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The story of a little powerwall
#21
127 out of 300 cells is pretty good ratio. I would say around 30-35% is normal.
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#22
Now i do know why people hate (and ignore) my "lucky" thread. Big Grin

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#23
First system integration test.

BMS communications: online.
Temperature sensors: chilly room temperature.
Charge relay: clicking, controls leaking.
Discharge FET: switching.
Heaters: heating.
Charger: charging after a good whack.
Power priority sequencing: seems correct.

Next steps - fix bugs, test with real cold, test with real panels, test at full load.

You can follow my projects at https://orbides.org
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#24
(11-20-2018, 11:52 PM)Artlav Wrote: Despite the lack of snow, it's well below freezing outside here.
And Li-ion does not like being charged in the cold.

The BMS logic would be "if below +5*C and sun is on, heater on. If above +5*C or no sun, heater off. If Tb<0*C, charge off".



Be careful not to mistake sensor temperature with cell core temperature.  A cell thoroughly cooled to -20*C might take an hour to warm up, while the sensor might register positive *C within minutes of turning the heating element on.  Also, the middle of the pack will likely be warmer than the cells at the edges.  Also, best to be conservative and disable charging if temp<3*C or so?

My recommendation would be to:
* surround sensor with cells on all sides, or
* implant sensor into a fake cell, perhaps filled with sand, and/or
* Software solution: 
   once charging was disabled due to low temp:
   do {
   heat to +5*C
   turn heater off
   wait 3 min
   }  loop while temp<3*C   // the sensor cooled rapidly, probably because the cell core was still cold.  Reheat again
   charging on
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#25
Da bomb have been planted for live fire exercise.
Tomorrow's forecast is a day full of crispy cold sunshine, conditions can hardly be more ideal.
Hopefully there will be enough power to both warm the cells up and charge them... Hopefully.

So far the BMS only does logging to microSD card, no way to see how it going live. Will have to check tomorrow.

To recap, 55W of solar input, ideally, four 7W heaters taking half of that, -15*C outside, +3*C needed to start charging with the remaining 25W or whatever the winter sun would provide.





(11-27-2018, 07:08 AM)ajw22 Wrote: Be careful not to mistake sensor temperature with cell core temperature.
The probes are deep in the middle and the heating is rather slow, so i expect everything to get heated more or less evenly.
And sure, the heaters would cycle around +5*C or so. Stop at +5, restart at +3, to be precise.
You can follow my projects at https://orbides.org
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#26
Data time!
So, at best i get about 45W solar input this time of the year, 10:30 to 16:30 with a fir tree hiatus between 11:00 and 11:30.
Clear sky, sun at 13* altitude maximum, panels angled accordingly.

It took the cells the whole night to cool down from +20*C to -10*C, at a rate of about 1*C every 15 minutes.
The first hour of sunshine was lost to a software bug, then the cells started heating.
At about 5 minutes per 1*C it took them an hour to reach +3*C and start charging. 22W of heating.
The heaters turned off a bit later, and cycled again twice for 15 minutes to keep the temperature between +3 and +5.
In total, about 40% of available energy over the day was lost or spent heating up and keeping the cells warm.
Interestingly enough, the middle two packs took 2 hours to cool down 2 degrees, while side ones took an hour and 10 minutes.

All in all, about 10% charge was gained over the day...
At the consumption of about 11% charge per day and one sunny day a week this won't really last.

Still, fun was had.


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#27
Very nice experiment Artlav! What is your location that you have these temperatures at this time of the year allready?

Br, Rik
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#28
So it's been 3 sunny days.
As i feared, one of these days is barely enough to account for a day's consumption, so the battery stayed around the same charge level for the first two, then went down by the end of the third due to some clouds in the morning.
On the bright side, it had no problems getting warm from -15*C.



(11-29-2018, 09:45 PM)RikH Wrote: Very nice experiment Artlav! What is your location that you have these temperatures at this time of the year allready?
Western parts of Russia. Haven't been much of a fall here, went from 89th of August straight to early December.
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#29
(11-29-2018, 05:23 PM)Artlav Wrote: In total, about 40% of available energy over the day was lost or spent heating up and keeping the cells warm.
Interestingly enough, the middle two packs took 2 hours to cool down 2 degrees, while side ones took an hour and 10 minutes.


Makes perfect sense.  The middle packs only lose heat on one surface (the top), whereas the packs on the side lose heat on 2 surfaces (side & top).  
I think you can drastically reduce the time&energy needed to heat up the battery by building one big pack with many more layers of insulation.  Though you may have overheating issues in the summer then.  Perhaps simpler to just put the whole thing into a cooler box, then take out in the summer.
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