2 12volt battery bank help

jiggyjonboat

New member
Joined
Jul 31, 2019
Messages
12
Hey guys. Ive just purchased 40 headway 38120 cells wanting to make 2, 20 cell batteries 12v batteries, question is do I do wire batteries up with 4 banks of parallel cells then wire for the 4s after? Or wire the batteries into series first then branch off for the 4 banks of parallel groups. ? . I want to get the cell placement.Down property so I dont mess up.
Any help would be amazing sorry for being a newbie but Ive looked everywhere for info on cell placement making packs with no luck
 

OffGridInTheCity

Well-known member
Joined
Dec 15, 2018
Messages
1,272
One consideration that jumps out at me would be the BMS aspect. Seems like if you do 4 banks (4 seperate 'batteries') then you need 2 BMS(s) per larger battery. If you do 1 bank per battery then you'd only need 1 BMS. I presume its a hi-amp BMS situation... A recent @jehugarcia youtub titled "4000A BMS?" shows a potential solution on headway 38120s.
 

Korishan

Moderator
Staff member
Joined
Jan 7, 2017
Messages
6,329
jiggyjonboat said:
question is do I do wire batteries up with 4 banks of parallel cells then wire for the 4s after? Or wire the batteries into series first then branch off for the 4 banks of parallel groups. ?

If you wire 4p then 4s, which is a 4s4p configuration, then you'd only need 1 bms, as OffGridInTheCity mentions. If you wire it up as 4s then 4p, which is a 4p4s configuration (I know, can be confusing on the naming conventions), then you'd need 4 bms units, 1 for each string of series.

I personally would recommend 1 bms, or 4s4p. Reason being, is if you have multiple bms sharing a common output, it's possible 1 of them could shut down prematurely, which would cause a shift in current flow from the other strings and over load those as well. Depending on the current draw, this "could" burn out the FETs on the bms, requiring replacement.

This is, of course, if you are using a cheapo bms from chinesium manufacturing. If you are using something quite a bit more robust and smarter, then that makes it more of a funds available question than anything else.

In both situations, 4s4p or 4p4s, they are electrically identical on the absolute outputs. Charging/Discharging would be more or less the same. The only issue that "could" come up with 4p4s, is that one of the cells in one of the strings could become a different voltage than one of the other cells in another string. This would cause a little imbalance in the system. This would be the "primary" reason to have a smart bms that talks to the others, or go with 4s4p and use 1 bms from the get go.
 

jonyjoe505

Active member
Joined
Feb 28, 2018
Messages
221
My 220ah lifepo4 battery bank is 2x110ah 4s80p batteries, I just connect both batteries to a y connector that is connected to a bms and then you connect the balance connectors on both batteries together and to bms. I had it like that for the past 2 years running perfectly.

I'm building a smaller30ah lifepo4 battery, it will be 3x 10ah 4s6p connected together with a connector that can handle all 3. Same principle as above.

Thats the best way to do it, it makes for a more compact footprint, also you can disconnect one of the batteries or add more batteries to increase capacity, and you only need one bms.

Just make sure all batteries are same voltage when you join them together.

This is a picture of a parallel board, but I make my own with xt60 connectors. Same principle. Once batteries are joined together, they charge/discharge as one big battery.


image_wrosee.jpg
 

OffGridInTheCity

Well-known member
Joined
Dec 15, 2018
Messages
1,272
jonyjoe505 said:
.... I just connect both batteries to a y connector that is connected to a bms and then you connect the balance connectors on both batteries together and to bms.
This sounds as if you are (by definition)connecting them in parallel... and I would thinkthesmall wire size for the balance leads could easily burn up under load.
Perhaps I missing something here :)
 

Korishan

Moderator
Staff member
Joined
Jan 7, 2017
Messages
6,329
The main connector is connected using a Y connector. The balance leads are connected using the smaller white connectors in the image (I'm guessing as such)
 

jiggyjonboat

New member
Joined
Jul 31, 2019
Messages
12
I would love to see a simple drawing of how this would look, I know it will be 4s4p but I want to see how you would wire cells and secondly wiring up the bus balancing leads
 

jiggyjonboat

New member
Joined
Jul 31, 2019
Messages
12
So if my set up is 4s4p. What is this considered ? I get the 4s part but what is the parallel portion of this and how would you label this set up ?

image_etzsvp.jpg
 

OffGridInTheCity

Well-known member
Joined
Dec 15, 2018
Messages
1,272
jiggyjonboat said:
So if my set up is 4s4p. What is this considered ? I get the 4s part but what is the parallel portion of this and how would you label this set up ?

This is 4s (4 in series) and 8p (8 in parallel).

image_vuhkdr.jpg


If you cut the battery in this picture in 1/2 from left to right - you would have 2 batteries of 4s4p.

This would 1 of the 2 batteries of 4s4p

image_nuuiml.jpg
 

Korishan

Moderator
Staff member
Joined
Jan 7, 2017
Messages
6,329
jiggyjonboat said:
So if my set up is 4s4p. What is this considered ? I get the 4s part but what is the parallel portion of this and how would you label this set up ?

I think you also need to read the FAQ on the main page as this question is answered in detail with several example images.
 

jiggyjonboat

New member
Joined
Jul 31, 2019
Messages
12
Wow guys thank you so much with the awesome info!
All your help is certainly pointing me in the right direction
Now all things considered since I purchased 40 headway cells
I would create 2 20 cell packs 2 bmss.. so all this said how would youcalculate the ah of one 20 cell 12volt pack 4s5p Eachbattery is 8ah.
Thanks again in advance
 

Korishan

Moderator
Staff member
Joined
Jan 7, 2017
Messages
6,329
Hrmm, posted the same reply 3 times :huh: :huh: :huh: , and apparently didn't read the FAQ yet. (that's where you can find how capacity is calculated)
 

jiggyjonboat

New member
Joined
Jul 31, 2019
Messages
12
Sorry about that, I have gone through the faq page and in your thread I see some info on ah calculations. Thanks so much. For all the good info.
 

jiggyjonboat

New member
Joined
Jul 31, 2019
Messages
12
Alright guys after a long time I got the packs together and took them out this past weekend and wow Im blown away these batteries are amazing. Anyhow I have a charging question, when I charge up the pack the negative battery side aka 1s as a bms or balancer is labeled, it always is undercharged and the other 3 banks seem to be relatively equal. Example first cell bank on the negative side is 3.3, next is 3.53, next 3.55, and last bank closest to charge lead positive is 3.6volts which total charge is 87_89% charged I want 95% and up. What do I have to do the make this charge properly, do I just have to manually charge the lowest bank with a 3.5volt charger? Any help would be awesome
 

Korishan

Moderator
Staff member
Joined
Jan 7, 2017
Messages
6,329
Sounds to me like your BMS isn't wired up correctly or is just plain not functioning. If it were working, then all cells/packs in series should be the same voltage.
 

jiggyjonboat

New member
Joined
Jul 31, 2019
Messages
12
Ok so correct me if Im wrong when the bms is wired up the pack negative is the starting point, and the first cell positive is directly under the negative which would be cell 1, next in series would be the next cell positive and so on till I reach the last positive which would be 4s and the last lead on the bms. If this is true its hooked up properly so that means bms isnt working
 

jiggyjonboat

New member
Joined
Jul 31, 2019
Messages
12
Secondly I checked the wiring its in fact correct
FYI I am using a Chinese bms made by deli green that might be the reason huh? Any suggestions on 4s and 8s bms. Do you recommend a higher balance amperage ? I would think the higher the amperage the quicker the balance. I reconnected my cell monitor 8 and checked each voltage 1s 3.35 2s 3.42 3s 3.5 4s 3.68 so I shit it down and let my small balancer bring the 4s down to a safer voltage. I need this baby to give me 100% soc not 89-91%
 

jiggyjonboat

New member
Joined
Jul 31, 2019
Messages
12
After you guys help me with my last replys. Can someone recommend their personal bms/10amp balancer for 4s and for 8s. Please guys Im going nuts here trying to figure this out.
 

jiggyjonboat

New member
Joined
Jul 31, 2019
Messages
12
Ok my only fix is balance charge the batteries with buck converters and all voltages set to 3.56 so its been working quite well I think thats my fix.
And to answer the question to the bms having to balance, the balancer is only on at a set point of a fully charged battery.so not all cells would get up to that set balance voltage. So manually charging all 4s banks is my option or spend 200$ on a quality bms
 

jiggyjonboat

New member
Joined
Jul 31, 2019
Messages
12
Ok another update. I removed the bmss on both my batteries. I top balanced all the parallel packs (meaning I charged then individually to a 3.6volt cut off) and went out with the boat and everything was within .03 volts which is awesome. So Im charging the packs with my big charger and everything is staying balanced. No balancers no individual charging so crazy. I even unplugged the bmss for now. So balance charging solved my problems so far.
 
Top