gt6k
Posts: 4
Joined: Sun Dec 06, 2020 2:18 pm

Re: Set a maximum charge limit

Although I think the 80% charging is the EV equivalent of a fad diet I will suggest a way to achieve it as it is the OPs right to do what he wants with his car.

The answer is that if you have a sufficiently smart charger such as the OHME one then you can set the charge level on that. It even allows you to set a charge % against individual schedule items.
eNate
Posts: 929
Joined: Mon Sep 09, 2019 5:33 pm

Re: Set a maximum charge limit

Another option is a spring wound mechanical timer. There are outlet box-mountable versions of these rated at 20A so would work at L1 charging. I installed one in my garage and connected it to the 120v receptacle I plug in to, but so rarely charge at home that I just don't use it. But given that at 16A I recoup 5% of my battery it's pretty simple to figure out how many hours I want to charge. The niftier thing about these timers is they have two switched terminals, one normally on and one normally off. So it can be wired to a split duplex receptacle so that (for instance) the top receptacle will remain OFF for 4-hours while the timer is counting down, the turn ON, but the bottom receptacle will remain ON for 4 hours while counting down then switch OFF.
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jadnashuanh
Posts: 5192
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Set a maximum charge limit

If the vehicle is fully charged, with the latest software, it should unlock the charging plug from the socket. If the vehicle is locked while it's charging, and with older software, you can't remove the plug unless you unlock the i3 first. But, normally, when you want, just press the release button and pull the plug out. Pressing the release button opens a switch, which causes the ACV to be shut off so you don't get a spark when you then break the connection by pulling out the plug.

The last 5-10% of charge tends to be much slower than the first 90%. If the batteries are super cold or hot, the first charge can slow until things are brought more into the optimum temperature. They can be heated or cooled, depending on what's needed.

Power = volts * amps. It's likely your level two EVSE can source more amps, but even if it couldn't, having twice the inlet voltage would mean twice as much power going into the i3 per time.
Jim DeBruycker
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)
Hacky
Posts: 4
Joined: Thu Jan 13, 2022 1:20 am

Re: Set a maximum charge limit

EvanstonI3 wrote: Sun Mar 28, 2021 6:38 am Not this again. This is not a Tesla. There is a Battery Management System that prevents us from "fully" charging or discharging the battery. About 10% of the capacity is held back as a buffer for this.
Dispite many other statements, the i3 does not hold back a buffer at the higher end of the cell voltage range. The buffer only prevents exhaustive discharge.

You can see, that it charges the cells up to 4.20 V when reading out data from the OBD2 connector. See also: https://www.goingelectric.de/forum/view ... 3#p1454473

I have seen this for the 94Ah and 120Ah models and I assume the same strategy for the 60Ah models.
alohart
Posts: 2576
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Location: Honolulu, HI

Re: Set a maximum charge limit

Hacky wrote: Thu Jan 13, 2022 1:36 amDispite many other statements, the i3 does not hold back a buffer at the higher end of the cell voltage range. The buffer only prevents exhaustive discharge.
You are basing your assertion on the output of the Electrified app that indicates a battery pack voltage of 402 V, and then you assume that this is the maximum possible voltage for a pack of 96 120 Ah cells. Pack voltage is temperature-dependent, so did you correct this voltage for the cell temperature?

In one of the screen shots in the forum that you referenced, the Electrified app displays a battery health of 99%. It also displays a charge level range from 12.0% to 96.3%. You state that no high charge level unusable buffer exists yet the Electrified app shows a 3.7% buffer. The fact that you believe the pack voltage but not the maximum allowed charge level of 96.3% reduces the credibility of your assertion.

Electric Vehicles and the BMW i3 (60, 94, & 120 Ah) by David Bricknell includes charts from testing labs that clearly show low and high charge level unusable buffers. His description of the battery management system clearly states that unusable buffers exist at both low and high charge levels. The difference between BMW's published gross and net battery pack capacity can't be explained by the existence of only a low charge level unusable buffer unless one assumes an unreasonably large low charge level unusable buffer. BMW's TIS Website describes the SME as controlling the charge level range of the battery pack without specifying buffer sizes which vary by battery cell capacity.

There's overwhelming evidence from credible sources disputing your assertion that no high charge level unusable buffer exists.
Aloha,
Art
[22-04-25 to now] 2019 BMW i3 Imperial Blue Metallic, Giga World, Tech + Driving Assist, Heat Pump, 428 Wheels
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SuperPoly
Posts: 39
Joined: Fri Oct 01, 2021 8:12 am

Re: Set a maximum charge limit

There is less regen braking above 90%, right? For that reason alone I would like to set a lower charge limit for my daily use. I use between 0-10% of the battery most days.
Arm
Posts: 225
Joined: Tue Jan 07, 2020 5:53 pm

Re: Set a maximum charge limit

You can limit the charge to 80% via the app but it'll have to be done manually. Rule of thumb...every 1% battery charge takes around 2.8 minutes on a 7.2 kWh charge rate for the 94aH pack.
Hacky
Posts: 4
Joined: Thu Jan 13, 2022 1:20 am

Re: Set a maximum charge limit

alohart wrote: Thu Jan 13, 2022 6:16 pm You are basing your assertion on the output of the Electrified app that indicates a battery pack voltage of 402 V, and then you assume that this is the maximum possible voltage for a pack of 96 120 Ah cells. Pack voltage is temperature-dependent, so did you correct this voltage for the cell temperature?
On the lower left corner of the screenshots you see the (max.) voltage on cell level and beside that with smaller numbers the range between lowest and highest cell voltage measurement across all cells. Both examples show 4.19V as highest cell voltage (cell levels ranging from 4.18V to 4.19V). So I do not talk about pack voltage here. The first example shows a battery temperature of 10.7 °C, so also nothing special that should be taken into account.

As you may know, most Lithium Ion cells have maximum charge voltage of 4.20 V as long as these are not special LiIon HV cells (which can be charged up to 4.35V). But those HV cells will not be found in normal automobiles due to lower cycle life times.

The 94Ah cells from Samsung SDI are specified for maximum charge voltage of 4.15V per cell and even those are charged up to 4.20 V on the i3.

So I see no indication for a buffer at the "top" end.
Arm wrote: Thu Jan 13, 2022 11:42 pm You can limit the charge to 80% via the app but it'll have to be done manually. Rule of thumb...every 1% battery charge takes around 2.8 minutes on a 7.2 kWh charge rate for the 94aH pack.
Where can you set a charge limit of 80%?

Do you have a screenshot?
Star63
Posts: 41
Joined: Mon Sep 10, 2018 12:05 pm
Location: Finland

Re: Set a maximum charge limit

On a Chech forum they wrote that the max. voltage of the 60 Ah battery is 392 V while the 120 Ah battery is 402 V. The 60 Ah i3 software didn't accept the higher voltage of the replacement battery (120 Ah) as such.

This would mean that the cell voltages are about 4.08 V vs. 4.19 V. ? And that the 120 Ah battery has a smaller buffer (if any)?
2014 BMW i3 REX => 2018 i3S 94 Ah REX
Hacky
Posts: 4
Joined: Thu Jan 13, 2022 1:20 am

Re: Set a maximum charge limit

Yes, I also found this post from "Havrla", who is well known in Europe for his 60Ah -> 120AH battery upgrade skills.

As I said already, I did not yet see OBD2 read outs for the 60 AH i3 and I have not yet seen the specs for the Samsung 60 Ah SDI cells. May be that for this one, there is some upper buffer.

EDIT:
Looks like a module (12 cells) of the 60Ah SDI batteries is specified for a maximum voltage of 49.2V (see here: https://secondlifestorage.com/index.php ... ons.10971/) which means 4.1 V per cell. So if the i3 60 Ah charges the whole battery up to 392V, that means 4.0833 V/cell and also in this case almost no top buffer.

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