This is a very interesting concept, giving me some ideas to make better use of the spare real estate on the negative half of the board. I’ll work on this over the weekend.
Another idea to consider, which is a combination of options 1 & 3 above. Load resistors have been changed from 3.3ohm to 1.5ohm with another 16 load resistors on the back of the board, bringing the total balance current & power to 5.6A & 23.5W @ 4.2V.
@Billy_Boes Your design is looking really nice but I have some concerns. You raise the amount of energy the circuit can handle on the same surface size, that means it will get hotter. Where does all that heat have to go? And something I don’t know but also concerns me is that this heat is produced close to the top of the cell. These cells can handle for sure some heat but is it OK to heat them up unevenly? Also think of the overpressure valve that is just below it.
I like the idea @Ross came with a separate board with resistors and a FET that you can connect to the board on the cell. Such a board you can place away from the cell for example on a heat sink. This also has the advantage of making the design scaleable you can connect to your design resistor boards that can handle different amounts of energy. So a user can select a board with resistors that fits his balancing needs and can still use the same cell monitor board.
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Albert,
Thanks for the feedback! I agree the additional heat generated will require either passive cooling via heatsink(s), active cooling via forced air or both. I’ve been testing different heat dissipation techniques with Stuart’s board design, and have been surprised how little air movement is required to keep the board below 65degC during balancing, even without a heatsink. The latest rules allow triggering a relay for the fan if/when the onboard thermistor rises above setpoint.
Regarding Ross’ idea to distance the balancing circuit from the battery, I feel that Stuart’s existing board design accomplishes that. I don’t believe there’s any advantage to splitting the module components into several PCBs. This greatly increases the cost without benefit.
I do, however, really like the idea of modular resistor banks, allowing the user to customize their balance current. Still thinking about how to best accomplish this.
You are totally right a bit of airflow will get rid of heat fast. Still I would like to advise you to do some calculations on the heatsink and airflow needed to get rid of the heat produced by only 1 monitor board dissipating 23watts.
Heat sink size calculator for Tcase max use 65 as this is the max temp for the board and resistors.
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I’m seeing some recent discussion of increasing the balancing current–is this really that important? If I look at a commercial BMS (e.g., the ubiquitous Daly), I see that its balance current is only 30 mA, even for models as large as 500A. The diyBMS, in its stock configuration, manages 40x the balance current. Surely this would be sufficient?
For new, well matched grade A cells a low balance current is more than sufficient. For second hand, grade B, bottom balanced or mismatched cells then a higher balance current is justified.
Thats a lot of heat!
Also - why are there two TX/RX JST sockets?
I believe it is so the boards can be used either way around (typical parallel vs series cell arrangements) and still be connected cleanly to the adjacent boards.
A lot of heat indeed! A good test of the PID algorithm! Heatsinks and forced air will likely be required.
Blaine nailed it. Only one each of the Tx/Rx sockets would be used, depending on cell arrangement.
Are you ok with your name and the project name/version being on the silkscreen? Any critique appreciated!
Perhaps change the silkscreen to something like…
“Based on diybms design by Stuart Pittaway”
Just to make it clear that it’s not something I’ve designed directly.
Will do, thanks for the input!
Why not place a 10W 2,7Ohm cement resistor over the PCB (like v3.0).
Lifepo4 cut Off is 3,65V means 1,35A @ 2,7 Ohm max. thats 5Watts.
And the Heat is not transfered to the PCB.
For me, the cost difference and susceptibility to vibration damage led me to stick with smd resistors.
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Looking forward to your controller, that woud ease installation by miles.
One thing i would like to bring up: maybe no all will have the chance to install active cooling or install it in a close box. May there be a chance to have a solderbridge to steer the restistors/balancing current?
Excellent points. Since JLCPCB will only populate one side of the board, it will be up to the end user to add resistors to the backside if they desire higher balance current. If heat sinks or fans are not used, Stuart’s pwm logic will modulate balancing to whatever board temperature you deem acceptable.
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Hey Billy_Boes,
How are you getting on? I would like to help, but i guess i’m pretty useless…
Any chance you’ve had a chance to try out the boards? I am willing to give untested boards a shot, too.
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I’m also interested in trying out the boards. I have soldering and software experience I can offer.
Thanks
–
Steve
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