Building for reliability: 24v 200-400Ah LifePO4

[Factory400]

I have been considering my options for an off-grid mobile power system for a while. On the topic of the battery storage, I recently decided on 24v LifePO4.

Requirements -
Basic use: 100Ah
Typical use: 200Ah
Typical use with hopeful rainy day energy budget: 400Ah
Max load 2.5kW
Average load 750w

Initial thought was to build the pack with large prismatic cells to get 200Ah. This is the lowest cost and easiest implementation, but I was concerned that if a cell (or BMS) goes bad - the whole pack is dead.

If I build 2 parallel packs with 100Ah cells and one goes bad - I would still have half the capacity to muddle through the days. As need increase, I could just add matching 100Ah packs for additional capacity and redundancy.

The question is this: Am I over-thinking this? In the context of reliability (not being in a zero-power situation in the middle of nowhere) is going with multiple parallel packs going to improve the reliability compared to a single large pack? My thinking is that if push comes to shove - I could re-arrange cells in the field to repair packs.

Most likely, I will be getting cells from the usual vendors on Alibaba (of which I am not very trusting). The BMS system will be rather high-end and wholly of partially designed/built by me. I am an electrical engineer that specializes in power electronics. I do not, however, have any practical experience with battery packs. Lots to learn.

 

[ghostwriter66]

YES to over thinking ....

With that said though .. I never build a pack that if ONE single cell goes down I loose the entire system ...

I always build it where if ONE goes down then I can cut the pack in half and drop down to 50% power ...

Of course you could always also just buy one spare battery and not worry about it ....

I am finding 200aH 3.2V batteries on Aliexpress for $108 delivered .. never used that vendor but talked to them and them seem legit ... https://www.aliexpress.com/item/4000885735662.html?spm=a2g0s.9042311.0.0.65c04c4dDV2jWP

 

[Factory400]

To build an 8S 200Ah pack with 200Ah cells - there would be no redundancy at all, no? One bad cell or BMS failure and the pack goes to 0% pretty fast.

Those cells are VERY affordable though....if I have the space and weight capacity - maybe shooting for 2 independent 8S 200Ah packs in parallel would offer all the power needed and if one fails - I still have 200Ah working without any effort.

Fairly heavy for a mobile system, but it may work. I thought of the idea of keeping 1-2 spare cells (actively monitored at a safe storage charge). Technically, that would allow me to swap a bad cell or two in the main pack. Of course, that requires considerable effort and time to get the system balanced and back up to a working pack.

One major variable that I do not understand is how to estimate the reliability of the cells (assuming they are treated appropriately electrically and physically). Are 'random' or otherwhise unexplained failures common?

 

[Kesa32]

Feedback on the cells vary , some mention the build info being scratched off , one even mentioned looking like they were a couple of years old ....still measured ok apparently

 

[Steve_S]

My suggestion just to chime in, is to go with two 24V 8S packs (1 BMS & Fuse for each) and parallel them together to share the Load & Charge capabilities. As you are in a MOBILE situation and will certainly have battery placement concern,s, I suggest you measure the space you have available for your batteries. You would not want to buy cells you cannot fit into the space, well, unless you want to do a Fred Flintsone (or BamBam) imitation. Also factor in that the cells will have to be contained in a box/case of some sort and shock vibration dampening for them is "good practice". There are many really cool & creative boxes, cases and even aluminium plate frames that people here have posted about in their own DIY.

Spare Cells are not a bad idea if your buying a large group of cells for big battery bank because there is "Murphy's Laws" to account for and of course if it's not available at the corner store, the likelihood of "S* Happen" goes up right... That being said, I would suggest IF you want to buy spares, get 4, because IF/When you do not need them, you can sell 4 off easier as that makes a 12V pack and there are always people looking to build a 12V pack. Or you can use it for your own 12V for something.

2 packs requires 16 cells (8 cells ea) and there are Bundle Deals available with some careful searching.
For example:
16X 105AH cells (2 bundles of 8 cells) = US $1,260.96 Delivered DPP ($78.81 per cell) Link to Luyuan 105s
16X 120AH cells (2 bundles of 8 cells) = US$1,292.04 Delivered DPP ($80.75 per cell) Link to Luyuan 120s
16X 202AH CATL Brand cells. no bundle = US$2,209.40 Delivered DPP ($138 per cell) Link to Luyuan 202s

Luyuan Tech is a sister company with Xuba Electronics and are Known Good Vendors and Amy Wan (the rep) is generally considered a rockstar here.

If you wish to reach out to her via the Messenger on Alibaba, be sure to tell her your from DIYSolarForum.

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