Work in progress.
I don´t see much open source on the BQ75614 (TI BMS IC - 250mA Balance) ?
Im going lighter this time around. It will probably end below 2kg. (Edit: The batteries alone is 1960g. 28x70g each)
It should last a lifetime, hence the modularity (Format).
The BQ75614 need a external MCU, so I thought nRF5340 would be a good candidate. I do have a gadget that tunes antennas in that range.
Im wondering. Has anyone used the first cell for MCU current. The MCU current draw is so small, that the BMS should balance it out and in a way take the current from all the cell´s.
Do you have the full schematics?
@TheMoment I am just exploring the format / design aspects. In the past I worked on the nRF5340 and antenna design, just have to dig through some folders. I guess I needed a break from The Field Stack stepper project, since it has taken much time. I also see it in a larger context. The modularity of that STspinG4 controller makes it suitable for driving a BLDC, which will need a battery pack in any circumstance.
The BMS design is also meant to become modular. The core BMS IC (BQ75614 or similar) will be on a separate PCB Module with the com lines RX/TX/fault etc. broken out. This allows for a software lib. compatible with lots of MCUs, like SimpleFOC, but targeting TI´s BMS IC´s, although further support for other brands/parts is naturally also possible within that format.
If one needed CAN FD interface, the SAME51 MCU module would make sense. Possibly smaller MCU with CAN could do.
My main goal doing a “custom” battery design, is to integrate it with SimpleFOC motor control, where the onboard FETs in the battery itself can be useful with regards to cutting power when breaking and or burning current through a brake resistor.
Edit: A neat thing with these BMS ICs is that they are stackable. I have not begun to explore it in detail, just important to remember and make that feature accessible. Possibly (not sure) they can be daisy- chained using a single MCU.
Looking through some doc’s it does seem like the BQ75614 is out side stacking category.
It also looks like the BQ79600 com extender is needed for stacking / daisy chaining.
Edit: Further reading the doc’s, unveil the wireless stacking protocol. I think this approach, for small to medium size battery-stacks is the most appealing, especially if the goal is to use nRF5340. How exactly the stack should be setup/programmed I’m not sure yet, but I suppose it will be through an app.
By using both sides for components the CAP and resistor circuits can be stitched on the inner layers.
It is in a way a standalone BMS module. The MCU is likewise modular.
Thermal-wise, my focus has been to connect the thermal pad to large GND planes.
Im still not sure if we can use the build-in (10mA MAX) LDO for BLE SoC (MCU). or if the isolator is needed.
Edit: The length is not final, depending on how the cell contact will be done the dimensions will change.
If im not mistaken, the 21700 cell wrap is 0.2mm thin. Those solder blobs would therefore need to be taler then that. Ideally one should use a 0.3mm stencil. Part of the paste is flux, but the blob will cone up, it should be taler in the center.
The two large PCBs will be torqued down by lots of 2.5mm screws. Possibly the lid can also be bonded in with some sort of pressure foam layer sandwiched in
Here is a good candidate. 5000mAh 15amp discharge.
As you can se, the + and GND terminals are both quite flush, they should both be reachable with solder blobs…
The PCB itself is springy…
Hmm… LibreSolar Open BMS software repo. W. Bq769x0 support.
Im thinking some sort of cover on that MINI blade fuse.
The Micro Blade Fuse has the same 30Amp rating.
These 2.5 mOhm FETs are 200amp rated.
CHANGELOG: Moved Wire Lugs down a nudge.
Dont know why Libre Solar BMS design has all those large FETs, I suppose it is for quite a large battery. In my case 30AMP will be absolute MAX current draw for short bursts, w. 10-15 AMP Nominal. Of course one can parallel two of these to get higher current (depending on cell´s).
you are going to reflow batteries?
I’m counting on the pressure from all those small screws to hold everything tight together. The whole concept kinda depends on that to work. I guess it is somewhat experimental.
The solder blobs could be cut or sanded a bit down do get a clean flush surface. Maybe even use a 1mm thick 8mm diameter coin for the plus terminals.
Sure there are limitations to the design. Take the trace from the FET´s to the FUSE. It will be around 32mm wide, and with 70um copper, 24 amp will raise the temperature 2C degrees.
Having the wire terminals on the side, is mainly suited for two wires tightly connected with a plug for easy and fast mounting. But of course it can be stacked on the wide part.
I’m inclined to make room for two fasteners in the far and. From center screw to edge of gasket there is 21mm, so a fastener with a flange of Ø5 will make that distance less then 2CM. Still, the lid should be firmly in place, while also pushing down on the cells, it should be able to let out pressure, if for some reason a cell may go into runaway mode, worst case scenario.
// By using drones, lithium cells, MCU´s etc., in war scenarios, they are effectively delaying the transition to a sustainable, non-fossil fuel, society, AKA hydrogen driven vehicles. But of course, they don’t see a business in that.
// The price for each missile, grenade etc. could be used so much better for some humanistic goal. Like developing the African Farming, so that they can produce their own grain, and should need to import from Ukraine.
// Its the same for many other products, and it is in-fact a kind of exploitation. Imagine all those tanks, and missiles, that could have been tractors, maybe even hydro-driven. Maybe they are afraid of that scenario? Maybe it´s the hydrogen we are afraid of? I mean, imagine a tank of it exploding.
The Fat Getters & the Black robes, Floating Cities, AirCraft Carriers. NewClear Fusion Reactors. Absolute Sovereignty, Self Governance.