first you have to determine if only the output mosfets are turned off or if only the charging mosfet is turned off. in this case we know the output mosfet is turned off since he has no voltage on the output of the battery.
if you measure the gate voltage on the mosfets, both the charging mosfet and the output mosfet, then you can determine if both of them are turned off at the same time. in that case it is likely that the circuit current is turned off.
that could be caused by an open circuit in the thermal breaker or there is no power to one or the other of those two ICs on the pcb that control the balancing and the HVC and LVC functions of the BMS.
you can check the thermal breaker by measuring the resistance across the breaker. this has to be done when the BMS is disconnected at the sense wires. if the breaker is still closed as it should be in normal operation then the resistance measured will be 0 ohms. if you measure open circuit here then that would immediately identify the problem. it can be solved by placing a jumper across the thermal breaker since the breaker really provides no protection to the BMS or the battery. it is essentially useless.
continuing when there is no voltage on both charging and output mosfet gates. if you look at the wiring that provides power to those two ICs, every 5th sense wire delivers power to each of the ICs and the ground of the second IC is from the 5th sense wire. you can see that in the traces on the pcb. so if the B0 sense wire, or the B5 sense wire or the B10 sense wire is disconnected then there is no power to one of those ICs so the BMS will keep both of the mosfets turned off.
it is simple enuff to just use the DVM, 20 V DC scale and measure the voltage on each of those two sets of traces to see if there is power to the ICs. if both are powered up then it is not shut off because of those sense wires being disconnected.
if both of the mosfet gates are turned off then there is somehting else going on that would require more examination but i will continue on the main and simplest path where only the output mosfet is turned off.
if you take your DVM, 20V DC scale, and measure the cell voltage of each cell where the sense wire plug socket is soldered to the pcb, you should be able to measure a voltage between each adjacent pin. that voltage would have to be above the LVC spec for the BMS, usually 2.7V. if there is one cell below that voltage or if there is no voltage present then the BMS will turn off the output mosfet to prevent damage to the battery. i think this is the most likely case since it is the most common case.
if the mosfets had been subjected to excessive current then the BMS would shut off the current before it could damage any of those 4 output mosfets. the turn off of the mosfets is orders of magnitude faster turning off than a fuse blowing.
there is no damage visible on the mosfets. the shellac that covers the pcb is still intact on top of the mosfets and it would have scorched and melted if the mosfet overheated. if the mosfet had shorted as they normally do on failure then the BMS would still conduct current and the mosfets would overheat and melt the solder underneath them.
so i know the mosfets are still functional, from looking at his picture. so i am certain that the BMS is not only not broken, it is functioning normally but has inputs that cause it to think it should be turned off. if there is no gate voltage on the output mosfet.
if there is gate voltage on the output mosfets then the cause of the battery not expressing voltage at the terminals is because of the wiring or the the output plug itself, not the BMS. so then just even a simple examination of the wires from the B+ terminal of the pack and the P- terminal of the BMS would show where the open circuit is located.
usually open circuits in the wiring are right inside the connectors. bent pins or clips in an XLR type connector or dislodged contact in the anderson connectors. or the solder could be broken that connects the wire to the connector. all very easy to locate and identify.
the same could be true of the sense wire connections. the sense wire could be torn off the top of the cell in the pack. it could be cut by accident in use or even in manufacturing and did not fail until use. i looked at the soldered connections of the sense wire plug socket and i could not see cold solder joints and i would not expect that either on a new pack in any case and not the sort of thing that would happen since the sense wires do not carry much current which is usually what creates cold solder joints. but the sense wire is connected to tiny clip that fits inside the sense wire plug and those connections could be open or the clip itself is not seated fully into the plug so the sense wire does not make contact inside the plug to the pins on the pcb.
that would be obvious when you measure the cell voltages on the pcb in the solder at the base of that sense wire plug socket.
this is more complicated than most people can grasp when first examining the BMS so i find it easier to ask people to examine the device themselves in a sensible progressive manner to isolate the failure. it does not require an EE degree. i do not have an EE degree. i did take the junior and senior level classes in EE when i got my masters in physics along with my physics coursework but that was 40 years ago and has no bearing on the procedures to follow in examining the BMS which anyone can do with a simple $4 DVM. i did go to grad school in EE for year before getting a job but i did not study any more EE then, i just took courses in electron transport theory and properties of semiconductor materials and plasma physics. maybe this guy will use this as way to develop an interest in learning how to do this stuff but that is not the american way. now everything is handed on a platter so there is no need to strain to learn new stuff. so i get annoyed with it i guess when i see that all the science projects are done by indian and chinese students and not the all american types that used to be common when i was young. now there is no need since there is always money to just buy new and throw away the old. we were expected to beat the russians and just by accident we invented the integrated circuit and everything that has followed since. facebook and the iphone and the mind that is never quiet enuff to see how the transport equation works or the solve the scattering matrix. yep it is sore spot.
