Lamps flicker when turn on the air conditioner

Been dealing with something similar in my shepherd's hut setup and it's been driving me mad.

When my small portable AC kicks in, the LED strips I've got running along the ceiling do this brief but noticeable flicker — lasts maybe half a second but it's enough to be irritating. My situation is a bit more modest than the OP's original post (single Victron MultiPlus-II 3000 rather than two in parallel), but I suspect the root cause is the same.

From what I've read, it's the inrush current on compressor startup that causes the voltage to dip momentarily. Even with a decent battery bank (I'm running Fogstar Drift 200Ah LiFePO4), there's still that brief sag before everything stabilises.

A few things I've been looking at:

• Cable sizing — my current runs are probably borderline, might be worth reviewing
• Soft starter for the AC unit — apparently these can dramatically reduce that startup surge
• Whether the MultiPlus charge/inverter settings need tweaking (ESS assistant configs etc.)

Has anyone actually fitted a soft starter to a mini-split or portable unit in a small off-grid setup? Curious whether it made a meaningful difference or whether it's one of those solutions that sounds better on paper.

Also wondering if anyone's noticed whether this is worse in colder weather when the compressor has to work harder on startup — seems like it might be more pronounced in winter for me but could be confirmation bias.

Would be good to hear from anyone running parallel inverters too, since I imagine the dynamics are a bit different when you've got two units sharing the load.

@SophieHobbs74 Classic inrush current problem. When the AC compressor kicks in it draws a spike — sometimes 3-5x the running current — and your inverter momentarily struggles to keep voltage stable, hence the flicker.

A few things worth checking:

• Inverter size — what's your VA rating? Undersized units suffer worst
• Battery connection — loose terminals or undersized cable will make this worse
• Soft starter — fitting one to the AC unit (MicroAir EasyStart is popular) dramatically reduces that inrush spike

I had the same issue in my static caravan before I fitted a soft starter. Made a significant difference. Victron inverters handle it better than budget units due to their power assist capability, but even they'll show voltage sag on a big spike.

Also worth checking your LED driver quality — cheap ones are more sensitive to voltage fluctuations than decent Meanwell-style units.

@SophieHobbs74 had exactly this on my boat last summer. Even with a decent Victron MultiPlus the LEDs would dip when the compressor fired up.

What sorted it for me was a soft starter on the AC unit — massively reduces that inrush spike. Micro-Air EasyStart is the one most people fit, though pricey.

Also worth checking your battery connections and cable gauge — any resistance in the circuit makes the voltage sag worse. I'd swapped to Fogstar cells and the problem improved noticeably just from having a more capable battery bank holding voltage better under load.

If you're on a budget, sometimes just moving sensitive lights onto a separate circuit helps too.

Hey @SophieHobbs74, I had almost the identical issue in my converted barn last year! One thing neither @RetiredPlumber nor @MasterAdventure has mentioned yet — it's worth checking whether your LED driver/transformer is a quality unit, because cheap ones are far more sensitive to those voltage dips than decent ones. I swapped out a couple of bargain LED drivers for some better Meanwell units and the flickering reduced noticeably even before I addressed the underlying inrush issue. Also, are your battery connections and cable runs properly sized? Undersized cabling adds resistance which makes voltage sag worse during those startup spikes. Sometimes it's not just the inverter struggling but the whole circuit compounding the problem. Worth ruling out the simple stuff first before spending money on soft starters or bigger inverters! What inverter are you currently running? That might help narrow things down.

Great points from everyone here. @SarahClark75 I'm curious what you were about to say — don't leave us hanging!

One thing worth adding is to check whether your inverter has a "PowerAssist" or similar feature enabled if it's a Victron unit. Also, the type of LED driver matters enormously — some cheaper LED strips have really poorly regulated drivers that are far more sensitive to voltage dips than others. Swapping to a quality driver with decent capacitance can make a noticeable difference even if the underlying inrush issue remains.

@SophieHobbs74 also worth measuring your actual battery voltage at the moment the AC kicks in. If you're seeing it drop below about 11.5V (on 12V system) during that spike, your battery bank may simply be undersized for that load, and no amount of tweaking will fully solve it.