2 x in 1 box LifePO4 15S build with JK-B2A24S20P BMSs

[LydMekk]

I will be adding a double DIY LifePO4 battery with 15S and using 2 pcs. JK-B2A24S20P BMS.
Couple of weeks until the BMSs is here (ordered one more as backup...)
Bought cells through Qishou - EU warehouse delivery.
Planning to use EVE 280K, 30 pcs. 2 banks of 15S.

This is to be an extension to my existing pack with 4 x 200ah (same 15S 48V config), need more storage.
Also the reason for 15S to ensure the same 48V nominal instead of going with 16S 51.2V.
I am not a huge fan of the current raised price level on these things compared to a year or two since but what can you do...

I will add to this thread when stuff happens and product appears ^^.
There WILL be pictures.

 

[sunshine_eggo]

I will be adding a DIY LifePO4 battery with 15S2P and using a JK-B2A24S20P BMS.
Couple of weeks until the BMSs is here (ordered one more as backup...)
If 18650 batteries doesn't answer (again...) the cells will probably go through nkon.nl in the EU.
Planning to use EVE 280K, 30 pcs. 2 banks of 15S paralleled.

This is to be an extension to my existing pack with 4 x 200ah, need more storage.
Also the reason for 15S to ensure the same 48V nominal instead of going with 16S 51.2V.
I am not a huge fan of the current raised price level on these things compared to a year or two since but what can you do...

What chemistry are your current 200Ah batteries?

 

[LydMekk]

Normal LifePO4s. Everything will be the same chemistry.

 

[Steve_S]

LFP = 3.2V per Cell Nominal. a 48V (51.2V Nominal) Battery Pack IS 16S not 15S, you are shortchanging the pack and will not get the performance & wh as you expect. The Voltage Curve is VERY Flat with LFP where the Deliverable AH's comes from (3.000-3.400 Volts per cell) or 48.0V to 54.4V from the "48V" Battery Pack. With 15S LFP you are creating a Handicapped Battery Pack and the BMS will not see this as correct either.

PS: I am the same Steve_S that edited their manuals & docs and helped them, for a while.
Hope it Helps, Good Luck.

 

[LydMekk]

Ok, 32 pcs. EVE 280K cells from Qishou ordered.
So 2 spares, had to order in 4 pcs. units.
Andy gave these several good reviews.
So far recommended. Let's see how the shipment goes (EU).

 

[jimf909]

LFP = 3.2V per Cell Nominal. a 48V (51.2V Nominal) Battery Pack IS 16S not 15S, you are shortchanging the pack and will not get the performance & wh as you expect. The Voltage Curve is VERY Flat with LFP where the Deliverable AH's comes from (3.000-3.400 Volts per cell) or 48.0V to 54.4V from the "48V" Battery Pack. With 15S LFP you are creating a Handicapped Battery Pack and the BMS will not see this as correct either.

PS: I am the same Steve_S that edited their manuals & docs and helped them, for a while.
Hope it Helps, Good Luck.

View attachment 141531

Can you help me understand why a 15S battery pack is a Handicapped Battery Pack compared to a 16S battery pack? Sure, a 15S has slightly less capacity (6.25%) but the inverter is still happy with the 48 volts it supplies. What's wrong - or handicapped - with a 15S if charging/cutoff parameters are set knowing that it's 15 cells? It just seems like a battery pack that has 15/16ths the storage capacity for 15/16ths the cost (ignoring the minuscule fixed cost variance of the BMS. Thanks.

 

[toms]

Can you help me understand why a 15S battery pack is a Handicapped Battery Pack compared to a 16S battery pack? Sure, a 15S has slightly less capacity (6.25%) but the inverter is still happy with the 48 volts it supplies. What's wrong - or handicapped - with a 15S if charging/cutoff parameters are set knowing that it's 15 cells? It just seems like a battery pack that has 15/16ths the storage capacity for 15/16ths the cost (ignoring the minuscule fixed cost variance of the BMS. Thanks.

You are correct, there is no problem running 15S LiFePO4.

I doubt those commenting otherwise have ever actually seen a 15S LiFePO4 system in action.

When the REC-BMS was first available it was only for 15S systems, plenty of people ran LiFePO4 at 15S (and i know of some that are still going strong, well over a decade later).

As long as your inverter and chargers can run from 45V there is no issue. (for example SMA can operate from 41V to 63V)

 

[sunshine_eggo]

Can you help me understand why a 15S battery pack is a Handicapped Battery Pack compared to a 16S battery pack? Sure, a 15S has slightly less capacity (6.25%) but the inverter is still happy with the 48 volts it supplies. What's wrong - or handicapped - with a 15S if charging/cutoff parameters are set knowing that it's 15 cells? It just seems like a battery pack that has 15/16ths the storage capacity for 15/16ths the cost (ignoring the minuscule fixed cost variance of the BMS. Thanks.

Capacity:
6.25% lower
2.5V * 15 = 37.5V -> this is notably lower than a typical 48V inverter will be happy with. Most default to 42V leaving a small amount of capacity of a 15S battery unusable (an extreme case - most wouldn't choose this as a cut off, but many inverters can't be changed).

Current
6.25% higher for any given power because the operating voltage is lower.

16S more closely matches the operating range of lead-acid batteries. These systems have been designed with that operating range in mind for decades.

 

[TomC4306]

You are correct, there is no problem running 15S LiFePO4.

I doubt those commenting otherwise have ever actually seen a 15S LiFePO4 system in action.

When the REC-BMS was first available it was only for 15S systems, plenty of people ran LiFePO4 at 15S (and i know of some that are still going strong, well over a decade later).

As long as your inverter and chargers can run from 45V there is no issue. (for example SMA can operate from 41V to 63V)

He proposes ADDING a 15s battery to an existing 4 battery bank. (presumably 16s)
My opinion is that this will NOT work.
OP..care to clarify?

 

[TomC4306]

Can you help me understand why a 15S battery pack is a Handicapped Battery Pack compared to a 16S battery pack? Sure, a 15S has slightly less capacity (6.25%) but the inverter is still happy with the 48 volts it supplies. What's wrong - or handicapped - with a 15S if charging/cutoff parameters are set knowing that it's 15 cells? It just seems like a battery pack that has 15/16ths the storage capacity for 15/16ths the cost (ignoring the minuscule fixed cost variance of the BMS. Thanks.

15s can work fine IF all the batteries in the pack are 15s.
If you are mixing 15s and 16s the combined pack will only operate with the capacity of the 15s.
And probably worse than that because the 16s batteries will never fully charge.

 

[SparkyJJO]

Normal LifePO4s. Everything will be the same chemistry.

Are they 15s or 16s? Can't mix them.

 

[sunshine_eggo]

Deleted.

 

[jimf909]

Based on the title and the OP's comment that they ordered 32 cells and will have two spares it appears that the OP is running a 15S 48 volt system.

 

[TomC4306]

Based on the title and the OP's comment that they ordered 32 cells and will have two spares it appears that the OP is running a 15S 48 volt system.

We don't know that yet. OP hasn't told us what the existing four batteries are

 

[LydMekk]

Can you help me understand why a 15S battery pack is a Handicapped Battery Pack compared to a 16S battery pack? Sure, a 15S has slightly less capacity (6.25%) but the inverter is still happy with the 48 volts it supplies. What's wrong - or handicapped - with a 15S if charging/cutoff parameters are set knowing that it's 15 cells? It just seems like a battery pack that has 15/16ths the storage capacity for 15/16ths the cost (ignoring the minuscule fixed cost variance of the BMS. Thanks.

YOU are correct. Steve's statement above is not correct.
A battery pack with 15S instead of 16S will be slightly less in amps (equivalent the 280ah from the "missing" cell).
There are NO other differences.
Ofc. it will discharge slightly faster than a 16S cell but again: only by the amount of the "missing" cell.
And Steve, a 16S pack IS 51.2V not 48V as a 15S battery config.
Mkay?

 

[LydMekk]

We don't know that yet. OP hasn't told us what the existing four batteries are

Yep, we do
I posted so in my original first post in this thread.
Old stuff: all 15S 48V.
Therefore I need also the same config on my this new planned battery build so the same voltage are maintained and charging will not be an issue.

 

[SparkyJJO]

Ah, missed that.

Then yes, you need to stick with 15S. Makes sense.

 

[sunshine_eggo]

YOU are correct. Steve's statement above is not correct.
A battery pack with 15S instead of 16S will be slightly less in amps (equivalent the 280ah from the "missing" cell).

Nope.

It will be slightly less in AMP-HOURS. It will actually require more amps for a given power. You effectively say that below with "discharge faster," but terminology and units matter.

There are NO other differences.

Lower operating voltage. Potential loss of a small amount of unusable capacity.

Ofc. it will discharge slightly faster than a 16S cell but again: only by the amount of the "missing" cell.
And Steve, a 16S pack IS 51.2V not 48V as a 15S battery config.
Mkay?

Advantages of a 15S LFP battery:
has a nominal 48V

Advantages of a 16S LFP battery:
6.25% more capacity.
6.25% lower current for a given power (also may improve cycle life slightly).
Voltage better matches the typical operating ranges of 48V power systems.

When not influenced by your current 15S setup, there is no benefit to selecting 15S.

Mkay?

 

[jimf909]

A benefit of a 15S setup is lower cost. It's minor but a benefit nonetheless that reflects that natural tension in building batteries: more batteries offer more capacity but cost more.

As for 16S better matching 48 volt operating ranges, it seems that 15S will work fine with currently available equipment and are a better match for legacy gear that was used with FLA batteries.

I'm not a proponent of one over the other but I believe it's important that people aren't given the impression that 15S can't be part of a highly functional 48 volt system. 16S has some advantages just as 48 volt has advantages over 24 volt (but the 16S advantages over 15S are much smaller compared to 48v v 24v).

 

[42OhmsPA]

Lots of good info and discussion already.
Something I didn't see mentioned, and consider a positive for 16s, if 1 cell were to fail you could still reconfigure for 15s (assuming you use a 'smart' BMS) and keep running after adjusting settings.

 

[sunshine_eggo]

A benefit of a 15S setup is lower cost. It's minor but a benefit nonetheless that reflects that natural tension in building batteries: more batteries offer more capacity but cost more.

Lower total cost, but higher per unit energy cost. Since a 15S BMS costs the same as a 16S BMS (especially if a universal is chosen)...

15S + BMS is more $/kWh than 16S+BMS

15S yields less kWh/$

Do you want to save money or get more for your money? I'm in the latter camp.

Furthermore, the 15S will have a reduced cycle life due to higher current and greater DoD for a given application than 16S. Not a lot, but one can't argue the opposite. This further increases the cost of the 15S...

As for 16S better matching 48 volt operating ranges, it seems that 15S will work fine with currently available equipment and are a better match for legacy gear that was used with FLA batteries.

literally the opposite of what has been said/documented.

15S operates 4.5V below the low limit and 4.45V under high limit of FLA.
16s operates 2.5V outside the low limit and 0.8V under high limit of FLA.

Less voltage is outside of FLA range with 16s than 15S. Not sure how 15S is a "better match."

I'm not a proponent of one over the other but I believe it's important that people aren't given the impression that 15S can't be part of a highly functional 48 volt system.

I am a proponent of 16S. It's literally all upside for 16S. I don't believe I have presented anything that can be used to give an impression that 15S can't be as you state. It absolutely can. It's just always second choice.

16S has some advantages just as 48 volt has advantages over 24 volt (but the 16S advantages over 15S are much smaller compared to 48v v 24v).

If there are no existing restrictions, there is no technical reason to select 15S over 16S. There may be a financial reason, but if $150/battery is where one is trying to save money, the system margins are probably too tight.

Now... all of that said... if I had an option where 15S was 25% cheaper per kWh than 16S, sure. I'd probably go for it, but I would wrestle with the decision.

Back to the OP... they have no choice. I wouldn't consider mixing 15S/16S, nor would I scrap or replace a perfectly healthy 15S with 16S just on principle.

 

[jimf909]

@sunshine_eggo , thanks for the helpful response to my earlier question and other comments. You've covered the differences pretty well although I'll have to sketch out the comparison the FLA voltages to get my head around it.

 

[sunshine_eggo]

@sunshine_eggo , thanks for the helpful response to my earlier question and other comments. You've covered the differences pretty well although I'll have to sketch out the comparison the FLA voltages to get my head around it.

12V FLA cut off: 10.5V
12V charging: 14.8V (varies, some are lower and some are as high as 15.0V - Rolls Surrette)

4* 10.5V = 42V
4 * 14.8V = 59.2V

LFP:
Cut off: 2.5V
Charging: 3.65V

15S:
37.5V vs. 42V min (-4.5V)
54.75 vs. 59.2V (-4.45V)

16S:
40V vs. 42V (-2V)
58.4V vs. 59.2V (-0.8V)

I acknowledge that these are absolutes, and few will want to run any chemistry down to the extreme minimums; however, in an emergency, one might want to use every last electron.

 

[Steve_S]

And Steve, a 16S pack IS 51.2V not 48V as a 15S battery config.
Mkay?

Don't get SNARKY MKay ! We always refer to these are 48V systems regardless of 51.2V or not...
ALSO I am NOT some idiot NOOBY !
Charge Profiles, presets etc are designed for specific voltage curves 12V, 24V, 48V which is the common default industry standards.

YES a 15S will work BUT there will be side effects & issues as discovered by several others which can and sometime DO limit operations. I will not get into this Rabbit Hole again... it has been previously beaten to death.

15S "working voltage range for LFP" = 45V to 51V
16S "working voltage range for LFP" = 49V to 54.4V
Working LFP Range = 3.000-3.400 Volts per cell.
Allowable LFP Range = 2.500-3.650 (the SAFE Voltage Range for the cells that does not cause harm)

Also:
15S (200AH) = 9600Wh
16S (200AH) = 10,200Wh

As for the Snark... you are on your own...
Some lessons can be expensive & painful. Lessons are standing by.

 

[jimf909]

12V FLA cut off: 10.5V
12V charging: 14.8V (varies, some are lower and some are as high as 15.0V - Rolls Surrette)

4* 10.5V = 42V
4 * 14.8V = 59.2V

LFP:
Cut off: 2.5V
Charging: 3.65V

15S:
37.5V vs. 42V min (-4.5V)
54.75 vs. 59.2V (-4.45V)

16S:
40V vs. 42V (-2V)
58.4V vs. 59.2V (-0.8V)

I acknowledge that these are absolutes, and few will want to run any chemistry down to the extreme minimums; however, in an emergency, one might want to use every last electron.

Thanks for walking through this information for me. Much appreciated!