MPPT behaviour when battery side disconnects

DIY
1

Dear Victron staff and experts

I am running a small system with 2xmppt100/50, each one with 2s panels canadian solar 435W (80-90V and 13A). I have 2pole DC mcb 16A on pv side and 25A on battery side. Last week, one of 25A tripped, leaving mppt floating, with PV attached.Yes, was charging with 28A in full sun. I was scaried, thinking will be destroyed. I was able to communicate with him via isolated (!) VEdirect cable, put the charger off, on remotely. The mppt was networking with smartshunt and the other mppt. I did’t know what happened, the mppt has 0W, no alarm, the batt voltage was cloned from the other mppt via bluetooth, charger on, seems normal… I was guessing the mcb tripped. Indeed, it was. My mistake, should be 32A instead of 25. Until I replace the mcb, I put 24A limit max charge current.

Anyway, when I went home, got the MCB back on and everything normal again.

Good news Victron MPPT survived with PV connected and no battery, even some users , on another forum, claim that event destroys him… see this: https://diysolarforum.com/threads/victron-isolated-mppt.85721/post-1233681

So, instead of blaming users not to be professionals, after months with no advice and solutions, I think we deserve some attention, for everybody’s safety. After all, we all love Victron, don’t let us down

2

Moved to a new topic as this is not related to the subject it was posted on (PV voltage passing through to the battery).
Since you put a 25A breaker on a 50A charger, did you configure the charger limit on the MPPT as well? These two should be done together so the charger knows not to exceed limits, that it tripped seems like it was not?

An MPPT can have solar attached without a battery, it will just not draw anything from the panels.
The problem is a sudden disconnect of the battery during power delivery, this leads to an overvoltage condition, which again is typically survivable if the system has been sized correctly.

3

Yes, now I have limit the max charge current on each mppt. A thread already exist: Switch on MPPT battery side - will it damage the MPPT?

Good to know Victron mppt’s are not destroyed when battery disconnect. Maybe it’s by design or just luck.

Anyway, we learn together, making mistakes…

More important it’s what’s happen when an mppt with > 80V on pv side get shorted between pv+ and batt+ and all the voltage from panels flows to a 12/24V battery. Worse case scenario is house fired up or few thousand loss… This is the big question

4

what’s happen when an mppt with > 80V on pv side get shorted between pv+ and batt+ and all the voltage from panels flows to a 12/24V battery

Voltage doesn’t flow, current does. I doubt a few panels could raise the voltage much since they can only source Isc, which is probably much less current than when the MPPT was doing it’s job of converting 80V to 12V volts. I wager the designers aren’t counting on luck. The circuit protection after the MPPT should prevent fire & monetary loss.

5

Dear Paul, thanks for your support.

We don;t know internal diagram of a Victron mppt charger, but maybe is something like that

image
image677×342 31.5 KB

Q1 and Q2 are driven with PWM pulses by a microcontroller according to MPPT algorithm. It’s basically a DC to DC converter. Voltage panel x Current panel = Voltage battery x current battery. Almost, minus 5% loses.

On victron mppt, PV- = Batt-, common ground (minus). Q2 is actually an active diode (‘freewheel’ diode). Q1 is switching on/off in normal functioning. During On state, current flows from panel to load/battery via inductor L. During Off state, Q2 is On, current flows via Q2, L to load

image
image529×397 26.8 KB

Basic buck converter

The problem is when Q1 is shorted. Voltage from PV panel is applied directly to Battery plus. Of course no more dc-dc conversion, but PV panel is still capable to inject 10-12 A. During normal operation, current injected into battery could be 2x-3x higher.

The voltage won’t jump instantly to the panel’s open-circuit voltage; instead, it will gradually rise. If the voltage continues to rise unchecked, damage will eventually occur. Overvoltage on all devices connected to DC-bus…

How we manage this situation? Yes, an overvoltage alarm via VRM or victor connect, a buzzer, siren , a relay to disconnect all mppt from busbar. Even if a million to one chance to happen, lot of damage can occur. Or maybe we should take Stalin’s words: “The death of one man is a tragedy, the death of millions is a statistic” (wikipedia says is misattributed to Joseph Stalin)

My first thought is to use Multiplus NO output to drive an intermediary automotive relay , 60-80-100A, when an overvoltage is detected, to disconnect mppt from battery… or is just paranoia?

6

Yes I’m familiar with the topology, my point was you don’t get full voltage out at short circuit current. Seems a Smart BatteryProtect would work as a load disconnect or to disconnect solar from the battery if you’re worried. Still, voltage doesn’t flow.

7

Worth reading: https://diysolarforum.com/threads/victron-isolated-mppt.85721/

1 Like