For the battery experts - Which is better, using single 100Ah LiFePO4 cells or multiple 5Ah cells?

[Crossy]

Our grid-tie solar is working fine, it just sits on the car-port and offsets our bill nicely.

 

Of course the engineer in my head is bored, and this, coupled with the future possibility (likely probability) of PEA installing electronic/no-reverse meters, I'm looking at "hybridising" the system whilst at the same time adding more solar capacity. This would also add a degree of "UPS" functionality and reduce the necessity for our (noisy) genset.

 

A hybrid system of course needs batteries which are the subject of this thread (discussion applies equally for those going totally off-grid too).

 

I can assemble 100Ah packs of 32650 cells for about the same cost as single 100Ah (golf cart) cells.

 

So, which is better, using single 100Ah LiFePO₄ cells or the "Tesla" method of using multiple 5Ah cells?

 

A few thoughts:-

• Smaller cells cost less each, so gradual expansion of the pack can be done on an "available funds" basis.
• Smaller cells should allow for better cooling.
• The big cells need less effort to assemble packs.
• Failure of a single small cell just removes a small chunk of capacity, loss of a big cell removes a big chunk (or even the whole pack).

 

Thoughts welcomed of course (or I wouldn't be posting here) ???? 

[VocalNeal]

Eggs? Basket?

[ravip]

I found this write up and I am not sure it applies to your question 100%... but an interesting thought.

 

Beware of Series Connected Batteries!
There is a potential issue when multiple lithium-ion batteries are connected in series. For example, two 12 Volt 100 Ah batteries, each with their own build-in BMS, connected in series to make 24 Volt 100 Ah. Now assume one of those two batteries is near-empty, the other pretty full, and you put a load on the batteries, to discharge them. The near-empty battery will reach the point where the BMS decides “enough is enough” first and it will switch off that battery, in effect disconnecting your entire battery bank, even though the other battery is still full.

https://www.solacity.com/how-to-keep-lifepo4-lithium-ion-batteries-happy/

 

[Crossy]

22 minutes ago, ravip said:

Beware of Series Connected Batteries!

 

Yes!

 

It's also worth noting that many 12V (4S) BMS's don't like being more than 2 in series (24V system).

 

I intend using 16 x LiFePO₄ groups with a single 16S x 200A BMS for a 48V system to avoid this issue. 

 

The actual size of the groups doesn't matter so long as they are the same capacity and chemistry.

[tjo o tjim]

Tesla’s approach works because they are able to properly bin across millions of cells, and they need performance charge/discharge, which requires good cooling.  For your use, the 100AH prismatic would likely be a lot more reliable. 

 

I’m looking at doing a 4x100Ah system at 48V myself to offset the refrigerator load.  It pulls 4kWh/day... We have a net metering agreement (in the US), but we aren’t allowed to modify it, so we end up short after charging our EV.  A stand-alone system not connected to the grid isn’t regulated, so it is easy to add on.

[Muhendis]

With regards to batteries of cells and assemblies in series. Yes, they certainly can and do fail because of uneven charge/discharge characteristics of individual cells (and of course BMS controlled battery assemblies). That is why each individual cell is monitored and "managed" to ensure that all cells charge to the same level and discharge likewise. 

As far as 100Ahr or 5Ahr choice is concerned, you pays yer money and takes yer choice. Obviously there are far more connections to make with hundreds of smaller cells to connect together and plenty more opportunities for joint failures. Such a task would need plenty of enthusiasm and time. I would go for the prismatic 100Ahr cell solution as in this vid. https://www.youtube.com/watch?v=RB_O_5Ppv7g 

 

[Muhendis]

Additional things which might affect purchasing decisions as follows:

The bigger the battery, the smaller the depth of discharge (DoD) and therefore the longer the life expectancy of the battery. (Aim for 30%)

The lower the charging voltage, the longer the life expectancy (4.2v rather than 4.35v for instance). In other words, the battery does not get fully charged to 100%. To achieve this you need a fairly flexible charge controller/BMS and an increase in the overall battery size.

Keep your battery well below 40 C (25 C is a good choice) above 40 will shorten life.

What these actions do for the battery is reduce the stresses both physical and chemical, on the structure.

It has been said that a cycle life in excess of 50,000 is possible.

[tjo o tjim]

2 hours ago, Muhendis said:

The bigger the battery, the smaller the depth of discharge (DoD) and therefore the longer the life expectancy of the battery. (Aim for 30%)

With LiFePO₄?!  All the recommendations I have been seeing is 90% DoD is fine, and a few have no objections to 100%.  One manufacturer even warranties the battery for 10,000 cycles to 90%.

 

Limiting charging voltage is always verbotten with other chemistries, but I that is really the BMS’ job to address.  Agree on temperature though.  Rated at 27C, ok to 35C, 40C will cause significant degredation was the recommendation I was given.

[Muhendis]

4 hours ago, tjo o tjim said:

One manufacturer even warranties the battery for 10,000 cycles to 90%.

That is highly unlikely. Which manufacturer is that?

[Muhendis]

4 hours ago, tjo o tjim said:

Limiting charging voltage is always verbotten with other chemistries,

Yup. Absolutely in the case of FLA's failure to charge to 100% will cause sulfation sooner or later. Some batteries have a memory effect which needs fixing by complete discharge etc. but LiFePO4's are not at all fussy and will operate quite contentedly with no ill effects if only partially charged. This makes them ideal for solar.

Depth of Discharge vs Cycle Life

 

The above graph was constructed for a Lead acid battery, but with different scaling factors, it is typical for all cell chemistries including Lithium-ion.

[Covedian21]

On 4/19/2021 at 6:01 PM, Crossy said:

 

Yes!

 

It's also worth noting that many 12V (4S) BMS's don't like being more than 2 in series (24V system).

 

I intend using 16 x LiFePO₄ groups with a single 16S x 200A BMS for a 48V system to avoid this issue. 

 

The actual size of the groups doesn't matter so long as they are the same capacity and chemistry.

This is not a simple question. There is many variables to figure out which is “best”. For a long term solar energy storage project, reliability does come in to play.

 

Having 20 x 5 amp hour cells all connected in parallel is a lot of connections.

 

A Single 100 AH cell is just 2 connections. Each connection could be a possible failure point.

 

Tesla uses lots of small cells welded in parallel banks, and it does work.

 

The Chevy Bolt is a series string of 3 x 60 AH cells in parallel. Total capacity is close, but the packaging is very different.

 

For the Tesla setup, they have an integral fuse at each cell, so if something does go bad, one cell might disconnect.

 

With so many in parallel, losing a single failed cell is not a big loss of capacity. With only 3 cells, if a cell fails, it is 1/3 of the capacity.

 

And in your example, if you do lose a 5 AH cell, you lose 5%, Lose that 100 AH cell, you lose all of it, -100%.

 

The single large cell will take up less space, have less connections, weigh less, and take far less labor to assemble the 100 AH pack.

 

The bank of small cells will be larger, heavier, have a lot of connections, and take much more labor to assemble.

 

There are also other pros and cons, but those are the easy ones right from the start. Many of the large cells can only handle about a 1 or 2 C rate.

 

Many small cells can take a 5C rate. So that single 100 amp hour C2 cell could put out up to 200 amps. But C5 rated cylindrical cells cells, might be able to crank out 500 amps.

 

But to do that, all of the cells would need to be very well matched. You can’t always just multiply the current when paralleling a huge pile of cells. They may not perfectly share all that current across all 20 cells. So in reality, maybe 400 amps at best, but still possible to beat the single large cell.

 

The gang of small cells has a lot more surface area to dissipate heat. But you need to either leave air space or run a cooling jacket like they do in the Tesla. DO you need that kind of current? For solar energy storage, we typically run well under a 1C rate.

 

I run under a 0.1C rate to get 10 over hours out of the battery bank. But if you want to accelerate an EV, then you may need those peak current for short bursts.

 

If you give your labor a value of zero, and the cells add up to the same price, it is no big deal. But even factor in your time building the battery bank at just $10 an hour, and how much do you then save with the 100 AH cells?

 

This is what a good friend who's working in the field has told me. 

Edited April 21, 2021 by Covedian21

[Tubulat]

I still working/thinking to instal a 5kWh hybrid installation on my carport, and get some quotations from installers in the surrounding of Hua-hin.

Later i was considdering, why i do not the installation by myself but there are many problems to overcome especially if its the first time, and also realize that i am retired.

Also the Lipo4 batteries I found on Lazada gives headaches, I prefer 48V 240Ah but it looks like difficult.

Anyway what i like to mention is this website i found for many info: https://diysolarforum.com/

Maybe it can helps by your decission.

oh, i see it is not allowed to writing down the website, well it is 'do it yourself (abbreviation)solarforum dot com'

 

 

Edited April 26, 2021 by Crossy
Fixed the link

[Crossy]

10 hours ago, Tubulat said:

oh, i see it is not allowed to writing down the website, well it is 'do it yourself (abbreviation)solarforum dot com'

 

For some reason that site only allows you to embed as a simple link, not a Thaivisa rule.

 

I've fixed it for you.

 

 

[Crossy]

10 hours ago, Tubulat said:

Later i was considdering, why i do not the installation by myself but there are many problems to overcome especially if its the first time, and also realize that i am retired.

 

If you are a reasonably competent DIYer with sensible knowledge of electrics it certainly isn't beyond you, particularly if on a car port (not too high up). You will of course need help the panels are 'kin heavy!!

 

If you're going hybrid and not exporting you should have no regulatory issues here.

 

My own thread (currently just grid-tie) here:- 

 

Why not start a thread, we've not had a hybrid installation detailed yet.

 

[Tubulat]

11 hours ago, Crossy said:

 

If you are a reasonably competent DIYer with sensible knowledge of electrics it certainly isn't beyond you, particularly if on a car port (not too high up). You will of course need help the panels are 'kin heavy!!

 

If you're going hybrid and not exporting you should have no regulatory issues here.

 

My own thread (currently just grid-tie) here:- 

 

Why not start a thread, we've not had a hybrid installation detailed yet.

 

I like to do but my English is not sufficient enough.

I've reread your entire report, and hope to gain sufficient experience with it.

I still have doubts to do it by myself or have it done by an installer.

I got a quote for a 6.3Kw, with 14 Longi 450 panels, Goodwe 5048 inverter, but excluding batteries for 225,000 thb

The batteries Lipo4, 42V 200Ah, I like to buy at Lazade for appr. 56,000thb

 

[Crossy]

3 minutes ago, Tubulat said:

I like to do but my English is not sufficient enough.

 

It looks pretty darn good to me ???? 

[rvaviator]

For a solar system I would go for large cell capacity LFP cells - 40Ahr to 100Ahr.  2P or 3p and then series up to give you the required system voltage.  Good BMS with WiFi or Bluetooth interface so you can keep an eye on things.  When people say that they monitor each cell - They tend to mean each cell block in parallel.  No issue with cycling LFP cells from 20% to 95%.   One thing you should do - is to clamp the cells together ... That will improve the cell life. The cells will try and expand during charge and discharge. By clamping them together you will prevent the swelling which is good for life time.

[ourdon]

What a good discussion !!! ????

I will have to agree with the gentlemen that suggest large cell capacity cells.  

1.   A lot of small cells connected together is a lot of connections.

2.  Each connection is a potential failure point

3.  Murphy was an optimist

 

[Muhendis]

Connecting cells together in parallel then managing the block as a whole is fine if there is only the one block but if that block is one part of a string there is the potential for failure. Consider what will happen in the event of a single cell failure in one block and the failed cell were to drag the others down in that block. The entire battery would then suffer. I prefer to connect cells into a string and then parallel the strings monitoring individual cells and balancing charge/discharge. If one cell goes south then the problem is picked up by the BMS and the rest of the strings can work as normal. Reduced output from one string would show as reduced capacity for the battery.

[Crossy]

16 minutes ago, Muhendis said:

Connecting cells together in parallel then managing the block as a whole is fine if there is only the one block but if that block is one part of a string there is the potential for failure. Consider what will happen in the event of a single cell failure in one block and the failed cell were to drag the others down in that block. The entire battery would then suffer. I prefer to connect cells into a string and then parallel the strings monitoring individual cells and balancing charge/discharge. If one cell goes south then the problem is picked up by the BMS and the rest of the strings can work as normal. Reduced output from one string would show as reduced capacity for the battery.

 

So multiple battery packs (strings of cells) in parallel with a BMS per pack?

 

This was my original thought before the "Tesla" idea infiltrated my brain ???? 

 

Two (or more) 100Ah x 16S sets each with it's own BMS and individually fused, interconnected only at the final (after the BMS) terminals.

[Muhendis]

1 hour ago, Crossy said:

 

So multiple battery packs (strings of cells) in parallel with a BMS per pack?

 

This was my original thought before the "Tesla" idea infiltrated my brain ???? 

 

Two (or more) 100Ah x 16S sets each with it's own BMS and individually fused, interconnected only at the final (after the BMS) terminals.

Yes, but I'm not clear about how you would connect the strings together post BMS.

Just for further info, a picture.....

[Muhendis]

1 minute ago, Muhendis said:

connect the strings together post BMS.

By this I mean I would guess that the charger/BMS might include isolation of each string but I would have to look into this more thoroughly to be sure.

[Crossy]

I was thinking of doing this:-

 

 

Whichever BMS reaches charge/discharge cutoff first will disconnect it's string leaving the other to continue charging/dicharging.

 

I'd be wary of "large" circulating currents if one connects a discharged pack and a charged pack together.

 

I've even seen someone on the RV forums saying you can do the same with LiFePO₄ and Lead-acid as the LiFePO₄ BMS will cut out before the lead-acid is fully charged/discharged. Probably not something I would be recommending but it does sound feasible.

 

[Muhendis]

I had a quick look and gave myself a mental kick up the ......... when the following reminded me about the charge and discharge MOSFETS in the BMS.

 

I'd err towards caution - everyone knows deep down that parallel batteries are not guaranteed to share charging and load currents evenly so, I'd use parallel arrangements of series batteries each protected by it's own BMS. So if you have a 3s battery then that has its own BMS. If you have another 3s battery then that should have its own BMS: -

With 4 parallel sets of 3s you'd have 4 BMSs and only make parallel connections at the ends of each series chain.

Of course this is an expensive solution but it has to be considered as viable if the cost and risk warrant it. If the cost and risk don't warrant it then just parallel 4 batteries and hope for the best with a single BMS.

 

The MOSFETS and diodes are the blocking devices so those slow MCB's would not be necessary. 'course you have to choose a BMS which will handle all those amps but paralleling them should be fine. Don't forget the fuses.

[maxpower]

The series parallel combination of packs and how this influences detection and repair of failed cells depends on many factors including connectivity method cell quality and monitoring. Complex packs built with low quality cells cheap BMS and no state monitoring are not the best choice for long term reliable solar storage.

The video below from Seplus shows how LiFePO4 cells are arranged into a pack. It also shows how higher capacity is achieved by connecting individual packs in parallel and monitoring with second level BMS at the charging point.

Keep the battery arrangement as simple as possible and employ good quality BMS and monitoring.

 

 

 

 

 

 

[Muhendis]

That Pusun stuff looks pretty useful, and a bit expensive too.

[Muhendis]

8 minutes ago, Muhendis said:

a bit expensive too.

Alibaba 100Ahr 51.2v 65,000 Baht

[maxpower]

1 hour ago, Muhendis said:

That Pusun stuff looks pretty useful, and a bit expensive too.

 

All quality well supported storage systems are expensive take a look at Weco Dyne or Schneider for a decent shock.

The video is a good primer for those wanting to DIY some reliable storage and maybe avoid the bodges.   

[Muhendis]

Bodges?

What are they?

[maxpower]

14 minutes ago, Muhendis said:

Bodges?

What are they?

 

I believe its a Brit word used to describe a poor job.

 

Yes confirmed by our resident Brit - bodge = make a mess of it.
 

Edited April 28, 2021 by maxpower