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PW Life: Cost Savings vs Self Powered or Balanced?

Mar 15, 2021
64
30
California
I guess my question is what is best for the lifespan and efficiency of the batter over time, to allow the battery to charge and discharge more in self powered or balanced (where the battery may discharge to less than 50% to before charging back up to 100%) or cost savings (where the charge percentage may discharge to around 80% before charging back to 100%)?
 
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BGbreeder

Member
Jun 19, 2020
143
79
Bay Area
That is the trillion dollar question.

We are all part of the experiment and gathering data.

There are small studies that suggest being 100% full all the time is bad, high discharge is bad, and that "deep" discharge is hard on the battery electrodes, but there are bucket loads of crappy papers out there that make assertions not supported by their experiments or data.

In all seriousness, this is being worked out in real time by Tesla car and Powerwall owners. Tesla feels good enough about their batteries to warranty 70% capacity at the end of ten years without putting footnotes on details like a percentage of charge. Beyond that, I think this is really a wait and see. I think of batteries like tires; take care of them and they will last a longer time before they wear out, or abuse them and buy new ones much sooner. Either way, they will wear out. Fortunately for end users, the controllers on battery systems keep end users from doing things that are egregious (e.g. huge discharge currents, aka "jack rabbit" starts), so the battery packs are more or less bubble wrapped.

Details really matter in batteries; design, chemistry, processing, manufacturing, thermal status in use, charge control, accuracy of charge control to name a few. Almost all of it is not under the control of end users. Sony had a huge recall of lithium ion batteries a number of years ago that basically came down to how they cut an edge of an electrode in manufacturing. I think that perfection is hard.

I do think that the large volume of cells and battery packs that Tesla is producing works in favor of high actual MTBF. Based on Tesla's Chinese production, I would expect Powerwalls to transition to different battery chemistry in the future.

YMMV...

All the best,

BG
 

RKCRLR

Member
Apr 13, 2020
285
106
Garden Valley, CA
The Powerwall is warrantied for unlimited power cycles for 10 years in solar self consumption/backup modes. However, it is warrantied for 37.8MWh in aggregate throughput in all other modes. Tesla doesn't mention any restrictions on how to charging/discharging is performed.
Until there is actual data on Powerwall life vs how it is charged/discharged I'm going to assume the 37.8MWh is good number and I'm not going to micromanage my system.
If you discharge the Powerwalls an average of 50% daily that is 15 years of life. Discharging an average 0f 40% daily is 19 years of life. At that point I suspect the Powerwall is more likely to die of old age than how many cycles or how it was charged/discharged.
 

h2ofun

Active Member
Aug 11, 2020
1,615
262
auburn, ca
The Powerwall is warrantied for unlimited power cycles for 10 years in solar self consumption/backup modes. However, it is warrantied for 37.8MWh in aggregate throughput in all other modes. Tesla doesn't mention any restrictions on how to charging/discharging is performed.
Until there is actual data on Powerwall life vs how it is charged/discharged I'm going to assume the 37.8MWh is good number and I'm not going to micromanage my system.
If you discharge the Powerwalls an average of 50% daily that is 15 years of life. Discharging an average 0f 40% daily is 19 years of life. At that point I suspect the Powerwall is more likely to die of old age than how many cycles or how it was charged/discharged.
Do we not think in 10 years there will not be a new killer battery technology and we will just throw the PW's away and replace them
 

h2ofun

Active Member
Aug 11, 2020
1,615
262
auburn, ca
If my Powerwalls die after 10 years then that is quite possible what will happen. But I'm not throwing them away until they die.
Depends. If newer batteries are a LOT better, and one cannot add anymore because of code changes, who knows. Just like some are replacing their old lower wattage PV panels even though they are not dead
 
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RKCRLR

Member
Apr 13, 2020
285
106
Garden Valley, CA
Depends. If newer batteries are a LOT better, and one cannot add anymore because of code changes, who knows. Just like some are replacing their old lower wattage PV panels even though they are not dead
I'm currently at 50% reserve and will likely have a surplus at true-up so I'll adjust my reserve even higher. I'm locked into NEM2 for 20 years. The rates would have to change so drastically in 10 years that it would justify replacing working batteries with new batteries and changing to NEMX at that time. I don't see it happening.
 

h2ofun

Active Member
Aug 11, 2020
1,615
262
auburn, ca
I'm currently at 50% reserve and will likely have a surplus at true-up so I'll adjust my reserve even higher. I'm locked into NEM2 for 20 years. The rates would have to change so drastically in 10 years that it would justify replacing working batteries with new batteries and changing to NEMX at that time. I don't see it happening.
Never say never. :)
 
Mar 15, 2021
64
30
California
My question was just more of one of curiosity. I go back and forth between Balanced and Cost Savings. The only difference other than all solar first charging the battery or net solar charging the battery, is how much power the house uses from the battery.

The battery also charges slower on balanced since it is only net solar instead of all solar going to the battery.
 

holeydonut

Supporting Member
Jun 27, 2020
1,027
606
East Bay NorCal
I don’t think anyone wants to be reaching for a disconnect that is surrounded by a run-away Li-ion fire...


I think this disconnecting means is for the safety of the person working on the ESS. I didn't really think this "line of sight" thing was for fire safety.

But yeah, if my Powerwalls burst into flames it's taking out the entire wall that has my precious fire safety placard. So if I have a Li-ion fire, I think the firemen are just going to stand back and watch my house burn.
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
7,749
8,559
Riverside Co. CA
I think this disconnecting means is for the safety of the person working on the ESS. I didn't really think this "line of sight" thing was for fire safety.

But yeah, if my Powerwalls burst into flames it's taking out the entire wall that has my precious fire safety placard. So if I have a Li-ion fire, I think the firemen are just going to stand back and watch my house burn.


(this was ment as a tongue in cheek joke about disconnects, electricity, etc)
 
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