diy solar

diy solar

Monitoring pack pressure for safety

wow.. you are overdoing it. Totally waste of money. It is your money tho. W/e makes you happy go for it
Yea, expensive. If the spring constant is known a fairly good measure of pressure should be had by just measuring spring height. I would probably go the load cell direction if I wasn't retired and poor.
 
i do feel a bit silly doing this; it’s to collect data ?‍♂️ no other way to satisfy my curiosity.

planning on posting the data for free anyways because i wanna keep the knowledge accumulation ball rolling ?
 
Hi all,

Nice to meet you !

I was thinking about this a bit out of the traditional spring / sponge / plywood solution.

The pressure resistors commonly found seems to fit the 12 psi measurement and digitalization via arduino board.The rough ideea is to fit them to measure pressure than send it to a arduino board that in turn will trigger a step by step motor to turn a pressure screw and modify the pressure to the needed level ?. Basically a feed-back loop. In the same time arduino can monitor both pressure and temp and send alarms or release pressure etc…

When i will receive my first lifepo pack i will start playing with ??
 
Build battery, place in sturdy box, cover with clean mason’s sand. Compression, thermal sink, and fire protection.
 
Hi all,

Nice to meet you !

I was thinking about this a bit out of the traditional spring / sponge / plywood solution.

The pressure resistors commonly found seems to fit the 12 psi measurement and digitalization via arduino board.The rough ideea is to fit them to measure pressure than send it to a arduino board that in turn will trigger a step by step motor to turn a pressure screw and modify the pressure to the needed level ?. Basically a feed-back loop. In the same time arduino can monitor both pressure and temp and send alarms or release pressure etc…

When i will receive my first lifepo pack i will start playing with ??
Well ..... springs would be a LOT easier .... but that definitely sounds like fun.
 
due to a number of factors, availability and ease of engineering mainly, i have decided to go with an S type strain gauge 200kgf, four units.

it has one M12 attachment on each side. instead of using a threaded rod to compress pack, using four of these with threaded rod on each side.
that way, it's as if there's just four threaded rods holding the pack together between two metal plates (and some insulation..)
but each threaded rod can report the tension on itself.
read by load cell amplifier

for assembly i would observe the four readings and tighten until readings match calibrated target.
like airplane orientation readout.

14282-03.jpg
14282-02.jpg



so there'd be four total. upper left, upper right, lower left, lower right.

each one has 200kgf full scale capacity. due to the geometry splitting the load and 150% load capacity rating i feel pretty decent about it being able to hold.

got four 300Ah cells with threaded studs pre-laser welded. planning on relying on oshcut to get some stainless steel sheets laser cut and maybe bent to create the battery case.

it's gotta happen! for science.

adding this diagnostic feature will increase the BOM by about 300usd for four load cells and four load cell amplifiers. this is for research and data collection. however.. it absolutely will be able to detect a very clear signal if the cells were to swell in a failure mode.

edit: this is the work in progress CAD model
View attachment 69996
I really love your plan, the idea is great.
Any update on progress or data?
I'm curious if it's worth investing the 300 bucks.

Comming back to the original threat question. I think a pressure sensing system to trigger an alarm is to slow and delayed when messuring the bloating of the cells itself.
The idea of measuring the bulging of the vent cap would be more sensitive but still delayed in comparison to excessive heat that appears out of a sudden.
Adding several temp sensor probes to the cells (you could stick them into some foam layer in between the cells) will tell you much more quickly and precisely if something is going wrong in your pack.

In addition one could combine that with a sensitive CO2 sensor inside the pack's box. The cells are not 100% gas tight, which means that as soon as there will be a certain amount of CO2 inside your box, there is something shitty going on inside one of the cells.

BTW none of the EV battery pack producers monitor pressure or CO2 levels inside their battery packs. It's all done with temperature probes... and I mean a lot of them.
 
Appreciate all of this insight (and it’s exactly what I was hoping for when I started the thread).

As far as a bad electrical connection being a far higher risk of starting a fire than a shorted cell, we are in total agreement.

As far as a fire starting from a bursting cell, that’s not really the failure I was worried about.

I’ll lay out my working hypotheses and would appreciate your insight:

1/ a cell shorts (possibly from wiring failure, possibly from internal terminal failure due to mechanical stress, possibly due to some other mechanism such as dendrite formation possibly combined with a manufacturing defect (Chevy-Bolt-like, and yes, I understand the chemistry is different).

2/ high inter-cell discharge generates a excess heating of the generator me shorted cell.

3/ as the shorted cell heats up, it heats up the adjacent cells above normal operating temps.

4/ as the adjacent cells heat up above normal operating temps, they bloat beyond normal operating levels, resulting in pack pressure exceeding normal operating ranges.

So it was pack pressure exceeding normal operating range that I was interested in, in the hope that the pack pressure might increase from that effect before the primary shorted cell can reach the temps needed to start combustion.

I have no idea whether cell pressure/bloat even increases with increasing temps, let alone whether any such effect would be ‘fast enough’ to be an early indication of a shorted cell heating up y to k the point it could start a fire.

But it was not the risk of a bursting cell I was going after as much as the risk of an individual cell which becomes shorted (for whatever reason).

Maybe I missed something in this thread but if the above is your concern wouldn't you be better and more easily served by a temperature sensor?
 
I really love your plan, the idea is great.
Any update on progress or data?
I'm curious if it's worth investing the 300 bucks.

Comming back to the original threat question. I think a pressure sensing system to trigger an alarm is to slow and delayed when messuring the bloating of the cells itself.
The idea of measuring the bulging of the vent cap would be more sensitive but still delayed in comparison to excessive heat that appears out of a sudden.
Adding several temp sensor probes to the cells (you could stick them into some foam layer in between the cells) will tell you much more quickly and precisely if something is going wrong in your pack.

In addition one could combine that with a sensitive CO2 sensor inside the pack's box. The cells are not 100% gas tight, which means that as soon as there will be a certain amount of CO2 inside your box, there is something shitty going on inside one of the cells.

BTW none of the EV battery pack producers monitor pressure or CO2 levels inside their battery packs. It's all done with temperature probes... and I mean a lot of them.
have not assembled yet.

need to make the end plates. already got all the load cells and threaded rod and of course 302Ah catl cells

no intention of building more than one pack with Load Cells, for now. BOM increase undesirable, and it's kind of a research peek in the dark. so i cannot recommend it because i have no experience with it yet.

for fire time ouchy detection, Thermometer as you say and this also Laser Particulate Sensor, PMSA003I, which is used in the Purpleair citizen air quality monitoring network https://www.adafruit.com/product/4632 and https://map.purpleair.com/map single PMSA003I sensor 45usd, after living through multiple ca wildfire smoke seasons and watching the network based on PMSA003I laser particulate sensors (2 in each purpleair unit actually for detecting hardware failure, divergence signals one broke) i trust this sensor to notice smoke before me

1642107228585.png

the large opening on the left side is the Air Intake fan. smaller one on right is exhaust. it blows air at a steady rate through a dark room with a laser shining through the air stream, and a photodiode counts the flashes of lights caused by the thousands of particles flying by. Just Like sitting in an a dim room with curtains drawn and a single beam of sunlight illuminating the dust floating by.
1642107779655.png
For Temperature, my favorite is One Thermometer Per Cell (OTPC) and shutdown immediately if any single cell is more than X degree above the rest (e.g. 5-10C above any other cell means shutdown) should be very sensitive and few false positives if tested and implemented accordingly.

perhaps check this short vid

video go over cooling the Tab/Terminal of cell vs cooling the Wall of the cell and say cooling the tab/terminal is much more effective, because there is a direct metal thermal path into the cell from the tab (tab traditionally is one major spot where heat evolves also)

so one thermometer on each cell, either directly mounted on terminal or lug or cable/busbar.
https://www.adafruit.com/product/4890N e.g. this NTC for 0.95usd/unit affixed to cell
1642107205174.png

this topic interests me, diagnostic system design is fun

my personal goal in building DIY is long operational life, minimal maintenance, early warning to any failure
 
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wow, i missed this at first. the video about surface vs tab cooling says

at 6C discharge the Surface Cooled cells had 9.2% capacity degradation after 1000 cycles
at 6C discharge the Tab Cooled cells had 1.2% capacity degradation after 1000 cycles

anyways, thread topic is monitoring pressure of pack for safety. so temperature is a little on the side maybe :)

temperature and internal pressure of cell may be connected more than i assume. wish to learn more!

takeaway: if monitoring cell temperature and want fastest update response to "real value" (internal temp), thermally couple the sensor on or near terminal.
 
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