# Forums

I need some feedback on my assumptions here. If your are charging at 240 V 40 A for an hour, then your are using about 10 kW for the hour or 10 kWhr. This is what you get charged for by the electric utility regardless of how much is stored in the battery. The MS will slow the battery charge rate when the battery is getting full, but looking at the charging page, it still draws the 10 kW. So if your driving averages say, 100 miles per day, doesn't it make more sense to start charging when the rated miles are low, e.g. 20-40 miles and stop at 120-140 miles. This would be a cheaper charge than to charge it to 240 every night. Right or wrong or somewhere in between?

Musterion | February 24, 2013

David,

I'm a bit confused by your question because if the charging rate goes down, this corresponds to a smaller current and hence power <10 kW. In any case the conclusion is wrong. To 1st order the amount of energy you want to replace in the battery is what you will be charged for by the electric company (e.g. 85 kWh for full charge from empty). To 2nd order there is additional overhead discussed extensively in other threads. These energy losses like inverter efficiency <100% and ohmic losses in the power lines do not transfer energy to the battery but you pay for them too. In the case of large ohmic losses from Joule heating of your wiring, this is where charging slower could help you save some money (since the energy dissipated as heat goes as the square of the current). But none of this has to do with the SOC of the battery. Fire back if I misinterpreted your question.

nickjhowe | February 24, 2013

@David - you've misinterpreted something. The S will vary the current it draws as it charges the battery. You can change the maximum current the car will draw, but the total KWh are dependent on how much the car needs, and how long you leave it plugged in.

As @Musterion says, there are some losses that mean if the car needs, say, 40KWh it will draw closer to 45 or 50 from your house supply.

From everything we've seen the charging efficiency doesn't change whether the car is 10% charged or 90% charged, so to answer you question - it doesn't matter.

jat | February 24, 2013

Yes, the current drops off as the car starts accepting less so you aren't wasting energy just because the battery is full.

What I have measured so far, which is still just an estimate because I don't have enough data to correct for all the variables, is that charging at 40A is about 86% efficient overall (for comparison, I see about 87% on the LEAF @ 20A).

All else being equal, higher current has higher losses due to internal resistance in the battery, inverter, etc which are proportional to the square of the current -- however, even at 40A those losses are pretty low since those resistances are measured in milli-ohms (significantly less than just 1 person's body heat).

David Trushin | February 24, 2013

I admit to not really understanding this process, but it seemed to me that since the car said it was drawing 240 V at 40 A in the last hour of charging, then most of that was dissipating and not going into the battery. What I'm hearing is that the last hour of charging is really not at 240 V 40 A, but is less according to what the battery is drawing. That makes sense and so I withdraw my question. Thanks.

jat | February 24, 2013

@David Trushin - with standard charge, it doesn't drop off if you are charging at 40A. It does slow down if you are on a Supercharger, and I presume it might with HPWC but I can't tell since I don't have mine yet and nobody has posted details. It definitely drops off when doing a max-range charge, to the point that going from 91% to 100% takes 1.5 hours at 240VAC -- the last time I checked before it cut off, it showed 9A. So, the car definitely slows down how much current it is drawing and isn't wasting anything over just normal losses.