Does anyone know the voltage and ah ratings for tesla powerwall. Is the 13.5 kWh the max charging cell voltage multiplied by the cells ah rating? Or is the 13.5 kWh derived from each cells nominal voltage multiplied by ah? I just used tesla as an example, but if anyone knows the answer for any type of powerwall I would appreciate it.
powerwall spec determination
- Thread starter Hodge
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sunshine_eggo
Happy Breffast!
No. Datasheet says 50V, but it's not clear if that's a peak charge voltage or nominal voltage. I would assume it's 12S, and that's a peak charge voltage (4.17V/cell, 12S). This is also consistent with Tesla powerwall typically being comprised of two Model S 6S modules in series.
No.
Close.
Wh = "cell" nominal voltage * quantity in series * "Ah"
For a 280ah 24V LFP battery (8S):
3.2V * 8 * 280Ah = 7168Wh
"cell" and "Ah" is in quotes because that refers to whatever comprises an actual cell or group of parallel cells. A 280Ah EV cell is a single prismatic cell, but a powerwall has many, many smaller cells in parallel that forms each "cell".
Ampster
Renewable Energy Hobbyist
I believe the earlier models were higher voltage but since they are not user accessable it probably doesn't matter. The Powerwall 3 is LFP and can take DC solar input in addition to AC coupled solar like the older versions.
So a nominal 24V powerwall is actually 25.6 volts for those cells. When they calculate/advertise the battery capacity it would appear that they are taking each batteries cell at its high end float charge for their calculation.
The reason I am asking all this is because I am in the middle of a battery rebate application, and my system is flooded lead acid batteries. The nominal voltage of each cell is 2V but each cells high end float charge is 2.25V, which is a big difference when calculating in battery capacity for the dollar amount for the rebate. So in my case, they would have to do the math themselves, but in these powerwalls, the math is already done for them and the capacity is stamped on the side of the powerwall. I was wanting to know what voltage they are using for their math when advertising its capacity. Is it nominal voltage or the high end float voltage?
sunshine_eggo
Happy Breffast!
Revised for clarity.
A "24V" 8S LFP battery has a nominal voltage of 25.6V. While it's nominal voltage differs from 24V lead acid, it's full working voltage range matches lead acid VERY well and is almost a "drop-in" equivalant.
No. 25.6V is a nominal voltage, 8 * 3.2V but the cells may be charged to 3.65V safely.
I was speculating on the meaning of the 50V. I would regard it as peak voltage for the reasons given if they are still constructed from 2x 6S Models S modules in series.
With lead acid, your capacity is calculated based on 2V/cell. Period. ALL batteries are rated via nominal voltage.
Battery capacity in kWh = Cell capacity in Ah * cell nominal voltage * # of cells in series.
If you had a 1000Ah 24V lead acid, 1000Ah * 2V * 12 = 24,000Wh or 24kWh, 50% of which is usable.
The voltage of the Tesla powerwall is mostly irrelevant to you. The stated energy content is correct. It doesn't matter if it's 12V, 47V, 50V, 150V, etc.
What is the Ah and voltage of your battery?
Ampster
Renewable Energy Hobbyist
More important is probably the definition on your rebate and whether it is $/kWhs. LFP cells have a nominal voltage of 3.2 volts and NMC are 3.7 volts..There is no standard "powerwall" unless you are referring to a Tesla Powerwall and as mentioned, that is about 13kWhs.
Earlier @sunshine_eggo explained the logic to compute capacity.
Mine are 1944 ah for a 48V system, with 9 batteries in parallel. It is more than 50% useable as these cells are vented flooded lead acid batteries, not valve regulated lead acids. Its pretty comparable to what a BMS would cut off a lithium bank off at. I thought it was around 50% too, until I talked to the engineers who load test and set the battery curves for these cells.
Anyways, I don't care what my total % usage is as I will never remotely get close to using anywhere near its capacity. I was just wondering, when they advertise/stamp on the side of these lithium powerwalls (not necessarily tesla) if they are taking the high end voltage for their calculation. In my case, I have to do the math for these guys. They originally used 1.75 volts for their calculation because I gave them a discharge curve that was based off a 1.75 volt starting point. I was able to get them to use 2V volts for their starting point, but was curious if 2.25 volts was doable or not. The engineer I talked to said the cells would stay at 2.1 volts after I lost the grid for hours. But It sounds like battery banks are determined by nominal voltage. I appreciate your input. thanks.
sunshine_eggo
Happy Breffast!
I don't care who or what they say. There is no magic to these things. Properly maintained FLA has a much greater potential for extended life, but the issue is many don't properly maintain them, and AGM end up lasting longer.
The only reason I can think that someone would use 1.75V for nominal lead acid is to de-rate them... and allow you to use > 50% and still seem like you're getting something.
1944Ah * 48V = 93.3kWh, 46.7kWh usable
One inconsistency with lead acid is the rate rating. MOST batteries rate Ah at the C20 value, i.e., 20 hours at continuous 5A discharge can deliver 100Ah. Use more current, you get less Ah. Use less current, you get more Ah. In the EU, "solar" batteries are rated at their C100 value, i.e., whatever current they can deliver for 100 hours yields X Ah. In higher load applications like forklifts or for high power, short duration backup, they may use the C10 rate, i.e., the current they can deliver for 10 hours. This Ah rating is lower than C20 rating. Many quality batteries intended for this application list MULTIPLE ratings to help the customer determine what is best for their application.
FWIW, Nominal Voltage exists for this purpose. This is also, roughly, the "average" voltage experienced by the battery throughout the entire discharge to empty. If you actually measure Wh live, and calculate out the voltage, it always comes out really close to the defined nominal.
Put another way, if someone used the peak charge voltage to define battery kWh, and you measured the kWh extracted during a discharge, you would NEVER achieve the quoted value.
1.75 volts was used because these batteries are made to be used in a nuclear power plant and 1.75 volts was a very conservative value to determine the battery curve from.
Brucey
Solar Wizard
Ok so this was a typo and you meant 83kWh?
sunshine_eggo
Happy Breffast!
Ah. Sounds like you're trying to apply an edge case practice to what everybody else does. Lots more batteries in cars and off-grid power systems than are are nuclear power plants.
No, not a typo
We were talking about the 1.75 volt starting point. NCN-27 cells are strictly used in the nuclear industry. I was stating why they may have used 1.75 volts as a starting voltage.Ah. Sounds like you're trying to apply an edge case practice to what everybody else does. Lots more batteries in cars and off-grid power systems than are are nuclear power plants.
sunshine_eggo
Happy Breffast!
Wow. So you have:
1944Ah * 48V * 9 = 839.8kWh
Presumably used and likely at compromised capacity.
Do you have on the order of 84kW of PV to properly charge that battery? Do you have an equivalent generator to charge and equalize them when needed, or can you draw that from your grid, presuming you have a 1750A capable charger?
I must now go and collect he bits on the floor from my brain exploding. I'll close with this...
Regardless of voltage is used, the kWh rating of a battery bank is the kWh you would measure it if you discharged it according do the battery specifications (the given Ah capacity at a given rate). While use of nominal voltage to calculate kWh is the norm in the vast majority of cases, it ultimately comes down to the former.
Good luck with whatever comes next for you.
Take care.
I'll pray for you.
Wow, so you can do math. Yes I have around 840 kWh worth of storage. Why would I need 84 kW worth of solar to charge it? I’m grid tied. Plus I would have NEVER have gotten a battery bank this big if it wasn’t for the rebate I’m getting offered. And please, pray for someone else as I’ll be alright.
sunshine_eggo
Happy Breffast!
Math is kinda important, so I try to get it right.
You need 84kW of power to properly charge that bank to prevent sulfation.
Since you didn't know why I asked that question, it's painfully clear that you don't have any clue as to proper FLA maintenance.
I'll take your advice. I'll pray for your batteries. They need divine intervention to protect them from you.
These batteries will never get discharged. I will get the rebate money, then after a bit I will scrap them and buy a way smaller lithium setup. I do understand what you’re saying. But these batteries are literally going to sit on float charge in my pole barn for a year or so then they are done. There’s no discharging and recharging of them.
sunshine_eggo
Happy Breffast!
Ah. All part of a scheme. Gotcha. Good luck.
Ha yeah something like that.
sunshine_eggo
Happy Breffast!
Hmmm.... Wonder how much you can get for the lead when it comes time to recycle them. That's got to be a metric crapton of lead.
Using a low end of $0.10/lb (google says $0.10-0.30/lb), and using the weight/capacity of my Rolls S-605 FLA, that comes out to about $4.46/kWh, that's nearly $4K of recycled lead.
Aw... who am I kidding? If the SG is still good, you can probably sell them as operation batteries for WAY more.
We might find out here in a year or so.
Aw who am I kidding. We’ll find out soon as I also have a couple lots of 1200 ah batteries that I’m going to sell here soon.
No different than the 30% scheme you got when you initially put a solar/battery system in. This is just a bigger scale. And yes I know I said I will probably get rid of it in a year. We’ll see I guess.
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