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Parallel-serial-parallel common?
#1
I'm just starting to design, document, and prototype my own build. I've seen a few systems or products that seem to be made up a set of parallel 48v battery packs, and this looked pretty attractive to me as an interesting way to have a lot of flexibility for your overall capacity. The main thing I've seen is some server rackmount based battery packs, mainly from commercial offerings.

For me, an enclosed server rack would be perfect for keeping everything firmly secured, locked, and keep prying hands (kids) out, while letting me have everything stored in our detached shop. I was aiming to try and do 10p14s20p, where each 14s20p pack would be in a 2U server case and be about 3kW of capacity.

From my perspective, the main benefits of this approach is:

1) Start small, build up your capacity as you need more
2) Easy to take a unit of of use for maintenance
3) Growing the overall capacity lowers overall amp load per pack

Downsides could be:

1) BMS needs grow. At 10 units, I'd end up with essentially 140 cells to monitor, balance, etc. Looking at something like Libresolar in each unit vs needing like 140 Longmons.
2) Starting small will be very limited for amp load on each unit.

Any other downsides to this approach?
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#2
This is what most large capacity users have done. Daromer has at least 4 strings setup so far (if I remember correctly). HBPowerwall also has several strings
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#3
Are there any issues with fusing on individual units where one unit may be charging another if voltages start to skew or capacities of the units differ too much?
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#4
packs/strings/cells/batteries in parallel cannot have different voltages. Unless resistance in the wiring/connections is super high. If that happens, you have other issues.

The only time that they would be different voltages is if they are disconnected and used/charged separately. As long as the systems are within a few volts of each other, there shouldn't be much of an issue. I wouldn't do anything over 5V on the whole string. <0.5V per pack level connections. This is, pack1 of string1 paralleled with pack1 of string2 should be <0.5V difference.

Not sure if you plan on this level of connections or not. There are a few who have done this and used IDE ribbon cable to make those connections. If you don't plan on this level of connections, then you just need the connections for the main lines heavy enough to carry the full load. In this configuration, you'd need a bms for each string. In the former method, 1 bms can handle all strings as long as they are all connected with the balancing leads.
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#5
For some reason I was thinking mixing capacities wouldn’t work. I’m looking at each unit doing a BMS locally, based on, believe it was some of your work, mikethezipper’s, and libresolar using the BQ769, with either a CAN or RS485 to a master.

Thanks for the help, sure I’ll have more questions as I plan things out. Smile
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#6
Mixing capacities of a full string isn't much of an issue. It's just like mixing cell capacities to make parallel packs. A parallel pack is just like a huuuuuuge cell.


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#7
(07-01-2019, 03:54 AM)Korishan Wrote: Mixing capacities of a full string isn't much of an issue. It's just like mixing cell capacities to make parallel packs. A parallel pack is just like a huuuuuuge cell.


YW, and we'll be here to answer best we can  Smile

Agree that mixing cells within a pack is not an issue.  However, each pack in the series should have the same overall capacity.    As an illustration,  if pack1 has 20 cells @ 3000mah each that's a total of 20 * 30,000 = 60,00mah.   If pack2 is made up of 2000mah cells, then it needs 30cells for the same capacity (e.g. 30 * 2000mah = 60,000mah).
In practice - the recommendation is to distribute the cells among the packs of the string if you can so that both pack1 and pack2 have same mix of cells AND same capacity - e.g. in this example, pack1 and pack2 would have 12 * 3000 + 12 * 2000 - 24 cells.  

Strings of packs (e.g. a  battery) are also paralleled (as discussed) but each string (e.g. battery) should be same overall capacity.  

If you put 3 batteries in parallel where #1 = 60ah and #2 = 60ah and #3 = 30ah - then #3 will drain out while #1 and #2 still have 50% capacity - and then #1 and #2 will flow into or thru #3 to compensate.  My understanding is that its not fatal but adds unnecessary stress?  to the overall battery.   Maybe someone could comment in more detail about what exac

Back to your original question as to pack size, design, parallize.   In my case - practicality dictated my current design of 14s120p  * 3 in parallel.
- I started with 1s60p packs as the base unit.   Then as my battery grew it would have required 84 longmons - so I combined 1s60p with another to create a 1s120p unit (phsycially) and cut my pack number in 1/2 - requiring only 42 longmons.  
- The larger number of cells in parallel make the pack a little more robust - e.g. a cell here or there can fail and the overall percentage is so small the pack is able to maintain better.
- At 5,000g / 11lbs - it's still easy enough for me to carry / manipulate it into place.

The bottom line is that practicality affected my overall serial / parallel - and if you can start with a 'base unit' that can be combined in the future with other base units - that might serve you well.
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