EV charging - More efficient at higher (48) or lower (32) amperage with L2?

[DougSpindler]

Please help me settle this debate - Is it more efficient to charge a Rivain or Tesla at a higher 48a or lower 323a with a L2 charger?  Is charging at a lower amperage easier on the batteries?

I have a Rivian and when I charge at 48 amp my garage get very warm from the EV charger and the charging cable is warm to the touch.  While charing I hear what I think is cooling pump motor running in the vehicle which I am guessing is used to cool the batteries while charging.

I am of the opinion the heat loss and having to cool the batteries is less efficient due to all of that wasted heat energy.  And charging at a lower amperage is much more efficient as there is less heat loss even though it might take a bit longer to charge.  I’m also thinking charing at the lower amperage is easier on the batteries and will prolong there life….  But it’s probably just a little.

My opposition is saying charging at a lower amperage is 75-80% efficient where charging at 48a is 94% efficient.  Opposition is also claiming that excess heat that’s warming the garage and pump cooling the batteries is negligible and can be ignored.  As for prolonging the life of the batteries the higher current doesn't make a difference.

Thoughts?


 

[The Soulman]

What does the first law of thermodynamics tell you?

[DougSpindler]

You tell me.  What does it say about the efficiency and battery life?

[ConKbot]

Treez tier question.
7.5 to 10kw is tiny compared to the battery capacity, so your battery charge efficiency is probably steady between the two. How are the chargers in the vehicle cooled? Battery coolant circuit?
Also given the battery efficiency remaining steady, 50 percent more power in results in 50 percent more out. Do you really even care about efficiency? Or thermal loading and peak temperature in the battery?
Does anyone have any sources for any of these efficiency numbers? 7.5kw in at 75 percent efficiency seems unlikely in both terms of regulations, and feasibility of dumping almost 2kw out of the charger.

[DougSpindler]

Here’s the thing.  If I charge at 48 amps the EVSE gets very warm and heats my garage to 80 degrees if not more.  If I charge at 30a there is no temperature  difference.  That wasted heat energy has to be reducing efficiency.  Which is why I’m asking.

[PlainName]

Move the EVSE to your living room and run it at 48A in winter, 30A in summer?

[The Soulman]

You tell me.  What does it say about the efficiency and battery life?

Apparently charging at 48A produces a lot of heat, this is energy coming from your outlet and not getting into the battery, poor efficiency.

Battery life I'd say lower current = better battery life, read the user manual on your vehicle for guidance as this might be dependent on temperature and charge state.

[The Soulman]

Here’s the thing.  If I charge at 48 amps the EVSE gets very warm and heats my garage to 80 degrees if not more.  If I charge at 30a there is no temperature  difference.  That wasted heat energy has to be reducing efficiency.  Which is why I’m asking.

Correct.

[mikeselectricstuff]

Considering the EVSE is basically just a relay, if it's getting that hot then something's wrong and a potential fire risk.

[ejeffrey]

The battery don't care, 48 amps is nothing compared to what it handles under fast DC charging or regenerative braking.  Granted frequent fast DC charging is a concern for battery life but the difference between 32 and 48 amps is negligible.

Your I^2 losses will grow to be a larger proportion of the total.

If your vehicle will tell you the sensed AC voltage you can see how much it drops under load.  That would let you estimate the total ohmic losses.

[Lstkartagi]

>> At L2, no difference for battery health. 
As noted in post above, L2 is nothing in battery wear compared to DC fast charge.

I doubt it is an issue with Rivian, but for the Hyundai IONIQ 5 which is also rated to 48A L2, there have been a lot of failures of the ICCU module which includes the AC L2 charger, plus some IONIQ 5s had related issues such as hot charging ports on the IONIQ 5.  While I always charged my Mach E at 48A without thinking twice, I backed off to 32A (chargepoint flex lets you set the charge current by the analog signal communication with the EV) on the I5 trying to go easy on the charger.

So, some of what you hear in cooling might rather be the AC L2 to HV charge module in the Rivian being cooled.  But, if it's engineered well, there would be no concern for longevity of the charger module.

>>the EV charger and the charging cable is warm to the touch
Assuming no bad connections or L2 EVSE defects, this is normal.  EVSE always have the cable to the EV sized for use in air.  My first Clipper Creek L2 EVSE at 16A to my most recent chargepoints (32A and 48A) are like that.  Some even say that you must uncoil the charge cord for use to get proper cooling.

>>My opposition is saying charging at a lower amperage is 75-80% efficient where charging at 48a is 94% efficient
Seems like an alleged "fact" that maybe Rivian could confirm.  Otherwise, something you can try to measure.  This is a very common question / topic (L2 at 32A, 40A, or 48A, Is one better?) at almost all BEV forums.  So you could learn a lot from other forums such as the Mach E forum (a lot of electrical engineers and other high tech types hang out there) or one of the Rivian forums (google shows several threads on L2 charging).

Depending on your skills and available equipment (or, stuff you might buy for the job), you can try to answer some of these questions yourself.  There are many ways to measure the energy from your home electrical system to the charge station (the EVSE, the L2 charger is in the BEV).  Many charge stations report power consumed, but are often kind of course both in time and resolution.  There are any number of energy meters you can connect near your charge station that give very accurate data.  For example, a SENSE energy monitor (while pricey) can be mounted with the current pickup coils only on the two 240V "hot" wires to the charger.  With only the EVSE as the load, the SENSE power graph is relatively high resolution in both time and power level.

On the Rivian side, you can measure the energy change in kW-hrs gained.  However, efficiency is not a trivial measurement, at least because while most of the energy is charging the Rivian HV battery, some is running the low voltage systems including those pumps, computers, telematics, BT, Wifi and the like, etc.  Also, some energy loss (probably very little) is warming the cable from the EVSE to Rivian.  Again, also as mentioned above, depending on how comfortable you are with measurement equipment, you could measure the L2 voltage into the EVSE, the L2 voltage at the EVSE out into the charge cable, and then the L2 voltage at the Rivian inlet to see the various losses due to resistance (ohmmic loss).  It is probably not worth the effort unless you are into it as a hobby electronics academic project.

I was horrified in 2014 when I realized my first PHEV Chevy Volt, Clipper Creek 16A 240V EVSE charge cable was getting warm.  But, after working numbers in a spread sheet, I realized that the loss was trivial, both in energy and cost, in fact over years, less than would have been the cost of increasing the size cable to reduce the loss.

[DougSpindler]

Thank you very much for the reply.  Much appreciated.

[NiHaoMike]

Since the efficiency of a PFC boost converter is lower at lower input voltage, in theory an EV charger could offer a mode that gates the boost converter on only at the high parts of the AC sine wave, boosting efficiency at the cost of slower charging and lower power factor. (Some PC power supplies do that at low load.)

[Siwastaja]

Battery losses are I^2*R and thus efficiency drops linearly with increased current, BUT, AC charging is so slow anyway that it does not matter. Basically the battery efficiency is like 99%, and if it drops to 98%, no big deal.

Fast charging increases battery wear, ESPECIALLY near full state-of-charge, and ESPECIALLY at low temperatures. But again, AC charging is so slow anyway it does not matter.

Now the internal charger will definitely heat up more at higher charge rate but this isn't just because the efficiency is dropping; even if the efficiency is constant, higher power means more heat, but for shorter time.

It is likely the internal charger is less efficient than the battery pack, and slower charge, within reason, could improve the efficiency by a few percentage points. 94% is fair assumption, 75-80% is not, the difference would be much smaller. Whenever not in hurry, it would be a good idea to charge at a bit less than full power. Don't go excessively low - that would again decrease the efficiency of the charger (and possibly worsen the power factor). Generic experience says that optimum efficiency of power supplies / chargers would be around 50%-70% of the full rated power. If that 48A is the maximum supported by Tesla's onboard charger (I have no idea), then "usually" charging at 32A doesn't sound like a bad idea, but any gain in efficiency and lifetime would be nearly negligible.

[tom66]

The answer is it depends on the model.

Most electric cars use around 150-300W to run the charging systems. This provides power for the BMS, coolant pumps, contactors, computers and whatever else needs to run to support the charging.  If it's very hot outside, they might need to provide additional fan or air-conditioning cooling circuits.  If it's very cold, they might need to run a battery heater.

In general, these losses get less significant as the battery charge rate increases.  However, you then have the secondary effect of charge system heating requiring additional cooling.  So it's going to very much depend on the vehicle's designed, and the environment in which the car is charged (a garage is probably more efficient than outdoors).

Whilst I^2 * R losses are a factor, most batteries have very low losses here. Charging efficiency should be in excess of 99% at the currents we are talking of. The biggest loss by far is the charger loss itself, the conversion from AC power to DC.  In general, these chargers get more efficient with higher load, up till around their maximum rated load.

[Siwastaja]

Most electric cars use around 150-300W to run the charging systems. This provides power for the BMS, coolant pumps, contactors, computers and whatever else needs to run to support the charging.  If it's very hot outside, they might need to provide additional fan or air-conditioning cooling circuits.  If it's very cold, they might need to run a battery heater.

Yeah. You can, possibly significantly, reduce the energy consumption caused by this constant load part by simply stopping charging a bit earlier, like at 98% SoC, to avoid ramp-down of charging current in CV phase/balancing. But every car is different. It seems my 62kWh Nissan Leaf takes around two hours to finish from the start of current ramp down, and while I'm sure it doesn't waste the higher end (300W) of your numbers while in CV/balancing, it's still surely tens of watts which do not end up in the battery pack, if from nothing else, but from fact that the charger power electronics are likely not designed to be efficient over such wide power range. OTOH, Nissan Leaf is quite simplistic in design, does not use coolant pumps, and any "computers" that run are not anywhere near the numbers Tesla probably still uses (although I guess the days of always-on 200W phantom consumption caused by six general purpose computers running Ubuntu are long over).

[tom66]

Most electric cars use around 150-300W to run the charging systems. This provides power for the BMS, coolant pumps, contactors, computers and whatever else needs to run to support the charging.  If it's very hot outside, they might need to provide additional fan or air-conditioning cooling circuits.  If it's very cold, they might need to run a battery heater.

Yeah. You can, possibly significantly, reduce the energy consumption caused by this constant load part by simply stopping charging a bit earlier, like at 98% SoC, to avoid ramp-down of charging current in CV phase/balancing. But every car is different. It seems my 62kWh Nissan Leaf takes around two hours to finish from the start of current ramp down, and while I'm sure it doesn't waste the higher end (300W) of your numbers while in CV/balancing, it's still surely tens of watts which do not end up in the battery pack, if from nothing else, but from fact that the charger power electronics are likely not designed to be efficient over such wide power range. OTOH, Nissan Leaf is quite simplistic in design, does not use coolant pumps, and any "computers" that run are not anywhere near the numbers Tesla probably still uses (although I guess the days of always-on 200W phantom consumption caused by six general purpose computers running Ubuntu are long over).

You would be surprised at how much does get wasted. I've checked my ID.3 and when in charge mode, the DC-DC current measured at around 10 amps.  That's at least 130 watts.  At 7200 watts of charging power, that's around 2% loss in support systems.  (The DC-DC isn't 100% efficient either, but this is probably negligible extra loss.) 

Balancing depends a lot on the car too.  Hyundai vehicles tend to balance around 60-70% SoC, and will continue that process throughout charging.  Not sure about my ID.3, but it's usually linear in charge rate - adding 1% takes just as long at 10% as it does at 95%.  It charges at the full 7kW up until the battery is full. Perhaps this is because the battery is not actually full at 100% SoC, but around 90-95%, as regen is still possible at around 30kW when the battery is at 100%.

One of the issues with EVs built by legacy automakers is that they are still built a bit like petrol cars.  With a petrol car there hasn't been a particular obsession over 12V losses because whilst it does increase fuel usage it was a small line item in overall losses.  For instance my old Golf GTE kept the electric power steering controller energised while charging... Why?  Because it is on the same bus as the ECU which talked to the BMS and charger, and it was presumably simpler to do that.  Little decisions like that have a big impact.