Input filter capacitance of MultiPlus-II 48/5000/70-50 120V need for pre-charge resistor calculation

Been down this rabbit hole myself when I commissioned the Victron MultiPlus-II 48/5000/70-50 on the narrowboat last spring — though we're talking the European 230V variant here rather than the North American UL1741 version someone's been discussing elsewhere.

The pre-charge resistor question is genuinely interesting regardless of which version you're running, because Victron are characteristically tight-lipped about the actual input filter capacitance figures in their documentation.

What I eventually pieced together from the Victron community forums and a bit of careful measurement: the 48V/5000VA MultiPlus-II has a fairly substantial input capacitance — somewhere in the region of 3,000–4,700µF depending on which revision of the board you've got. That inrush without pre-charge will absolutely weld your battery contacts or trip protection on a well-configured BMS before you've even got going.

My manual pre-charge setup used a 22Ω 50W resistor with a simple momentary pushbutton, held for roughly 3–5 seconds before closing the main contactor. Calculated using the standard RC time constant approach (5τ for near-full charge), it worked reliably with my Fogstar Drift 280Ah cells.

A few things worth considering for anyone doing this properly:

• Contactor sequencing matters enormously — get it wrong and you're back to square one
• Some BMS units (Daly in particular) handle pre-charge internally, so check yours first
• The Cerbo GX doesn't help you here — this is purely hardware-side

Has anyone here actually measured the input capacitance directly on the 5000VA unit with a proper LCR meter? I'd love to compare

@RetiredNurse the 230V variant I've got on the static caravan pulls a ferocious inrush through those input caps — I measured the transient with a clamp meter and it was eye-watering even with a modest bank.

What I'd suggest: Victron's own documentation is surprisingly coy about the exact input filter capacitance figure, but the Cerbo GX logs will show you the voltage sag profile during connect, which gives you enough to back-calculate a reasonable pre-charge resistor value.

Practically speaking, most folks running large lithium banks — I'm on Fogstar Drift cells — use somewhere in the 20–50Ω range briefly before hard connection. Your BMS pre-charge circuit should ideally limit inrush below 25A to protect the MOSFET stack.

Worth posting this over on the Victron Community forum too — their engineers actually respond there.

Good thread this. Worth flagging that Victron don't publish the exact input filter capacitance figures in the public datasheets — you'd need to either measure it directly with an LCR meter on a de-energised unit, or work backwards from the inrush current measurements @MasterSolar mentioned.

For pre-charge resistor sizing, the standard RC time constant approach applies — most people go with something in the 50-100Ω range to limit peak inrush whilst keeping charge time reasonable. The resistor needs to handle the dissipation during that initial charge cycle, so don't skimp on the wattage rating even though it's only momentary.

Have you considered using a proper pre-charge contactor arrangement rather than a resistor alone? On the narrowboat application especially, repeated connect/disconnect cycles from engine alternator charging can stress those input caps over time.

@SparkyGaffer is right that Victron keep those figures close to their chest — I've been through the datasheets and the CCGX comms manual looking for exactly this and come up empty.

What I did for my static caravan setup was measure the inrush empirically using a current clamp with peak-hold function, then back-calculate an effective capacitance from the RC time constant. It's not precise but it gets you to a sensible ballpark for sizing your pre-charge resistor.

Rough method:

• Measure peak inrush current and time-to-peak
• Use C = t / R (simplified)
• Add a safety margin of at least 2x

For the 48V variants specifically, I'd not rely on figures extrapolated from the 230V models either — the DC-side filter topology differs between voltage classes.

Has anyone tried contacting Victron technical directly? They're sometimes forthcoming on application-specific queries.

@BayJason honestly the easiest workaround is just slapping a pre-charge resistor in series with the positive before you close the main contactor — I used a 33Ω 50W wirewound on my garden office Victron setup and the battery BMS stopped having an existential crisis every morning. 🔌

@VoltBarry that's what I did on my static — ran a 47Ω 10W resistor in series on the positive for initial connection, let the caps charge up over a few seconds, then bypassed it with a proper isolator. Sorted the inrush completely.

Worth noting though, if you're after a more calculated approach rather than just picking a sensible resistor value, Victron's own recommendation in the MPII install manual is to limit inrush current to something reasonable — I've seen folk work backwards from that using assumed cap values in the 1000–4700µF ballpark for units this size, which at least gives you a starting point.

Not gospel, but it's enough to size the resistor without blowing anything up on first connect.

Great thread, this caught my eye as I went through exactly this headache on my van build last year.

@HY_OffGrid's approach is sound but worth mentioning the time constant side of things — even without knowing Victron's exact input capacitance figure, you can watch the inrush current settle by monitoring voltage across the resistor with a multimeter. Once it reads near zero, your caps are charged and you're safe to bypass.

I've seen figures of around 2000-4000µF quoted informally on various forums for similar Victron units, though nothing official from Victron themselves. Using that ballpark with a 47Ω resistor gives you a time constant somewhere in the 100-200ms range, which feels about right practically speaking.

A pre-charge relay wired to a small timer module makes the whole process automatic if you're doing repeated connection cycles — well worth the tenner it costs.

Quick question before I go further down this rabbit hole on my own narrowboat setup — does anyone know the actual input filter capacitance figure for the 48/5000 variant specifically?

I've been hunting through Victron's documentation and the datasheet is suspiciously quiet on this. Trying to calculate peak inrush properly rather than just guessing at a resistor value and hoping for the best.

@HY_OffGrid did you measure yours empirically or work from a known capacitance figure? There's a big difference between "resistor survived" and "resistor was correctly sized" 😅