48V powerwall with parallel ebike packs

[mschaus]

Hey everyone, I wanted to share what I built this past year, a little different than many I've seen here. Big thanks for everyone's informational posts and YouTube videos I learned so much from.

I had a bunch of 13s ebike packs with 18650 cells. I decided that since they were already nice packs I would use them the way they were assembled rather than harvesting the cells and building one giant pack. I ended up with 19 packs in parallel, all together adding up to 13s89p and about 280Ah @ 48.1V = 13.5kWh.



I replaced the BMSs with 20A Daly models and tidied them all up in shrink wrap.


And built them all into a large aluminum case. Fuse blocks with ATO fuses rated to 58V were used to tie them all together.


Combine this with a 1500W pure sine inverter and manual transfer switch (the one sold by Goal Zero with 4 circuits) and I have an awesome backup power plan!

I have the full writeup on my personal blog: https://mschausprojects.blogspot.com/2023/01/diy-powerwall-parallel-ebike-batteries.html

Excited to share and hear what people think! Cheers!
Mike

 

[italianuser]

I think you did a great job and I know there's a lot of work in making this setup. I went to see your blog post to get more details. If it works I think it's good, and it does work!

You mixed 4P and 5P batteries, did I read that correctly? It surely works but, if so, some packs could take more stress than others and wear out earlier.

I just have few technical notes for others maybe going in your same direction, because when using 18650s it's better to keep safety standard real high.

- The fuse box is rated for 32V and 30A (per circuit). I suppose they set this limit calculating possible heat dissipation in the worst case. The box comes from China, has no CE markings (not good for me in Italy) and I'm not really sure about the producers claim "flame resistant".
- The fuses. I'd surely go for fast fuses with arc extinguishing sand inside.
- I'm not sure about the class D fire suppressant used to wrap the cells. Many studies show that a slow and constants flow of class D suppressant is required to mitigate fire effects. Initially the suppressant will stop the fire but not the thermal runaway which will start the fire off again after a few seconds. The producer states "Covering the fire load with CellBlockEX granulate displaces oxygen and separates it from the fuel. The fire is suffocated"; I didn't test their product, I'm sure it's good, it but from what studies say I understand you need a good quantity of suppressant and that a film covering cells would not do the job. I also checked out their test and it doesn't explain the testing mode, there's only diagrams and photos which show what seems like a bag of granules used to suppress fire. It doesn't seem to me a valid acceptance test because there's some fundamental data missing (quantity of granules used, release mode, etc). Nevertheless I think their product works. Some studies here and here.

A part from that I love all the details you cared about (connectors, panels, display, etc), great job, congrats!

 

[OffGridInTheCity]

Using/paralleling existing packs is a perfectly good approach. @jehugarcia youtubes have been trending this way for several years - here's a recent variation. Its the same concept as you're approach.

View: https://youtu.be/3fegpSVegw8

If you did mix 4p/5p batteries as @italianuser mentions it will be interesting to see how it holds up. You say it's for backup power, so that implies limited use which mitigates the issue.

In any case, welcome to the forum. Nice looking work and thanks for sharing

P.S. Look forward to any updates on how it performs over time and any tweaks/maintenance you do.

 

[mschaus]

Thanks for the feedback and support! This forum is great. On Facebook groups people are frequently jerks to each other.

4p and 5p packs mixed — yes this is the case, and yes it’s really only for low-cycle backup and tinkering so I wasn’t too worried. But now I am very curious! I will do an experiment with load sharing and capacity measurement of a parallel 4p and 5p pack using a proper DC load tester.

Cellblock (Class D fire suppressant) — yeah I’ve never seen it tested on an actual built battery pack of significant capacity. But I have some and it’s certainly better than nothing, which is what most people have. Final extra bit of safety after proper wiring, fusing, pack quality, and metal case. Though I could probably do more.

I have it set up in a large ziplock bag so it fully covers the top inside of my case with about 2” thickness. This is the same principle used in large battery storage cabinets and “drum plugs” used to cover a battery in a storage drum in an emergency. The idea is that the bag/wrapper above the battery melts/burns from the heat, releasing the Cellblock little balls to fall down and flow around where the fire is. I’ve called up Cellblock and they confirmed putting the loose fill material in plastic bags is just as good for the delivery mechanism.

Fuse blocks — indeed the blocks are not fancy. Inside is just chunks of metal to make connections so I’m not sure how a name-brand one is much different, but of course I want to learn. I suspect the most important part is the fuses, which is why I got properly rated 58V fuses from Digikey. I called up Blue Sea Systems (high quality supplier) to ask about the 32V rating on their fuse block and they confirmed the limit is the fuses, not the block.

Will update as I learn more!

 

[mschaus]

Update: Now at a full, round number, 20 battery packs. All together 13s94p, 293Ah @ 48V = 14.1kWh.

Load sharing -- I did a measurement of the current of two different design packs while discharging together. Here is a photo of the setup and a plot of the results. I used a time-lapse video to collect the data for me. The batteries are in parallel being discharged by a DC load with an additional current meter on one pack so that we know how much current is coming from each.


On average, the current from each pack was indeed proportional to their capacities as expected, but I did not expect so much variation! Especially at the very beginning of the discharge curve when somehow the smaller pack gave more current.

I suspect this is because each pack had different types of cells. The smaller pack has Samsung 18650INR-35E cells (3500mAh rated), while the larger pack has Panasonic NCR18650GA (3300mAh rated) cells. Different types must have slightly different discharge curves.

For an application with low discharge rate I'd say this is no problem since even if there were a 20% increase in current flow at some points it will be no problem for an individual pack.

Even more detail and photos at my blog if you are interested: https://mschausprojects.blogspot.com/2023/02/load-sharing-parallel-lithium-ion-battery-packs.html

Thanks for asking the interesting question!