Why not 2 BMS on 1 battery?

[dreuge]

What would be wrong with adding a 2nd BMS in parallel on the same battery to increase the max allowable current? I do not see any show stoppers, in fact I see it as a better option than what some do by building to separate 4S batteries. For example with 16 8 cells, one 2P4S w/ 2 parallel BMS's vs two 4S batteries each with its own BMS.

 

[MisterSandals]

For example with 16 cells, one 2P4S w/ 2 parallel BMS's vs two 4S batteries each with its own BMS.

How many cells do you expect to use in making a 2P4S battery with 2 BMSs?

 

[Short_Shot]

But you could just buy a higher output bms.

 

[dreuge]

How many cells do you expect to use in making a 2P4S battery with 2 BMSs?

Sorry, I got projects mixed up. Yes 8 cells.

 

[dreuge]

But you could just buy a higher output bms.

For 12V systems, the options are limited for a good bus with low temp cut-off, Bluetooth and preferable RS485 communications.

 

[John Frum]

2p4s is 8 cells.
The BMSs will not be totally in sync for voltage so one will trigger first on high/low triggers.
Depending on the amount of current flowing the late BMS may trigger on over-current instead of a high or low trigger.
The balance leads will be stacked and the balance current of one BMS may influence the voltage reading on the other.
I suspect this because I have read that at least some BMSs don't do voltage sense and balancing concurrently.
Might want to disable balancing on one BMS.

 

[Fil-Am Gabe]

I was thinking about doing the same thing. I have 4 48v overkill solar 100a bms but only can afford the cell cost of to make 2 280ah batteries at themoment. I actually just want it for surge for pumps and not continuous.

Any thoughts?

 

[dreuge]

Connecting BMSs in parallel has been working very well for me. The only thing different in the BMS settings are the "Total battery Capacity". You should not assume that it will be 50%-50%. But this is easy to figure out as it is directly related to the relative resistance of BMS with leads from battery to load. In your case, set this value for each bms1, bms2 = 280AH* current1/(current1+current2), 280AH* current2/(current1+current2). If your leads and bus are identical then will be close to 140AH each. In my case, I have slightly different battery cable length resulting in current1/(current1+current2) = 53.4%, so my capacities are set to 299AH and 261AH for a total of 560AH.

 

[x4dow]

problem with parallel BMS,s is that if one stops before the other, the other will take double the current after the 1st shuts down.

 

[dreuge]

problem with parallel BMS,s is that if one stops before the other, the other will take double the current after the 1st shuts down.

I don't see this as a problem. The cases described are when 1 BMS is at or just below the max current needed. For example, I started with a single JDB 120A BMS for a 2P4S 560AH battery. I bought a 2nd BMS but with the addition of RS485 readout, keeping the 1st as a back up spare. My loads are typically 5A - 20A (mainly DC loads on a sailboat) except when I use the microwave which uses another ~100A. The microwave is used only minutes per day, but with 1 BMS, I had to be mindful of the battery load before operating it as one time the BMS did shutdown the battery because of over current. With 2 120A BMSs and a load of 120A, the max current through each BMS would be ~60A. That's operating at 50% max current for each BMS as apposed to 80% max current for a single 150A BMS. Going with 2 BMS is like having an automatic redundant back up system. If I were to purchase a 150A BMS, I would likely get a spare too, and see no reason not connect it in parallel.

I readout both BMS data via raspberry pi with a simple python program that constantly monitors and plots the data for the past 2hr and 48hrs. I use vnc to remotely access the BMS performance from anywhere(nice when I'm not on the boat). Attached is a screen shot taken remotely this morning(i.e. 100 miles away). It shows my refrigeration system turning on/off within the past 2 hrs. The lower plots show the past 25hrs (but eventually will show 48hrs). There is a little blip at -120A on the Battery Current plot which was when I used the microwave to heat up some soup for yesterday's lunch and later the plot shows a charging cycle at 40A once I connected up to shore power. Not shown, are 2 additional plots that are scrolled down. These are the 48hr Residual Capacity and and overlap of all 4 cell voltages.