Pylontech + MPII: Effective charge voltage exceeding 52.4V and reaching 54.4V

DIY
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1

Hello everyone,

I’m experiencing a voltage issue with my Pylontech battery charging.

My setup:

  • Multiplus-II 48/5000/70-50 (only AC-in connected), with ESS and grid feed-in
  • 2x MPPT 150/45
  • 4x Pylontech US2000C
  • 5 kW PV system
  • Note: Unless I’m mistaken, everything is configured according to the Victron/Pylontech documentation (absorption at 52V, float at 51V, DVCC enabled, etc.). I double-checked everything today. Communication between all devices (including the BMS) seems to be working fine.

The issue:
The system has been running smoothly for two years. Fifteen days ago, I shut it down (to add a circuit breaker—no other hardware or software changes that I recall), and restarted it two hours later.

Since then, in late morning when the batteries are at ~100% and before my automated loads start (so batteries are full and consumption is low), the battery charging sometimes continues (with several tens of amps). The MPPTs then cut out (suddenly dropping from 1-2 kW each to 0W) and restart, repeating this cycle every ~10 seconds. I quickly suspected a maximum voltage issue.

After investigating, I noticed in the advanced graphs that the Effective Charge Voltage, which had previously capped at 52.4V (normal, as Victron overrides the Pylontech BMS CVL, as explained in the docs), now reaches 54.4V (see attached images). The timeline matches the restart and the onset of the issues.

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My questions:

  1. How is the Effective Charge Voltage calculated? Why does this value fluctuate? (Linked to temperature, voltage drop detection?) I thought it was the minimum between the voltage value potentially set in DVCC (which was previously disabled in my case) and the 52.4V limit for Pylontech (Victron’s override of the BMS value).
  2. As a temporary precaution, I enabled the voltage limit in DVCC (52V), and the Effective Charge Voltage drops back to 52.4V (52V + 0.4V). Is the operation now strictly equivalent to what I had before the issue? (From my research, I don’t think so.)

Thank you very much for any help! :folded_hands:

2

Dc side? Do you have a multimeter at hand? Check the voltage drop of that breaker, if that may be not well connected, or have a high resistance due to its internals, voltage readings may be affected.

(And depending on where that breaker is added may, or may not mess with things)

3

At most, I have a voltage difference of 0.5V between Pylontech-Voltage and MultiplusII-Voltage in the advanced VRM curves (I haven’t had the chance to test this with a voltmeter yet). This maximum voltage delta logically appears during phases of high charging or high discharging. At night (with low currents), everything balances out. These values don’t appear to have changed before or after installing the latest circuit breaker (which was correctly added on the DC side, as you suspected).

All the connections are properly tightened (double checked).

The only odd thing is that I sometimes hear a faint crackling noise in one of the circuit breakers (I can’t remember if it’s the one I added last or not, since I have two identical ones). It’s a Schneider A9N18363.

4

Never a good thing.