Expanding Li battery (Nissan Leaf)

RoboChicken

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Feb 16, 2021
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8
Hello,

last year I sold my three 48V625Ah lead acid batteries and bought a Nissan Leaf Gen2 battery pack. 48+1 cells for 14S 24kWh
BMS is a batrium WM5.
I knew that I will soon upgrade the battery pack, the first pack was to see how it works. During the winter time here in Germany there was no need for a larger battery. Pack works fine, monitoring with the Batrium seems to work and the SolPipLog software logs and switches everything on a Raspberry Pi.
Last week I got an offer for another 2 complete Gen2 Nissan packs, each 48 cells. Instantly bought them, they will be delivered by the end of this week.

Do you have any thought on how to expand the current pack ? 1st pack has SOH of 86%, the 2 newer ones will be 91% SOH
Should I go for one big pack 14S20P ? Monitored by 1 WM5 batrium BMS
Pro: only 1 pack, only one BMS
Con: 1 BMS (WM5) has to do a lot of balancing. It does no do much balancing with the current pack, so it might be OK.
Cells are at least 5% off (SOH), I do have 49+48+48 cells (Nissan cells consist of 2 pouch cells in one case) = 145 cells total. One 14S20P = 140 cells (each 2 pouches)
I could put the 5 left over cell within the weaker string

Or go for 3 packs 14S6P in parallel ? They will be monitored by 1 batrium WM5 and 2 MM8, so each pack one BMS.
Pro: dont know, what advantage might there be for having 3 packs in parallel ?

Picture shows the first pack during assembly. I 3d printed the battery pole covers and used copper bars

thx for input
 

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OffGridInTheCity

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Welcome. Healthy packs just don't need a lot of balancing. I have 84 packs of 260ah and they range from 30mv to 60mv max difference each day as they go thru the charge/discharge cycle.. with no balance. After 5 months or so they 'drift' further apart up to 90mv max difference - and then I use Batrium auto-level for a few days or to bring them back to the 30-60mv max difference.

If you can get healthy cells (which you should have anyway), and do an initial balance, then on a regular basis you don't need much balancing - and if you're patient with balance, I would guess you could operate with 1 large battery if you wish.

My last balance was Oct 27th, 2020. Approaching 4 months since the last balance - here's a snapshot from this morning with bounce-back from 49.5v low/cut-off from yesterday's cycle. The packs are still ranging between 40mv and 60mv during the daily cycle with an occasional 70mv max difference starting to show on the edges.
1613491196764.png
 
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RoboChicken

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Feb 16, 2021
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Ok, the plan now is one battery, one BMS. The new/used cells have arrived today (2 full sets of Nissan Leaf Gen2).
Will spend the weekend sorting everything, 3D printer already made the pole caps this week. Now I have to join all 3 sets. Will do seven "bricks" and hopefully find a way to put all that into a big steel case.
 

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jdeadman

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The Other thing you can do is pre test each Cell (pair of cells) for capacity and Build your next battery with balance in mind from the get go. Match each cell as best as possible and then you will need little balancing.
 

RoboChicken

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Measured the voltage of all cells, they are very close. I do not have the hardware for a cap test of each of the 290 single cells.
I am still a bit shy combining the 1st pack which is >6% less than the newer 2 sets. I do have 5 spare cells when going 14S20P, they will be distributed on the lower SOH bank.

Started assembling the cells today. Will do seven 20P packs. The pole caps are 3D printed, as well as the white spacer which replaces the original bracket with 2 screws, that is no longer needed. Cells go on a long threaded rod. The smart (lazy) mechanic uses a cordless screw driver to easily rotate the rod through all cells. End plates are still missing, they will be done next week (my neighbour has a waterjet cutter)

Next step is to disassemble the 1st battery pack which currently is online. Will try to find a cloudy or rainy day.

Does anybody here has Nissan cells ? What is the best voltage range for PV use ? I currently use them 3.3V to 4.05V. The lower end will be chosen by the voltage difference, so they do not drift apart too much. What about the upper voltage, Can I safely go to 4.1V ? Or what s the highest safe voltage for Nissan cells for regular use ?
 

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slimf

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I have gen 1 packs. 21P - that’s 147 modules sourced from four different leaf cars over 3 years. I tested every module with a CBA IV. Took an eternity but was important as I was able to build seven roughly matched stacks.

my individual module capacity varies from 30ah up to 40 somthing. So a big difference - but it doesn’t matter as long as each of your 7 stacks are closely matched in total capacity. Testing is important especially if the cells are older and degraded.

photo shows powerwall with most modules added - still two rows to add to bottom shelf then fit extra busbars.

the current 13p stack you can see with bus bars is operational.
 

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OffGridInTheCity

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Couple of thoughts.....
1) Testing - An ICharger X8 ( https://www.amazon.com/iCharger-Battery-Features-Capabilities-Current/dp/B07RNKLR8T ) is capable of testing these. You can do simple 50w or with a little more work, setup regenerative discharge and do over 100w. I think (personally) I'd recommend you do a lithium-ion test process - e.g. test capacity, let sit a few weeks looking for any self-discharging cells and check IR and then distribute/parallel the capacities evenly.

2) No Testing - If you think there's a 6% capacity difference between the original cells and the new ones - you could evenly distribute the old cells (in parallel) with new ones - if you get what I'm saying - and this will smooth things out.

3) Voltage range...
>highest safe voltage for Nissan cells for regular use
These are lithium-ion, so its 4.2v (same as 18650 lithium ion cells and other lithium-ion) However, if you max at 4.2v each cycle the life span will be reduced.

After experimentation, I run between 3.5v (low - discharge curve knee is 3.4-3.5v) and 4.0v (hi) but I rarely get up to 4.0v most days. In the spring and summer I hit 4.0v occasionally but in the fall/winter I don't. Overall, I'm running a 41% DOD average per year. I've designed the battery bank / shooting for the 75-25% SoC line from the famous Battery University Chart showing 88% capacity at 5,000 cycles. I'm at 900 cycles with no detectable degradation so I can't say one way or the other yet. :)

Here's the Battery University graph near the bottom of this page - https://batteryuniversity.com/index.php/learn/article/how_to_prolong_lithium_based_batteries ... :)
1613858130502.png

If you map % SoC to the voltage range you can figure out where you are operating on this chart.

4) PV -> Battery ratio. I'm doing doing daily cycles of PV -> Battery -> Inverter and in this scenario there's a ration between PV in and battery bank size and DOD. In my case, its 13kw PV -> 81kwh battery bank and with this, I can consume 100% of the PV I produce within the 3.5v -> 4.0v range (41% DOD average for a year). I could go 3.6v to 4.1v or 3.4v to 3.9v etc... there's a little wiggle room here. Depending on your situation, you may have different aspects in play - and you may or may not have the option to go as low as 40% DOD overall. I specifically enlarged my battery bank till I reached the 40% DOD in the hopes I'll literally get to 10,000 cycles with 60% of original capacity left and re-coop some of my $$ :)
 
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RoboChicken

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Thank you for the info. The used SOC range here varies, I do not have full cycles every day, so a 30-95% window will mostly be used around 50-80%. In the summer the battery will be at >90% a few hours a day, for a very few weeks. During winter it will not sit at 30% or less.
The battery will be mostly used within the sweeter spots of the chart (above), similar to your numbers. Of course I would like to keep the battery for 10 years or more, lead easily did 10 years. Due to the hight energy costs (well, taxes on energy) here of 0.29€ per kWh or $0.35 the battery will pay for itself in a short time.

The cells came with a cap/voltage reading, they should be close enough. But the 1st pack I have, the old one has less capacity and some cells are drifting apart at low voltage. At 3.7V or higher they are 10mV or less apart. At 3.3V one pack is 70-90 mV lower. The others only 20-30mV max. apart.

I will follow the suggestion here to do a cap test on them. That will indeed take time, my icharger (oder model) is a bit slow, maybe a second one should be considered.
What is the voltage range for the Nissan cells for a cap test, standard 2.8V to 4.2V for lithium ? Just asking to be sure.

Maybe there will be a suitable charger/cap meter in the future with more power to do a cap test every x years. We then will see how the cells change over time.
 

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OffGridInTheCity

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>What is the voltage range for the Nissan cells for a cap test, standard 2.8V to 4.2V for lithium ? Just asking to be sure.
I do 18650 cells and use OPUS(s) - they default to 4.2v -> 2.8v. However, you don't have to do 4.2v -> 2.8v, you could do 4.15v -> 3.0v (for example). Just be consistent so you have apples to apples comparison - so you can distribute the cells evenly (if needed). I use iCharger X8 to test my overall 260ah packs.

The reason I mentioned iCharger X8... in addition to 50w / 100w (so you can do it in reasonable time frame), you can set explicit 'automatic' cut-off voltages if that's a concern - or just go with the defaults.

>At 3.7V or higher they are 10mV or less apart. At 3.3V one pack is 70-90 mV lower. The others only 20-30mV max. apart.
As mentioned above, I routinely operate with 40mv-90mv max difference between packs. Its not the max diff OR that this varies a bit thru the charge/discharge cycle - so much as does the max diff widen noticeably within a few days or weeks - which indicate a self-discharge.

As long as things are stable to each other over several months - then the only issue with mv differences is that it limits your top (and bottom) voltage ranges by that max difference. For example, with 90vm max difference - I can only charge the battery up to 4.11v * 14 = 57.54v (instead of the the theoretical max of 4.2 * 14v = 58.8v) else I risk one of the packs going over 4.20v.

In my case I have 84 packs with 6 different cell manufacturers - so variance (max mv differences) in the discharge discharge curve might be exasperated by this. I don't have experience Leaf cells! Maybe others can comment, but they are lithium-ion and you got them in different purchases/sources... so some variance may be normal. I think that if you test, weed out any self-dischargers and evenly distribute the cells (in parallel) and make sure IR is very similar/good - you should expect a stable battery bank.
 
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