Who has experience with (blue) aluminum cells in a mobile system?

[mndare]

mndare, new member here, who purchased 280Ah cells from Amy
May I ask how you built your packs?
My 8 are going into series for 24v unless I can be convinced otherwise
thinking about using aluminum straps 1"x 1/8 as vertical spacers between cells
Then pulling pack together with threaded rod and end caps

Hi,

We have just put are cells into fibreglass or lexan boxes built to size. The cells are strapped together with SS strapping but that is it. The issue of keeping the cells under compression is only needed if the cells swell ( they are being damaged by over/under charging is the only reason for that) so we do not do that part and never have. Excellent BMS use and reasonable battery loads means that s an overkill in my opinion. We are 12 volt as the cable distances are not enormous and the battery cable system setup on our boat are very large gauge already. Also my system is huge 17 Kilowatts but the max load is a small fraction of a 1C load so again the cells are pampered and never even get warm. Don't charge/discharge into the "knees" of the charge/discharge profile and use a god BMS and secure well on your boat. Can't imagine buying a lead accid battery for a boat...

 

[KauaiMolokai]

Hi,

We have just put are cells into fibreglass or lexan boxes built to size. The cells are strapped together with SS strapping but that is it. The issue of keeping the cells under compression is only needed if the cells swell ( they are being damaged by over/under charging is the only reason for that) so we do not do that part and never have. Excellent BMS use and reasonable battery loads means that s an overkill in my opinion. We are 12 volt as the cable distances are not enormous and the battery cable system setup on our boat are very large gauge already. Also my system is huge 17 Kilowatts but the max load is a small fraction of a 1C load so again the cells are pampered and never even get warm. Don't charge/discharge into the "knees" of the charge/discharge profile and use a god BMS and secure well on your boat. Can't imagine buying a lead accid battery for a boat...

Hi mndare, do you have a website for your sailing adventures? I'd love to see some details

Rock on

 

[mndare]

No website. We sail the west coast of BC so very rugged, very remote at times and lots of storms and adverse weather, Baba 30, loaded with solar, wind gen, hot pressure water, water maker etc. a solid boat for a semiretired couple in 50's. do 1-2 month trips with friends with their own boat every spring/summer. Sailing for 30 years on west coast Canada

 

[KauaiMolokai]

Awesome. Thanks.

Yeah you have some rough weather off BC for sure. And cold!

Staying on solar topic, if the batteries work in your application that says a lot. Its real world and demanding. I appreciate you posting.

Enjoy every day!

 

[mndare]

I left lead acid behind in 2013 and have never looked back

 

[Christo]

Hi,

We have just put are cells into fibreglass or lexan boxes built to size. The cells are strapped together with SS strapping but that is it. The issue of keeping the cells under compression is only needed if the cells swell ( they are being damaged by over/under charging is the only reason for that) so we do not do that part and never have. Excellent BMS use and reasonable battery loads means that s an overkill in my opinion. We are 12 volt as the cable distances are not enormous and the battery cable system setup on our boat are very large gauge already. Also my system is huge 17 Kilowatts but the max load is a small fraction of a 1C load so again the cells are pampered and never even get warm. Don't charge/discharge into the "knees" of the charge/discharge profile and use a god BMS and secure well on your boat. Can't imagine buying a lead accid battery for a boat...

Thanks much for your response, good advise from someone who is actually using the equipment
As a " nervous nellie beginner" I tend to overthink too much
Practical, real experience will cure that I hope, without "letting the smoke out"

 

[grizzzman]

Here is good info regarding cell construction and heat.Aluminum vs plastic cells

 

[ghostwriter66]

Here is good info regarding cell construction and heat.Aluminum vs plastic cells

Once the page opens you have to double click on the Life Expectancy Chart of Aluminum Lithium Batteries header or the page will not load correctly

 

[Dzl]

Here is good info regarding cell construction and heat.Aluminum vs plastic cells

The article makes a claim-- "Aluminum cells have TWICE the life of plastic encased cells"-- but it doesn't provide any evidence, data or context to support the claim, the whole article only has a single sentence that even relates to the top line claim:

When batteries charge or discharge they create heat. Heat that is trapped inside the battery will cause the battery life to go down. A plastic encased battery of a given Ah rating will have a shorter life than a metal encased cell. That is because the plastic is a poor thermal conductor.

I'm not rejecting the claim they make out of hand, but I also wouldn't put much faith in that article or writer.

I would expect that if there was even some truth to the notion, aluminum cell manufacturers would be stating it in big bold letters in every piece of marketing material, Alibaba listing, and datasheet, but in my experience this is not the case.

From my understanding based on what I've read and watched here and elsewhere, for our applications (fractional c-rate charging/discharging) LiFePO4 shouldn't normally get more than a few degrees above ambient. I highly doubt the difference between a few mm of plastic and a few mm of aluminum would cut the lifespan of the batteries in half, especially at 0.5C, 0.2C or 0.1C. And I sortof doubt it would have any signficant real world effect.

The article also notes that small Ah cells are important for cycle life. I imagine this is also true in theory (for high C rate applications) as larger cells have a lower ratio of surface area to volume (less surface area to dissipate heat), while I think this is probably a relevant consideration for EV's, I wonder how relevant it is for our fractional C applications.

Heat and active cooling is much more of a factor in Electric Vehicles where high C rates are more common (and its worth noting that this article is from an EV site), and active cooling is often necessary, but I wonder if it has any relevance for our application?

@Will Prowse I believe you have some insight into this topic, and I seem to remember you may have found some studies in the past that addressed fractional C charge rates and LFP cell temperature and/or cycle life.

Spoiler: Content of the article so ya'll don't have to do the double clickthrough thing

Life Expectancy Chart of Aluminum Lithium Batteries
Posted by Carl Clark on 8/20/2018 to Lithium Battery Info

Aluminum Encased Batteries Have TWICE the Life of Plastic Encased Batteries​

Measuring the life expectancy of a lithium battery is difficult to pinpoint. So we thought that we would like to expound on the subject to help an anticipated lithium user understand the issue and conclude how to get the maximum life from their battery pack.

Understanding Charge Cycle Life – Lithium vs. Lead Acid

Typically the life of any battery is measured in the number of charges the battery has before it deteriorates to a point where it can only hold 80% of its capacity when it was new. This number is called the batteries ‘Charge Cycle Life’.

As a comparison I would like to start with a lead acid battery. Lead acid batteries have a charge cycle life of between 350 charges all the way up past 600 charges. With the lower charge cycle life usually found in car starter batteries and the higher charge cycle life found in laboratory or solar storage applications. But here is where things can become a little misleading. The honest way of measuring a battery’s life can be manipulated. Because of the Peukert effect which exists on all lead acid batteries you cannot discharge more than about 55% of the batteries new 20 hour capacity rating. A 100 Amp Hour ‘Ah lead acid battery will only yield about 55 Ah before it is considered fully discharged.

Lithium batteries are not affected by Peukert’s Law to the extent that lead acid batteries are so you can discharge a lithium battery down to 20% of its full State of Charge ‘SOC’ before the battery is considered discharged. With many lithium batteries if the discharge is more that 80% DOD it will not hurt the batteries life but is generally accepted that the lithium battery is fully discharged at 80% DOD. To run honest tests to determine the exact life of a lithium battery the battery must be fully charged and then fully discharged to 80% DOD.

Charging and discharging thousands of times takes a lot of time therefore most companies will do it for a number of complete cycles and then extrapolate the remaining life expectancy based upon some typical known curves for the chemistry of the battery.

Lithium Titanium Cells and Polymer Battery Drawbacks

Lithium Titanium cells have the longest life. But they are 3 to 5 times more expensive than the common Lithium Iron Phosphate ‘LiFePO4’ battery. Lithium Nickel Cobalt Magnesium (also called a polymer battery) or NCM batteries exhibit about half the life of the LiFePO4 cells. NCM batteries are lighter in weight and lower in cost and are often seen in electric vehicle applications where cost and weight are paramount.

How Companies Cheat on Charge Cycles

Some companies cheat and they say that their LiFePO4 batteries will last over some ridiculous number - like 8000 charge cycles. Those numbers are not realistic if they fully discharge the battery down to 80% DOD, which they do not. However, there are several other factors that play into the battery life scenario. One of them is the case of the battery. When batteries charge or discharge they create heat. Heat that is trapped inside the battery will cause the battery life to go down. A plastic encased battery of a given Ah rating will have a shorter life than a metal encased cell. That is because the plastic is a poor thermal conductor.

The smaller the Ah rating of the battery the easier it is for the battery stack to dissipate the heat away from the core of the stack and therefore will have a longer life. Honest manufactures of LiFePO4 plastic encased batteries which I call a prismatic cell will normally claim that their cells will have more than 2000 charge cycles. A prismatic plastic encased battery is illustrated in the graph that is in this dissertation.

Why Aluminum Encased Lithium Batteries Perform Better

A typical aluminum encased battery that has better heat dissipation will start out higher. Dependent upon the size of the cell the expectant charger cycle life of a smaller LifePO4 cell can be as much as 4000 charge cycles and follow the same trajectory of the plastic prismatic cell shown below. The simple fact is that if you have an application where you want a long life, like for solar storage, and you do not discharge the cells below their nominal 3.2 voltage you will have a cell that will probably outlive you. Depth of discharge has a very profound effect on a lithium battery’s life.

​

What is the Life Expectancy of the Aluminum Batteries?

- Factor 1: Depth of Discharge…Percentage of Capacity Used Per Cycle.

- DOD, short for the Depth of Discharge, is used to describe how deeply the battery is discharged. If we say a battery is 100% fully charged, it means the DOD of this battery is 0%, If we say the battery have delivered 30% of its energy, here are 70% energy reserved, we say the DOD of this battery is 30%.

- Factor 2: Discharge Rate, The AMPS Divided by the Capacity

- To Read the Chart Above, Here are Some Examples

A. All Numbers are at a 1C Discharge Rate, Which is Aggressive…

B. At a 90% Depth of Discharge, Our Batteries Will Provide 2400 Cycle to 80% State-of-Original-Capacity (SOIC)

C. At a 10% Depth of Discharge (Say, Engine Starting) Our Batteries Will Provide 35,000 Cycles Until 80% SOIC. Beyond 35,000 Cycles the Battery is Still Functioning But Gradually Losing Capacity, but not Performance

Conclusion: By recharging a lithium battery more frequently, thus reducing the DOD (depth of discharge), the battery cycle life is Increased. Solar is very often a beneficial method to reduce DOD and increase life!

 

[Dzl]

I found one of the sources for information on LFP cell temps vs ambient temps. It came from the Nordkyn Designs series on LFP, which I consider one of the best (if not the best) comprehensive resources on LFP system design out there.

From the article (in a section comparing the merits of prismatic vs cylindrical cells):

Wound cells, and small cylindrical cells in particular, are cheaper to manufacture than the larger prismatic ones for a given capacity. They also have a higher volumetric energy density, but their round cross-section prevents from packing them together without gaps and this advantage doesn’t extend to the assembled battery. The gaps between the cells can present an advantage for cooling when thermal management is necessary due to very high currents, but, in marine applications, the currents are modest and the battery cells never seem to get more than a few degrees above ambient temperature. Mechanically, cylindrical cells are very robust and very resilient to mechanical damage from shocks and vibrations, which is good in electric vehicles.

-----------------------------------------

And in searching for more data, I came across this journal article (PDF) which investigates the effect of various discharge rates on cell temperature.

They find:

The results of this study demonstrate that the increased C-rates from C/10 to 4C results in increased temperature profiles on the principal surface of the battery. Also, at the lower discharge rates (below 1C), the surface temperature remains close to an ambient temperature, but at higher discharge rates (above 1C) ; the surface temperature quickly increases for all C-rates. The most noteworthy surface temperature distribution is observed to be 58.1°C towards the end of 4C discharge.

For C/10 discharge rate (i.e. for 10 hour discharge), there is no change in the surface temperature of the battery and it remains close to the ambient temperature (lab temperature). This trend is the same for all lower discharge rates (below 1C)

It should be noted this testing was done on a 20Ah pouch cell, not a high capacity prismatic. And that for the fractional C tests, the tests ran from 2 to 10 hours (until the battery was depleted).

 

[grizzzman]

Well the article being discussed IS from "Electriccarpartscompany" so... With that said the number one killer of LFP is heat and in my opinion, worthy of finding ways to reduce what kills them. Like it or not even at lower "C" rates the core gets hot. Heat seeks cold. Anything you can do to help lower temperatures will assist in longer life.

 

[Dzl]

Well the article being discussed IS from "Electriccarpartscompany" so...

Well yes, but we are discussing it at "diysolarforum" so I figure its fair to question how relevant advice that may be based on 2, 3, 4, 5, 10c charge/discharge rates is to our application.

With that said the number one killer of LFP is heat and in my opinion, worthy of finding ways to reduce what kills them. Like it or not even at lower "C" rates the core gets hot. Heat seeks cold. Anything you can do to help lower temperatures will assist in longer life.

I agree with everything you are saying in theory. And I'm not saying it should be ignored or discounted, especially if you intend to charge/discharge at > 0.5c or 1c regularly and continuously.

But regarding whether plastic vs alu casing would have a meaningful effect on temperature at the c-rates relevant to us, I'm not convinced.

If you look at the test data I posted above, at a 0.1c discharge rate the cell temperature never really rises above ambient, at 0.2c the cell is maybe 2-3 degrees above ambient, at 0.5c its still only 2-3 degrees above ambient so it gets there faster. Now I would like to see some data with larger cells over longer periods of time. But from the data I have, I wouldn't imagine the casing material being alu or plastic would have a meaningful effect on cooling when there is only a few degrees between ambient and cell temperature. If I were to guess cell size would have a greater effect on internal temperature than casing material, but again, probably not enough of a difference to worry about for our applications unless you have special needs.

If heat is a super big concern for your application, I think there are probably a handful of more effective ways to address it, but maybe case material could be one strategy if there is a big difference between ambient temperature and cell temperature. I may be wrong but my guess is that for most of us, heat from without would be a more frequent and common problem than heat from within.

This is just my opinion, based on the minimal information I have seen, the data posted above, and what I've heard from others. I wish the ECPT article would've provided some data or linked to some data to support what they are saying, or at least given some context as to what scenarios/usage they were referring to, because I would like to know if there is data that suggests internal heat is an issue at fractional C charge/discharge rates.

 

[Dhowman]

And in searching for more data, I came across this journal article (PDF) which investigates the effect of various discharge rates on cell temperature.

One great article! Thanks for posting it. Was especially interested to see the calculated vs observed results were practically identical.

But even ore so, the thermal scans. Seems the hot spots start at the NEG cell terminal, so if you want to know if you're overheating, just point your infrared thermometer at the NEG terminal post and, voila, you have the max temp of your cell.

 

[grizzzman]

Well I want to use my system to brew coffee and use the microwave. Not everyone is using a .1 C rate. Thanks for your opinion.

 

[Dzl]

Well I want to use my system to brew coffee and use the microwave. Not everyone is using a 0.1 C rate.

If you look at the data, 0.5C still keeps you within a couple degrees of ambient for about ~100 minutes. And 1C remains close to ambient for the first 15-20 minutes.

Based on your sig, your battery bank is 150Ah@24V, meaning 0.5C is ~2000 watts, 0.25C is ~1000 watts. Do you anticipate exceeding these values for extended periods of time?

My guess is that your microwave and coffee machine won't be pushing these limits for long enough to raise the internal temperature significantly, if they were your batteries would depleted pretty fast, and you'd be pushing the limits of most prismatic cells continuous C rating. If cell life is your primary concern (enough of a concern to worry about plastic vs alu casing) I would imagine you wouldn't want to be pushing 1C very often.

With that said, the data I'm basing this opinion on is based on 20ah cells, ideally I'd like to see some data from 100 or 200ah prismatics since larger cells theoretically have a harder time dissipating heat.

And, as I said before, we all have different needs and priorities so maybe your use case requires you pay more attention to heat than the average solar user. But based on this limited data, I don't think running your microwave for a few minutes or coffee machine should be cause for concern. You could probably even run them both at the same time without issue. On the other hand, it never hurts to err on the side of caution, especially with a large investment, and especially when all we have is limited data, so I don't want my comments to give the impression that I'm opposed to that.

 

[Sojourner1]

Here are some readings that I recorded from my system and you can see cell temps. There are 3 temp readings.
1. Cell temp from sense board mounted on top of each cell.
2. Thermometer mounted in the battery compartment.
3. Outside thermometer, ambient temp in misc note section.

This one is when using a honda eu2000i generator for charging , orange colored section is generator time.


This here is a full day charging using only solar with other loads.


Just to give an idea of cell temps in a fractional C use. Should note that these are plastic prismatic 100ah cells.

 

[Dzl]

Here are some readings that I recorded from my system and you can see cell temps. There are 3 temp readings.
1. Cell temp from sense board mounted on top of each cell.
2. Thermometer mounted in the battery compartment.
3. Outside thermometer, ambient temp in misc note section.

This one is when using a honda eu2000i generator for charging , orange colored section is generator time.
View attachment 9578

This here is a full day charging using only solar with other loads.
View attachment 9579View attachment 9580

Just to give an idea of cell temps in a fractional C use. Should note that these are plastic prismatic 100ah cells.

Thank you so much for taking the time to record and share such detailed log data. It looks like at the low charge rates recorded (roughly ~0.2C and ~0.05C) there is a minimal to non-existant rise in temperature (temps rise, but at a slower rate than compartment temps).

It's interesting that the cell temps are consistently lower than the compartment temperature. Why do you think this is? Thermal mass of the batteries holding a more moderate temperature throughout the day? Heat generating electrical devices (controllers, inverters, converters) heating up the air in the compartment?

 

[BiduleOhm]

It's interesting that the cell temps are consistently lower than the compartment temperature. Why do you think this is? Thermal mass of the batteries holding a more moderate temperature throughout the day? Heat generating electrical devices (controllers, inverters, converters) heating up the air in the compartment?

It's likely the calibration of the temp sensors.

 

[Sojourner1]

The batteries do stay relatively cool even in summer. That compartment does stay cooler unless the inverter/ charger is used for long heavy loads which is minimal, the IC is located above batteries.
I'll open my computer later and pull a couple reading from the past at different times of the year.
After 4 years though of full time use in a fractional C use cell temp rise is very minimal. I have not seen any of my cells get higher than 85f and that was when charging with IC at 100a and during the final absorb stage then the temp will drop some.

Depending on the season of the year the cells stay between 45f -75f.

 

[Dzl]

It's likely the calibration of the temp sensors.

This may be the case (or part of it). But I find it interesting that the compartment temp is higher than both the cell temp and the ambient outside temp (of course the data is from early spring so inside ambient might be higher than outside ambient due to heating), of course this could still be due to calibration, but I think it could also be the result of the electrical compartment being heated by the various efficiency losses of the components in a small trapped air space, and the amount of energy needed to raise the air temperature would be comparatively minuscule compared to the amount of energy needed to raise the temperature of a ~125lb battery bank would it not? (I'm just speculating here)