Charge limit voltage (CVL) precision

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mysik asked Jun 10, '20

Charge limit voltage (CVL) precision

I have a question about Charge Voltage Limit (CVL) when using BMS connected via CAN bus. In my system (VenusGX and Multiplus II) I'm using BMS connected via VE.Can (Batrium BMS). I would like to set Charge Voltage Limit (CVL) to 56.75V, but it looks like precision is fix to just one decimal place, so I need to set it to 56.7V or 56.8V. Is there a way to have 2 decimal places (e.g. 56.75V). All settings in VE Configuration tool have precision to 2 decimal places, why CVL (and CCL as well) is different???

Kind Regards

BMS voltage VE.Can DVCC

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Answer 1 · 2020-06-10T19:19:57Z

nebulight answered · Jun 10, 2020 at 07:19 PM

I'm not a batrium user, but have looked into it previously. I believe this limitation is on Batrium as their GUI sets the limits, correct? Maybe talk to them to see if they can increase it? However .5 volts over 16 cell groups is such a small amount that I'm not sure it would really make a difference (56.75 is 3.546875v per cell where 56.7 is 3.54375‬ and 56.8 is 3.55).

Since you can set your balance start voltage, I don't really see why you would need to higher than 56v.

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mysik commented · Jun 13, 2020 at 06:13 PM

That is not Batrium limitation. In Batrium config you can change scaling to 100 and that will give you the 2 decimal places ( you can even change it to 1000 and have 3 decimal places if you wish). So this functionality is already built in. The problem is that if you set the limit 56.75V in Batrium, VenusGX will read it as 567.5V. Venux only reading one decimal place.

And BTW my system is 14 cells not 16, and the difference is 0.1V not 0.05V. It is especially important when you setting your bypass voltage. With charge voltage you need to set it up high enough to balance cells, but not too much to make sure you are not just burning energy and reducing you cell monitor life span.

Also you have different cells chemistries and 0.1V for cobalt type liion cells might not be a problem but for LiFePO4 where discharge curve is very flat it can make a huuuge difference.

At the end of a day, in VE config I can set my voltage limits with 2 decimal places, why I can't do the same through CAN Bus?

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Daniël Boekel (Victron Energy Staff) ♦♦ mysik commented · Jun 15, 2020 at 08:52 PM

Hi @Mysik

0,1V in a 14S battery is 0,00625 V per cell.

Please keep in mind that both the measurement of the inverter and of the BMS will be less accurate than that.

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