I want to make my own BMS

[Ampster]

I have no credentials, but I have 3 contactors that I got at a good price and will continue to use them as my fail safe until the Joules come home.

 

[Factory400]

I have no reason to spout my credentials. But while working on manned flight space hardware reliability was utmost important. Junction temperatures was a key design parameter. Of all the components in the BMS, power fets have some of the highest junction temperatures. Hence they can have the greatest probability of failing.

Of course - but that can all be managed. We have data about Tjmax and all the data about how to avoid that (or stay within a specified user defined limit). Engineers in modern power electronics applications know how to drive and protect the FETs so that they are operated within the defined boundaries. I worked at an aerospace company a while back that did power electronics for space hardware and aviation - FETs were never the problem. In testing, if the FET's were damaged it was only a result of a poorly designed driver system but that is still really really uncommon. The engineers know the parameters ahead of time. They are not inventing anything new, just making adjustments to the driver and protection circuits that have been around for a very long time in order to match the capabilities of the chose MOSFET.

If you do stupid things, you get stupid results. You can blow the gate out of a FET. You can violate the SOA. All sorts of things can break a FET.
The important part is the rest of the design that surrounds the FET. That is where the reliability comes from.

 

[Factory400]

I have no credentials, but I have 3 contactors that I got at a good price and will continue to use them as my fail safe until the Joules come home.

Contactors are not a problem at all.....if you have the space and budget.
EV's and other high voltage applications use them to guarantee an air-gap disconnect for safety reasons.

In the BMS world, I like MOSFETs for the size benefit.

 

[Ampster]

In the BMS world, I like MOSFETs for the size benefit.

We don't disagree.

 

[Factory400]

We don't disagree.

@Cal does bring up a good point in that a MOSFET solution is only reliable if it is designed properly - otherwise it is a smoke generator. The MOSFET's are just a single part of a more complex circuit.

I suspect that @Pidjey understands the intricate details of designing a high-current MOSFET switch in a way that is reliable for his application.

 

[Bob B]

I'm not a design engineer and never had any desire to be one .... but my common sense is violated when I start thinking about removing battery voltage from devices who's documentation say .... don't do that .... always disconnect the input voltage first or damage may result.

That is the one of the reasons I prefer Cal's approach to remote shutdown if possible ... when not possible, I would want to shut down the input power to a charge controller instead of interrupting the battery ... The inverter I chose has a power switch, so very low current to turn that off.
The other reason I would prefer to switch remote or input power is that that is normally the point where it is necessary to switch the least power.

I installed, programmed, and maintained commercial building control systems for many years .... We installed hundreds of thousands of low voltage, low power relays. Our go-to relay was the RIB style relay which also had low coil current. It was extremely rare to have a problem with these.

Places where higher power was being switched is where the failures were.

 

[Factory400]

I'm not a design engineer and never had any desire to be one .... but my common sense is violated when I start thinking about removing battery voltage from devices who's documentation say .... don't do that .... always disconnect the input voltage first or damage may result.

That is the one of the reasons I prefer Cal's approach to remote shutdown if possible ... when not possible, I would want to shut down the input power to a charge controller instead of interrupting the battery ... The inverter I chose has a power switch, so very low current to turn that off.
The other reason I would prefer to switch remote or input power is that that is normally the point where it is necessary to switch the least power.

I installed, programmed, and maintained commercial building control systems for many years .... We installed hundreds of thousands of low voltage, low power relays. Our go-to relay was the RIB style relay which also had low coil current. It was extremely rare to have a problem with these.

Places where higher power was being switched is where the failures were.

I think there is no question that the ideal operation is to communicate with the various devices in the system so that they can gracefully turn themselves on/off or adjust some parameter on their own terms.

In the event that communication fails or the device does not respond as desired - the BMS should have the ability to disconnect to avoid permanent damage. In a perfect world, the BMS would never need to do a hard disconnect - it is the last line of defense to protect the cells.

 

[John Frum]

I think there is no question that the ideal operation is to communicate with the various devices in the system so that they can gracefully turn themselves on/off or adjust some parameter on their own terms.

In the event that communication fails or the device does not respond as desired - the BMS should have the ability to disconnect to avoid permanent damage. In a perfect world, the BMS would never need to do a hard disconnect - it is the last line of defense to protect the cells.

Its the last line of defense for over/under voltage.
I have a class-t fuse installed as close to the positive terminal as possible for the over current last line of defense.

 

[Factory400]

Its the last line of defense for over/under voltage.
I have a class-t fuse installed as close to the positive terminal as possible for the over current last line of defense.

Indeed -

A BMS can (optionally)have an electronic fuse that can react to over-current events with a shutdown via MOSFETs or contactor - the fuse is the ultimate backup to that circuit.

The e-fuse can be set to trip at a current somewhat lower than the fuse. If it fails to do its job - it blows the fuse which you then have to replace. Assuming the e-fuse works as intended - it will self reset and you move on with your day without needing to bolt in a new fuse.

 

[Bob B]

I think there is no question that the ideal operation is to communicate with the various devices in the system so that they can gracefully turn themselves on/off or adjust some parameter on their own terms.

In the event that communication fails or the device does not respond as desired - the BMS should have the ability to disconnect to avoid permanent damage. In a perfect world, the BMS would never need to do a hard disconnect - it is the last line of defense to protect the cells.

I'm not talking about using communications ... I don't like that for last resort shutdown either. My rule is that critical functions are always hard wired .... So, I am talking about using a small control relay (Could be an SSR) for the shutdown.

 

[Factory400]

I'm not talking about using communications ... I don't like that for last resort shutdown either. My rule is that critical functions are always hard wired .... So, I am talking about using a small control relay (Could be an SSR) for the shutdown.

In my world of system design - any control signal is classified as 'communication'. A TTL pulse over a wire, relay closure, CAN bus, Ethernet, etc are all communicating something.

Are you suggesting that the inputs to the solar charge controller be disconnected with an SSR/Relay device? I was referring to the to the common remote on/off features found on many devices.

 

[BiduleOhm]

I'm not talking about using communications ... I don't like that for last resort shutdown either. My rule is that critical functions are always hard wired .... So, I am talking about using a small control relay (Could be an SSR) for the shutdown.

Yea but that still counts as com...


Edit: well, @Factory400 was faster...

 

[Bob B]

In my world of system design - any control signal is classified as 'communication'. A TTL pulse over a wire, relay closure, CAN bus, Ethernet, etc are all communicating something.

Are you suggesting that the inputs to the solar charge controller be disconnected with an SSR/Relay device? I was referring to the to the common remote on/off features found on many devices.

My RV will have a 120V converter charger, a DC-DC charger, and a SCC. I want to select devices that can be remote controlled with a "dry contact" input .... or, I will shut down the input power to those devices rather than disconnect the battery from them.

I already have an inverter with a power switch which I modified by put a "dry contact" in series with the switch. The BMS I am using has independent 12VDC outputs to control a relay or SSR for charge and load .... no "communications" involved ... all "hard wired."

 

[Bob B]

Yea but that still counts as com...


Edit: well, @Factory400 was faster...

I am talking about a relay controlled by a 12 VDC output from the BMS .... That is hard wired in my book ... no communications.

My definition of communications would be data sent and received via a protocol such as Modbus or CAN.

 

[John Frum]

I want to select devices that can be remote controlled with a "dry contact" input .... or, I will shut down the input power to those devices rather than disconnect the battery from them.

That is the word I was looking for earlier in the thread.
I called it zero potential which didn't click for most.

 

[BiduleOhm]

My RV will have a 120V converter charger, a DC-DC charger, and a SCC. I want to select devices that can be remote controlled with a "dry contact" input .... or, I will shut down the input power to those devices rather than disconnect the battery from them.

I already have an inverter with a power switch which I modified by put a "dry contact" in series with the switch. The BMS I am using has independent 12VDC outputs to control a relay or SSR for charge and load .... no "communications" involved ... all "hard wired."

Yes, but most inputs on devices are actually MCU inputs so a lot of things can go wrong between your signal and what need to be turned off.

am talking about a relay controlled by a 12 VDC output from the BMS .... That is hard wired in my book ... no communications.

My definition of communications would be data sent and received via a protocol such as Modbus or CAN.

But that's com, very simple one obviously, the protocol being only two different voltages.

 

[Bob B]

Yes, but most inputs on devices are actually MCU inputs so a lot of things can go wrong between your signal and what need to be turned off.




But that's com, very simple one obviously, the protocol being only two different voltages.

In my mind, I need to differentiate between an actual communications protocol being used for control ... and what I have always called hard wired control.
I think you guys are splitting some kind of hairs to decide that 12VDC on / off is a communications signal. To me, that is "hard wired" and the most bullet proof type of control that can be achieved.

For my inverter, I am interrupting the power switch. That seems to me to be the most reliable way to control it that is possible other than disconnecting the AC power input.
IF the BMS shutdown is because of an emergency event ..... having the remote shutdown to the charge controller not work would require 2 points of failure.

 

[Factory400]

In my mind, I need to differentiate between an actual communications protocol being used for control ... and what I have always called hard wired control.
I think you guys are splitting some kind of hairs to decide that 12VDC on / off is a communications signal. To me, that is "hard wired" and the most bullet proof type of control that can be achieved.

For my inverter, I am interrupting the power switch. That seems to me to be the most reliable way to control it that is possible other than disconnecting the AC power input.
IF the BMS shutdown is because of an emergency event ..... having the remote shutdown to the charge controller not work would require 2 points of failure.

In the world of EE - 'hardwire' is only used to characterize a connection that is made with a wire.

'communication' is ANY signal that is intended to control something and does not imply digital, analog, MODBUS, Ethernet, or really anything other than some method of conveying a command to do something like 'Turn Off'

A conversation about semantics is not really helpful though....if you make up your own definitions, that is on you.

 

[Factory400]

For my inverter, I am interrupting the power switch. That seems to me to be the most reliable way to control it that is possible other than disconnecting the AC power input.
IF the BMS shutdown is because of an emergency event ..... having the remote shutdown to the charge controller not work would require 2 points of failure.

Interrupting the power switch is exactly what I was referring to earlier.
On many modern devices, however, the power switch is just supplying a little voltage to the input of a microcontroller that in turn runs some code that shuts down the inverter. Most of the time that is good - but I would not rely on it to save my batteries from destruction. Way too many ways for that to fail IMO.

 

[Bob B]

In the control world .... the definitions are as I already explained. I think you EE guys are making clarity impossible ...LOL