Going back to post #1, it appears that the intention is to switch off the power if either battery pack approaches some lower charge level. OR is there some other purpose in mind??
I an guessing that normal operation is not to simply apply 144 volts to the drive motor. I have raced that way at one rental track,, max power at all times..But that is not recommended most of the time.
So I am guessing that there is some sort of controller between the battery string and the motors. THAT would be the point to get the low-charge shutdown control input. Avoid the contactor drain, it does nothing to move you down the track faster.

 

Going back to post #1, it appears that the intention is to switch off the power if either battery pack approaches some lower charge level. OR is there some other purpose in mind??
I an guessing that normal operation is not to simply apply 144 volts to the drive motor. I have raced that way at one rental track,, max power at all times..But that is not recommended most of the time.
So I am guessing that there is some sort of controller between the battery string and the motors. THAT would be the point to get the low-charge shutdown control input. Avoid the contactor drain, it does nothing to move you down the track faster.

The intention is to run the motor at 144v+ but it's necessary to monitor the entire battery pack and that's why there two bmss. If you look for any single bms out there that's 40s they are hard to find and if anything have here short comings and are typically very expensive. With 2 bmss I need to make sure that if either bms detects any issue with either battery it will turn off complete power to main load.

 

Re post #20. Whatever logic you use to detect a fault you still need the 12 volt supply to close the main contactor. This 12 volt supply needs to be capable of supplying about 3.5 amps for a short time (Which is up to 130 mS according to the datasheet.) and then a continious holding current of 130 mA. A large capacitor to supply the current pulse would probably be too large to fit in the space available. Another solution would be to use a 10 cell Ni-mH battery on trickle charge or 3 or 4 cell Li-ion pack that was charged at the same time as the main batteries. You could then use a smaller 12 voly power supply as it would only need to provide the holding current of 130 mA.

Les.

 

Re post #20. Whatever logic you use to detect a fault you still need the 12 volt supply to close the main contactor. This 12 volt supply needs to be capable of supplying about 3.5 amps for a short time (Which is up to 130 mS according to the datasheet.) and then a continious holding current of 130 mA. A large capacitor to supply the current pulse would probably be too large to fit in the space available. Another solution would be to use a 10 cell Ni-mH battery on trickle charge or 3 or 4 cell Li-ion pack that was charged at the same time as the main batteries. You could then use a smaller 12 voly power supply as it would only need to provide the holding current of 130 mA.

Les.

Les I realized that the current dc to dc I have has a max output of 1.5a and yet it was still able to open and close the contactor

 

If you are prepared to ignore the datasheet rating then just use one of your converters with the output set to 12 volts and supplied by one of the 72 volt batteries. You will then probably have to use one or two opto isplators to isolate the signals from the two BMS modules as their output signal will probably be referenced to the negative connection of the battery. You could then logically OR the fault signals from the BMS modules. The output of the logical OR to break the supply to the contactor using a power transistor or mosfet You will need to supply us with the information on the BMS modules so we can tell you how to wire it up.

Les.

 

Certainly using opto-isolators of some kind will be the best choice. IF it is possible to use DC solid state relays driven directly from the BMS output, those could serve to directly control the contactor.
But now a question: Is the main motor directly driven from the 144 volts out from the battery pack, with only the contactor to switch power on and off? Is there no speed controller in the system?? That seems quite amazing to me.
Looking at the circuit from post #1 I see that we are only shown one part of the system, because the TS has decided that is where the issue lies.
That is not where the issue lies, because the requirement is to stop operation if one battery pack stops providing power. Probably there is in a power control segment, which we do not see, a much simpler way to force a switch-off than using a heavy duty contactor that burns a significant amount of power. So we really need to see the whole system to provide a reasonable suggestion of a solution.

 

Probably there is in a power control segment, which we do not see, a much simpler way to force a switch-off than using a heavy duty contactor that burns a significant amount of power. So we really need to see the whole system to provide a reasonable suggestion of a solution.

Agreed. If we had visibility of the BMS spec we'd be better off to advise - I asked several times but the TS hasn't responded. My initial thought was if the two BMS are in series what's the leakage when one shuts down? Is the contactor needed anyway, since a shut down BMS wouldn't be conducting, but could be reverse biassed, and would that affect it (reverse bypass protection diode could - maybe - be expected to take full current load of operatng BMS?)

 

Agreed. If we had visibility of the BMS spec we'd be better off to advise - I asked several times but the TS hasn't responded. My initial thought was if the two BMS are in series what's the leakage when one shuts down? Is the contactor needed anyway, since a shut down BMS wouldn't be conducting, but could be reverse biassed, and would that affect it (reverse bypass protection diode could - maybe - be expected to take full current load of operatng BMS?)

Sorry I didn't share the bms here it is

https://a.aliexpress.com/_mNtaWoS

 

Certainly using opto-isolators of some kind will be the best choice. IF it is possible to use DC solid state relays driven directly from the BMS output, those could serve to directly control the contactor.
But now a question: Is the main motor directly driven from the 144 volts out from the battery pack, with only the contactor to switch power on and off? Is there no speed controller in the system?? That seems quite amazing to me.
Looking at the circuit from post #1 I see that we are only shown one part of the system, because the TS has decided that is where the issue lies.
That is not where the issue lies, because the requirement is to stop operation if one battery pack stops providing power. Probably there is in a power control segment, which we do not see, a much simpler way to force a switch-off than using a heavy duty contactor that burns a significant amount of power. So we really need to see the whole system to provide a reasonable suggestion of a solution.

Yes there is a controller as well that is necessary to power the vehicle. However the main focus is on the battery pack, if there is any issues within the batteries there needs to be a way to shut off power quickly. The best way to monitor a battery pack is with a bms. And the bms will turn off if it senses any problem.

 

USUALLY any "battery pack issues"are caused by mechanically caused short circuits, and often handled by a fuse. If a battery becomes discharged then disabling the motor driver is appropriate, likewise if a battery simply becomes overheated. A manual shutoff switch is the way to disconnect the batteries, rather then a contactor that presents a constant power drain. And it does need to switch off if one of a series arrangement becomes discharged to avoid reverse charging of the discharged battery. The signal from one of the BMS packages to the drive controller should be fast enough to protect the pack that has become discharged. OR, is it that the Gokart will be driven by those who have no understanding of anything about the battery system, such as renters at a commercial Kart rental track??

 

USUALLY any "battery pack issues"are caused by mechanically caused short circuits, and often handled by a fuse. If a battery becomes discharged then disabling the motor driver is appropriate, likewise if a battery simply becomes overheated. A manual shutoff switch is the way to disconnect the batteries, rather then a contactor that presents a constant power drain. And it does need to switch off if one of a series arrangement becomes discharged to avoid reverse charging of the discharged battery. The signal from one of the BMS packages to the drive controller should be fast enough to protect the pack that has become discharged. OR, is it that the Gokart will be driven by those who have no understanding of anything about the battery system, such as renters at a commercial Kart rental track??

No I forgot to mention that the max discharge output of the bms is only 100 amps and we would like to discharge 400+ amps. That being said finding a bms to discharge that high of amps is difficult, and they are often not reliable, and bulky.

 

What do you anticipate that the BMS will do during discharge that will be a benefit? That is the puzzle that I do not understand. Certainly it can be a benefit during charging, but if you are drawing 400 amps for any length of time you are beyond the max. You already stated in post #1 " Both BMS will not be used for discharge and only for charging." And they will certainly try to shut down if 400 amps is sensed.

So it is very unclear as to what you anticipate the BMS doing for you during the discharge. An explanation is in order about that.

 

What do you anticipate that the BMS will do during discharge that will be a benefit? That is the puzzle that I do not understand. Certainly it can be a benefit during charging, but if you are drawing 400 amps for any length of time you are beyond the max. You already stated in post #1 " Both BMS will not be used for discharge and only for charging." And they will certainly try to shut down if 400 amps is sensed.

So it is very unclear as to what you anticipate the BMS doing for you during the discharge. An explanation is in order about that.

If any of the cell banks goes below the threshold it will cut off our if one of the temperature sensors hits a threshold it will shut off. And also when charging it will take care of the full charging process

 

Those bms units don't normally manage the charging process. They keep the cells balanced, and disconnect if the charging voltage is too high. The charging voltage/current regulation is done by a separate charger. I also have reservations about how the batteries are connected in series. I think you may have bypassed the protection MOSFETs.

 

Of course, in racing priorities are a bit different than everywhere else: I recall "Be First or Blow It UP Trying as being the mantra of a few racers.