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BMS amperage

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NMNeil

Solar Enthusiast
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Mar 14, 2020
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As I posted elsewhere, due to my own carelessness I connected the balance lead from cell #12 to the connector for cell #15 and visa versa.
Now the BMS is showing the wrong voltage on a couple of cells; so I've fried it.
Looking for a replacement for the 100A JBD unit and I'm finding lots of 80A replacements but very few 100A that are in stock.
As I have 3Kw of panels and 3Kw MPP all in one units, do I really need a 100A BMS?
3Kw at 48 volt gives 62A, and I've never seen that much output from my panels, maybe 45A on a good day, and of course the same amperage applies to the MPP, which would shut down on over current, that is if my 80A fuse didn't pop first.
So how do you size a BMS? accepting that I would need to apply a margin of error for exaggerated current handling claims by the BMS sellers.
 
If you can wait, get JK BMS from aliexpress, hankzor store. Takes a few weeks to arrive. Whatever you plan to pull from the battery is what you should use. I had a 100amp on my 6k inverter before and it was fine but it all depends on your loads. 80a might work.
 
A BMS is typically sized for the battery and NOT the Inverter. For a 100AH Battery (100AH Cells) a 100A BMS goes with that, a 280AH Pack with 280AH Cells 200-250A BMS is acceptable. The BMS should be able to handle the 1C Rate of the cells.

Using 100AH as an example:
A 100AH Battery Pack @ 1C will output 100A for 1 hour BUT it will only take 0.5C Charge of 50A for 2 hours. This is an LFP and how it works.
 
Steve_S said:
A BMS is typically sized for the battery and NOT the Inverter. For a 100AH Battery (100AH Cells) a 100A BMS goes with that, a 280AH Pack with 280AH Cells 200-250A BMS is acceptable. The BMS should be able to handle the 1C Rate of the cells.

Using 100AH as an example:
A 100AH Battery Pack @ 1C will output 100A for 1 hour BUT it will only take 0.5C Charge of 50A for 2 hours. This is an LFP and how it works.
Click to expand...
So if I have 2 banks of 280Ah cells in parallel giving me 560Ah I'd need a 500A BMS at a minimum?
That makes no sense.
 
NMNeil said:
So if I have 2 banks of 280Ah cells in parallel giving me 560Ah I'd need a 500A BMS at a minimum?
That makes no sense.
Click to expand...
BMSs are sized for the charge current going in and the load current going out of the battery.
For exampe This BMS
overkillsolar.com

16s BMS 100a for LifePo4 without Wires

Without wires soldered to the board. You will need to solder your own wires. Do not bolt connections directly to the circuit board. BMS For 16 cell, 48v volt Lithium-iron-phosphate DIY batteries 10…
overkillsolar.com overkillsolar.com
can pass 100amps continuous in either direction reliably.

100 dc amps * 48 volts low cutoff * .85 efficiency factor = 4080 ac watts.

That means you could use a high frequency inverter of up to 4000 watts.
 
Last edited:
The 280ah eve cells that are popular here can be charged and discharged at .5c(140 amps) without undue stress.

The other important measure of a BMS is its balance current.
When you mention 250 amps I think Daly.
Daly BMSs have very wimpy balance capacity.
 
NMNeil said:
So if I have 2 banks of 280Ah cells in parallel giving me 560Ah I'd need a 500A BMS at a minimum?
That makes no sense.
Click to expand...
NO, Each battery has a BMS which should be matched to the size of the Battery Pack itself, regardless of how many are in Parallel.
100AH Battery Pack = 100A BMS, 280AH Battery Pack = 250A BMS (200A is readily available, 250A+ not so much), there are a few.

Clarification:
A Battery = Cells + BMS in one box. 12V=4 cells, 24V=8 cells, 48V=16 cells.
A "BANK" is a collection of complete batteries (Cells & BMS) linked in either Series or Parallel.
Batteries linked in Parallel increase Stored Capacity (AH) while Batteries linked in Series increase the voltage of that string (not recommended).

Higher Amp-hour batteries (usually above 200-250A) will typically use a Relay Based BMS and not MOSFET, as the Relays handle the current. For example if you are using Winston 1000AH Cells to make a battery pack, you are absolutely going to use a relay / contactor based BMS. Don't ask how much $$$$$$ LOL

TSWB-LYP1000AHC​

Thunder Sky Winston Water Based Lithium Yttrium Power Battery​

Nominal Capacity：TSWB-LYP1000AHC
 
Steve_S said:
Higher Amp-hour batteries (usually above 200-250A) will typically use a Relay Based BMS and not MOSFET, as the Relays handle the current. For example if you are using Winston 1000AH Cells to make a battery pack, you are absolutely going to use a relay / contactor based BMS. Don't ask how much $$$$$$ LOL

​

Click to expand...
You would also need copper wire as thick as your arm to carry 1000 amps, even though the maximum the inverter is able to draw is around 60 amps for a 3kw unit and the same 60 amps which is the maximum output of my 3kw of panels.
Still makes no sense to me.
 
Steve_S said:
NO, Each battery has a BMS which should be matched to the size of the Battery Pack itself, regardless of how many are in Parallel.
100AH Battery Pack = 100A BMS, 280AH Battery Pack = 250A BMS (200A is readily available, 250A+ not so much), there are a few.

Clarification:
A Battery = Cells + BMS in one box. 12V=4 cells, 24V=8 cells, 48V=16 cells.
A "BANK" is a collection of complete batteries (Cells & BMS) linked in either Series or Parallel.
Batteries linked in Parallel increase Stored Capacity (AH) while Batteries linked in Series increase the voltage of that string (not recommended).

Higher Amp-hour batteries (usually above 200-250A) will typically use a Relay Based BMS and not MOSFET, as the Relays handle the current. For example if you are using Winston 1000AH Cells to make a battery pack, you are absolutely going to use a relay / contactor based BMS. Don't ask how much $$$$$$ LOL

TSWB-LYP1000AHC​

Thunder Sky Winston Water Based Lithium Yttrium Power Battery​

Nominal Capacity：TSWB-LYP1000AHC
Click to expand...
Wow. I was hoping to get these Winston Thundersky 3.3V 1000Ah cells to make a massive 16S home battery, but I guess the BMS needed and wires need would be impossible? Would this still be the case if I only want to draw no more than 30 amps?

batteryhookup.com

4x Thundersky Winston 3.3v 1000ah Lifepo4 Prismatic - 13.2kWh

Free shipping anywhere in the lower 48 states. These batteries are monsters. 3.3v 1000ah 3300wh each. You get 4 of them for a total of 13.2kWh. They were in service for 4 years as backup only. They have not been capacity tested but all cells have good voltage and we do not see any swelling. We...
batteryhookup.com batteryhookup.com

1686868287049.png
 
BlueMarblePA said:
Wow. I was hoping to get these Winston Thundersky 3.3V 1000Ah cells to make a massive 16S home battery, but I guess the BMS needed and wires need would be impossible? Would this still be the case if I only want to draw no more than 30 amps?

batteryhookup.com

4x Thundersky Winston 3.3v 1000ah Lifepo4 Prismatic - 13.2kWh

Free shipping anywhere in the lower 48 states. These batteries are monsters. 3.3v 1000ah 3300wh each. You get 4 of them for a total of 13.2kWh. They were in service for 4 years as backup only. They have not been capacity tested but all cells have good voltage and we do not see any swelling. We...
batteryhookup.com batteryhookup.com

View attachment 153046
Click to expand...
Size your BMS, wiring, and overcurrent protection for your needs.
 
Steve_S said:
NO, Each battery has a BMS which should be matched to the size of the Battery Pack itself, regardless of how many are in Parallel.
100AH Battery Pack = 100A BMS, 280AH Battery Pack = 250A BMS (200A is readily available, 250A+ not so much), there are a few.

Clarification:
A Battery = Cells + BMS in one box. 12V=4 cells, 24V=8 cells, 48V=16 cells.
A "BANK" is a collection of complete batteries (Cells & BMS) linked in either Series or Parallel.
Batteries linked in Parallel increase Stored Capacity (AH) while Batteries linked in Series increase the voltage of that string (not recommended).

Higher Amp-hour batteries (usually above 200-250A) will typically use a Relay Based BMS and not MOSFET, as the Relays handle the current. For example if you are using Winston 1000AH Cells to make a battery pack, you are absolutely going to use a relay / contactor based BMS. Don't ask how much $$$$$$ LOL

TSWB-LYP1000AHC​

Thunder Sky Winston Water Based Lithium Yttrium Power Battery​

Nominal Capacity：TSWB-LYP1000AHC
Click to expand...
@Steve_S do you have any experience with the Winston Thundersky battery. I am looking at the battery hookup offer of $1900 for four cells 3.3v 1000Ah. Good price for building a home storage option, but quality may be poor?
 
BlueMarblePA said:
@Steve_S do you have any experience with the Winston Thundersky battery. I am looking at the battery hookup offer of $1900 for four cells 3.3v 1000Ah. Good price for building a home storage option, but quality may be poor?
Click to expand...
I wouldn't. 4 year old battery. No idea how they were treated. BMS for 1000ah? Set of 4 for 48v is almost $8,000. I'd rather get 4 sets of 16 lf280k new eve for the same price.
 
DIYrich said:
I wouldn't. 4 year old battery. No idea how they were treated. BMS for 1000ah? Set of 4 for 48v is almost $8,000. I'd rather get 4 sets of 16 lf280k new eve for the same price.
Click to expand...
I see. This is helpful. I am still learning about batteries.
 
When you get to building a battery pack that is above 250A Delivery Capacity the BMS used should be a Relay Based BMS and NOT a FET Based BMS. Relays are capable of handling High Amperages of course the cost for this increases along with their capacity. Appreciate that @ 250A, a 12V system can handle 3000W, 24V for 6000W and 48V for 12000W, excluding surge

While at First Blush, 1000AH Cells sounds good BUT... They can take 500A for Charging. It is ONE battery assembly and if something goes wrong, well... you get it. You are also looking at a fairly hefty BMS relays. YES, you could use a 250A Max FET Based BMS but that will of course limit the charge & discharge capacity of the cells.

Battery Packs that are installed in Parallel share/divide Charge & Discharge, if one or more fail for any reason the other packs keep going, which is fault tolerance & fall back all in one. With LFP, you can safely add other battery packs in parallel over time as well without suffering any deleterious effects. They are NOT like Lead Acid in that repect.

Remember that each battery in a bank must be fused.
 
Those cells are 41 kg each, 90 lbs. The BMS only has to meet your load requirements, the battery capacity is irrelevant.
For example a 100 amp rated BMS will operate with 100Ah cells, or the same BMS with 280 Ah cells.
The problem with the very large cells are extremely high current under fault conditions, ( rated for 3000 amps continious) and possible mechanical damage due to the size and weight.
 
I don't get this. The limiting current draw on most systems is the inverter capacity. Size the BMS for the inverter capacity or if you like, twice the inverter capacity for extra resilience and you are good to go. Why would you size a BMS for the whole battery capacity at an arbitrary 1C? That's just a number and no relationship to the load on the battery?
The lithium will supply any load you place on it so 1C is not relevant. A dead short is several thousand amps. Why are you not you sizing the BMS for this? The obviuos answer is you won't be applying a dead short. Nor will you be applying a load greater than the rating of the inverter so size the BMS to suit the load not an arbitrary 1C
 
TimE said:
I don't get this. The limiting current draw on most systems is the inverter capacity. Size the BMS for the inverter capacity or if you like, twice the inverter capacity for extra resilience and you are good to go. Why would you size a BMS for the whole battery capacity at an arbitrary 1C? That's just a number and no relationship to the load on the battery?
Click to expand...
Thank you. I have learned that sizing the BMS has more to do with the load and not with the battery capacity. !!
 
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