fet based bms current path efficiency

[John Frum]

A good number of people on the forums are interested in bms's that are not fet based or not in the current path.
I wonder how efficient a fet based bms is at passing current.

Here is my idea off the top of my head for a test bed.
hook up the bms balance leads to a balanced battery that is not in the current path.
Hook up the b- and p- and a shunt in the current path of a very beefy battery charger.
Pass current through a test load.
Measure the heat produced by the BMS with an IR camera.
Measure the current and voltage via the shunt.
No heat, no loss; right?
I would guess some math could be done to compute the efficiency from the heat generated.

 

[John Frum]

I guess you use a copper busbar as the baseline stand-in for the bms.

 

[gnubie]

Pass reasonable current through the BMS while measuring the voltage drop across the BMS, and the current flow through if you can't ensure a given current from a lab power supply etc, and then you can calculate the loss fairly easily.

Extra reading

 

[RCinFLA]

In my opinion, the cutout MOSFET's are terrible on the Alibaba BMS's. True current carrying capability, for more then a minute or two, is about 20% to 40% of what they are claiming. Much of the material cost is in the series MOSFET's and they are skimping as much as they can to minimize cost.

For example, one BMS claiming 320 amps and says the series resistance on the passthrough to be 0.15 milliohms. At 320 amps that is a power dissipation of 320A^2 X 0.00015 ohms equals to 15.4 watts with no heat sink and enclosed in an external aluminum housing. They will get blistering hot, which also drives the series resistance of MOSFET's up by 50% making even more heating. That is assuming 0.15 milliohms is not a stretching of the truth. Most units do not state what the series pass resistance is. Then there is the two #6 wires soldered to PCB on input and output sides.

Best solution would be disabling charging capability at the inverter/charger at low temp without cutout series MOSFET's in BMS. I live in S Fla and don't have to worry much about low temp. Overvoltage charging can also be handled with separate monitor sense and fuses for overcurrent protection. Problem is this is all dependent on inverter/charger used.

There was also one BMS claiming 16s capability. The photo showed 16 battery wires connected and 16 bleed resistors installed on PCB The picture had the MOSFET part number readable and showed SUP70N03 part number. That part number is a 30v part and two 30v MOSFET's in series does not equal 60v breakdown like stacked diodes peak inverse voltage. A 30v breakdown is not really enough margin for a 24v battery system let alone a 48v 16s system. Hopefully the switch opening is never needed on those units.

Other then the heating, the voltage drop is not much of an issue to efficiency, maybe slightly on 12v systems.