PIP-4048MS and PIP-5048MS inverters

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Post by weber » Sat, 14 Feb 2015, 07:30

Here's a diagram showing the BMS communications topology.

Image

"IMU" stands for "current monitoring unit", based on "I" being the quantity symbol for current, since we already have "CMU" standing for "cell monitoring unit".

In the MX-5, the IMU also performed insulation testing, but in this solar application the reed relays were replaced with MOSFETs and their drivers, for switching contactors.

In this solar application the IMU acts as the BMS master, and the system controller. It accepts regular status bytes from the string of CMUs twice a second, and acts to protect the cells by turning off load or source contactors if required.

The IMU tells the CMUs what the current is, twice a second, so they can compensate for their cell's internal resistance in calculating undervoltage or overvoltage stress. All the communication mentioned so far is at 9600 b/s.

The IMU also sends commands to the PIP-4048MS inverter/charger at 2400 b/s. Initially it was planned to run a PI-control loop for charging, based on the stress level of the cell with the highest voltage. And indeed this was successfully implemented for charging from the AC input. However it was discovered that this was not possible with the MPPT charging from the PV array.

So the only things the IMU sends to the PIP now are commands for various non-default settings, on startup, since it seems the PIP will forget its settings and return to factory defaults if it is turned off for long enough.

It's quite tricky how the IMU can send 2400 b/s commands to the PIP and 9600 b/s commands to the CMUs because in fact it only has one serial output. The two Industrial Fibre Optic LEDs on the IMU are simply driven in parallel with 10 ohm current sharing resistors. So the CMUs see all the 2400 b/s data sent to the PIP and the PIP sees all the 9600 b/s data sent to the CMUs.

It turns out that, to a 9600 b/s receiver, 2400 b/s data can appear as one of only four different byte values, none of which need to be used by the CMUs and so CMU-1 simply deletes them.

To a 2400 b/s receiver, 9600 b/s data can look like any value, however the probability that they will accidentally compose a valid PIP command, let alone one with a valid CRC-16 followed by a carriage return, is small enough to ignore.
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Post by weber » Sat, 14 Feb 2015, 07:43

Ths photo shows how simple it was to add a fibre-optic input to the PIP-4048MS. The Industrial Fibre Optic (IFO) phototransistor is simply soldered in parallel with the phototransistor (the inboard side) of the PIP's input opto-coupler.

The IMU had no way to read ACKs or NAKs coming back from the PIP. But in any case, we found that although an ACK might come back almost immediately, there was still some time after that before it would accept a new command without corrupting it. This delay was typically one to two seconds, depending on the command. I asked MPP Solar if there was any way to predict this, and have received no reply.

Image

This photo shows the IFO receiver held in place with neutral cure silicone.

Image
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Post by PlanB » Sat, 14 Feb 2015, 17:47

The CoconutHeadphones what a delicious turn of phrase. You're right the occasional example wouldn't go astray. We've done a lot of trial & error here also,reminds me of this:



You do nice BMS drawings too what app is that you use? Some of us are stuck in a time warp with Corel, don't know what I'm gonna do when my last XP machine dies?

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Post by weber » Sun, 15 Feb 2015, 01:13

PlanB wrote: The CoconutHeadphones what a delicious turn of phrase. You're right the occasional example wouldn't go astray. We've done a lot of trial & error here also,reminds me of https://www.youtube.com/watch?v=czro0qB72Ng

You do nice BMS drawings too what app is that you use? Some of us are stuck in a time warp with Corel, don't know what I'm gonna do when my last XP machine dies?

I try. Thanks for the kind words. They make it all worthwhile.

I use Microsoft Word for drawings. Currently Word 2007. Frequently used commands are Insert/Illustrations/Shapes and Home/Editing/Select/Select_Objects. And when a graphical object such as a text-box, rectangle, line or arc is selected, the "Format" set of commands becomes available on the ribbon. And of course you have context-menus available via right-click. The most annoying thing is how it keeps taking you back to word-processing mode, and you have to recognise the different highlighting of objects and the different cursors that tell you which editing mode you're in.

If anyone knows how to convince Word that you're never going to do any actual word-processor-style text-editing in a given document -- that you're only ever going to edit text in text-boxes -- then please let me know.

[Edit: Thanks for a blast from the past with that cartoon. Duh, open sasquatch?]
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Post by rhills » Tue, 17 Feb 2015, 07:32

Hi PlanB,
PlanB wrote:You do nice BMS drawings too what app is that you use? Some of us are stuck in a time warp with Corel, don't know what I'm gonna do when my last XP machine dies?
I use Inkscape quite a bit. It's Open Source (ie free, as in beer), available for Windoze, Mac and Linux.

Like any powerful drawing program, it takes a bit of learning, but there's lots of good manuals, tutorials etc online for it. It also lends itself well to building libraries of drawing objects (eg electronic component symbols) that you then just drop onto a drawing and push around into place.

Probably harder to learn than Word Draw, but a whole lot more powerful also.

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Post by weber » Fri, 27 Feb 2015, 05:20

Some great real world experience of LiFePO4 cells in a solar power system here, with recommendations on boost and float voltages.

http://www.technomadia.com/2015/02/livi ... ry-update/

Even more reason for me to tweak that float voltage down from 3.375 Vpc (54.0 V) to 3.3625 Vpc (53.8 V) as per Kurt's friend's recent research. The boost to 3.45 Vpc (55.2 V) can stay, with bypass resistors turning on at maybe 3.44 V.

[Edit: Added bypass voltage]
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Post by Johny » Fri, 27 Feb 2015, 12:39

Wow - what a great write up. Thanks for posting that link Weber.

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Post by offgridQLD » Fri, 27 Feb 2015, 14:30

Yes the RV lifepo4 post was a interesting link.

I think his Suspect #3: Lack of Balance / Laggard Cell) is where a lot of his capacity might be hiding.

If his capacity test involve him taking the cells down to until the BMS shut down. Especially as his bms wont even balance until 3.7v and he is using conservative charge voltages. There is a good chance cells would be out of balance.

Though the heat is just as likely. Not to sure about the 50% less life with 10C heat gain. Was it that pronounced over that kind of temp rise in other study's?

If so I might install a little inverter split system AC in my power room (set to 23c)

Kurt
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Post by PlanB » Fri, 27 Feb 2015, 14:45

All the drama over Leaf batteries in Arizona a few years back seems to suggest that temperature is a big factor. Tesla have certainly gone to a lot of trouble with thermal management. You got the monolith on the southern veranda Dave?

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Post by weber » Fri, 27 Feb 2015, 15:52

offgridQLD wrote:Not to sure about the 50% less life with [10°C temperature] gain. Was it that pronounced over that kind of temp rise in other study's?

There is no doubt whatsoever about this. Even without testing, it was utterly predictable, because this is a near-universal relationship between temperature and the rate of chemical reactions of all kinds, which has been known for 130 years, and found to apply to every other battery chemistry. Antiscab and I have mentioned it on the AEVA forums several times. It's called Arrhenius' equation.
PlanB wrote:You got the monolith on the southern veranda Dave?
It's actually on the eastern verandah, for various reasons of convenience, but this is well shaded from the early morning sun by another building, and (unfortunately for the PV array) by trees. The major heat generating components are well above the cells and there is an air vent at each end of each row of batteries.
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Post by offgridQLD » Fri, 27 Feb 2015, 16:09

I'm not doughting the relationship between heat and battery life. I was just questioning the 50% reduction in life with 10 deg c over 23c the battery manufacturer mentioned to him. Having the cells at 33c 24/7 is kind of hot I guess.

My cells are bang on 24c today. Though on a hot day or days (consecutive hot days enough time to heat the thermal mass of the pack) where the ambient temps are 35+ then the cells will eventually start pushing 30c.

Kurt
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Post by PlanB » Fri, 27 Feb 2015, 16:42

The lack of hard data makes us play hunches. In summer down here in Sydney I don't charge the Leaf pack soon as I get in at the hottest part of the day, I plug the J1772 in & set the timer for 2am. Who knows if it does any good?

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Post by weber » Fri, 27 Feb 2015, 16:45

offgridQLD wrote: I'm not doughting the relationship between heat and battery life. I was just questioning the 50% reduction in life with 10 deg c over 23c the battery manufacturer mentioned to him.

I knew that's what you were questioning. That's why I quoted it, and followed it by "There is no doubt whatsoever about this", and gave a link to an article about Arrhenius equation. But I see I must quote the second paragraph of that article here:
Wikipedia wrote:A historically useful generalization supported by Arrhenius' equation is that, for many common chemical reactions at room temperature, the reaction rate doubles for every 10 degree Celsius increase in temperature.
And there is absolutely nothing special about 23°C in terms of cell life. If you run them 10 degrees below that, they will last twice as long, all other things being equal. Trouble is, the beneficial chemical reactions are just as beholden to Arrhenius equation as are the destructive ones, which manifests as increased internal resistance at lower temperatures. Although this is not usually an issue for Lithium cells at the C-rates used in solar power systems, until you go below 0°C.

BTW, the symbol for degree Celsius uses an uppercase C and can be obtained by typing
& deg ; C
without the spaces.
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Post by weber » Fri, 27 Feb 2015, 17:39

Just in case that's still not clear: It is a fair approximation for _any_ battery chemistry, that its life will be halved for every 10°C rise in temperature.

There's plenty of hard data on this. Just google:
LiFePO4 aging temperature
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Post by offgridQLD » Fri, 27 Feb 2015, 17:41

Thanks for the tip on °C I was wondering how you got that. Image

A chart showing all the contributing aspects of capacity loss over the life of a cell would be interesting. What would be the points. Temperature, DOD, Time spent at particular %SOC, charge rate, discharge rate and total energy throughput. Excluding mechanical variation like vibration.


In the end a cell that is never used in a controlled environment might retain it's capacity for the longest (lifepo4 cell in the fridge at a set SOC) never cycled.

I have rough pack temp logging ( adhesive probe stuck in the middle of my pack)from day one. At the end of 1 years logging. I guess I could load all the data into a spread sheet and get some kind of a weighted average temperature number. Though I don't see the number being much off the manufactures temp used for specs. Though the nights are warm in QLD so I might get a surprise.

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Post by weber » Fri, 27 Feb 2015, 18:19

My reading of the graphs in this paper (obtained by the previously mentioned google search)
https://hal.archives-ouvertes.fr/hal-00876555/document
is that the effect of storing at 65% SoC instead of 100% SoC was approximately (over an 8 month trial) a 25% increase in life, but this can be wiped out by a mere 3°C increase in storage temperature.
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Post by offgridQLD » Fri, 27 Feb 2015, 19:15

Sounds like a refrigerated power room isn't a bad idea in QLD. Some of the small - 2kw inverter units now are only a 200w load with a coefficient of performance of of 5. It would help the inverter and charge controllers out to.

Not a big strain on a system to keep everything cool. Wasn't so easy with lead acid batterys but not with lithium the room can be totally sealed up.

Kurt
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Post by Rusdy » Fri, 27 Feb 2015, 19:21

weber wrote: Some great real world experience of LiFePO4 cells in a solar power system here, with recommendations on boost and float voltages.

http://www.technomadia.com/2015/02/livi ... ry-update/


My guess the temperature is the real killer. I've read a whitepaper here (http://www.battcon.com/PapersFinal2008/ ... ROOF_9.pdf) that confirms all of this temperature problem. Also, why Nissan Leaf battery suffers from temperature (as it uses Manganese spinel), where as Tesla doesn't (as it uses NCA).

Quoting the whitepaper:
"
Metal dissolution from the positive is exacerbated by temperature. Manganese (spinel) is particularly soluble, resulting in rapid capacity fading. Iron phosphate also shows solubility above 40 °C to 45 °C, making it less suitable for high-temperature applications such as telecom outside plant. NCA and to a lesser extent NMC exhibit the lowest solubility of their metals, which translates into longevity, especially at elevated temperature.

"

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Post by weber » Fri, 27 Feb 2015, 20:08

Rusdy wrote:Also, why Nissan Leaf battery suffers from temperature (as it uses Manganese spinel), where as Tesla doesn't (as it uses NCA).

I suspect the main reason is that Tesla's cells are actively cooled while Nissan's are not.
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Post by Johny » Fri, 27 Feb 2015, 20:14

Keep in mind that the RV guy is charging his LiFePO4 mostly at the hottest part of the day - "when the sun shines most bright". In contrast we charge EVs at the coldest part of the day and usually discharge mainly morning and evening.
I have had a lot of issues with low charger voltage not quite getting all cells to the balancing point. (I have 12 chargers on board and voltage drift is an issue yet to be fixed.)
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Post by offgridQLD » Fri, 27 Feb 2015, 20:42

I'm not sure if the cells being charged is the issue.

My understanding is just having the cell stored at a elevated temp results in shorted life regardless of then being charged or discharged.

I understand charging the cells increases the temp. Though very little at the rate we are charging them at. Temp gain just from the ambient temps around the cells would most likely contribute more.

Kurt
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Post by weber » Sat, 28 Feb 2015, 00:41

offgridQLD wrote: Sounds like a refrigerated power room isn't a bad idea in QLD. Some of the small - 2kw inverter units now are only a 200w load with a coefficient of performance of of 5. It would help the inverter and charge controllers out to.

Not a big strain on a system to keep everything cool. Wasn't so easy with lead acid batterys but not with lithium the room can be totally sealed up

Are you thinking of the 2.5 kW Daikin Ururu Sarara 7? A lovely piece of kit. Its COP is nearly 6! But it is expensive.

But why cool the whole room? How about an insulated battery box with a little baby heat-pump? Maybe the kind those PC overclocking dudes use? Or a Peltier device?

Wait a minute. Batteries have substantial thermal mass, and in a solar power system self-heating should be minimal, and charging at low rates is endothermic, and it's colder at night. So maybe all you need is insulation and a thermal diode! Remember, you read it here first.

You think I jest? A thermal diode is just a fan that only runs when outside is cooler than inside, with gravity louvres on the box's intake and outlet vents.
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Post by offgridQLD » Sat, 28 Feb 2015, 02:46

I have looked into the Daikin Ururu Sarara 7. I agree they look great if the sales material is anything to go on. For the one air conditioned room inside our off grid house that model will get some careful consideration. Though I haven't committed to anything yet.

Though that model is way overkill - paying for features I wouldn't need just to cool a power room .

Mitsubishi do a 2kw system SRK20ZMXA-S / SRC20ZMXA-S Reliable units and well priced. Very quiet to inside and out.

Though you did get my attention with the idea of a insulated (perhaps cool room material) box with some kind of refrigeration....looks across at the new $80 efficient Aldi freezer to see if 16x400ah cells would fit with the thermostat modified Image


Oh man not another project.... Image

Kurt
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Post by antiscab » Sat, 28 Feb 2015, 18:36

I too wonder what mass a fridge could take - I have a *large* quantity of nimh cells that are very temp sensitive, that I would like to use in a house storage project.

If I can keep their temp below 20 deg C, they're actually quite manageable

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Post by lopezjm2001 » Tue, 03 Mar 2015, 04:50

offgridQLD wrote: I'm not sure if the cells being charged is the issue.

My understanding is just having the cell stored at a elevated temp results in shorted life regardless of then being charged or discharged.

I understand charging the cells increases the temp. Though very little at the rate we are charging them at. Temp gain just from the ambient temps around the cells would most likely contribute more.

Kurt


Image

The graph above shows the storage life of Lifepo4 cells at elevated temperatures without even using/abusing them Image

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