My Open Source Solar BMS and Digital MPPT

[Korishan]

Most of you have used cells in the 2000mAh range but when new those cells where around 2400 to 2600mAh that means they are very close to the end of usable life so if new they where good for 500cycles and if now in average they are just good for another 100 cycles

Yes, this is true, mostly. This is assuming we are using the cells from 4.2V to 2.7V full range. However, most are using the cells from 4.0V to 3.2V, which drastically increases their cycle life. Hence the reason for so many cells in the packs.
Daromer has been running at least 1 string for almost 4 years (if I remember correctly) now. There's others who have ran theirs for 2 or more years as well.

We have to remember that the life expectancy of the cells is dependent on how hard we charge/discharge them, as well as what the working range in voltage is. These two factors helps us to get far more life out of the cells than what the datasheet states.

However, you do have a valid point. This is one reason why I am going to be building my packs with ease of replacement and monitoring so I know which cells need to be replaced. And this will be for new or used cells.

[camthecam]

How depressing...

[emuland-metroman]

here lifepo4 18650 from scap..(only 100 cartridge 5P ready of 1000)

i ll take a look of Your BMS because i like ti have items that do only One job.

great prpject!

[daromer]

Welcome.

[electrodacus]

Yes, this is true, mostly. This is assuming we are using the cells from 4.2V to 2.7V full range. However, most are using the cells from 4.0V to 3.2V, which drastically increases their cycle life. Hence the reason for so many cells in the packs.
Daromer has been running at least 1 string for almost 4 years (if I remember correctly) now. There's others who have ran theirs for 2 or more years as well.

We have to remember that the life expectancy of the cells is dependent on how hard we charge/discharge them, as well as what the working range in voltage is. These two factors helps us to get far more life out of the cells than what the datasheet states.

However, you do have a valid point. This is one reason why I am going to be building my packs with ease of replacement and monitoring so I know which cells need to be replaced. And this will be for new or used cells.

When you say cycle life you always need to mention the depth of discharge but for those type of cells the rule is close to being proportional so say DOD is 50% then you will have 2x the cycles at that DOD but then total energy stored over the life time will be the same if you do 100 cycles at 100% DOD or 200 cycles at 50% DOD or 2000 cycles at 5% DOD

If you barely use the battery then they will last even 10 years the degradation over that period in this case will be mostly related to calendar degradation so cost amortization may be even higher.

This high amortization cost for high energy density cells is also the reason why an EV like Tesla will be more expensive than an equivalent ICE
If you look at the warranty of model 3 long range that has the new cells (supposedly better).
Model 3 has a 192000km or 8 year warranty whatever comes first and is based on a 80kWh high energy density NMC
That model 3 has a range of 500km on a full charge (almost 100% DOD excluding the small top and bottom reserve) so 192000km / 500km = 384 cycles
Now the old cells if you look at Panasonic spec sheet is capable of 500 cycles at 100% DOD and by that time it will only have 70% of original capacity.
The warranty also claims 70% of original capacity in 8 years or 192000km thus it means they took in consideration the worst case that is 384 cycles about 90% DOD as there is some 5% top and bottom limits. People will also not drive like that they will likely charge to 80% and almost never get close to 0% SOC so they should be fine with around 15% or so degradation from cycling and the other 15% degradation for aging assuming around 2% per year calendar degradation that will be typical for EV due to higher battery operating temperature.
That 80kWh battery still cost around $16000 (that excludes the BMS and other stuff around) thus you pay that extra compared to an ICE car and when you consider that extra cost it will be $16000 / 192000km = $0.083/km or put in another way $8.3/100km and that is more than what my inefficient 6 cylinder ICE will require in gasoline cost. That is just the battery amortization cost not counting the electricity to charge the EV. This will be about the same for all EV's Tesla was just an example.
Put in a different way $16000 / 70kWh x 384 cycles =  $16000 / 26880kWh = $0.59/kWh and that will be the cost amortization for that battery over the life time of the battery.

If they could offer a better warranty they will do so but they can not as those are the limitations of the specific battery technology. That is more than adequate for the average driver so is a good choice of battery for the specific application but as you seen in my earlier comment LiFePO4 can do much better at just 0.16cent/kWh so LiFePO4 is much better for stationary energy storage.
LiFePO4 will not be so great for an EV as is about 2x lower energy density so you will have a car that costs the same but has half the battery capacity and half the range so people will not by this. BYD uses LiFePO4 in their buses and taxis since those type of vehicles will need to put much more km than a consumer personal car.

For me cost amortization over the life time of the product is very important and I calculate that for anything.

That is how my small energy efficient 65m^2 (700sqft) house cost a total of $42 / month in therms of cost amortization for both electricity and PV electric heating about half of that amount for each and it is the most cost effective solution even when compared to natural gas.

[Power 2 Spare]

Sent you a private message, hope you don't mind.

[daromer]

Eletrodacus: Same as I do. Its always about total energy or cost over time that is essential and not if they will do x or y cycles or they have x or y capacity but over time capacity or cycles

Community is still a bit young and many still stare alot over cycles or voltage ranges or capacity in cells but forget about how much it will yield over set amount of time.

[Charly144]

Electrodacus. Maybe i am blind but i don’t find a detailed description of tthe bms0 somewhere... can you pls give me a hint!

Thx
Karl

[w0067814]

That is how my small energy efficient 65m^2 (700sqft) house cost a total of $42 / month in therms of cost amortization for both electricity and PV electric heating about half of that amount for each and it is the most cost effective solution even when compared to natural gas.

Slightly off topic...
I linked to another forum where electrodacus is talking about his house here, where Dan is also talking about going off-grid in sunny Wales.

[jdeadman]

For me it's about using what is going to be discarded. I ran my entire Camp all summer on 3Kwh of Storage with only 200W of Solar but Since I was only out there 4 days out of 7 it was all I needed. this was my test bed. 7s56p and it worked flawlessly. This bank also ran 80%DOD All last winter before taking it out to camp. so 8 Months of 80%dod and 4 months of 30% dod and the cells were just finished being re-tested and all but 9 cells were the same capacity or better than originally tested.

So this leads me to believe that as long as you are using good testing cells that are fairly close to their rated capacity a low dod Wall should last for a long time.

Now this is just a hobby right now for me. but in the next 10 years or so I plan of buying some land and building a retirement home which will be offgrid. I have been researching what batteries I want to use and since it's so far off I have time to play now.

Just a side note the 8 months I had my camp wall running at home I only ran my fridge, freezer and Internet in a time shifting mode. where during Peak pricing It ran off my wall and during low cost power I charged that bank. No solar at all. I used what I had gotten for free for everything other an the busbars, solder and battery holders. ( I think I spent ~100$ on that and test equipment) and I was saving about $20/month just doing that. That savings was the difference from last years bill to this years bill where the prices did not change much.