MPPT 450/200 does not respect current limits

Q&A and troubleshooting
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1

Dear community,
I am using an MPPT RS 450/200 with a relatively small battery (3 × Pytes E-Box 48100R, max. 150 A charging current). The maximum charging current is set both in ESS (with 1 x MP II/5000 and Cerbo GX) and in the RS itself. However, the device does not adhere to this limit; it constantly overshoots and then gets reset to zero by the BMS, so with sunshine it keeps oscillating back and forth.

I have already tried interventions via Node-RED — no reaction.
Has anyone experienced a similar problem, or maybe even found a solution?

Thanks!

2

Are your batteries actually tripping?

3

The batteries are not disconnecting completely (no hard trip / relay opening).
What seems to happen is that the BMS sets the allowed charge current to 0 A when the MPPT overshoots. The RS then ramps back down, and when the BMS allows current again, the cycle repeats. So it’s more of a soft-limit oscillation than a full battery trip.

4

Ok. And you have no loads on at the same time?

Ideally even when the battery sends 0ccl the system should still cover loads directly from solar.

What should ideally happen is the system charges to the set absorption voltage and hold it there for whatever time and possibly drop to float (this is battery specific though i haven’t worked with pytes.)

Cant say i am impressed with the information on the victron pytes page. I would set the charge voltage slightly lower than the 53.8v it asks for though.
Does the pytes shift the Charge Voltage limit at all?

5

Yes Pytes does this a lot and Pytes also needs that high voltages to balance and synchronize

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@franky1981 Do you have DC feed-in enabled?
This can cause that the current limit is ignored but since Pytes also shifts the CVL that shouldn’t to much.
If you have it enabled, try to disable it for a while to let the cells balance.

What are the cell voltages at that moment?

6

Thanks for the input!

  • DC feed-in is not enabled in my system, so that can be ruled out.

  • I had actually set the absorption voltage too high (57.5 V in RS 450 Settings). I’ve now lowered it to the official Pytes range (~56.0–56.2 V, float ~55.8 V). I’ll monitor if that stabilizes the charging behavior and reduces the overshoot.

  • So far the battery parameters shown in GX (max charge current ~150 A, CVL) look consistent.

Appreciate the suggestions – I’ll report back with results after some sunny hours.