Can the output of one MPPT go to the input of a second MPPT

woodysranch

I have a portable 200 Watt Mono PERC panel with these specs Voltage … Open 26.72V, Working 21.5V; Current … Open Max 9.35A, Short 9.79A

I have a standalone Power Station by BougeRV model Fort1000 1120Wh. The power station has a PV input with specs max input 8 Amps, 12 to 30 Volts. The Power station is completely enclosed, you cannot access the battery terminals directly, you must connect the Panels to the PV input.  I would like to connect the panel direct to the PV input on the power station, but I am worried that it could exceed the max input current of 8 amps, so I thought adding a Victron MPPT and setting the charge current to 8 would protect the Power Station

BougeRV customer service folks tell me the power station has a built in MPPT, but are also advising me that I must not connect a panel that would exceed the input limits of the power station.  Here is what they said to me.

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The power station itself has a built-in MPPT. If controllers that already have MPPT are connected together, mutual exclusion will occur in the power station as the final end of the solar input, causing the power station to trigger the protection function and stop working, causing unnecessary damage.

Your MPPT can change the current control within 8A, and the voltage will drop accordingly, but the final key is the problem of the two MPPTs.

With the Victron connected, it will be limited to 8A, but it does not ensure that a solar panel above this current will cause irreversible damage to the power station when connected.
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A few months ago when there was sun, no snow and when I was not smart enough to think there could be a problem I connected my solar panels to a Victron 75/15, set max amps to 8, connected it to my Power Station, put the panels in the sun and observed that the current was constrained to 8 Amps.  Everything was working and the power station was recharged, then went to float mode.

Can anyone tell me what BougeRV is talking about with respect to Mutual exclusion.

Can anyone explain what could possibly be wrong with using the Victron 75/15 to provide a controlled input to the power station of 8 amps. 

 

 

Graham Parkinson

As you found out, it worked (mostly, at least for as long as you tried it).  What they were saying is that there is potential for unexpected behaviour.

Having two active MPPT devices in a circuit is really sorta similar to having two automatic transmissions on a car.  It's possible that both of them could decide to buck or boost against each other (like two transmissions working against each other).

In reality, you could just ensure that your panel was not facing the sun's daily track square on, that would also limit the current to below it's theoretical maximum.  On the other hand, panels rarely produce their rated output current anyways.   

If you wanted to be sure, perhaps experiment with a resistor with enough resistance and power dissipation rating to reduce the short circuit current of the panel from 9.5A to 8A would be enough.  

woodysranch

Thanks Graham,

What I was thinking was since the Victron 75/15 MPPT can set/control the max charge current via to 8 amps, or any current up to its max of 15, then the Power Station's built in MPPT should be able to control the max input current at its spec'ed 8 amps.  It shouldn't be a big ask for it to do this - isn't that one function of any MPPT?   

As you have said, and I already surmised, most days, if ever, the panels would never achieve the max current and the spec of 9.5ish amps is not that much about what the power station can handle anyway. Good idea with the resistor.

I'm partly surprised that Tech Support did not offer more detail on this. Every time I asked more questions such could they suggest their own products that would work with the power station, they directed me towards 100 watt panels even though it was supposed to handle up to 200 watts.  I'm partly not surprised because there is an obvious language barrier (China with a US address) and believe the person at the other end is customer support with limited knowledge, not technical support, and they just wanted me to go away.