arnis
Posts: 869
Joined: Sat Jan 23, 2016 3:21 pm
Delivery Date: 23 Jul 2014
Leaf Number: 015896
Location: Estonia, Europe

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

Sat Mar 03, 2018 4:53 pm

What happens with SOC, SOH and Hx values if two batteries are connected simultaneously?
Well, SOC will be wrong. It will drop like it would according to wasted energy. But then it will stop
dropping at 3-4 bars left. And then it keeps that for long. Until it starts dropping again.
SOH value will recalculate up and up. Until 103%. Which is maximum. Even though actually, there
is more kWh of energy available as there are actually two slightly worn batteries.
Hx value will recalculate to realistic value. It appears, that Hx value is the resistance of vehicle's
battery pack
. 100% is factory default value. In case of two batteries, it will skyrocket. But it will be
around the average of two separate batteries (so in case of two new batteries, it will be 200%).

Image
Short range EVs <30kWh -- Medium range: 30-60kWh -- Long range: >60kWh
Charging: Trickle <3kW -- Normal 3-22kW -- Fast 50-100kW -- Supercharging >100kW

jkenny23
Posts: 80
Joined: Sat Aug 19, 2017 12:49 pm
Delivery Date: 19 Sep 2017
Leaf Number: 009318

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

Sat Mar 03, 2018 7:45 pm

arnis wrote:What happens with SOC, SOH and Hx values if two batteries are connected simultaneously?
Well, SOC will be wrong. It will drop like it would according to wasted energy. But then it will stop
dropping at 3-4 bars left. And then it keeps that for long. Until it starts dropping again.
SOH value will recalculate up and up. Until 103%. Which is maximum. Even though actually, there
is more kWh of energy available as there are actually two slightly worn batteries.
Hx value will recalculate to realistic value. It appears, that Hx value is the resistance of vehicle's
battery pack
. 100% is factory default value. In case of two batteries, it will skyrocket. But it will be
around the average of two separate batteries (so in case of two new batteries, it will be 200%).


Does regen improve any over stock "new" battery state? Regen seems to be limited by reduced Hx values, so I wonder if the converse is true? That's one of my biggest complaints is the super weak regen ability of the Leaf. Almost all of my driving is city driving, so I'm often having to stop for traffic and traffic lights anyways, my efficiency would go up significantly if I could utilize more regen, but the Leaf is just too timid. Compared especially to my Prius which gives you full ~20-30kW of regen at any speed, until the tiny battery is full anyways. Perhaps a second motor attached to the rear wheels plus a dedicated controller for "external" regen would be the only option for improving it significantly?

User avatar
davewill
Posts: 4904
Joined: Thu Mar 17, 2011 6:04 pm
Location: San Diego, CA, US

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

Mon Mar 05, 2018 9:20 am

I'm pretty pretty sure that none of those values are going to change as they are computed and measured by the BMS, which is inside the LEAF battery pack. The car will probably just start thinking you are doing a bang up job on driving economy.

I'm skeptical that charging of an add-on can be done using the LEAF's internal charger. I find it more likely that the secondary will need it's own charger. For one thing, without that you would have no way to deal with charging if the two packs ever fell out of sync voltage-wise.

You might be able to wire it to the same inlet port like the BRUSA integrations that were done, although it will be a little trickier dealing with the case where the secondary needs to charge, but the primary is full and doesn't start the EVSE.
2014 Rav4 EV, Blizzard Pearl White
2011 LEAF SL w/QC, Blue Ocean, returned at end of lease

User avatar
IssacZachary
Forum Supporter
Posts: 804
Joined: Fri Jan 13, 2017 9:57 am
Delivery Date: 15 Nov 2016
Leaf Number: 420789
Location: Gunnison, CO, USA

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

Mon Mar 05, 2018 10:45 am

davewill wrote:I'm pretty pretty sure that none of those values are going to change as they are computed and measured by the BMS, which is inside the LEAF battery pack. The car will probably just start thinking you are doing a bang up job on driving economy.

I'm skeptical that charging of an add-on can be done using the LEAF's internal charger. I find it more likely that the secondary will need it's own charger. For one thing, without that you would have no way to deal with charging if the two packs ever fell out of sync voltage-wise.

You might be able to wire it to the same inlet port like the BRUSA integrations that were done, although it will be a little trickier dealing with the case where the secondary needs to charge, but the primary is full and doesn't start the EVSE.

Agreed that an external secondary battery shouldn't affect main battery numbers because those are measured off the main battery's BMS, not the motor controller or battery charger. Basically the BMS will just think you're apparently going downhill because you're using less kWh's per mile out of the main battery.

Charging another secondary traction battery off the main charger has been done already. Yes, if they are out of voltage there could be a problem. But that would only be a problem before or after charging or running the car. During charging or driving, there is no way both batteries could "fall out of sync voltage-wise" because they would be connected in parallel. It would be like two pressure tanks hooked up at the same time, neither is going to be able to have more or less voltage than the other. However, when you shut off the car and charger both batteries will disconnect. If one has more leakage current than the other or one battery gets hotter or colder than the other it could end up with a different voltage than the other once you go to turn on or charge the car. This could cause a sudden, but temporary rush of current from one battery to the other. The solution is to put an Arduino system that measures the main bus and the auxiliary battery's voltage and connects the auxiliary traction battery only when both voltages are the same. If you get home and plug in right away both battery's should have the same voltage and both begin charging unless one got a dead cell or loose connection or something in the second battery, which you wouldn't want to charge anyway.

I've contemplated doing a second battery with LiFePO4 cells. They have a narrower voltage range than the Leaf's Li ion cells, which could actually be advantageous. That way the main battery would always charge up to a higher voltage and would likely discharge to a lower voltage than the auxiliary battery. That way the Arduino system would always start with the auxiliary battery disconnected, and after a while of driving the main bus would always end up matching the auxiliary battery's voltage. Then the auxiliary battery would connect until it's depleted. Then it would be disconnected and leave the main battery to finish discharging. Charging would be the same, only in reverse.
2013 SL 50,000 miles.
12 bars until 44,300 miles on June 2, 2017. :D
11 bars current. :)
The Nissan Leaf is the fourth best long distance car for highway driving. >>Best Long Distance Cars<<

jkenny23
Posts: 80
Joined: Sat Aug 19, 2017 12:49 pm
Delivery Date: 19 Sep 2017
Leaf Number: 009318

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

Mon Mar 05, 2018 12:16 pm

IssacZachary wrote:I've contemplated doing a second battery with LiFePO4 cells. They have a narrower voltage range than the Leaf's Li ion cells, which could actually be advantageous. That way the main battery would always charge up to a higher voltage and would likely discharge to a lower voltage than the auxiliary battery. That way the Arduino system would always start with the auxiliary battery disconnected, and after a while of driving the main bus would always end up matching the auxiliary battery's voltage. Then the auxiliary battery would connect until it's depleted. Then it would be disconnected and leave the main battery to finish discharging. Charging would be the same, only in reverse.


I would be very cautious of using a second battery with a voltage curve drastically different than that of the Leaf's battery; basically the current out of the secondary battery will be very different depending on what the state of charge of the 2 batteries are at. I've done a couple such experiments on the small scale, putting a high capacity power cell in parallel with a lower capacity higher internal resistance cell with a different discharge curve, and while the experiment was done at high rate of constant discharge, it did show that the power cell (akin to the Leaf battery) had most of the load for the first half of discharge, but the lower capacity "auxiliary" battery took a majority of the load toward the end of the discharge (details here: https://secondlifestorage.com/t-Differe ... Experiment)

User avatar
davewill
Posts: 4904
Joined: Thu Mar 17, 2011 6:04 pm
Location: San Diego, CA, US

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

Mon Mar 05, 2018 12:29 pm

IssacZachary wrote:... The solution is to put an Arduino system that measures the main bus and the auxiliary battery's voltage and connects the auxiliary traction battery only when both voltages are the same. If you get home and plug in right away both battery's should have the same voltage and both begin charging unless one got a dead cell or loose connection or something in the second battery, which you wouldn't want to charge anyway. ...

Yes, the voltage monitoring is assumed, I think. But if you can't plug in right away for some reason, or the Arduino decides to crash, and you do end up charging the main battery without charging the secondary, you would have no way to get the secondary charged short of draining the main down to the volatage level of the secondary. In an extreme case, it could even be impossible to get them in sync again just by draining the main. If the idea is get this reasonably foolproof, it needs a better plan than just assuming it won't happen.
2014 Rav4 EV, Blizzard Pearl White
2011 LEAF SL w/QC, Blue Ocean, returned at end of lease

User avatar
IssacZachary
Forum Supporter
Posts: 804
Joined: Fri Jan 13, 2017 9:57 am
Delivery Date: 15 Nov 2016
Leaf Number: 420789
Location: Gunnison, CO, USA

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

Mon Mar 05, 2018 1:14 pm

jkenny23 wrote:I would be very cautious of using a second battery with a voltage curve drastically different than that of the Leaf's battery; basically the current out of the secondary battery will be very different depending on what the state of charge of the 2 batteries are at. I've done a couple such experiments on the small scale, putting a high capacity power cell in parallel with a lower capacity higher internal resistance cell with a different discharge curve, and while the experiment was done at high rate of constant discharge, it did show that the power cell (akin to the Leaf battery) had most of the load for the first half of discharge, but the lower capacity "auxiliary" battery took a majority of the load toward the end of the discharge (details here: https://secondlifestorage.com/t-Differe ... Experiment)


True. The LiFePO's would have to be able to handle perhaps full current. This may be possible if the auxiliary battery is also around 24kWh or more, or made of high discharge cells. Of course the cables have to match the current.

I'd love to throw 60kWh of LiFePO's on a trailer and be able to go anywhere.

davewill wrote:
IssacZachary wrote:...In an extreme case... the idea is get this reasonably foolproof...

Just don't discharge below 0%. Or disconnect the auxiliary battery when it gets down to 10%. If the auxiliary battery is small enough you may not need to even worry about mismatched voltages after it's installed.

On the other hand I've always wondered what if you took a second used Leaf battery and wired the car to switch between batteries. Apparently this has been done. That way mismatched voltages are of no problem whatsoever. Then I also wonder what would happen if you add bigger cells, or cells in parallel, to the cells in a Leaf battery.
2013 SL 50,000 miles.
12 bars until 44,300 miles on June 2, 2017. :D
11 bars current. :)
The Nissan Leaf is the fourth best long distance car for highway driving. >>Best Long Distance Cars<<

mux
Posts: 128
Joined: Sat Jan 13, 2018 3:52 am
Delivery Date: 13 Oct 2011
Leaf Number: 6177

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

Mon Mar 05, 2018 2:01 pm

As long as you wire entire packs in parallel, there should never be an issue with charging and discharging. I don't see how it's in any way safe to do hot-swapping (or hot-switching, I guess). What if your code glitches? What if your arduino's voltage sensing drifts? What if a cosmic ray hits your microcontroller? These kinds of things are done in automotive hardware and battery switching, but they require intrinsically safe coding practices and hardware redundancy. If you do it wrong, what you get is something close to short circuit currents running between packs.

Just put packs in parallel. It's the only safe way to go about adding capacity, outside of replacing the main battery pack.

Also, out of curiosity, IsaacZachary, why would you want to use LiFePO4s in automotive? To me that seems really strange, but I might be missing something. Here's what I know about this tech - and I consider myself fairly knowledgeable considering batteries are core to my line of work:

- LFP used to have a flatter discharge curve, but NCA pretty much has the same characteristics, yet at a higher voltage
- LFP used to have better power density, but that's... just not true anymore, it's a lot worse than any modern chemistry now.
- LFP used to have higher cycle life, but that's totally not true anymore
- LFP used to be considered more environmentally safe, arguably the cobalt in NCA still makes it a bit more toxic but with 811 NMC coming up this is moot as well
- LFP cannot thermally runaway. It technically does have this advantage, as it makes it a somewhat inherently safe battery, but this aspect is so incredibly moot these days. NCA is so close to inherently safe that you have to massively abuse it to get it to do anything strange. The higher internal resistance and lower energy density (thus higher system complexity) of LFP makes it, IMO, more dangerous than more modern chemistries.

The low energy density (~2.5x difference with NMC) also makes that you really do need a trailer to pack 60kWh of LFP (=1150 lbs!!), whereas you can probably fit 60kWh of NMC (=450lbs) in the stock battery shell and a few briefcase extenders in your trunk, likely without even increasing total vehicle weight if you replace the original pack. Much more convenient, and always available, not just when you're pulling a trailer. Not to mention the cost difference and the sheer amount of extra work. 60kWh of NMC is still somewhat affordable, LFP is... just crazy. You can buy a new 2018 Leaf for that kind of money.

LFP really only makes sense as a lead acid replacement in front, not as a main traction pack.

I don't believe that one crazy video where the guy, out of frame, pretends to switch battery packs on a Leaf. Sure it could work, but I don't consider it demonstrated or particularly useful. Also, I don't consider the Leaf pack to be particularly desirable as a range extender pack. It's really heavy for its capacity and the chemistry they use inside is really inferior to even stuff like Yuasa LEV50s. As stated before, they somehow managed to get only 24kWh into a volume that can comfortably hold 45-50kWh, and still get significant voltage sag on just 80kW of load and very high wear even in mild climates. The Leaf battery is just not very good overall, I wouldn't use it for anything tbh.

User avatar
IssacZachary
Forum Supporter
Posts: 804
Joined: Fri Jan 13, 2017 9:57 am
Delivery Date: 15 Nov 2016
Leaf Number: 420789
Location: Gunnison, CO, USA

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

Mon Mar 05, 2018 2:13 pm

mux wrote:Also, out of curiosity, IsaacZachary, why would you want to use LiFePO4s in automotive?


Thanks for the incouragement! (Really! I'm not being sarcastic.)

Only because

  1. I can get them cheap right now. Cheaper by the kWh than any other battery chemistry, although not by much.
  2. Because they are safer in that they don't ignite or have a spillable liquid electrolyte.

Other than that, those are the only two reasons I've contemplated using them.
2013 SL 50,000 miles.
12 bars until 44,300 miles on June 2, 2017. :D
11 bars current. :)
The Nissan Leaf is the fourth best long distance car for highway driving. >>Best Long Distance Cars<<

User avatar
davewill
Posts: 4904
Joined: Thu Mar 17, 2011 6:04 pm
Location: San Diego, CA, US

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

Mon Mar 05, 2018 3:44 pm

mux wrote:As long as you wire entire packs in parallel, there should never be an issue with charging and discharging. I don't see how it's in any way safe to do hot-swapping (or hot-switching, I guess). What if your code glitches? What if your arduino's voltage sensing drifts? What if a cosmic ray hits your microcontroller? These kinds of things are done in automotive hardware and battery switching, but they require intrinsically safe coding practices and hardware redundancy. If you do it wrong, what you get is something close to short circuit currents running between packs.

...


He's describing a second pack in parallel. I agree you don't need that kind of monitoring if you're inside the main pack's contactor and the cells are never disconnected from one another, but if that's not the case, then something has to verify that it's safe to close the secondary pack's contactor.
2014 Rav4 EV, Blizzard Pearl White
2011 LEAF SL w/QC, Blue Ocean, returned at end of lease

Return to “Engineering”