Charging rate inconsistency between ChargePoint and the Car

The difference may be due to inefficiencies in the in-car charger. One article I read reported perhaps 90% efficiency for level 2 chargers, and your numbers show 87%.

 

Don’t know much about this. Here is a somewhat dated reference which provides a bit of info about AC charging. https://ieeexplore.ieee.org/document/7046253

 

I assume the "Chargepoint" you are referring to is an AC EVSE? If so, typical AC charging efficiency for most EVs is in the 70-85% range. DC fast charging efficiency can be as high as 95+%. Most of the losses are from the AC->DC conversion. My Niro EV measured around 85% last time I checked. There will be slight differences in efficiency depending on charging speed - 120V vs 240V and the maximum current used. Using 120V with low amps is typically the least efficient because it takes the longest amount of time and you're wasting electricity running the car's electronics (BMS, cooling, etc.) the entire time it's charging. Also, most people forget that home wiring also loses some energy due to resistance which will not be reported by the EVSE and can only be seen on your main home power meter. At the end of the day, the power meter is what counts since that is what determines your electricity bill.

I always cringe whenever I read an article or watch a video that calculates EV cost of ownership with bogus electricity costs based solely on the miles/kWh reported by the car's trip computer - completely ignoring charging losses. Luckily the EPA MPGe ratings do include charging losses from the car's onboard AC charger.

 

I recently read an article, and i would linkbit here if I could vaguely remember where I read it, that the Niro EV is most efficient charging at its maximum possible capacity on AC- this is 7.6kw (EDIT - just checked the spec sheet - says 7.2 KW, but I'm virtually positive I've seen 7.6 KW at a public charger) so basically 32 amps at 240 volts. The article reported that the efficiency in charging at this rate was virtually 90% and lost efficiently as you stepped away from that 7.6 kw mark. This is because of the fact that the AC charger and all systems are still working when charging below that 7.6 kw rate. Also, charging efficiency will be affected by the battery temperature. The lower that pack temp, the higher the internal resistance of the cells and therefore losses in charging due to heating of the pack. This is also seen in DC charging, but is also managed by the BMS charge control which will limit the number of amps delivered to the battery when it's cold and its internal resistance is higher. This is also evidenced by the fact that charging power will increase when DC charging as a cold pack begins to warm.

So a long answer to your question is that if you were AC charging below 7.6 kw you can expect lower efficiency simply because of the hardware in the car requiring more relative energy to run than at its maximum, and if the pack was cold or cooling due to cold outside temperatures, then this would further exacerbate this inefficiency.

Hope that was helpful, now to try and dig up that article...

 

@CanuckTom The Niro absolutely gets 7.6kW max charge speed. My JuiceBox 40 has a graph visualising the charging speeds over the charging duration. Since it's a 40 amp home charger, I regularly see 7.6kW.



I agree with everyone that the kWh difference was from inefficiency loss. Just the physics of 1) electricity passing through wires and 2) converting from AC-to-DC.

 

Two reasons:

1) Losses aren't necessarily higher, but the systems the car needs to utilize to charge use a fixed amount of energy, so therefore, lower overall efficiency due to not utilizing the maximum available input.

2) Line losses across the charging cable are there in any condition, but higher voltage and higher amps - therefore higher overall power (KW) makes for less losses across the charging cables. This is why setting up your home charging to meet the highest possible input your car will take, will provide the highest possible efficiency.

 

I suspect the DC-DC converter in the charger has lower efficiency when its input voltage is lower. I think line losses are probably insignificant, especially since we are talking about efficiency from the EVSE onward.

 

Is the ChargePoint level 2 or level 3? In other words AC-DC loss or no DC-DC? I was assuming this was DavidT's home level 2 charger.

 

The Chargepoint is level 2, but it is merely an EVSE, not a charger, the charger is in the Niro. Inside the in-car AC charger, there are 2 main parts to get to the voltage required to charge the battery - first, rectifiers to convert incoming AC to DC, and second a DC to DC converter to increase the DC voltage to that required of the batteries (356V).

 

The max charge rate I get with a 240v 32a AC EVSE at home is 7.5kW, with a warm pack. The max charge rate I have seen at a 50kW DC fast charger is 48kW, with a warm pack. I got the 32a EVSE because the literature suggested I would not get higher charging speed from a more expensive 49a EVSE...

 

87% charging efficiency would not at all be unusual. In fact, I've seen 85% cited for an average efficiency. I've seen claims for as high as 92% efficiency, but if my understanding is correct, the charging efficiency drops as the car approaches 100% charge, so that 92% efficiency would be maximum efficiency, not average charging efficiency.

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