Good day all

The only lifepo4 cells it seems to be found in the country currently is some 55ah cylindrical cells.

I have drawn up a picture of a 12V, 110ah battery bank by connecting the cells first in parallel and then in series.

My question is, would it matter if the adjoining modules are connected via busbar on only one side as in the picture? Should it rather be connected on both negative terminals to both positive terminals of the next module, or should I drill a hole in the middle of the busbar to try to keep resistance the same to each terminal?

On 2023/06/02 at 2:05 PM, JohanBar said:

My question is, would it matter if the adjoining modules are connected via busbar on only one side as in the picture?

Hi  @JohanBar, as long as the busbar connecting the positive and negative between the paired cells together can carry the 110A current with ease (should not be on the limit), then it should work fine.
You could also consider a different battery layout where you use 3x longer busbars with 4 holes for the inter connections an 2x shorter ones for the end connections. Something like shown below. Compression of the cells (if you want to) might also be easier with the suggested layout.

Enjoy your build.

Blessings, Deon

Edited June 3, 20233 yr by DeonBez

Thanks @DeonBez Busbars are short ones I will be receiving from supplier.

Is the above valid for the balance leads of the bms as well? Will a connection to the terminal on one side be sufficient or does it need to be smack bang in the middle between terminals?

Also what would be my limit in adding cells in parallel in this way? (Taking into account cell voltage balance and not adding too old and new cells together) The reason for this is I can still have one bms that takes care of all the cells.

21 minutes ago, JohanBar said:

Busbars are short ones I will be receiving from supplier.

@JohanBar the busbars coming with the cells might only be rated for 55A or just a bit more than the cell Ah rating. 

Please share the cell details / model / datasheet or link to give a more informed input as the continuous charge/discharge C rating of the cells can also play a role.
 

23 minutes ago, JohanBar said:

Will a connection to the terminal on one side be sufficient or does it need to be smack bang in the middle between terminals?

As long as the busbars are suitable to carry the higher current after paralleling some cells, measuring the balancing voltage on either of the terminals should be sufficient.
A build of the perfectionist will have it in the middle though 😉
 

32 minutes ago, JohanBar said:

Also what would be my limit in adding cells in parallel in this way? (Taking into account cell voltage balance and not adding too old and new cells together) The reason for this is I can still have one bms that takes care of all the cells.

Before answering, please indicate which BMS model do you plan to use?

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Please share the cell details / model / datasheet or link to give a more informed input as the continuous charge/discharge C rating of the cells can also play a role.

https://lithium-cells.co.za/products/55ah-cylindrical-lifepo4-cells

Seems each cell can give a max of 1C or 55ah, charge is half of that.

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As long as the busbars are suitable to carry the higher current after paralleling some cells, measuring the balancing voltage on either of the terminals should be sufficient.

Excuse my ignorance, but wouldn't the busbar's resistance cause a voltage drop which would relate to an over/under voltage measurement of that specific cell?

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Before answering, please indicate which BMS model do you plan to use?

Was planning on using the 150A JK BMS, so also planning to switching to 48V at some point, and be able to use the same BMS as it can handle 8-20s and in this way keep the amps running through it well under the 150A limit.

Edited June 3, 20233 yr by JohanBar

3 hours ago, JohanBar said:

Busbars are short ones I will be receiving from supplier.

@JohanBar, I would expect the busbars from the supplier to be only rated for 55A or a bit more. Except if you have the datasheet for it that indicates it can handle 110A, I would recommend getting busbars with the correct rating.

2 hours ago, JohanBar said:

Excuse my ignorance, but wouldn't the busbar's resistance cause a voltage drop which would relate to an over/under voltage measurement of that specific cell?

If the busbars used are rated to carry 110A and made from copper or aluminium, then the voltage drop across it should be so small that it should not influence the cell balance measurement. 

2 hours ago, JohanBar said:

Was planning on using the 150A JK BMS, so also planning to switching to 48V at some point, and be able to use the same BMS as it can handle 8-20s and in this way keep the amps running through it well under the 150A limit.

The JK BMS is good in the sense that it has built-in active balancing with high enough current to cater for nice balancing. 

I see you mention a JK BMS 8S to 20S model... just note that the 8S will only work for a 24V battery and higher and not for a 12V battery as mentioned in your initial post/drawing. For a 12V a 4S model is required. 

2 hours ago, JohanBar said:

https://lithium-cells.co.za/products/55ah-cylindrical-lifepo4-cells

Seems each cell can give a max of 1C or 55ah, charge is half of that.

I noticed that the cycle life of these cells are only 2000 cycles which is much less that the normal 3500 or even 6000 cycles specified for some of the prismatic LifePo4 cells. Just wanted to bring it under your attention 🙂

 

Edited June 3, 20233 yr by DeonBez

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@JohanBar, I would expect the busbars from the supplier to be only rated for 55A oe a bit more. Except if you have the datasheet for it that indicates it can handle 110A, I would recommend getting busbars with the correct rating.

I understand, will do thanks.

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If the busbars are rated to carry 110A and made copper or aluminium, then the voltage drop across it should be so small that it should not influence the cell balance measurement. 

Excellent.

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The JK BMS is good in the sense that it has built-in active balancing with high enough current to cater for nice balancing. 

I see you mention aJK BMS 8S to 20S model... just note that it will only work for a 24V battery and higher and not for a 12V battery as mentioned in your initial post/drawing. For a 12V a 4S model is required. 

Yes I am actually building a 24V battery, I said 12V just to keep the drawing simpel, as I am sure the principles remain the same for longer series strings.

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I noticed that the cycle life of these cells are only 2000 cycles which is much less that the normal 3500 or even 6000 cycles specified for some of the prismatic LifePo4 cells. Just wanted to bring it under your attention 

Yes I guess you get what you pay for. Cycling them once a day every weekday will still get me 7 years of use, so I see it as an okay trade-off for what I pay. Also this is my introduction to DIY battery building, so I can spend R7000 in total with BMS in stead of R23k+ to get my feet wet. I do think in 7 years time, available technology would have changed quite a bit.

Edited June 3, 20233 yr by JohanBar