Charging rate question

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blaxerB

Member
Joined
Oct 25, 2014
Messages
13
Hello all, almost a new i3 owner here, I am told my car is at the port. Anyway I have been reading as much as I can about the i3 and started to wonder about charging. I have a very short commute to work, about 20 miles total a day and don't really drive much other than that. I have seen that the i3 has settings for charging, how many amps to pull, time of pre-conditioing etc. Does anyone think it would be a good idea to dial back the amps since I will not have all that much charging to do? I am thinking in terms of "treating the batteries" and KLE as nicely as possible, would a slower charge be less strain on well.. everything? Or should I just use "full power" and let the software sort it all out?

TIA!
Blax...
 
The vehicle logic pampers the batteries so you don't need to. The limits in the menu, especially when using the supplied level 1 (in the USA) EVSE are to ensure that you don't pop a circuit breaker or fuse, not to protect the vehicle since there could be something else plugged into that circuit other than the car. Same thing could be true of the level 2 chargers, but often, those are on a dedicated circuit, so full works. Note, the car doesn't use the maximum rating all of the time - it tapers it off at the end, then stops, and, monitors the temperature of the batteries in the process, heating them or cooling them as needed to also help protect them.
 
blaxerB said:
Does anyone think it would be a good idea to dial back the amps since I will not have all that much charging to do? I am thinking in terms of "treating the batteries" and KLE as nicely as possible, would a slower charge be less strain on well.. everything? Or should I just use "full power" and let the software sort it all out?.
Surprisingly, folks with the resources to measure the draw from the wall during charging have found that the relatively fixed overhead of the EVSE monitoring the charging logic is quite inefficient, so charging at a higher rate generally results in significantly greater wall to wheel efficiency. This has been observed across nearly all EV platforms. In other words, charging at the maximum rate available when possible saves you money, and saves us all CO2 emissions.

Coddling the battery pack is not an issue at all when you consider that it is designed to provide 17 times the max charging rate to the motor for acceleration, and is capable of regenerating (accepting a charge of) 7 times that same charging rate during deceleration, and does both hundreds of times during a normal drive.

50 kW DC Fast Charging is a bit of different situation that you might want to avoid when you can. Thermal management of a 20 minute continuous 50 kW charge is a completely different challenge than a 10 second 50 kW deceleration.
 
Ultraturtle,
your state" charging at the maximum rate available when possible saves you money, and saves us all CO2 emissions", can you elaborate?
We had planned on installing a level 2 charger but we find that the very basic charger is enough for us. Given that our power is Hydro generated and thus don't involve carbon emissions, I am trying to get a handle on the exact fallout from not installing that level 2 charger. What do you think?
 
AviatorMan said:
Ultraturtle,
your state" charging at the maximum rate available when possible saves you money, and saves us all CO2 emissions", can you elaborate?
We had planned on installing a level 2 charger but we find that the very basic charger is enough for us. Given that our power is Hydro generated and thus don't involve carbon emissions, I am trying to get a handle on the exact fallout from not installing that level 2 charger. What do you think?

I think what is being said is that for low and high voltage AC charging, the power consumed by the car during charging (by ancillaries such as cooling fans and pumps, and by the on board charging units themselves, not the bit going into the battery) is constant i.e not proportional to charging rate. So charging quicker consumes less than slower charging, because the ancillaries are on for less time.

Bill
 
AviatorMan said:
Ultraturtle,
your state" charging at the maximum rate available when possible saves you money, and saves us all CO2 emissions", can you elaborate?
We had planned on installing a level 2 charger but we find that the very basic charger is enough for us. Given that our power is Hydro generated and thus don't involve carbon emissions, I am trying to get a handle on the exact fallout from not installing that level 2 charger. What do you think?
What Bill said.

240 Volt 30 amp charging takes place at five times the rate of 120 Volt 12 amp charging, so spends roughly 1/5 the time powering ancillary loads. EV owners on other forums have measured efficiency differences of several percent between L1 and L2 charging.

As for where your power comes from, we are talking about the marginal, not total case. That is, where the next electron is coming from to power an additional load you are considering, not the ones you already use regularly. Grid power consists of hydroelectric generation already running at full capacity, supplemented by natural gas, coal, nuclear and other sources. Unless you have your own micro hydro generator operating at excess capacity, your next electron could well be coming from natural gas or coal. Charging off peak has the potential to lessen the impact, but the additional draw of less efficient charging makes less clean energy available for other users, forcing them to draw from dirtier sources.

All that said, if L1 charging works for you, stick with it. It is only a few percentage points less efficient and doesn't matter in the grand scheme of things, as there is embedded energy and carbon costs in the manufacture and transport of that L2 EVSE.
 
ultraturtle said:
AviatorMan said:
Ultraturtle,
your state" charging at the maximum rate available when possible saves you money, and saves us all CO2 emissions", can you elaborate?
We had planned on installing a level 2 charger but we find that the very basic charger is enough for us. Given that our power is Hydro generated and thus don't involve carbon emissions, I am trying to get a handle on the exact fallout from not installing that level 2 charger. What do you think?
What Bill said.

240 Volt 30 amp charging takes place at five times the rate of 120 Volt 12 amp charging, so spends roughly 1/5 the time powering ancillary loads. EV owners on other forums have measured efficiency differences of several percent between L1 and L2 charging.

As for where your power comes from, we are talking about the marginal, not total case. That is, where the next electron is coming from to power an additional load you are considering, not the ones you already use regularly. Grid power consists of hydroelectric generation already running at full capacity, supplemented by natural gas, coal, nuclear and other sources. Unless you have your own micro hydro generator operating at excess capacity, your next electron could well be coming from natural gas or coal. Charging off peak has the potential to lessen the impact, but the additional draw of less efficient charging makes less clean energy available for other users, forcing them to draw from dirtier sources.

All that said, if L1 charging works for you, stick with it. It is only a few percentage points less efficient and doesn't matter in the grand scheme of things, as there is embedded energy and carbon costs in the manufacture and transport of that L2 EVSE.

Thanks for all of the information, very helpful.
Got off my lazy ass and did the google thing, on page 11 of:
An Assessment of Level 1 and Level 2 Electric Vehicle Charging Efficiency
found at:
tinyurl.com/lffl8zo

it says "Level 2 charging would likely yield approximately 2.1% efficiency gains." Extrapolating that to my situation the money cost is $5/year about.

My electricity fuel mix is:
Generation Type Percentage
Hydro 89.8%
Nuclear* 4.4%
Wind 3.9%
Coal* 0.8%
Other** 0.6%
Landfill Gases 0.5%
Total 100%

Looks like I'm sticking to Level 1.
 
Thank you everyone, it's really nice to have a place to ask questions and get helpful, pleasant, thoughtful responses with no "internet attitude" :)
 
Relating to speed of charge. I have three phase available in my house ( not currently used) and wonder whether a higher rate charger could be used than the standard 32kw BMW wallbox and if so whether this would be good or bad for the battery pack in the longterm?

As an aside I like to keep my cars for a long time and as the i3 doesn't rust or have ICE age related issues this could be a VERY long time, how easy/difficult is it to replace the battery pack?
 
The charging circuit in the i3 is limited to about that 32A acv input...having more available won't help. The internal circuits are setup to only allow a max of about 240vac two wire input...more within certain limits could damage the i3. The only way to get a faster charge cycle would be to use that higher voltage you have available to power a DC fast charger. The i3 can handle up to 50Kw on that port (about 400vdc) while the acv inlet is limited to 7.4Kw or so. But, a DC fast charger will cost an order of magnitude or so more than an acv EVSE. The one BMWUSA is promoting to its dealers is a 25Kw unit and has a suggested retail of around $10K, not counting installation. So, faster is possible, but not by having an EVSE bigger than 32A, at least in the current battery offering.
 
Jadnash thanks for the reply - more a technical question as I'm happy with the 3or4 hr charge time. But it's food for thought next time round and as ev's will have longer ranges with battery packs holding two or three times current charge.
 
The 2.1% efficiency gain seems low. Anecdotally, my L1 charger gets quite warm when charging as compared to my L2. Also, since most L2 chargers will have over-spec wiring doing a direct "home run" to the panel versus a random 110V outlet. Would be great if someone did meter-to-battery testing to pick up these potential extra pieces of loss.
 
The conversion from 120vac to 400vdc isn't as efficient as 240vac to 400vdc. Then, since things take longer to charge, it's possible, but not certain, that the vehicle may need more time with the battery cooling turned on, further decreasing the efficiency. The slower charging rate may not produce enough heat under some conditions to require it, whereas it's more likely to need it with the faster charging rate at 240vac.
 
jadnashuanh said:
The conversion from 120vac to 400vdc isn't as efficient as 240vac to 400vdc. Then, since things take longer to charge, it's possible, but not certain, that the vehicle may need more time with the battery cooling turned on, further decreasing the efficiency. The slower charging rate may not produce enough heat under some conditions to require it, whereas it's more likely to need it with the faster charging rate at 240vac.

jadnashuanh,
I posted the 2.1% and it seems low to me as well. EPA appears to assume the charger is 90% efficient when they calculate the wall-to-battery loss component baked into the eMPG ratings. They appear to be relying on a study ( TinyURL.com/mdwhjns ) that seems to say:
"The experimental result indicates 92.5% of the efficiency at input voltage 220Vac and 88.3% at input voltage 110Vac under 3.3kW of output power."
Which is 4.2%. If all that is true, my $5 money cost is now $10/year.

As you point out, battery cooling will be an energy loss that will factor, perhaps differently, into the charging rates. I also wonder if an i3 battery that is always slow charged at 120v will last a couple % longer or shorter than one charged at 240v. For me this all keeps coming back to staying with my 120v charger for now.

From the EPA-
Note: EV energy use estimated by ORNL (Oak Ridge National Labs) as follows:

Electric motor efficiency—including inverter and gear reduction losses—assumed to be 76.4%–80.2%, using estimates from Miller et. al. (SAE 2011-01-0887) and adjusting downward by 4% for parasitic losses.
Battery and battery charger efficiency are assumed to total 81% (roughly 90% each) based in part on estimates from published studies (Chae et. al., 2011; Gautam et. al., 2011).
 
The i3 can handle up to 7.4Kw input...the bigger the input, the more heat produced in the conversion process to DCV, and the more cooling that may be required, potentially skewing the efficiency ratings (that listed in the study was about half the size of the unit in the i3). From a practical point, to most people a $1-2 per month in increased electricity costs are probably not a big deal. But, from a network standpoint, every little bit helps.

I'm wiring in a volt/amp meter on the feed to my EVSE, mostly out of curiosity. I may get it in today - I modded the cover plate and have the module, a switch and fuse mounted, but haven't wired it up yet. While it has no recording capability, I thought it would be interesting to be able to view the instantaneous inputs to the EVSE during charging. The parts only cost me about $30 and my labor is free.
 
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