LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[dlentz]

Originally Posted by goboatingnow

Right now the indications are that approx 8-10 % of capacity is lost per annum irrespective of normal usage patterns.

Dave

That would certainly leave a lot of OEM EV manufacturers out on a limb warranty wise. Considering many EV manufacturers consider EOL at 80%, I can't believe Tesla/GM/Nissan plan on replace all of their batteries every 2-3 years or that they are ignorant of this.

Jack Rickard on EVTV recently tested some first generation CALB cells for loss of capacity from storage. Calb was/is trying to move old stock and was offering them at a reduced $. I don't recall the numbers, but he did find a loss, but I'm thinking it was in the 3-5% range on brand new 3 year old cells.
Dwain

[Maine Sail]

Originally Posted by dlentz

Jack Rickard on EVTV recently tested some first generation CALB cells for loss of capacity from storage. Calb was/is trying to move old stock and was offering them at a reduced $. I don't recall the numbers, but he did find a loss, but I'm thinking it was in the 3-5% range on brand new 3 year old cells.
Dwain

The write up is HERE

I suggest reading the text not watching..... If you want to watch the video on this start at about 75 minutes...

[goboatingnow]

Originally Posted by dlentz

That would certainly leave a lot of OEM EV manufacturers out on a limb warranty wise. Considering many EV manufacturers consider EOL at 80%, I can't believe Tesla/GM/Nissan plan on replace all of their batteries every 2-3 years or that they are ignorant of this.



Jack Rickard on EVTV recently tested some first generation CALB cells for loss of capacity from storage. Calb was/is trying to move old stock and was offering them at a reduced $. I don't recall the numbers, but he did find a loss, but I'm thinking it was in the 3-5% range on brand new 3 year old cells.

Dwain

Remember EV. Companies will spec batteries carefully to ensure the loss of capacity remains within the overall stated capacity. The other thing is really no manufacturer has good data on EV battery life as yet. Tesla in particular is suspected to have a lot of detailed Li data which it has not released to the scientific community. ( I wonder why )

Also remember that my predicated annual losses are based on predicting forward based on test results over a shorter period

If I look at the individual results I can see anything from about 4% to over 10 % note that this capacity loss is based on a combination of storage time, standby time and load support activity ( including different C discharge and pulse performance ). It's not a test purely on storage alone.

I remain however of the view that the lifespan as opposed to the cycle life is not as good as the snake oil salesmen have claimed.
( anyone with an iphone will tend to agree )

[transmitterdan]

Actually, my iPhone and iPad experience is there has been no noticeable loss in capacity. I worried about it when I bought the iPad (first version) but the dang thing keeps working. I thought they were crazy for locking the battery inside but it's still working fine.

There is no doubt that Li degrades over some time/usage pattern but we know so little about the physics no one knows how to accelerate the aging process. If Tesla knows the physics then I can understand why they would keep it a secret. The path to getting max life and lowest cost is to understand the physics. Most intellectual property laws don't protect knowledge, only the application of knowledge and even then it's not uniform the world over.

[deckofficer]

I just have to share this with the lithium gurus on this thread as it is a gold mine of information and pictures.

A member on the Tesla car forum is building a stand alone solar system for his fully electric home. 30 kw of solar feeding (2) 85 kwhr Tesla Model S battery banks that he buys from a auto graveyard for $20k apiece. Not only is the energy density much higher than our prismatic LiFePO4, but 85 kwhr for $20k is a lot cheaper than our cells. So anyone who wants to keep the weight down on their multihull, this would be the way to go.

Lets say you want a housebank of 400 ahr, that is 5.2 kwhr at 12 volts (13 volts). That is $1225 worth of those Tesla cells, so 16 cruisers would pool and buy the pack and split up the modules. Our best price is $1760 for the Winstons. But this is where it gets good, 5.2 kwhr for 48 lbs of cells vs 126 lbs for the Winston cells we are using.

T1 Terry and Maine Sail, it really is worth a look.

Pics/Info: Inside the battery pack

[hellosailor]

And somehow, he won't have to monitor or balance a couple thousand small cells, but with only four big ones, you need a BMS? How's that work?

[OceanSeaSpray]

Very interesting photos, but talk about complicated!! When I think that some here spit the dummy at the simple thought of protecting 4 cells properly...

Unless you want a high voltage system and then all the interconnect is already there, you would be building your own battery out of a multitude of tiny cells after breaking up the monster.

There is an interesting write-up about this battery chemistry here.

[OceanSeaSpray]

Originally Posted by hellosailor

And somehow, he won't have to monitor or balance a couple thousand small cells, but with only four big ones, you need a BMS? How's that work?

It has got 16 BMS boards inside, that's how it works. And it is not the nice forgiving LiFePO4 chemistry.

[roetter]

Prof. Dahn’s presentation has been mentioned here before. One of his PHD students is now at Tesla.

In the presentation below he explains that the interface between the electrodes and the electrolyte is very reactive at high voltages and causes plating that lead to aging. Rapidly cycling Li batteries says little about the life expectancy.

Quote: “Electrodes in Li batteries are very reactive when charged, not reactive when not charged.”

Why do Li-ion batteries die and what to do about it.

I believe the take away is - just like Main Sail says- "Stay away from high voltages. 3.45V per LiFePo cell recommended by Main Sail is a good recipe for a long life.

This also explains why the battery manufactures can claim such a high charge voltage. If you charge to 4V at 1C and then immediately discharge at 1C, then you spend very little time at high voltages.

However, on a boat we are lucky to do 0.1C charging and 0.02C discharging. If we then follow our old instincts and try to charge the batteries to 100% every day and then spend a long time up there, we murder them. My batteries have been "floating" the last few days between 80% and 60% SOC and I am getting anxious to get them to 100%. Old habits die hard. My brain is telling me they are fine that way, and my stomach is telling me to get them fully charged now.

Interestingly, on the Tesla Mortors Club forum in a post someone called a BMS a “Battery Murder System”. Again, to balance the cells the BMS keeps the cells at a very high voltage for a very long time to bleed off the high cells. I think that is why he calls that murder.

[transmitterdan]

Originally Posted by dlentz

That would certainly leave a lot of OEM EV manufacturers out on a limb warranty wise. Considering many EV manufacturers consider EOL at 80%, I can't believe Tesla/GM/Nissan plan on replace all of their batteries every 2-3 years or that they are ignorant of this.

The OEMs know about this so they don't use all the bank capacity on day one. If they did they would have exactly the problem you mention. So they start out on day one using only somewhere between 50-75% bank capacity (it's a trade secret how much they keep in reserve on day one). Then as the bank ages they have enough reserve to get them well past the warranty period.

It is supposition on my part but I'm guessing that's what caused the early promoters of the Chevy Volt at GM to over estimate the range and to say it would not need any internal combustion engine. They probably could get the higher range day one but could not hold it for 8-10 years. Once they understood the loss in capacity over time they probably had to back down on the range and add the gasoline generator.

The loss in capacity over time is real. No one understands exactly how or why it happens. Heck almost no one understands exactly how Li batteries work in the first place. That's why there are so many different chemistries.

[T1 Terry]

We have a pack still in operation with over 1200 cycles on it, still no capacity loss, it not only a matter of "it's all about the batteries" as Plasma Boys says, it's also all about how they are treated at the initial charge, then every recharge after that. I 100% agree with the "Battery Murder System" claim and have done from the very beginning, don't do it if you want a long cycle life, as simple as that.
The next thing is to understand that cells aren't all created equal and different construction types suit different purposes. Cylindrical cells are for full capacity cycling at their rated discharge, not for stand by use, light discharge rates or long life, prismatic cells respond well to low discharge rates, partial capacity use and low voltage float charging. Cylindrical cells are one single set of plates wound into a cyl, the inner cools a lot less than the outer layer but there is a very limited path for the heat to dissipate, so it's the heat that kills them. The prismatic cells, particularly the Winston and Sinopoly brands, have individual plates bolted to a lump of copper or aluminium, depending if it's the anode or cathode, and this acts as a heat sink for each individual plate. The separator is a single sheet woven back and forth between the plates, this rests against the side of the case and also helps to dissipate the heat, so as long as those 2 areas of cooling remain available and the cell voltage kept below the upper knee, the cell remains within it's operating temp and seems to happily cycle with very little degradation, as long as the cell voltage remains between the upper and lower knee. Now for the battery murdering system, that is designed to hold the cell in the upper knee voltage, then dissipate energy via a controlled short between the 2 electrodes, turns the electrical energy into heat energy, then dumps all that heat into the copper block all the negative plates are attached to..... so where can they dump their heat??? The only place left to dump all that heat is into the electrolyte and separator, the copper sheet temp rises and the copper starts to corrode, stuffing the cell..... murder by a thousand cuts.
If you want long cell life, do not store them for long periods above 3.2v, that self discharge of a stored fully charged cell is it slowly eating itself, exactly the same as storing them below 2.8v for long periods. I have both thundersky and Sinopoly cells here with 6 yrs calendar life on they, still in their packing crate, still at 3.2v, that kinda blows the constant 2% to 3% self discharge being a linear thing out the window eh :lol:


T1 Terry

[Maine Sail]

Originally Posted by T1 Terry

WNow for the battery murdering system, that is designed to hold the cell in the upper knee voltage, then dissipate energy via a controlled short between the 2 electrodes, turns the electrical energy into heat energy, then dumps all that heat into the copper block all the negative plates are attached to.....
T1 Terry

Bingo! Most BMS systems don't begin shunting or balancing until you are already in the voltage range that is doing damage to the cells. They need you to get up into the knee range in order for their system to work. Guess what? When you get into the upper knee you may actually need balancing... It is a self fulfilling prophecy. Simply stay out of the knee and balancing is pretty much a non issue... Charge into the knee and go figure, you may have a need to balance....

While I use a BMS system, for LVC and HVC, I do not, have not and will not use the system for its balancing aspect. My system charge voltages are solidly well below the "balancing/shunting" voltages of my BMS system. If I need to re-balance, perhaps once every 4 years, I will do it manually and not use the BMS for this...

P.S. Glad to hear I am not the only one not seeing much decline in calendar life...

[transmitterdan]

Reading the thread on the tear down of the Tesla pack was interesting. Apparently Tesla agrees with Maine Sail. According to that thread the BMS can only shunt in the mA range which suggests they rely on initial balance almost the whole life of the pack as it would take eons for that shunt to balance such huge groups of cells.

They also run coolant (antifreeze) through the pack. On a boat this might be a way to get around the cold charging problem. One could put a resistive heater in the coolant loop and use the charger to warm the pack before sending current to the cells.

[deckofficer]

Anyone that has very limited space and wants a price better than $1 per ahr at the cell level might consider these....

60Ah 3.2V 2C LiFePo4 Lithium Prismatic Batteries HiPower LiPo Li-Ion EV Battery USA Stock 60Ah 3.2V 2C LiFePo4 Lithium Prismatic Batteries

Maine Sail or T1 Terry can correct me if I'm wrong, but I don't think there would be any problems running these cells in parallel to get the desired ahr bank, plus since they are small would fit in some small areas.

Plus I can't help but like a business that is owned by someone with his track record for doing the right thing long before others. About Us!

[Jd1]

Originally Posted by deckofficer

Anyone that has very limited space and wants a price better than $1 per ahr at the cell level might consider these....

How are these less than $1 per ahr ????
They are 60 ahr cells and are being sold for $110!