NMC Bank, Bus Bars and Batrium Love

[yabert]

Heard the same. Have no idea. These are PHEV cells that I believe are Yttrium doped and good down to -20°C, but I don't have any hard data. I'm assuming they're only 0°C tolerant as is typical.

I never drive a Ford Fusion Energy or other PHEV with those cells, but I can confirm that a Chevy bolt can regen ''a lot'' at -10°C.
It surprising to me to see up to 20 kW of regen (0.3C) at low temperature for this NMC battery. I bet GM know that don't destroy the cells.
So I expect similar spec for Ford cells.
To me, anything under 0.1C at low temperature will not hurt the cells.

 

[sunshine_eggo]

Most Li NMC cells should be fine to at least -5C. But the charge rate should be less than 0.5C. Many EV grade cells are using a different electrolyte specifically to help in low temperatures. But it may be difficult to know if your cells have this. With this MP based ester electrolyte, Li NMC cells can charge at 0.5C down to -20C and still function at lower current down to -40C. The Chevy Bolt that my cells come from had no charging restriction to -10C, but would run a heater below that while charging. So they are probably an in between cell type. And of course, Chevy has a 10 year warranty to deal with. Where I am in So Cal, the outside temp rarely dips below 0C freezing, and even when it did, the inside of my garage stayed over 5C. But I still set my BMS to stop charge at 2C to be completely safe. my charge rate never exceeds 0.15C even in the best conditions. In the car, these cells are rated for 1.0C charge rate. But that is also with an active liquid cooling system.

Good to know. Thanks. With the cell arrangement, 0.5C is about 1500A. I'll never be able to push more than about 250A.

As for the cell balancing talk, even with NMC cells in our use, it is best to only run the balancer at your top 1/4 of your state of charge range. when the cells are full, pull them as close as you can. But while discharging, let them spread. In a perfect world, each cell group will have put out the same amp hours. A weak cell group may lose 30% of it's charge, while a strong cell group may only lose 20% of it's charge. If you keep trying to balance, it might give you a tiny bit more run time with an active balancer, but not with a pas.ve type. it will waste energy pulling down the strong cell to have it keep up as the voltage falls. Don't worry about it, let the weak cell fall faster. It will determine your usable energy when it hits low cell cut off.

What you describe is what is happening. The active balancers do almost nothing when the dV is low.

When you go to charge up again, the lowest weak cell is also going to rise faster as it has less capacity. All of the cells get the same amount of current. They should all reach full at the same time again. So only have the balancer trying to balance as it reaches the top of your charge range.

This didn't happen. The active balancers aren't powerful enough to make a meaningful difference. They're just gently working all the time @ XXmA to bring dV to within 0.01V.

You likely missed it in my word stew, but I interleaved cells across all modules such that each 21P module/cell has very similar capacity, i.e., each 21P cell includes 4-5 cells from 5 different packs. Also the active balancers are voltage proportional, i.e., 3A @ 1V or .03A @ .01V, and they don't operate below 0.01V delta. The Batrium .5A passive balancers are off most of the time. I have the autolevel function enabled to catch the Batrium's buggy cell readings that jump 0.1V above actual. The activation of the balancer for just a second or so brings the offending cell in line. I'll occasionally disable autolevel and have the Batrium balance at 3.92V. Ultimately, there is little balancing going on.

This is what autolevel did on the way up from 0% to about 72%. dV is .03V:



Active balancers did almost nothing as the dV was .03 pretty much all day, .09A max balance current:



Even if the active balancers ran at .09A all day, that's only 0.72Ah or 0.16%.

My goal was to make the 21P "cells" as identical as possible, and I'm confident I got darn close even considering these are harvested from 5 different PHEV packs.

I know I made many comments about benefits of balancing throughout the range. That was really more about the deployment than ongoing efforts. It definitely made sense to balance here:

and here:

Once I got clued into autolevel...

 

[sunshine_eggo]

I never drive a Ford Fusion Energy or other PHEV with those cells, but I can confirm that a Chevy bolt can regen ''a lot'' at -10°C.
It surprising to me to see up to 20 kW of regen (0.3C) at low temperature for this NMC battery. I bet GM know that don't destroy the cells.
So I expect similar spec for Ford cells.
To me, anything under 0.1C at low temperature will not hurt the cells.

Thanks. IIRC, Bolt batteries are made by LG.

They're made by Panasonic, and the only thing I've seen kill them is AZ heat since the battery is air cooled with ducting from the interior. I sourced these from milder climates, and capacity tests were extremely encouraging.

 

[GXMnow]

The active balancing in my JK-BMS works a bit different. It's big disadvantage is that it can only work on a single cell at a time. But it's advantage is that it can pull or push a full 2 amps even if the cell delta is only 1 mv. Here is a screen shot.

The highest cell is blue, and the lowest cell is red. The delta here is just 0.005 volts, and that is higher than normal for me. I have it set to not balance below a delta of 0.008 because of the bus bar and cable difference was causing it to try and balance cells 10 and 11 because there is a 1 foot cable instead of a bus bar between those cells. In this shot, the charge controller has dropped to float so the cells are full (to my setting) and it should reset the SoC to 100%, but it just won't for some reason, so it is showing just 38%, but I know that is way off. 4.124 volts per cell is about 92 to 94% charged. I need to dial it a bit lower, 4.08 to 4.1 per cell is the 90% sweet spot. Too bad this cheap BougeRV charge controller only has 0.4 volt steps. Next step lower is 0.4 volts lower. 57.3 / 14 = 4.09 per cell. I set it down to that just now, and I'll where it falls tomorrow.

This is only 1/2 of my battery bank, which is why it shows a capacity of 360 amp hours. Each cell group on this BMS is 6 x 60 amp hour cells in parallel. The sun has just now dropped low enough that the Enphase panels are no longer making enough to run the house so my battery inverter has stepped in. It is not pulling 200 watts from the battery to keep the grid at about zero watts. During the peak rate time, I actually let it export up to 80 watts.

Surplus EV cells are over kill for our use. They should last a very long time at the low currents we are running them at. I wan over 7 years, but I bet they go a lot longer. The discharge rate I am running is like driving the car at a steady 4 miles per hour. And the peak charge rate is barely into Level 2. In the car (Chevy Bolt EV), they are only running 3 of these cells in parallel with 96 in series. At 30 amps level 2 charging at 240 volts it comes to 7,200 watts to the 96S3P pack. That would be 25 watts per cell. My peak charge rate hits 65 amps at 55 volts or 3,575 watts into 14S12P of cells. That is just over 21 watts into each cell. Even if I max out the 6,800 watt inverter, I am still only at 40 watts per cell. At full acceleration, the car hits nearly 700 watts per cell. I wonder why it needs liquid cooling?

 

[sunshine_eggo]

@GXMnow I hear you. Fortunately, my cost on these was pretty comparable to new LFP. Cells are 6-10 years old already with all testing above 80% and most testing above 90% capacity. I'm assuming at least another 10 years if not more given the absurdly low C rate. Even at the aforementioned 250A, that's only .04C once I have the full battery deployed.

My active balancers are the very primitive single cell balancers in a chain. No configuration. They can carry current between any two adjacent cells with each capable of up to 3A @ 1V dV.

 

[sunshine_eggo]

1 (-ish) year update.

Nope. Didn't finish the second battery.
Yep. Still a rat's nest.
Nope. No fire.

Has worked really well. Been very happy with it, and having that kind of capacity is really nice. The second battery has been sidelined for the same reason as the first... system is working well enough that I don't need it at all... Projects are finally getting knocked off one by one, so it might trickle to the front in 2023.

Is an unseasonably hot summer with temps > 100°F @ 6500 ft elevation. Have an alarm set for 40°C to get my attention, and the BMS sets charge current to 0A @ 37°C. In an uninsulated shipping container, it's hitting 40 most days in the afternoon. Since utilization is low while off-site, I've cut peak voltage to 3.80V/cell - about 65% SoC to help minimize impact of high temps and high SoC.

 

[GXMnow]

I am still very happy with my Chevy Bolt NMC cells. They are working great and still running dead cold at the C rates we use. My peak current has never exceeded 100 amps on my 720 amp hour battery bank. On the hottest days yet, it does exceed 90F in my garage, but the cells are a little cooler from the overnight low. The current does not make enough heat to even keep up with the daily temp rise. Yesterday, it hit 1.6F outside, and the cells still didn't top 35C (95F).

My cell balancer has not activated in over a year. The worst I have seen the cell voltage balance go is only 5 mv (0.005 volts) from the lowest to highest cell. I have the balancer set to activate at a cell delta of 0.005 volts (5 mv).

Stopping at just 3.8 volts per cell is certainly reducing cell stress. That is not much above the storage and transport state of charge. If you don't need the capacity, that will be very nice to the cells and they will last a good long time like that.

Due to the recent hot weather here, and needing to run my A/C a lot, I have expanded the range I am running the NMC cells a bit more. I am now letting them discharge down to 50.5 volts or 3.607 volts per cell. That is about 42% SoC. The XW-Pro is then charging up to 56.7 volts or 4.05 volts per cell. That is about 85% SoC. But then if we still have sun coming in, the DC Charge controller can push the pack voltage up to 57.5 volts or 4.107 volts per cell. That's around 92% SoC. But it will only get that high if it is sunny into early evening, and the A/C is not running. After 4 pm, the XW goes into invert mode, and if the A/C is running, the DC solar is not enough to keep the battery from discharging, so the voltage is falling with the A/C running, not climbing. Today, it only hit the 56.7 volts as the XW stopped charging. The load of running the A/C kept it from going any higher. And before the sun was fully down, the battery had already dropped to 54.6 volts or 3.9 per cell. With any luck, that will be enough to run the house load through the night until the sun comes up. It has not made it the last 4 days, but this looks like it might. It is 11:12 pm as I am typing this, and the battery voltage is down to 53.2 volts. The XW claims it charge 10.0 KWHs into the batteries, an the DC charge controller is reporting another 10.2 KWHs went to the DC bus. So that is over 20 KWHs yet again. But also again, over 3 KWHs of DC charge power was inverted by the XW to drive loads, and never actually went into the batteries. I figure I can safely use 18 KWHs out of my 36 KWHs of battery without shortening the cell life too much. Especially at the low charge and discharge rates I am using. How long will they last???

 

[sunshine_eggo]

Whelp... Time for another discharge test. Weather looked like it was going to cooperate and temps were more mild. With the adoption of Starlink, my DC loads are higher, and I don't have a protection scheme in place, so I only went down to 11% SoC rather than cut-off. I would have lost my internet connection and been unable to turn the MPPT back on.

Here's the whole discharge:



Took about 44 hours with the normal ~275W "background" loads and me remotely engaging a 1500W heater and a 12V charger. The usage of high drain items are indicated by the red arrows.

Here's the start:


and the end:



Start voltage: 4.16-4.18 (almost 100%)
End voltage: 3.47-3.50 (about 15-20% SoC)
Start SoC: 100% (VRM shows 99%, but BMS was reporting 100.0%)
End SoC: 11%
Ah consumed: 399.40Ah

Total cap calculated based on SoC:

399.40Ah/.89 = 449Ah, which is consistent with the programmed 450Ah.

I likely have 5-10% more total capacity BEYOND that, based on cell voltage, so again, I'm pretty happy. The low temp charge protection scheme is working, and there's been no measurable capacity loss since installation in 7/2022. Worth noting that these cells are from vehicles that are 7-11 years old, so calendar aging and cycle wear are both evident. New, the cells would yield 26.6kWh. This test yielded about 24.6kWh - about 92.5% SoH.

Cell delta was pretty constant throughout: .02-.04V

Of course, since it's working so well, and it meets our needs, there's no urgency to deploy the second battery of comparable capacity...

Lazy bastid...

 

[GXMnow]

That looks like a very good successful test. Not bad at all for cells over 7 years old, and no idea how many cycles before you got them. In our use case, EV cells are just coasting along. It will be fun to see how long they end up lasting.

My Chevy Bolt cells were essentially new. One of these days, I should do a full cycle test and see what I really get. By the state of charge graphs and how much I do cycle, my capacity seems to be close to the factory LG Chem ratings. I should be close to 720 amp hours, but I never really pull them below about 35% SoC, about 3.6 volts per cell, 50.4 volts for the whole 14S bank. I have my XW-Pro revert to grid power at 50.5 volts unless the grid fails. With my daily 40% cycles, I am hoping for close to 10 years. My original bank has been going for over 3 years and the second half is over 2 years now.

 

[Andrewr05]

Very informative thread.

As you've stated a few times in this thread, nowadays with the ever decreasing cost of new LiFePO4 cells these NMC cells aren't as cheap to implement as they once were, but these still seem like a great deal.

I've been eyeing some of those NMC EV batteries for some time, but couldn't bring myself to pull the trigger.

I have a very small NMC setup of eBike batteries at the moment and that's what leads me to this thread reading up on cell voltages.

There really isn't a whole lot of centralized DIY-er information on NMC cells out there like there seems to be for LFP chemistry.

 

[MattiFin]

Very informative thread.

As you've stated a few times in this thread, nowadays with the ever decreasing cost of new LiFePO4 cells these NMC cells aren't as cheap to implement as they once were, but these still seem like a great deal.

I've been eyeing some of those NMC EV batteries for some time, but couldn't bring myself to pull the trigger.

I have a very small NMC setup of eBike batteries at the moment and that's what leads me to this thread reading up on cell voltages.

There really isn't a whole lot of centralized DIY-er information on NMC cells out there like there seems to be for LFP chemistry.

https://secondlifestorage.com/index.php is lot more active on NMC EV batteries.

There is for example ongoing open-source project to connect with the CAN bus on entire 400v EV HV packs

 

[Andrewr05]

https://secondlifestorage.com/index.php is lot more active on NMC EV batteries.

There is for example ongoing open-source project to connect with the CAN bus on entire 400v EV HV packs

Nice, I'll have to check it out.