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tuxedo0801 avatar image
tuxedo0801 asked

Voltage loss compensation when controlled via BMS?

Hi there,

I have a 3xMP2-48-5000 setup with multiple Victron MPPTs as well as a BMS/can powered battery.

ESS and DVCC is enabled.

Battery tells via BMS to Victron, that it requires 55,2V. And MPPTs output voltage is set to 55,2V (I can see this via Victron Connect).

Problem is: I have around 100mV loss between MPPTs and battery. I'm already using the thickest possible cables. Cables are as short as possible.

This causes a bit of a problem charging the battery up to 100% respectively it's 280Ah.
It's charging easily to 99%, but then it struggles... I'm pretty sure this is due to the fact, that MPPT produces 55,2V and battery receives only around 55,1V and therefore not getting full.

I can of course reconfigure the BMS to request 55,3V instead of 55,2V... But then MPPTs will produce 55,3V, battery still receives 100mV less, means 55,2V and required 55,3V... So still the same issue.

As MPPTs are controlled by the BMS: How can I compensate the 100mV? Or is there a thing I was not thinking of?

br,

Alex

p.s. This is a "DC coupled PV, feed in excess" setup. Once I enable "SVS" in DVCC and battery is 99..100%, feed in to grid stops :-(

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1 Answer
snoobler avatar image
snoobler answered ·

Hitting 100% on pylontech type batteries is very difficult. It has to do with how the BMS feeds the SoC value to the GX. In most cases, it is based on what voltage the cells attain.

You changing the charge voltage to 55.3V doesn't move the 100% value, at least not for any BMS I've seen. 100% is based on cell voltage.

Assuming 16S, 3.45V may never attain true 100%, and below that, the cell voltages may not be high enough to trigger balancing. Try 56.0V and see if you hit 100%. That's an average of 3.50V/cell.


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tuxedo0801 avatar image tuxedo0801 commented ·

I'm using a Gobel Power battery, which has a Pace BMS inside. The BMS SOC value jumps to 100% if

a) pack charge voltage is reached. This is by default 55,2V (=3,45V per Cell)

AND
b) charge current drops below 2amps

Only if both happen, BMS assumes "100% SOC".
But from MPPT's output terminal to batteries's input terminal, I have some voltage drop (cables, fuses, ...). About 100..200mV.

So condition a) is seldomly reached.

I can change the "pack full charge voltage" in the BMS settings, but that affects the CVL that is sent to the MPPTs. And in combination with the voltage drop between MPPT and Battery, this does not help.

I today tried to charge up to 3,6V resp 57,6V (which is still fine for my cells). The system overshoot a bit and I reached the pack full charge voltage as well as the drop below 2amps for charging. So I got 100%. But again: There is a voltage drop. After the overshoot during high-amp charge from lot of sun from the roof, the drop is again visible.
I assume that fulfulling condition a) is kind of random if I cannot somehwere set an offset to compensate the voltage drop.

Any ideas?

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