Same thing happened with my Sky. Guess we’re all getting to the year of battery life. Funny thing is that my Sky is dead - no updates since yesterday morning - but I have received no alarms. Nothing from the app or on the web. Waiting on the Amazon van to bring new batteries today.
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They drop so quickly that it is darn near impossible to throw an alarm to warn that they are about to expire…
I didn’t even get an offline alarm. I did receive a notification and email when it came back online after the battery change out.
That is a normal characteristic for lithium polymer batteries… they deliver to the end and then just die. Sort of like a total cardiac arrest.
Most battery types go through a gradual charge decline and provide some warning of their demise.
Rechargeable lithium polymer batteries are the same way… sudden death. Lithium ion are more user friendly, as are zinc/carbon, lead/acid, alkaline, NiCd, NiMH and several others.
Just swapped out the SKYs batteries (Ultimates). Lasted pretty much 8mths (power save mode disabled).
Remaining voltages:
Top group of 4:
1.64v
0.61v
1.11v
1.08v
Bottom group of 4:
1.15v
1.60v
0.61v
1.13v
I should have kept the vertical pairs together as that’s probably more relevant.
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The upcoming solar panel will save you all that trouble. The field tests were awesome. Keep tuned to WF.
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Check it out: Solar power for SKY
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Brilliant! ordered. 
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Just installed my solar power unit for my sky. I’m so happy. Took maybe five minutes and now I don’t have to worry about sudden lithium death syndrome anymore.
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I meant to also give a shout out to the amazing WeatherFlow team. They said they would deliver us a great product and they did. They said they would give us solar power and they did. Top notch work - congrats all around for everything you have done! I’m a big fan.
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I received my SPAs for my two SKY units but 5080 was showing below 3V so I decided to let it run until it died. It was a guessing game of, “will it die while I’m gone or or not?” This morning it was at 1.85V but went up before I left. Strangely, it died at 1.93V. I know what I’m doing when I get home.
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For those who are interested, here are the voltages of the Energizer Ultimate Lithium cells taken out of Sky 5080 after the unit shut down. These are in order.
Top set: 1.465 1.469 0.3936 1.246
Bottom set: 0.3527 0.3847 1.430 0.586
Totals: 1.8177 1.8537 1.8236 1.832
Note that the arrangement of these cells is a paralleling of a series set of cells rather than putting in series a set of parallel cells. The latter is what Tesla does with its battery packs and what I did with the car I converted from lead-acid batteries to LiFePO4 cells. Other than the disadvantage of more points of contact, I would expect that the Sky would have run longer if there were a conductive plate between the two sets of cells. This would have made sure that all four cells in a set went dead together, rather than what you see in this particular case.
If anyone wants to test this out make sure you do NOT use aluminum since aluminum oxidizes rapidly, forming an insulating layer of aluminum-oxide on the surface. Use a thin sheet of nickel or copper unless you just happen to have a sheet of silver laying around. 
I don’t plan on trying this since I now have the SPA for each of my units. 
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the question arises what will happen if you connect a battery with 0.38V in parallel with one of 1.430 Volt. That might not be good. But it might in practice also never happen (provided you start with good fresh batteries), as they probably will drain in such a way that the voltage is the same on both batteries.
Another quesition is, does it help, or will for example the bottom set drain to 0.4 V and the top set to 1.4.
If it does help, I’m interested as I’m still in doubt about the solar panel, mainly for esthetic reasons. If I decide to buy the panel, it would be mainly for environmental reasons.
I’m not going to have pointless discussions with you again. I removed my part of one pointless discussion a few days ago, did it a long time ago for another discussion, left another one up. but I’m going to try to ignore you. if you could do the same with my postings, that would be appreciated. ( @buckeyeRay 
)
Careful, you’ll get flagged like I did.
I haven’t tested paralleling primary Lithium cells so I’m not exactly sure what would happen. If I paralleled the set I removed above right now, probably not much would happen as far as safety is concerned since all the cells are depleted for the most part. If I had started out with a paralleling sheet the voltages would have quickly come in line and then the highest capacity cells would have carried the majority of the load. I don’t think the ESR of the individual cells would have a very large effect with such low current. In my set above it is obvious that the lower set would have gone below 1V first given that it had three really low cells. I wish I had thought of doing that in the beginning.
I have paralleled two 40Ah LiFePO4 cells with one near 100% SOC and the other below 40% SOC. I used 1AWG cable with a 500A 50mV shunt. The initial current was ~100A and dropped to the single digits within a few seconds, well within the design specs of the cell. FWIW, these numbers are from memory as I did the test nearly 10 years ago.
For the 2p20s pack I built, I did a rudimentary capacity check of the cells then tried making pairs with the same total capacity. It has worked great.
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My second set of Sky Energiser Alkaline batteries finally died after 6 months and 4 days.
I moved my Sky a few times during their life and the voltage changed slightly sometimes after movement.
This is my voltage chart from 1st December 2018 to 4th June 2019 at 1.78V and the lowest temp 9degC.
https://smartweather.weatherflow.com/station/4555/graph/12620/battery/5
The discharge curve to give notice of expected failure of those cells gave this voltage: to this number of days remaining:
new
3.25V:185days,
3.09:181,
3.0:175,
2.9:156,
2.8:132,
2.7:114,
2.6:87,
2.5:57,
2.4:47,
2.3:32,
2.19:25,
2.1:20,
2.0:17,
1.9:9,
1.85:8,
1.8:5,
1.78:0
The voltages of the cells in matching order was
Top set: 0.93, 0.94, 0.93, 0.94
Bot set: 0.93, 0.93, 0.93, 0.925
My examination of the compression force on the cells within the Sky confirmed there is room for some extra compression.
I couldnt find my sheets of brass so I thought I would try aluminium foil (Yes @gizmoev I hear you) folded to create a thick layer forcing the cells tighter AND into parallel configuration:
So I am now trialling that configuration.
cheers Ian
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That set of cells was very well matched. Way better than my set was. It will be interesting how the paralleled sets do.
@iladyman Gizmoev posted this remark some time ago:
Hi @sunny,
Thank you 
Yes I remember Gizmoev’s warning which is why I mentioned him.
Prior to Gizmoev’s comment I had not considered that corrosion would be a problem. I was thinking about doing it due to the fluctuating voltages I had experienced after movements. I am curious to test it. I am trying it because I couldnt find any better conductor with the time I had. It is a simple easy solution for anyone to implement so ‘IF’ it works then it might help those experiencing battery problems with unequal discharge, which might also be aggravating the lithium voltage cliff. Which might be due to unequal pressures between the cell pairs and the lack of a parallel connection between the pairs so increasing the pressure onto the cell pairs with several layers of foil might help. Aluminium is used in electrical connections quite often. In my younger days I worked as a linesman and we regularly used aluminium crimp lugs and other aluminium electrical components in high voltage work but we used special yellow grease to reduce corrosion. Aluminium corrodes from the moment it is exposed to air. It only needs to last 6 months and will not hurt anything if it fails. I like experimenting to see if the simplest solution works?
cheers Ian
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