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Unexplainable design fault
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Pentoad:
I have been designing a battery management system for several months now and have come up against a strange problem.

I have set the three multiturn pots, R2 R3 and R4 so that overvoltage triggers with a battery voltage of 4.2, undervoltage of 3.4 and a balance voltage of 4v


When I apply power and the battery voltage is below 4v, both the overvoltage and undervoltage outputs are triggered and in some cases the balancing will trigger also. If the supply voltage is above 4v, they don't trigger when the power is applied and the module works as it should across the entire operating voltage range of a lithium ion cell. I am beginning to suspect that this may be due to my poor choice of comparator, the LM239DT. I am using a 1.225v voltage reference and the common mode voltage range of the comparator is VCC+ -1.5 VCC + -2 as per the datasheet with a supply voltage of 30v. The confusing thing is that not all of the boards have this problem despite being exactly the same so I am not too sure what to do. It would seem like the comparator on the faulty boards is requiring a higher voltage to start up properly.



Prehistoricman:
Are you powering the comparators with the battery or H1P2?
Is H1P2 30V?

Why not measure the comparator inputs when you see this fault occurring? Then you can see if your components are misbehaving or if there is something else going on.
mikerj:
Why is the overvoltage detection measuring at 'Q2 Drain' (presumably battery +ve) but the undervoltage detection is measuring at 'K1 4'?

It's not obvious to me how a balancing circuit with one switching element can work?  You say the battery voltage is only 4.2v which is a single Li-Ion cell, so why is balancing even needed?

You'd also be wise to put a small amount of hysteresis into those comparators to prevent them chattering when the voltage gets close to the threshold.
SiliconWizard:
Same here: 1/ I don't understand where K1_4 comes from, and 2/ I don't understand how your "balance circuit" is supposed to work.
Pentoad:
There was a mistake in the diagram, K1 4 is connected to Q2 so I have sorted this. One of these modules is used on each cell in a pack and this diagram represents only one module. H1 is where 5v is supplied to turn on the module via an optoisolator connected to Q2 so that if the pack is not being used it doesn't get destroyed over time. H1 also outputs the voltage warnings and is isolated via the optoisolators.

H2 is where a removable jumper is connected to allow me to calibrate the module without the load resistor causing a voltage drop and upsetting the calibration.

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