question

philb avatar image
philb asked

Connecting a Victron MPPT to a Multiplus inverter - charger disabled.

I have a Multiplus 48/5000/70 operating as a UPS connected to a 48 battery bank. I have dissabled the charger of the Multiplus in VE Configure.

I have recently purchased a SmartSolar charge controller MPPT 250/60 to charge the batteries/supply the inverter. When connecting the the charge controller to the DC bus (with the batteries isolated) there was considerable arcing. I was expecting no voltage to be present as the Multi's charger was off. However there is 39Vdc from the Multi present on the DC bus.

Is this voltage normal with the charger is off? Is it safe to connect the Multi and the charge controller to the bus or can either of them be damaged?

MultiPlus Quattro Inverter Charger
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2 Answers
Murray van Graan avatar image
Murray van Graan answered ·

Hi @PhilB. You most likely measured the charge held by the large capacitor bank inside the multi which was charged when the batteries was connected.

It is safe to connect both the multi and MPPT to the DC Bus, obviously each device should be fused correctly.

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solardude avatar image
solardude answered ·

I always recommend a switch or contactor between the battery and each component. Turning the switch or contactor OFF will eliminate the big arc when connecting components. It also gives you options when servicing individual components without having to disconnect the whole system.

The spark is normal, but I don’t like it, so I put a switch or contactor on every battery system I install.

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Daniël Boekel (Victron Energy Staff) avatar image Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

If you don't see the spark (because it's in a switch or contactor), doesn't mean it is not there.

These arcs can destroy a switch or contactor really fast, always pre-charge, or for a one time connection: live with the spark.

On my test systems, I always make sure voltages are almost equal to prevent sparks.

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solardude avatar image solardude Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

Agreed, precharge is by far the best way to go. I install contactors is so the end user can service each item individually, and the spark doesn’t damage the terminals on the battery or the component. Almost every system I have worked on has pitted or damaged terminals on either the battery or the cable, definitely from arcing. My opinion is that a heavy duty switch can handle the spark better.

You got me curious now, I’m going to disassemble my main contactor and see if there is any damage.

But 100% agree precharge is best.

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thanar avatar image thanar solardude commented ·
You can easily precharge using an incandescent lamp of higher operating voltage. Prefer tungsten instead of halogen, since halogen when cold has a very low resistance which can actually also lead to arching.
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