Battery Pack Questions (new to this stuff)

benha

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Hello!

I’m working on a design for a submerged electrical device which requires an 8S6P battery pack. For space / weight balance reasons I’d like to split the pack into two separate 8S3P columns, each sealed into a PVC tube.

I’m very new to battery pack design, but my understanding is that it’s “safer” to construct an 8S6P pack than to parallelize two 8S3P packs, so here’s what I’m thinking:

If I get a piece of 16/12 SO cord and run it between those two packs, I could connect the packs at each of the series junction points so that even though they’re not in the same physical space, they’re electrically connected the way they would be if they were tabbed together.

Novice questions:

1. In concept, does what I described above make sense? Are there weird electrical consequences to having a wire bridge connecting two banks of tabbed cells? Is 16GA wire sufficient for a <30” bridge length?

2. Can I seal these things in tubes and never remove them during discharge and charging? Or is that going to create heat buildup problems?

3. If I do this, I’m assuming I can put the BMS on one side and the bridge between the two stacks of batteries allows it to do its work for the whole pack, yes?


Thanks,
-Ben
 
you can build 2 x 8s3p packs and just connect the two batteries with balance connectors and use a y-connector to charge both packs. You can put the BMS close to where the charger is where it can feed both packs equally. If you put the BMS on one pack and charge it from there, the other pack will charge slowly because of the distance.
Its how I charge/discharge my 2x110ah lifepo4. As long the wires are the same length/gauge it should be good. I never had problems doing it that way. The important thing is both batteries need to be connected together with the balance connectors.
When connected to the charger it will see one 8s battery pack, even though its 2 seperate battery packs. And the balancing will be equal on both packs since they are connected together.
 
benha said:
If I get a piece of 16/12 SO cord

What does "16/12 SO" mean?


Is 16GA wire sufficient for a <30” bridge length?
The current you expect between them determines what wire gauge you need. Normally the only significant current will be between the main + and - of the two packs.


2. Can I seal these things in tubes and never remove them during discharge and charging? Or is that going to create heat buildup problems?
You'd need to look at the cell spec sheet, and see what it's temperature would be at the charge and discharge rates you'd be using. Then see what temperatures it's designed to work up to. Then keep in mind that the spec sheet is probably setup for a single cell in open air at "ambient", which is often 72F. (it may say on the spec sheet how they did the testing).

Without the spec sheet, then you should assume that if used at their max ratings, they'll get hot encased in something without airflow or other cooling. So I'd recommend using them at half or less of their ratings, which should keep them cooler. (but it means having more parallel cells if you need to get more current out of the pack).


One way to keep them cool is to seal the cells into *metal* tubes using thermally conductive but electrically-insulative liquid or gel (whether it remains liquid or solidifies doesn't make too much difference in a very small tube). THen the fact that the tubes are submerged in another liquid will help them stay cooler. Plastic is a better heat insulator than heat conductor, so the PVC tubes will retain cell heat instead.

If the environment around the tubes is very cold, then you may actually need the cells to generate heat to operate correctly.
 
Great feedback, thanks!

amberwolf:
SO cord (technically SOOW cord) is a type of 'S'ervice cord that's 'O'il resistant inside and 'O'il resistant outside the jacket and meets the CSA 'W'eather resistant standard. Basically it's the kind of cord that's used to put a plug onto a power tool, build certain types of extension cords, etc.
16/12 indicates a version of SO cord that's got 12 independent conductors, each of which is 16AWG. If you're building an extension cord you'd use something like [16|14|12]/3 SO cord where you have a Hot, Neutral, and Ground conductor (the '3' part) of appropriate gauge for the task at hand.


In this case, I think what I was envisioning was overkill. If I understand what jonyjoe303 was proposing, I can basically create two 8s3p packs and use a balance connector setup to keep everything in sync. Image below to describe what I think this means. In essence, the BMS goes inside a harness that accepts balance connectors and power from each pack, and then has an output for Charge/Discharge.

BatteryPack.jpeg



Now, however, I want to touch on this idea of two 4s6p packs in series. Is that a more rational way to achieve this goal? Then each pack has its own BMS and is charged independently, and you just build a harness that serializes the outputs from those packs to create a higher voltage input to the ESC, right? Would this make more sense? Another image of what I think was being proposed:

SeriesPacks.jpeg


Thanks!
-Ben
 
Either of those configurations will work. The first one only needs one BMS. You could make the second configuration work with one BMS by extending the wires. The balance wires between cells don't need to be very big as they don't normally carry much current. 16ga would be overkill. 20ga is probably big enough. The main wires from the ends of the pack need to be big enough to handle the full current.

Having the cells packed in tubes shouldn't be a heat problem as long as the outside of the tube is exposed to air (or water).
 
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