2x 4s bms or 8s for 24v diy

[Skippyp]

Some good threads and just a few videos on this and I'm still unclear the ideal way to arrange my cells. Will have 8 272ah lifepo4 nominally tied to either an lv2424 or growatt (so 2.4kw or 3kw). A 250a 8s Daly bms gets me a solid 24v battery using less than half bms rated amps and room for inverter spike.
Alternatively I could use 2 120a 4s overkill units and then run two diy 12v batteries series. Overkill says 120a continuous is no problem, so that's a continuous 4.8kw available with 3kw intended thru inverter.
Daly is cheaper, simpler, and 250a gives me lots of margin. Overkill is highly recommended but requires 2, increasing the number and complexity of components. I suspect both bt apps are similar.

I could use some constructive recommendations on 24v cell/battery config. The system will have shore and/or 3kw generator power as well as ~1kw solar to the growatt/lv24.

 

[FilterGuy]

There are some pages toward the end of this resource that talks to pro's and cons of the two options.

Cell Configurations for 12V 24V and 48V LiFePo4 Batteries

This short deck shows 1P and 2P cell configurations for 12V, 24V & 48V LiFePO4 batteries. To get the deck, click on the orange button at the top of this page. Note: Reviews, Comments and corrections are welcome Document Revision History...

diysolarforum.com

 

[Skippyp]

Yeah that was a good thread. I'm still stuck on how easy putting 2 battleborns in series is no big deal when that's essentially the same. The drop in replacements are cells with bms inside a case vs DIY.

 

[FilterGuy]

Neither is 'wrong'. They both have pro's and cons.

 

[MTBioGuy]

The slide deck was helpful but could someone please offer me a little reassurance that my "plan" (below) will work?

I already have an Overkill 8s 100A BMS ready to go. Unfortunately, immediately after it arrived I stumbled upon a discussion on this forum that had me second guessing my system plan.

Intended application: Small RV Travel Trailer
- System: 8s, 272/280Ah cells, 24v, LiFePO4 with 2K/4K Inverter (24v).

- Max Current Needed: 1,000w microwave (my primary concern, short use... less than 5 minutes) plus (worst case simultaneous use... although unlikely as we tend to run only one device at a time) basic 6 gallon LP hot water heater & pump + basic LP refrigerator (8 cubic foot) + basic LP furnace fan. No future current capacity increases likely. 2/0 AWG wiring from batteries to inverter is already in place.

Question: will my new 24v Overkill 8s 100A BMS be able to handle that microwave?
I'm upgrading from a 630Ah, FLA, 3K/6K Inverter 12v system that was working OK with that microwave. My amperage needs have never been high... I mostly strive for maximum days without sun capacity.

? ? ? Should I have split my 8 (3.2v) cells back into a parallel 12v configuration and purchased TWO of the Overkill 4s BMS units instead?

I hope not since I have already purchased the 24v BMS, a 24v inverter and a 24v to 12v step-down converter.
Thanks

 

[Jimmynik1]

I read something somewhere that said something like microwave wattage is the cooking wattage and actual running wattage is higher.
If thats not the case and your just running the 1kw micro at 120vac through your 24vdc to 120adc inverter i think itll be no issue. Whether you go with the 2.4 or 3k constant both are more than double the rated wattage needed by the micro.
Sidebar, i plan to make basically the same system you have just with 16 cells still at 24v in a s/p config so either 560 or 600ah depending on whats available when i order and i plan to use 2 of either overkill or daly 8s100a and have done a ton of reading and asked several people and one 8s100a on its own is suitable for 2400w. So your micro is fine.

 

[MTBioGuy]

I read something somewhere that said something like microwave wattage is the cooking wattage and actual running wattage is higher.
If thats not the case and your just running the 1kw micro at 120vac through your 24vdc to 120adc inverter i think itll be no issue. Whether you go with the 2.4 or 3k constant both are more than double the rated wattage needed by the micro.
Sidebar, i plan to make basically the same system you have just with 16 cells still at 24v in a s/p config so either 560 or 600ah depending on whats available when i order and i plan to use 2 of either overkill or daly 8s100a and have done a ton of reading and asked several people and one 8s100a on its own is suitable for 2400w. So your micro is fine.

Thanks Jimmynik1
... but would you mind further explaining your line... "Whether you go with the 2.4 or 3k constant both are more than double the rated wattage needed by the micro." for this newbie?

 

[Jimmynik1]

Will have 8 272ah lifepo4 nominally tied to either an lv2424 or growatt (so 2.4kw or 3kw)

If your main concern is running your thousand watt microwave, and both of the inverters you mentioned above are 2400w and 3000w. Both of those are well above 1000 watt requirement for the microwave, even above the starting wattage.

 

[MTBioGuy]

I read something somewhere that said something like microwave wattage is the cooking wattage and actual running wattage is higher.
If thats not the case and your just running the 1kw micro at 120vac through your 24vdc to 120adc inverter i think itll be no issue. Whether you go with the 2.4 or 3k constant both are more than double the rated wattage needed by the micro.
Sidebar, i plan to make basically the same system you have just with 16 cells still at 24v in a s/p config so either 560 or 600ah depending on whats available when i order and i plan to use 2 of either overkill or daly 8s100a and have done a ton of reading and asked several people and one 8s100a on its own is suitable for 2400w. So your micro is fine.

How about another??
- You said: “I have done a ton of reading and asked several people and one 8s100a on its own is suitable for 2400w. So your micro is fine.”
Question: did you mean “one 8s cell bank” or “one 8s BMS” ?? Thanks!

 

[MTBioGuy]

If your main concern is running your thousand watt microwave, and both of the inverters you mentioned above are 2400w and 3000w. Both of those are well above 1000 watt requirement for the microwave, even above the starting wattage.

Oh... I see the problem... in my original post I listed my inverter as a "2K / 4K" model. I meant that it was a 2,000 continuous / 4,000 initial surge" model. Sorry about that. Thanks for your help.

 

[John Frum]

Oh... I see the problem... in my original post I listed my inverter as a "2K / 4K" model. I meant that it was a 2,000 continuous / 4,000 initial surge" model. Sorry about that. Thanks for your help.

The surge rating on a high frequency inverter doesn't really mean much.
Its usually ~20 milliseconds which is ~1 cycle of alternating current.
Low frequency inverters can have a surge capacity that is significant.

 

[Jimmynik1]

Oh... I see the problem... in my original post I listed my inverter as a "2K / 4K" model. I meant that it was a 2,000 continuous / 4,000 initial surge" model. Sorry about that. Thanks for your help.

I didn't even think I saw that part I saw the part where you said a 2.4 or 3K but either way you're fine.