BMS-controlled contactor for high-amp discharge

[harrisonpatm]

The MZJ (big orange ones) are not. They're waterproof and sealed with a gasket at the contact area, but not evacuated; I disassembled mine to investigate it awhile ago. I think they rely on two contact points and a relatively large opening to quench potential spark.

That's for the contact point, of course. For the coils, I haven't opened mine up to that extent. Maybe I will and post pics, I have a dead one lying around.

 

[jdmach77]

US $93.50 | 350A Relay BMS 3S 4S -32S 3000A Peak Lipo/Lifepo4 Battery protection board 7S 8S 10S 12S 13S 16S 17S 20S 24S 28S 32S 35S
https://a.aliexpress.com/_mMpL1L6

This might help

 

[LewTwo]

The MZJ (big orange ones) are not. They're waterproof and sealed with a gasket at the contact area, but not evacuated; I disassembled mine to investigate it awhile ago. I think they rely on two contact points and a relatively large opening to quench potential spark.

That's for the contact point, of course. For the coils, I haven't opened mine up to that extent. Maybe I will and post pics, I have a dead one lying around.

Correct and you can also get rebuild kits for some of them.

 

[A-DamW]

Non-isolated DC-DC, good to ~80V and ~1A
search "XL7015"
https://www.amazon.com/dp/B09TQ2B6WS/

Non-isolated DC-DC, good to ~86V and ~6A
https://www.amazon.com/dp/B09FNBSZTR/

ISOLATED range Input voltage: AC 70-277V OR DC 100-390V
search "AC DC Converter Module Universal"
https://www.amazon.com/dp/B07SJRX9R6/

I would choose the isolated, you can run full pack voltage and not worry about different ground levels.
Might have to experiment to see what the actual minimum DC input voltage it can run at(appears to be~100V)
You could run the switched HV BMS output directly to the DC-DC > contactor (maybe a capacitor between the DC-DC and contactor to buffer inrush current on the contactor coil)

 

[harrisonpatm]

@Eastwood brought this subject up, and it reminded me that I wanted to sketch out the basic wiring. The big question mark in the diagram is for bringing pack voltage down to coil voltage, if needed, and the various options for doing that were discussed in the thread.

 

[amberwolf]

Remember that a FET-based BMS switches the negative lead of the battery (ground), so any voltage conversion to the contactor coil would generally be done on the main positive battery lead to the coil.

For instance, you could use a small DC-DC that generates just enough current and voltage to operate the coil, with it's main positive input connected to the battery positive, and it's main negative input connected to the BMS P- point (that is switching main output negative). Whenever the BMS is on, the DC-DC is on, and thus the contactor coil is powered, and thus the contactor is on.

A manual shutoff switch simply has to cut either of the inputs to the DC-DC, so the DC-DC gets no power and does not turn on the contactor.

 

[harrisonpatm]

In planning my next build, I'd like to go with this concept of BMS-controlled contactor, so I've drawn up my planned wiring as such for this thread, for others that might be thinking of this route:


In this configuration, current from the battery will bypass the BMS to get to the controller. In the case of undervoltage, excessive voltage drop, or other battery issue, the BMS will cut output to the 120VDC to 12VDC converter, which will then cause the main contactor coil to open and cut power to the controller, thus preventing further damage to the battery during discharge. Charging is still done through the BMS, though, so it still will be able to trigger overcurrent/overvoltage protection.

The main element I'd like to focus on for critiques and comments is that I have the 12v converter wired to the battery's main + and -, via a switch. I can place this switch on the handlebars, but at my planned 28s, I'd rather not put up to 120VDC next to my hands while riding. More likely, I will use a circuit breaker-type cutoff switch in a discrete location, such as under the faux fuel tank, or next to my seat, something easy to reach; and have the contactor be powered on at all times.

My rationale is this: once purchasing the parts, I can test and measure the idle/parasitic no-load draw being used by the DC/DC converter. The DC/DC converter 12V output will be 0v, until switched on by a key, via a relay. If the idle draw is less than 5w, this is acceptable to me; I can put the converter input switch in a hard-to-reach spot and only turn it off if the bike is to be unused for a week or longer. If more than 5W, I should switch the converter's 120v input on every use, and will need to build and place the switch in a convenient (and electrically safe) location accordingly. Essentially, my plan is to have the 12V converter be "on" all the time, and only switch the 12vdc output on and off with each ride.

Let me know your thoughts on how this will work, if there are any suggestions

 

[amberwolf]

The BMS probably has an enable/disable function, either directly via an input, or via a temperature sensor, etc. If you wire *that* to your handlebar switch, it would be a low voltage (probably 5v) signal at extremely low current (less than mA's). Turning the BMS off this way would force it to turn off the DC-DC and thus the contactor.

I'm too tired to add this to the diagram in a useful way ATM.


Another thought, based on something that came up in another thread: If you use "typical" regen braking, and there is any risk that the BMS could shut off the connection to the battery during a regen event, you might choose to set the system up so it can't actually disconnect the controller in this event, but does disable the controller.

The reasoning for this is that regen braking**** usually creates the regen current by using the motor windings as a flyback transformer to spike / pulse the voltage higher than it would be as just a motor/generator, so it's higher than the battery voltage, and so causes reverse current flow. If you disconnect the battery during this process, the voltage spikes no longer have a load to keep them from going above what the controller electronics (and the DC-DC if it's on the same bus) can handle, and you can get POOF.

How it would disable it differs from controller to controller depending on externally-wired functions available. At worst case it could cut the KSI power to the controller, which turns off the LVPS that powers the brain. Best case is a wire that can be used to just tell the controller to stop. Middle case is a wire that disconnects the (5v) power to the throttle, and to any fixed or variable-regen control that isn't the main throttle. (if that control doesn't use power, just signal and ground, cut the signal wire).


**** some controllers (FOC, etc) create braking force by reversing the current vector, so it still takes power from the battery, and uses that to generate reverse torque in the motor. These don't have the same risk of flyback current spiking up the same way during braking, but there could still be a spike in voltage during disconnect as the circuit for current flow opens.

 

[harrisonpatm]

The BMS probably has an enable/disable function, either directly via an input, or via a temperature sensor, etc. If you wire *that* to your handlebar switch, it would be a low voltage (probably 5v) signal at extremely low current (less than mA's). Turning the BMS off this way would force it to turn off the DC-DC and thus the contactor.

I'll keep that in mind, but I've used JKBMS for my critical batteries, and a half-dozen other cheap BMS brands for various projects, but none of them have had the ability to enable/disable via a button. JKBMS can do it via bluetooth interface, but that doesn't help on a bike, I'd prefer a manual switch.

Another thought, based on something that came up in another thread: If you use "typical" regen braking, and there is any risk that the BMS could shut off the connection to the battery during a regen event, you might choose to set the system up so it can't actually disconnect the controller in this event, but does disable the controller.

The reasoning for this is that regen braking**** usually creates the regen current by using the motor windings as a flyback transformer to spike / pulse the voltage higher than it would be as just a motor/generator, so it's higher than the battery voltage, and so causes reverse current flow. If you disconnect the battery during this process, the voltage spikes no longer have a load to keep them from going above what the controller electronics (and the DC-DC if it's on the same bus) can handle, and you can get POOF.

How it would disable it differs from controller to controller depending on externally-wired functions available. At worst case it could cut the KSI power to the controller, which turns off the LVPS that powers the brain. Best case is a wire that can be used to just tell the controller to stop. Middle case is a wire that disconnects the (5v) power to the throttle, and to any fixed or variable-regen control that isn't the main throttle. (if that control doesn't use power, just signal and ground, cut the signal wire).


**** some controllers (FOC, etc) create braking force by reversing the current vector, so it still takes power from the battery, and uses that to generate reverse torque in the motor. These don't have the same risk of flyback current spiking up the same way during braking, but there could still be a spike in voltage during disconnect as the circuit for current flow opens.

This is a bit over my head, I'm going to need to reread it a couple times, but I do understand the gist of what you're saying; if the opens the contactor during operation, but the controller is still on, it will still attempt to regen with no battery, and that could damage the controller.

Since I don't fully understand it, tell me if this is a stupid idea: can i protect against this via a series of externally wired flyback diodes between the three phase wires? In the same way you might use snubber diodes on relay coils to reduce voltage spikes. Something that doesn't interfere with normal operation, but will protect the motor if the contactor unexpectedly opens.
Edit: ignore this, don't think it would work anyway, and I believe I found a much simpler solution below.

 

[harrisonpatm]

How it would disable it differs from controller to controller depending on externally-wired functions available. At worst case it could cut the KSI power to the controller, which turns off the LVPS that powers the brain. Best case is a wire that can be used to just tell the controller to stop. Middle case is a wire that disconnects the (5v) power to the throttle, and to any fixed or variable-regen control that isn't the main throttle. (if that control doesn't use power, just signal and ground, cut the signal wire).

Ok, I get it now. I was planning on having the controller's main on/off via a handlebar thumb switch, that way I can quickly kill the controller's main power via my thumb, same function as a regular motorcycle kill switch. All I'll need to do is place the contacts of a small relay in line as well, powered by the same 12V converter that is controlled by the BMS. That way, in case of fault, the BMS cuts power to the converter, which opens the main high-current contactor as discussed. In addition, this will also cut power to the small relay in line with the controller's on/off switch. That should successfully keep regen from engaging in time.

Since all this relay needs to do is carry the voltage and minuscule signal current for the controller's on/off, I just went to my stash of salvage circuitboards and pulled off a microwave relay the size of my thumb that should do the job. Rated for 5A at 250VAC, so yay, one less thing to purchase!

 

[amberwolf]

Just remember that if regen is already happening at the instant the BMS shuts off, the voltage will spike faster than the turn off time of the relay/etc. Whether this will be enough to cause a problem depends on too many things to know what will happen except by testing in each of the various situations. This is the situation that can be problematic.

If the BMS shuts off before the regen is activated, then the controller won't be operating (will be unpowered) and so can't engage regen.


But...AFAICS from your wiring diagram, you don't need a second relay to kill the on/offf switch line? Isn't taht already done by killing all power to the system which is already being done by the BMS shutting off it's output?



Regarding "snubbers" between the phases; I don't know if it would work or not--I'm not sure they have a hard enough cutoff point to not affect normal operation while still preventing overvoltage, and I'm also not sure if they could act fast enough (they have to snub faster than the FETs begin to avalanche). If you did use them, I'd put them at the FET bridges themselves rather than the motor end of the cable.


But anyway: Iff you're using FOC controllers that use negative current to brake the wheel, vs typical controllers that repeatedly and momentarily short the windings to create flyback spike currents/voltages, then you shouldn't have the problem the latter causes under disconnect of battery load.

 

[harrisonpatm]

But...AFAICS from your wiring diagram, you don't need a second relay to kill the on/offf switch line? Isn't taht already done by killing all power to the system which is already being done by the BMS shutting off it's output?

Not visible in this thread, but in my build thread, I have the contacts of the contactor bridged with a 2kohm resistor, always on, for my precharge. Therefore the controller can be turned on with the contactor off, so yes, i will also need a separate relay for the controller on-off.

Just remember that if regen is already happening at the instant the BMS shuts off, the voltage will spike faster than the turn off time of the relay/etc. Whether this will be enough to cause a problem depends on too many things to know what will happen except by testing in each of the various situations. This is the situation that can be problematic.

I was also pondering whether the relays shutoff faster than regen. Will certainly be taken under consideration if/when the build actually happens. This is why I posted the diagram, so people more knowledgable than me can bring up issues.

 

[amberwolf]

Not visible in this thread, but in my build thread, I have the contacts of the contactor bridged with a 2kohm resistor, always on, for my precharge. Therefore the controller can be turned on with the contactor off, so yes, i will also need a separate relay for the controller on-off.

Ah, yeah, that would be an issue.

I was also pondering whether the relays shutoff faster than regen. Will certainly be taken under consideration if/when the build actually happens. This is why I posted the diagram, so people more knowledgable than me can bring up issues.

Unfortunatley relays take time to turn off; field in the coil doesn't isntantly disappear once current is cut, takes time to collapse, and then it takes time ffor the mechanical contacts to begin to pull apart as the field weakens enough for that to happen.

There are solid state swtiches that shoudl switch faster, but for this kind of event they may still not be fast enough. It depends on how fast the voltage rises after battery disconnect, which will itself depend at least partly on what is happening at the wheel at that time.

I don't know the math to figure this stuff out, just have a "sense" of how it works in my head. :/ (seeing the current flows in various parts of a (relatively simple) circuit, etc, the way I "see" tension/bending/etc in a mechanical structure, etc. My brain is not always correct in the visions it shows me, but it's close enough much more often than not, and it gets better at it the more I learn).