FlexiBMS - 0.2 HW under work - Flexible configuration and charging BMS

what’s the point of having a cutoff after it already destroys my battery? the whole point is to stop it from completely draining my battery if i forget

if you’re using the esk8 to light up your house i don’t think it would be a huge inconvenience to press a button every hour so it didn’t shut off :woman_shrugging:

It’s not going to destroy your battery with idle current over ten hours. Unless you’re running like a 0.5Ah battery…

I’ll make it configurable, you don’t have to use it (no shutdown or load cut off time), but it’s there for those who want it. I think it’s perfectly reasonable feature. Set the time to something that makes sense to you.

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@SimosMCmuffin About the sleep state, what about making the power button 2 pole, and then it can also be used to wake up from sleep

If this delay is easily user defined them no problems, my boards are usually pretty well sealed with silicone after everything is working, and having to open it if get away from let’s say 2 weeks is a pain

Giving it some thought. I think it might better to just move the wake-up function completely to the external switch, but make it so that it can be just a normal off-mom switch or toggle off-on. Keep the switch simple as a single pole so you can use a following cheap switches for it: https://www.ebay.co.uk/itm/Waterproof-Silver-12mm-LED-Momentary-Push-Button-latching-Switch-1NO-4Pin-Cool/192049157592?ssPageName=STRK%3AMEBIDX%3AIT&var=491866747727&_trksid=p2057872.m2749.l2649 OR https://www.ebay.co.uk/itm/12-19mm-36V-LED-Power-Symbol-Metal-Momentary-Push-Button-Switch-For-Car-Boat/263292767049?ssPageName=STRK%3AMEBIDX%3AIT&var=562302410182&_trksid=p2057872.m2749.l2649

This would completely eliminate the need to be able to open the enclosure to wake-up the module and at the same time allow for the widest range of external switch types.

I’m gonna look into how to implement this.

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If I could add, it’s a really good idea if to power it off you have to hold the switch for X seconds. That way if a stick or rock flies up and hits the switch it doesn’t kill your brakes…

This is a planned software feature

EDIT: and also the load will be able to push current into battery through the Switch-module even if it’s switched off, so braking is essentially never disabled.

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the super debouncer

*Whlie still also enabling reading the state of the switch during battery power or usb power.

If the ESC is powered down, the output FETs won’t be able to deliver the braking current to the battery

The ESC will function off of the motor’s BEMF voltage and boost that voltage higher than the battery voltage allowing for braking to a lower speed, once the motor speed is too low then the ESC can’t boost the voltage no more and it’s DC bus voltage crashes and it shuts down.

If the ESCs DC bus voltage drops lower than the BEMF voltage, current will start to flow back into the DC bus through the FETs body diode. The motor starts to act as generator at this point.

EDIT: Try this: disconnect battery from an ESC, connect motor and spin it with a hand drill for example. The ESC will power up.

Oh no. That’s a pity. I would love to have only one communication line for all the variables on the board. That means one app (smartphone / smartwatch / smartwhatever) collecting and displaying all the data of your board (speed, current, cells voltage, real SoC, number of cycles…). Also I think that acting directly on the ESC output as switch off (by sending STOP command or something) is safer than simply power off the whole ESC. CAN forwarding has been implemented successfully by @rpasichnyk, @Ackmaniac and @emmaanuel on their own apps. Shouldn’t a big deal, should it ? I’ll be pleased to contribute as well :slight_smile: (in the scope of my skills :smile: )

Thanks for the pinout explanation of the connector. What the max current rating of those pins ? I mean, what would be the max charge current allowed ?

On the future of CAN, I’m planning on skipping on it on this first iteration, mainly due to the fact that the MCU doesn’t support it and if I were to change it, this would cause most likely a pretty big re-wiring job and atm there is no space for the CAN transreceiver or for the CAN connector. There is already a lot on the board that I want to test and see that the implementation works.

I can most likely fit the CAN in the future by changing MCU to one that supports CAN and the connectors to surface mount ones, which allows me to mount the CAN transreceiver underneath them then and share the USART pins with the CAN pins, so the connector can either go to a bluetooth module with the usart or to a another CAN device. That way it still supports CAN and Bluetooth directly.

“Current Rating 3A” per pin on the connector.

The reason why I put 4 parallel pins is to increase the current carrying capability through the connector. I’m speccing the parts for MAX 5 A charging current. Testing needs to be done to see how the ribbon cable behaves thermally with the max current. image

Thanks a lot for this constructive answer, love it.

That makes sense, definitely ! If you’re not against adding it in the future, I’m in :grinning:

So 4x 3A should theoretically allow a 12A charge but you limit to 5A, right ? Isnt it too conservative ? We have to be careful with the ribbon cable though, that’s right.

Thermal and electrical wize, I would advise (if the layout allows it), to put the GND at the other end of the connector. Thus you avoid accidental short-circuit (if 2 pins bent to each other) and also avoid having back and forth strong currents squeezed in the same side of the cable. Like this : connectors

Thermal wize, (red wires are the ones that generate the most heat while charging) :

What do you think about his ?

I do believe that 5 Amps is quite plenty of current to push through, if you consider that a 10S pack with 5 Amps is 180 Watts of charging power already, but once again this isn’t a hard limit. 7 Amps starts to close on some power handling capability of the charging current measurement shunt resistor.

Anyway the boost charger can’t support this high power, but you could push something like that with a hefty bulk charger.

I would somewhat question the possibility of the pins touching each other, because it’s on a 100 mil (2,54mm) pitch, so the pins aren’t exactly side-by-side and I don’t think you should be connecting the switch module while the battery is connected and the connector shroud should protect the pins from accidental bending. Fair point though, but I don’t see any immediate danger in this config.

Having the BAT+ and GND high current pathways next to each other also decreases the loop area, which then lowers the EM noise. The more sensitive analog signals are somewhat shielded from the noise by the GND being in between them and the BAT+. Thermal wise it’s not as optimal as the case you pointed, but I can’t really say anything about it’s performance yet, because I haven’t tested it. I’ll do a test today on my bench power supply to see how it behaves thermally with different currents.

Good point ! Didn’t think about that… :neutral_face: If you can, try different length of ribbon cable. I like this approach of experimenting stuff like that ! Great R&D !

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I whipped up a quick test piece, as pictured below.

Tested with 10 Amps on the workplace’s power supply and some heat was detected in the cable itself, but most in the connector, which was most likely due to the loosely fitting 4-pin male header connector (AKA not optimal connection).

I’ll try to do another test with better connectors at home and post some IR-pictures from that.

EDIT: Ribbon cable is usually 28 AWG and with the 4 wires together it’s equivalent to 23 AWG in surface area.

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Awesome tests ! IR pictured will tell a lot ! I need one at home.

Just out of curiosity I was googling around and found an article testing 28 awg ribbon cable in power carrying use. Seems somewhat old, but I believe it’s still relevant.

image image

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Fermilab??? Ahah you can trust these documents :smile:

So I guess 5A would be OK (they say that in a 40C ambiant temperature you add 15 C so it 's still in the boundaries, isn’t it ?