frank nbeans avatar image
frank nbeans asked

The Definitive Undersized Generator Thread

Hello friends,

I have a sailing yacht which has a fairly large DC Bank (660AH@12v AGMs) and an older 40A charging system with no inverter installed. It's my intention to replace it with a multiplus compact since my 800w @ 120v suitcase generator can't drive the 40A charger, and to run various AC appliances, no more than 1600w total. I imagine this setup is not uncommon on smaller yachts.

I have seen the guide which mentions minimum AC amperage for inverter boosting, and I've read "Multiplus magic" a few times through, but I'm still not sure my use case will work. The data seems to vary and much of it is hidden behind the "professional" portal. The questions are:

  • Can a 4-5amp (think eu1000i) 120VAC generator be used with a Multi 800? 2000? 3000? I realize the DC charge current will be limited, but is it even doable or will the Multi simply barf?
    • Some articles hint that it is, others say it's minimum input amperage is 13A@120VAC.
  • Can that same portable genny be used in conjunction with PowerAssist to deliver some efficiency-adjusted figure of 800w+inverter output? Or is this where the 13A minimum applies?
  • Is this setup at all typical/supported, or would this be considered very 'edge-case'?
  • Why hide the "real" minimum current figures behind the professional portal? It's a selling point for cheap yachties!

Really the reasons for this scenario are logistical. The tiny genny fits in a cockpit locker and is reliable, but we do have occasional need to run a hair straightener or other needless AC gadget if we're going out in a snootier anchorage. The dream is to be able to minimize the draw on the house bank by running the genny when we're using large AC devices, and when we need to throw some amps at the battery, without starting the diesel.

Thank you for your input (get it?)

GeneratorAGM Batterysystem design
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2 Answers
JohnC avatar image
JohnC answered ·

Hi Frank. Some comments, which I'll try to cover by your points..

* As far as I can see, the smallest Multi available in 120Vac/60Hz is the 2000VA model (1600W @ 77F). It's Minimum Shore Limit is listed as 8.4A with newer firmware (ac limit). The Multi won't barf, but the genny might at that level. The Multi may reject it's ac if it's not up-to-scratch in waveform, etc. That model's charging capability is 80A, which is a pretty good match for your 12V batts.

There is a 13.0A limit listed, but that's for the 5000VA model.

* For loads, the shore limit of 8.4A (1008W?) will be used first, and up to another 1600W will be added to it as required from the inverter. Even more for short term inductive loads.

* All supported.

* The Professional portal is really designed for suppliers/installers, but you shouldn't have an issue accessing it if you sign in as a self-installer.

So this lot may be too much for your little genny. Victron may well have more smaller Multis on the way, but are unlikely to want to provide that info publicly.

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ripper avatar image
ripper answered ·

I have an old Multi (not plus). It has no power assist but can only charge or invert. On charge, I can dial it down to almost whatever I like. I have a Honda EX7 which has only around 600 watts and is fully loaded via the charge current of 20A at 24V. Another inverter can then make 230V independently. Maybe the way to go for you if you only have high loads for short periods. I assume those skyllas or other dedicated chargers can be adjusted and inverters are available in any size there are Multiplusses. You loose efficiency because of up and down a couple of times more though.

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