIW, without a calibrated meter to check, neither could be correct! The car may be indicating the pilot signal's value, and not how much current it is pulling a the moment.

you're right... i did a % gain / time calculation where the % is calculated against the 19kwh theoretical 100% and i come up with about 1.4kw rate which would be 120V @ 12amp...

 

[alohart]

i did a % gain / time calculation where the % is calculated against the 19kwh theoretical 100% and i come up with about 1.4kw rate which would be 120V @ 12amp...

I think you need to consider the fact that charging isn't a 100% efficient process. If the efficiency is 85% (might even be lower for 120 v. charging), 14 amps would need to be input for 12 amps to actually charge the battery pack which is what you are measuring (12 / 0.85 = 14). The remainder is being lost to heat due to inefficiencies in the charging circuit.

 

[ritzo]

I think the "end of charge slow down" might not be as pronounced with L1 charging. Looking at my chargepoint graph at L2, I go from steady 5.1kW (not >7kW, but that's another thread) until 55 minutes prior to charging being done. BUT: it spends 25 mins of this ramp down STILL ABOVE 1.4kW. So, very roughly, the L2 spends an hour charging at about half (on average-- falling steadily) nominal rate. So you need to add 30 mins to the "simplistic" calculated charge time (what "should" take 30 mins, takes an half because at half rate).

But, for Level 1, it only spends 30 mins in the "top off" mode, so charge estimate should be extended by only 15 mins-- which is basically a rounding error when the overall time is so long and other variables are at work (line voltage not exactly 120, ambient temp changes will drive cooling needs and overall efficiency, etc.).

Finally, the 81% number is complicated number-- not some known constant. I think

overall efficiency is positively correlated with charge rate. Some of the electric "tax" on charging is relatively fixed in nature (or fixed + variable) like battery cooling / fans and the charging electronics being "on." So the length of charging itself extends the length of the charging (if you follow me). So basically, I think overall efficiency is < 81% at 8A, which likely explains your seemingly too long charging window.

Chrisn,
Great write up.
Everything I wanted to know and stuff I didn't know enough to want to know yet.