Leaning towards 18650's instead of other options

[madmax]

Im leaning toward my first Lithium build to move away from lead acid. Still very new to this, but trying to learn.
Originally I was considering biting the bullet and getting LTO's- But the cost is still very high.
My next considerations was prismatic cells- still more expensive than 18650's per KW, and I don't very much like the fact that most sources can come and go- meaning if you don't get them now, they may be hard to get in the future in the same form factor.

So Im leaning to the 18650's- seems pretty much tried and true.

Im building for use on the MPP solar LV2424- so it has a 29.2V max config, and I believe low voltage is 20V.

I've seen some folks build a simple 80P pack, and just series 7 of them. It seemed they were just using a 24V 7s BMS.

However, that doesn't seem to make it easy to add extra packs on, and some significantly less protection within the packs.


Can I Just do 7S80P for a 24V pack with its own BMS, and just keep adding more and more of these packs (with their own BMS) to add capacity?

Can I use Generic off the shelf BMS? Do I want balance versions?

Also, can I run packs of different capacities but of the same voltage? For example, 7S80P of 2A batteries will live in harmony with a 7S100P of 2A batteries and a 7S180P of 1A batteries. (I don't actually think there's an issue with this, as the BMS should cut out the packs with lower capacity first, though increase the draw on the other packs).

Short term goal is 3kw of storage. Im after long operational life, so Im thinking of 28.5v charge cutoff, and a 21v disconnect?

Help is appreciated!

 

[mandrews44]

Are your 18650's Lithium Ion? They have a cycle life of 600, not 3000+ we get with LiFePo4. LiFePo4 does come in cylinder cells like LTO that you see in Battle Born and simular batteries. Personally, I would not go the 18650 route unless they are truly LiFePo4's.
18650's require too many connections for my tastes and yes, they still need balancing which will be more difficult than going with prismatic cells that were created in the same factory and batch with the same internal resistance. LiIon is more voltitle and much higher risk of thermal runaway.

I would not recommend anyone using this chemistry to put on their wall.

Elon Musk is using an 18650 form factor but he is not using LiIon chemistry normally found in 18650's that are used today in flashlights and vapes.
Each of your packs that are of dfifernt capcacities will require a BMS. Unless you are buying bulk, factory verified same chemistry and internal reisistance, you will have a problem with balancing.

Fewer components = Better.. To me, 18650 for power wall = NO! Too many parts, the wrong chemistry can be deadly and lack of available batches to insure uniformity and ease of maintenance..

 

[madmax]

if tyou go with LifePO4, your stuck with the chemistry due to its nominal voltage.

Fewer components usually are better- except for when it comes for redundant failure. having 1 battery go when you only have 7, is a big failure rate. Having one battery out of thousands go, isn't a big deal. The complexity of thousands of connections is the most likely failure point.


I love LTO's, at least the concept of buy and forget, and lasting a lifetime. I think its a little bit more cost prohibitive at the moment. I did pickup one of the LTO starter batteries from battery hookups- but that's for a side project.

I did end up getting 14 NMC packs from their LG packs- those are nominal 3.7v packs, as you are correct, less connections are easier to do.

 

[Steve_S]

Tesla used 18650's and now uses 21700 cells which are changing shortly to a new configuration to a different kind of pack system.
Tesla in China (soon elsewhere) have LFP options starting with the Model 3.

There are many chemistries and their voltages vary a fair bit. The LFP is generally considered the closest to match with FLA voltage ranges. LFP also will NOT catch fire or explode if something goes terribly wrong.

Last point, LFP has been around for a long time and has improved over time and still improving regularly. They will not go away for a long time and will be available or many years ahead. LFP packs are very simple & easy to build and require no special gear of any sort, aside from a BMS to manage the cells.

Highly recommend you watch this video,

 

[mandrews44]

Awesome video!