Charging batteries "around" MultiPlus II charger

Been wrestling with this exact scenario on my setup, so thought I'd share what I've learned.

Running a smaller generator (mine's a 5kW Yamaha) with the MultiPlus II charger can be inefficient if you're not careful. The issue is that the Multi wants to ramp up charging aggressively, which means your genset has to work harder and burns through fuel quickly. In spring and autumn, when I'm not getting consistent solar, I've had to develop a workaround.

What I've found works is running the generator separately to top up via a basic MPPT-style charge controller set to a conservative amperage limit — around 30-40A depending on battery state. The MultiPlus stays in inverter mode or UPS mode during this time. Once the batteries hit around 80% SOC, I'll switch over and let the Multi finish the job if needed, though honestly the genset usually switches off by then.

The alternative approach some folks use is disabling the Multi's charger function entirely when running the generator, and instead using a dedicated charger (like a Meanwell or similar) set to a fixed output. Less elegant but genuinely more fuel-efficient on small gensets.

Has anyone else found a cleaner solution to this? I'm curious whether setting the Multi's charge current to a lower limit actually helps, or if it's just delaying the inevitable inefficiency. Also keen to hear if anyone's tried running their genset into the AC input with the Multi in passthrough mode — theoretically it should work but I've never risked it.

Has anyone actually tested the efficiency loss with a smaller genset running the MultiPlus II at partial load? I'm considering a similar setup—3kVA Yamaha paired with a 48/5000 unit—but wondering if it's worth keeping a separate charger as backup.

The issue I'm seeing is that smaller generators really suffer when the MultiPlus is drawing unevenly during the bulk phase. Mine tends to hunt around 1.5-2kW input, which isn't ideal for a 5kW unit.

@Trigger, did you end up measuring actual kWh consumed versus what went into the battery? I'm trying to work out whether a modest PSW charger (Meanwell or similar) would actually save fuel long-term, or if I'm overthinking it. Interested in what others have measured rather than guesswork.

I've got a 3kW genset paired with my MultiPlus II in the van, and yeah, the efficiency hit is real at low loads. The charger draws a baseline regardless of what you're actually needing, which tanks the genset's fuel economy.

What actually works better is running the gen until the batteries hit 80-90%, then switching to solar or shore power to finish the charge. The MultiPlus II's configurable charge curve helps here — you can dial in a lower absorption voltage to reduce that final-stage inefficiency.

Also worth checking: your genset's sweet spot. Mine runs cleanest at about 70% load, so I'll deliberately time charging to when I'm running other loads simultaneously (water heater, cooker). Kills two birds with one stone.

The trade-off is more manual management versus fully automated charging, but for van life that's often a worthwhile compromise.

Mate, the key issue is the charger's minimum operating current. Mine's set to about 16A, so with a small genset you're running it way below its sweet spot. The inefficiency comes from the charger constantly ramping up and down trying to match available power.

What I've found works better is setting a higher minimum charge current on the MultiPlus II — forces the genset to work harder but more efficiently. Or just run it when you've got decent battery depth to discharge through, rather than topping up partially. Sounds daft but it actually saves fuel in the long run.

Alternatively, if you're van-based like me, just chuck a solar controller in parallel for smaller top-ups. Costs a bit upfront but the genset stays for proper charging sessions only.

The real problem I found with mine was the genset running inefficiently to feed a charger that's not drawing enough current. You end up with the engine working harder than it should just to maintain voltage stability.

What actually worked for me was a hybrid approach — I've rigged a secondary charge controller (cheap 60A MPPT from Renogy) that sits between the genset and the MultiPlus, with the ability to bypass it entirely when loads are high enough. Bit unconventional, but it keeps the genset in its sweet spot rather than babying it at partial load.

@KenCross, the efficiency loss is measurable. I logged it over a month and saw roughly 15-20% wasted fuel depending on how light the load was.

The MultiPlus II is brilliant, but it's really designed for larger gensets or mains backup. Smaller rigs need a different strategy.

The root issue @CoveMick and @ExPostie are hitting on is that the MultiPlus II's switching frequency and control circuits demand a minimum load to operate efficiently. With a small genset, you're essentially paying the parasitic losses of both machines simultaneously.

What worked for me: I wired a secondary charging path using a cheap MPPT controller fed from a small solar array, even just 400W worth. Sounds counterintuitive, but it lets the genset focus on higher-current bulk charging whilst the MPPT handles float/absorption phases when the charger would otherwise be inefficient.

Alternatively, if solar isn't viable, consider whether a larger genset makes economic sense for your usage pattern. Running undersized kit harder for shorter periods often beats running oversized kit in its inefficient sweet spot.

What's your typical charge current requirement? That'll help narrow down whether it's genuinely a charger mismatch or just configuration tweaking needed.