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Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Posted: Wed Jun 06, 2018 1:26 pm
by AndrewLondon
HI all.
I am building extra battery for my Nissan Leaf as well, reading this tread gave me a lot information to think about.
My battery will be small, only 8.5 kwh. And build on 18650 cells. So problem with over current during rapid charging and discharging actual for me as well.
As we know, BMS of original battery can send commands to limit power of motor inverter - during discharge, or send signal to rapid charger to limit charging current.
So idea is to sens current to additional battery, and when necessary send canbus command to limit charge or discharge.

What you think, how possible it will be?

Another option, to false original BMS to think that battery to hot, and it will automatically will limit current from rapid charger, or to motor inverter.

Andrew

Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Posted: Thu Jun 07, 2018 12:15 am
by mux
In the Leaf, the rapid charger does its own current limiting; all the car does is set a maximum current (120A in the case of Leaf) and a maximum voltage (391V in case of the Leaf). The charger then does constant current-constant voltage (CCCV) charging just like any other battery charger.

If you put another battery in parallel to the main battery, the internal resistance will drop and you will be able to receive more current. The car won't complain and you don't have to do anything to enable this.

The same goes for discharging and regenerative braking; these are based on the Hx (internal resistance) signal, and this will change if you put more cells in parallel, so you will be able to regenerate earlier and harder, and you won't get power limited with a worn battery.

Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Posted: Thu Jun 07, 2018 12:24 am
by jkenny23
mux wrote:In the Leaf, the rapid charger does its own current limiting; all the car does is set a maximum current (120A in the case of Leaf) and a maximum voltage (391V in case of the Leaf). The charger then does constant current-constant voltage (CCCV) charging just like any other battery charger.

If you put another battery in parallel to the main battery, the internal resistance will drop and you will be able to receive more current. The car won't complain and you don't have to do anything to enable this.

The same goes for discharging and regenerative braking; these are based on the Hx (internal resistance) signal, and this will change if you put more cells in parallel, so you will be able to regenerate earlier and harder, and you won't get power limited with a worn battery.
I believe his concern (and mine though I am trying to solve by finding the right cells that can handle the full power envelope of the Leaf) is how to ask the Leaf to limit discharge power at lower SoC where the 18650 extender pack will be the dominant power source, since it won't be designed for high drain. Ditto for regen/quick charging in the opposite direction. It should be possible since the car already limits regen/power at very low SoC, just need to find out how to intercept those messages.

Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Posted: Thu Jun 07, 2018 12:52 am
by mux
Ah, I see. Thanks for clearing that up.

That is quite a bit more involved. AFAIK most of the reverse engineering to this end has not been fully figured out. I see a couple of approaches:

- Artificially raise/lower Hx values until the car starts throttling everything. This should affect everything: fast charging, regen and acceleration.
- Use cells with sufficient internal resistance that their contribution to the peak power is limited. This is very easy to do with a small 18650 pack simply by using standard welding tabs.

If you want to do it 'properly', this gets very involved and touches on the quick charge CAN bus which hasn't been touched mostly:

- You have to limit regen by changing the braking torque available messages from the BCM
- You have to limit maximum motor power by changing the discharge current available messages from the BCM
- You have to limit DCQC current by changing the accepted current messages on CHAdeMO CAN. Also tell the car that the charger can only supply that current.

The hardware for this is not quite available, but I'm still working on it. Well, in fact, I'm done with it, just have to find the time to document and film it. I haven't yet built it into my car between the battery and VCM, so I can't tell for sure that it works.

Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Posted: Thu Jun 07, 2018 2:07 am
by AndrewLondon
Yes, that is exactly I am thinking as well.
I am expecting my USB2CAN dongle soon, so I will be able to read/write can bus, and see how possible to
control all three parameters : regen, charging and discharge .

As Mux mention "- Artificially raise/lower Hx values until the car starts throttling everything. This should affect everything: fast charging, regen and acceleration." - this will be easy way of doing it.

Another options, as I mention before - to rise temperature on battery sensors, then BMS will think that battery too hot and will start reducing
charge / discharge / regen. Maybe it will be enough to rise temperature only on one sensor, and left another two/three still connected to battery.

Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Posted: Thu Jun 07, 2018 4:39 am
by mux
I can confirm that increasing the temperature to about 48-49 degrees does work. Only the highest temperature value counts, but they're both in the same CAN packet so you can just as well increase both. Just make sure that the actual battery temperature is always lower or equal to what you're spoofing it for.

Just a USB to CAN dongle isn't enough by the way. You have to intercept messages and change them on the fly. The bus will not accept ID spoofing (it'll throw all kinds of errors if you do - they're easy to clear but you won't be able to charge or drive while you do that) and it will throw errors if the BCM sends one value and immediately something else on the bus sends another value. You need something that reads messages from the battery, changes them and then sends them to the rest of the car.

Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Posted: Thu Jun 07, 2018 9:14 am
by lorenfb
mux wrote:
The same goes for discharging and regenerative braking; these are based on the Hx (internal resistance) signal, and this will change if you put more cells in parallel, so you will be able to regenerate earlier and harder, and you won't get power limited with a worn battery.
Are you implying that Hx actually represents the internal resistance, e.g. milliohms, of the battery or it's conductance (mhos)? To some, it's
another measure of battery health. To others, it's another variable that decreases with battery degradation over time. My Leaf data over
the years indicates that the internal resistance of the Leaf's battery has a negative temp coefficient and increases over time with battery
degradation, as one might expect, and doesn't correlate with Hx directly (dR/dT):

11/20/14 -13,700 miles, 76 mohms per LeafDD, 20 Deg, 73% SOC
11/27 -13,800 miles, 67 mohms per LeafDD. 25 deg, 63% SOC
11/30 - 13,900 miles, 56 mohms per LeafDD, 27 deg, 71% SOC
12/2 - 14.100 miles, 55 mohms per LeafDD, 28 deg, 67% SOC
12/16 - 14,500 miles, 89 mohms per LeafDD, 15 deg, 93% SOC
12/27/14 - 14,800 miles, 103 mohms per LeafDD, 11 deg, 24% SOC
3/10/15 - 17,400 miles, 60 mohms per LeafDD, 30 deg, 73% SOC
3/14 - 17, 550 miles, 56 mohms per LeafDD, 32 deg, 47% SOC
4/14 - 19,100 miles, 59 mohms per LeafDD, 25 deg. 38% SOC
5/4 - 19,989 miles, 64 mohms per LeafDD, 24 deg.  48% SOC
5/15 - 20,400 miles, 73 mohms per LeafDD, 20 deg. 41% SOC
5/22 - 20,700 miles, 58 mohms per LeafDD, 28 deg. 50% SOC
12/10/15 - 28,000 miles, 90 mohms per LeafDD, 19 deg. 92% SOC
4/5/16 - 32,000 miles, 74 mohms per LeafDD, 24 deg, 55% SOC
5/16 - 33,700 miles,89 mohms per LeafDD, 22 deg, 47% SOC
5/16 - 33.700 miles, 58 mohms per LeafDD, 31 deg, 76% SOC
10/5 - 39,300 miles, 100 mohms per LeafDD, 22 deg, 50% SOC
10/6 - 39,400 miles, 61 mohms per LeafDD, 30 deg, 51% SOC
10/7 - 39,500 miles, 80 mohms per LeafDD, 25 deg, 56% SOC
10/15/16 - 40,000 miles, 71 mohms per LeafDD, 27 deg, 45% SOC

If possible, please reference a source for a reliable definition of Hx.

Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Posted: Thu Jun 07, 2018 9:35 am
by mux
Well yes, that is part of the problem here. I don't know either how everything exactly correlates, but the charging speed is either dependent on many things at once OR has some hysteresis or mapping function that is a bit obscure. I haven't seen any reports of people reverse engineering it exactly. So it's going to be hard to know for sure how to limit charging/discharging speed.

Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Posted: Thu Jun 07, 2018 6:11 pm
by alozzy
FWIW, @GerryAZ once posted this comment on what he figures the Hx value is (I seem to recall reading something similar either in the help system of Leaf Spy or on the Wiki):

http://www.mynissanleaf.com/viewtopic.p ... 12#p493789

Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Posted: Thu Jun 07, 2018 6:24 pm
by lorenfb
alozzy wrote:FWIW, @GerryAZ once posted this comment on what he figures the Hx value is (I seem to recall reading something similar either in the help system of Leaf Spy or on the Wiki):

http://www.mynissanleaf.com/viewtopic.p ... 12#p493789
Yes, thanks. That's more logical, i.e. Hx is the ratio of the present battery conductance
(mhos) to a new battery's conductance. It basically follows SOH over time.