Very basic DIY BMS questions

fafrd

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Aug 11, 2020
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I see there are some existing threads on DIY BMSes but they are all focused on designs-in-progress and I did not think it would be appropriate to pollute those detailed threads with my very basic questions.

I’m using a 300A ‘dumb’ Heltec 8S BMS and everything is working fine.

But I’m starting to think about a future expansion of the system that has got me considering building my own very basic ‘BMS’ and I’ve got some basic questions for those who have gone down this path before me:

1/ I’m considering using a single large normally-off contactor to disconnect the battery (similar to the way the Daly BMS is architected). I understand there are trade-offs in terms of contactor consumption and $$$ but since I’m not expecting my cells to ever reach LVD of 2.5V or HVD of 3.65V and I am struggling to find a BMS supporting the high charge >100A charge currents I need, a simple safety-of-last-resort high-current DC contactor feels like the way to go. Something like this: https://www.amazon.com/Voltage-Contactor-Normally-SPST-NO-Energy/dp/B08C4S49SW

Is there anything else I need to be taking into consideration or will a relay like this work to disconnect the battery when needed?

2/ Once you’ve got a contactor/relay identified, the basic overdischarge/overcharge protection amounts to sensing individual cell voltages and comparing them to maximum and minimum thresholds (HVD & LVD). I’m sure there are optimized designs reducing component count and cost, but 8 dual-differential comparators like this: https://www.ti.com/lit/ds/symlink/lm393.pdf?ts=1634566574321&ref_url=https%3A%2F%2Fwww.ti.com%2Fsitesearch%2Fdocs%2Funiversalsearch.tsp%3FlangPref%3Den-US%26searchTerm%3DLM393D%26nr%3D22
Would allow you to have 16 digital control signals that you could use to open the switch whenever any cell increases past an analog programmable HVD or decreases below an analog programmable LVD voltage.

That’s all I’m really interested in. I’ve already got an active balancer that kicks-in whenever the battery is discharged nightly (my battery is bottom-balanced and drained every night). And I’m monitoring cell voltage and battery voltage separately (so I don’t need any ‘smart’ capability).

All I want is the ability to support both charge and discharge currents of up to 150A or 200A with a way to disconnect the battery if any cell drops or raises into the ‘danger-zone’ (below LVD or above HVD).

Is there any more that is needed than what I’ve outlined?

Of course cost in terms of $$$s or Watts may be prohibitive, but between spending money on a new all-in-one BMS or just rigging up a simple cell protection-only BMS, the latter is starting to have more and more appeal.

Appreciate comments from others that have already built DIY BMSs or looked into it whether there is something important I am overlooking in this very basic approach to cell protection…
 

fafrd

Photon Sorcerer
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Aug 11, 2020
Messages
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I’m not sure it would be worth going to the trouble of trying to use a more integrated BMS IC, but I found this: https://www.ti.com/lit/ds/symlink/b...=https%3A%2F%2Fwww.ti.com%2Fservice-worker.js

If you look at table 5-2, the BQ7790521 is factory-programmed to disconnect below 2.5V and above 3.7V (the exact disconnect voltages of my 300A 8S Heltec BMS).

2 of these ICs would allow me to protect up to 10 cells with an external disconnect switch.

Getting these mounted with all the needed support circuitry is probably prohibitive compared to just rigging up a bank of simple comparators, but I’m curious whether anyone else in the Forum has already headed down this path…
 

toms

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Jul 24, 2020
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I know several people that have been using a Junsi cell log for many years, with the alarm output switching a contactor.

Balancing is done manually as required, an active balancer would do this though.

If you don’t need any other features this sort of setup is fine.
 

fafrd

Photon Sorcerer
Joined
Aug 11, 2020
Messages
2,456
I know several people that have been using a Junsi cell log for many years, with the alarm output switching a contactor.

Balancing is done manually as required, an active balancer would do this though.

If you don’t need any other features this sort of setup is fine.
Happy to hear my thinking is not totally off base.

And thanks to the reference to the Junsi cell log. I don’t mind going deep DIY and getting out my proto-board, but a solution based on cobbling together off-the-shelf fully-assembled components is always my preference if such a thing exists…
 
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