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Pack Design
#1
Hello all, I’m considering putting together a battery pack to provide power to my RV. My initial design is to create multiple 7S5P packs with 3000mah18650s. This should provide me with 378wh pack (15ah x 25.2v) I am planning on creating 6 of these packs in parallel so that I initially have a 2kwh pack (90ah 25 2v) The goal is to use a 24v inverter to provide at least a 30A 120v AC power source to the RV.

I notice that a lot of people create large parallel packs (80P) and putting them in series and I’m wondering if there is any reason the smaller packs wouldn’t provide enough power to support my requirements.

Thanks in advance for any ideas/advice.
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#2
Many of the large projects 80P - are using second hand cells. I am guessing you will be using new cells, as there is no way you could expect to pull 3kw out of a 2kwh pack constructed from second hand cells. Even if you are using new cells, you are still expecting them to provide a discharge of 1.5a per cell.

The bigger the pack the less strain on the batteries. Also if you did have cells that could sustain the high current draw, your 2kw of storage would only last 45mins at 3kw. Even if 3kw was a peak and not a constant load, and even using new batteries, I would still aim to keep the draw at less than 0.5c (C standing for capacity). So if I wanted 3kw peak from a battery, even with new cells, that were rated for high current, I would still opt for at bare minimum 6kw. Even at this level, cell level fusing is not really going to be practical.

Then you still have to count for inverter losses as well. Even a good inverter most likely will not exceed 90% efficiency even at full load - so you need 3.3kw at the battery to make 3kw at the 120v side.

Then as a general rule of thumb, you should never run equipment at its max load either. 75-80% maximum. So if I expected to have peak draw of 3kw, I would opt for at least a 4kw inverter.
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#3
Actually, it's because BMS's are so expensive.
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#4
If when connecting all the packs together that each 5p is connected in parallel with every other pack, then electrically you could have a 7s20, 7s40p, etc. This would only need 1 bms while the packs are actually in use.
However, if you plan on connecting 'only' the absolute neg/pos ends of each pack in parallel, then you will need a bms for each pack.

Generally speaking, what you plan on building is what I call a System Voltage Pack. There is nothing wrong with this design as long as you know the requirements to use such a design. It is a bit more complicated in the end. And can be a bit higher price as noted above needing more bms if not connected a certain way.
However, this does give the advantage of taking a pack out and replacing it if needed without bringing the whole system down. But, you are talking about an RV, not a house. So that does make things a bit different here.

The reason most people build large parallel packs is because 1) it's a whole lot quicker and a whole lot less likely to screw up during assembly 2) it's cheaper to operate as you don't need as many cables or other electronics to monitor each one 3) because most are using reclaimed cells and the more in parallel the better. In fact, this is even true for new cells, too, as Geek mentions.
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#5
From the way the original post was written, it sounded like each 7S5P pack would be combined at the absolute positive and negative end. Like 6P7S5P rather than 7S5P6P. Plus the question about paralleling so many cells hinted at the packs being paralleled only at the ends. But maybe I read it wrong.

I don't think it would work in an RV, but the cartridge style powerwall would be the best way of going about with 7S5P6P. Otherwise, big packs work best for all the reasons Geek, Korishan, and I mentioned.
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#6
Thanks for the responses. I do indeed plan to add more capacity in the future. Say up to 10Kwh. But initially I was going to start with 2kwh. Yes they were going to be only paralleled at the ends as suggested in a 6P7S5P configuration. I am planning on using new cells for this project, Samsung 30Q I believe and was looking at a 4K inverter (Conext 4024). At present the RV is located in a remote location and is only drawing about 1kwh per day just running a security camera and mifi as well as an onboard charger for the house batteries. The Goal is to cut the grid tie and support this with solar.

Next question for the pack design is internal busing. I’m planning on using nickel trips and fusing cells to the nickel strips (8mmx.2mm). Then connecting 10ga copper wire to the nickel strips that are connected to XT90 connectors. Does sound reasonable?

Thanks again to all for sharing your knowledge.
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#7
(02-28-2019, 10:55 AM)Ebarnett Wrote: Thanks for the responses.  I do indeed plan to add more capacity in the future.  Say up to 10Kwh.  But initially I was going to start with 2kwh.  Yes they were going to be only paralleled at the ends as suggested in a 6P7S5P configuration.  I am planning on using new cells for this project, Samsung 30Q I believe and was looking at a 4K inverter (Conext 4024). At present the RV is located in a remote location and is only drawing about 1kwh per day just running a security camera and mifi as well as an onboard charger for the house batteries.  The Goal is to cut the grid tie and support this with solar.

Next question for the pack design is internal busing.  I’m planning on using nickel trips and fusing cells to the nickel strips (8mmx.2mm).  Then connecting 10ga copper wire to the nickel strips that are connected to XT90 connectors.  Does sound reasonable?  

Thanks again to all for sharing your knowledge.

I build some kits that able to upgrade at the later stages...
In your case you can start with 2 OFPPK kits, that will give you 8S 16P or 7S 16P with a little DIY, then when DIY upgrade to 8S 32P with 4 kits and to a full standard configuration with 8s 64P.  More detail here: http://bit.ly/416OFPPK
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