Cell module brownouts about diybmsv4 HOT 16 CLOSED

I should add, that I am using my own layout for the cell module PCBs. The circuit is identical to Stuart's apart from the addition of a 3A fuse at the power input, and a 100nF bypass capacitor between the MCU power pins.

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Hi Delboy711, thanks for the information.

What cells are you using and voltage range that you are dragging the cells below 2.7V ?

My existing circuit already has a 100nF bypass cap over the MCU - thats component C1

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@stuartpittaway My cell packs are a mixture of reclaimed 18650s. I noticed the problem when connecting a cell module to a pack at 4150mV with bypass threshold set to 4100mV. I also see the same effect when attached to my bench power supply.
During bypass the cell voltage does not drop more than around 50mV, and looking on my scope I have not been able to capture any spikes below the BOD threshold. Yet the MCU does go into lah-lah land as soon as bypass commences.

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This only happens on 2 of the modules or all of them? I also use a bench power supply for testing and haven't got it to brownout or crash like this

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Just the two. Some parts may be more sensitive to transients on the power rail than others.
In terms of what action you might consider I suggest removing the WriteConfigToEEPROM() statement in setup since it does not do anything useful and would save a few bytes.

I did wonder if Issue #18 might be related. Transient noise from the OP's charger could be affecting the MCUs.

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Interesting stuff. #18 may be related however that issue seems to also have larger problems like usb drop outs and mains noise on the power lines.

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Hi @delboy711 on the capacitor fix you put on the circuit, is this simply a 10uF between the positive side of the resistors and gnd ?

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@stuartpittaway No. I joined the ground side of R8 and R9 together, and put 10uF between there and ground. The idea being that there would be an RC time constant between the MCU power pin and the cap on both rising and falling edges as the MOSFET switches. It may be unnecessary to do it that way, but it worked so I stopped looking further.

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I'm probably being dim here but R8 and R9 are in the middle of the resistor string (one side has R6+R8+R10+R12 the other R7+R9+R11+R13) by joining r8+r9 together you will alter the load resistance. With the capacitor in place, wouldn't this always be charged from the VCC side of the resistors and the ground from the capacitor ?

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@stuartpittaway The resistance is unchanged. Since the resistors are all the same value those two points are both at the same potential, Vcc/2.
When the FET is switched off the cap will charge to Vcc, and when switched on it will discharge down to Vcc/2. Either way there is a time delay before the voltage settles which results in a slower change in voltage at the MCU power pin.
(In use, I observe that the cell voltage drops about 100mV when bypass is active.)

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Thanks for that, just for complete understanding, I've drawn a really crude picture, is this correct? Its probably worth me copying the solution if its helped so much.

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Yes. That is it.

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I have also issues with the B.O.D. set to 2.5V. The board measures a higher voltage and begins discharging until the battery block is completly drained. I will set the B.O.D. up to 3.0V and see if the issues resolve, as the balancing should stop at low voltages anyway.

The other pack has 20P3S and a voltage drop below the 2.5V is more unlikely. The 3P13S Pack has a very high likelyhood for package errors.

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Measuring of higher voltage (for real low voltage) is the result of the use of improper voltage reference it this case. For such low voltage is better to use something like REF3020, which work with voltage drop just few mV (It is 3pin voltage reference, not shunt type).

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REF3020

Hi DanoPTT, how would the change in voltage reference help in this situation? I've not seen a problem with the reference voltage on the modules as they currently stand.

The 4.21 design also uses a lower voltage reference 1.25V - using https://datasheet.lcsc.com/szlcsc/1808081832_UTC-Unisonic-Tech-TL432G-AE3-R_C171461.pdf

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Thanks.
I used the higher voltage reference LM4040BIM3-2.5/NOPB 2.5V, due to ordering error and used 2x 13 modules each in parallel on a power supply with a battery to smooth the voltage. The difference occours at voltages below 3200mV on the measurements. I will try a 1.25V reference next.

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