Battery design help...

zerberfert

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Joined
Apr 14, 2020
Messages
58
Hello again,

First a little about my batteries. They are from the ring doorbell system. They consist of LG D2's (blues) and Panasonic A's (Greens). Here are some pics.


image_wjremc.jpg



image_kydgmm.jpg


I am getting about 60% blues and 40% greens.

Lets talk about the IR of both (thanks Wolf).

The blues are averaging 36 mOhms and the greens 26 mOhms. The range of blues I am keeping 34-38 and the range of greens 24-28. Anything above those 2 ranges are being kicked out for other projects, not my powerwall. I was planning of putting the blues in the middle 3 rows and the greens in the out most 2 rows. The reason for this was because of the difference in IR between the 2 different batteries, even though it's not much. Thoughts on this?

My battery packs will be 14s115p. I could make it 120p but I am going to leave the last row in the pack open to be able add batteries to "bump" up an low performing pack. Hopefully it's not going to be needed.

Here is a pic:


image_psoata.jpg


Here is where I need some advice.Do I need to have all 4 bus bars (the red lines in the pic), or could I do with 2-3? This is going to be on the + side of the battery. What size fuse wire should I use for this? The way I drew the fuse wire setup in the pic, will that work? Or do I need to do something different?

On the - side I am going to useCell Level Nickel Fuse from batteryhookup.com. Again do I need 4 bus bars like in the pic for the + side, or can I get away with 2-3?


Cells are 3A cells and with 115p that is 345A per pack. I will start with a 5kw inverter and will probably use 3-4kw peak. 4000W/58V = 69A, 69A/115 cells = .6A a cell. Please check my Math. So, if yall figure that I need the 4 bus bars from the pic, could I go with 10 awg wire. A 10 awg wire can carry 30 amps, since there are 4 of them that should carry 120A. Almost double the 69A, calculated above.

Last set of questions. What size wire do I use for the bus bars? Also, do I use solid wire or stranded wire?

Thanks in advance,

Zerb
 

Korishan

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Each cell should connect directly to the buss bar. The only exception would be is if you had sub-bussbars. ie, you had a small nickel strip connecting 4 in parallel, then that strip gets fuse linked to the bussbar. Like how mike did on his latest design
 

floydR

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Aug 23, 2017
Messages
1,067
Wouldn't max current happen at the low end of your discharge range? for example 3.5v x 14 = 49v
4000/49= 81.63A ?
later floyd
 

zerberfert

Member
Joined
Apr 14, 2020
Messages
58
floydR said:
Wouldn't max current happen at the low end of your discharge range? for example 3.5v x 14 = 49v
4000/49= 81.63A ?
later floyd

I don't know, is that true? It's probably best for worst case scenario.


Korishan said:
Each cell should connect directly to the buss bar. The only exception would be is if you had sub-bussbars. ie, you had a small nickel strip connecting 4 in parallel, then that strip gets fuse linked to the bussbar. Like how mike did on his latest design

I need to use the 4 bus bars for sure then. Now it's just a matter of what size wire to use. Thanks.
 

OffGridInTheCity

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Dec 15, 2018
Messages
1,530
Take a look at these other threads going on today
* Bus bar wire guage -https://secondlifestorage.com/showthread.php?tid=9602 (shows example of size and source to buy)
-https://secondlifestorage.com/showthread.php?tid=9611 (more discussion)
* Cell level fusing -https://secondlifestorage.com/showthread.php?tid=9603 (discusses 30AWG and axial fuse as good candidates)

For solder - you typically run a 6AWG twisted copper wire inbetween rows of cells and solder wire from cell to bus - and on one side, very thin wire for fusing (or actual fuses). For this - you would go 2, 4, 6 ... (even rows) wide rather than odd 5 rows as you show in the picture. The even row approach lets 2 rows of cells share a single 6AWG bus. I can tell you this works for me and I deliver 400a @ 48v from 84 packs with no problem - doesn't even get 'warm' to the touch:)

If you go nickle strip/spot-weld (instead of solder)then the nickle strips will take care of connectivity to each cell. The fuse-style nickle strips will take care of fusing as well. You supplement this by adding some bus wires soldered to the strips.
Here's a great, recent, youtube using spot-weld / strips with fuse by @HBPowerwall -
 

zerberfert

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Apr 14, 2020
Messages
58
@ OffGridInTheCity - So 4 wide is preferred to 5 wide got it. If I use the fuse-style nickle strip on one side, I can just use regular nickle strip on the other? Fusing is only needed on 1 side?

Thanks for all the info everyone.
 

Korishan

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floydR said:
Wouldn't max current happen at the low end of your discharge range? for example 3.5v x 14 = 49v
4000/49= 81.63A ?

yes, this is correct. As voltage goes down, amps go up for a specific wattage. So buss bars should be gauged based on lowest voltage and most wattage that would be used.

example: 3.2V/cell lowest voltage, 14s would be 44.8V. Inverter of 4000W. 4000 / 44.8 = 90A
 
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