Battery Discharge level for Home Solar System

[Romer]

I set my Battery discharge level to 85% over the winter so if the Grid went down in bad weather, I would be able to extend my available power

This means I draw from Xcel after 10PM when they hit 85% until the panels come alive in the morning The charges are offset by Net solar bank credits

Last summer I had them at 50% and it drew from the batteries overnight without using the grid except in a few cases

With net metering, it would seem there is no impact on how much I pay. I believe that is true

All I can find is best Depth of Discharge rates are between 50-80%. My only thought is about lifetime impact on the batteries and not sure there is a difference for Lithium

Didn't find much of anything on Google

For now going down to 70% for no particular reason

Anyone dig into this topic?

 

[gungriffin]

Funny you should ask this. I was watching a video about something like this today, and I have looked into this quite a bit. Here is the summary that I would say. You will likely experience more loss from calendar aging of the batteries than losses from cycle count no matter what the DOD is. This is especially true because you are using such a low C discharge. A 1C discharge is equivalent to discharging the cells from 100% to 0% in 1 hour. 0.5C would be discharging them in 2 hours. Your cells are very very high quality Tesla cells. That means that even at 100% DOD, you should still get at least 3500 cycles with 100% DOD at 1C before you reach degradation equivalent to 80% of the cells original rating. That just means that using the cells way harder than you are now, you should be good for 10 years from the date of installation. This is all to say, I wouldn't sweat placing the setting to whatever the highest DOD that Tesla allows under the battery warranty.

Here is the link to what I was watching that I refer to above. It is on cell compression, but it also covers everything that I have said above. The additional good news is that most likely your cells are compressed to perfection due to being an incredibly engineered product. This will optimize the cycle count.


View: https://youtu.be/OsvPYw6hoHc

 

[Romer]

Thanks! That is a great video. They are LG Cells but should be the same

 

[DaveInDenver]

Keep in mind that a battery cycle is a full discharge or equivalent. So two 50% cycles is basically the same aging as one 100%. But it's absolutely the case that most of the time you never get close to cycle life due to other reasons, be it incomplete or improper charging, ineffective temperature corrections, etc. The old "never go below 50%" is for lead-acid and that's not even really true. In any case lithium doesn't care if you use 5% or 100%. In fact what is more harmful for lithium is not discharging them. Shallow discharges and constantly topping them is worse.

There's some texts the come to mind if you can find them.

Valve-regulated Lead–Acid Batteries by Rand and Mosley
The Battery Reference Book by Crompton
The Handbook of Batteries by Linden and Reddy
Handbook of Photovoltaic Science and Engineering edited by Luque and Hegedus (just a couple of chapters on storage)