My Onan produces 120v twice, basically like all the sub 10Kw Onan Diesel Genny's in larger motor-homes. Today I can only charge at 1/2 that charger capacity requiring me to use twice as much diesel ($$$) to charge my batteries, not to mention twice as much noise and twice as much exhaust that the wify loves to tell me all about. I already add a Gentauri to move the exhaust up and over the coach when boondocking more than a couple nights where the 20Kwh of battery isn't going to be enough.
I have been told, more than a few times on support calls, the Rosie is more like 2 different inverter/chargers tied together (3500w each) suggesting to me there might be a way to wire it to use both inputs to get the charger to output its maximum charge. Maybe it would require Firmware change and not different wiring. My buddy the electronics engineer is coming over for beers tonight so I will probably ask him to look at that from his knowledge base.
There are quite a few diesel pushers in this country with those same diesel Onan's that produce that same power as mine, so if it needed a firmware update there is easily a ROI to be discussed. Personally, that is the only thing now that prevents me from telling everyone that follows my blogs to install one of these and ditch the Victron Kludge..
I don't know the exact answer to your question.
I was curious though since I am considering getting a big rosie ( I have little rosie )
So I just asked AI and I had a suspicion it had to do with phasing from your generator.
The google AI says it did . Here is the explanation from AI
The core issue described in this Midnite Solar Forum post stems from a phase mismatch between how a sub-10kW Onan generator delivers 120V power and how the MidNite Solar Rosie inverter/charger expects it.
The Root Cause: Out-of-Phase vs. In-Phase 120V
The user's Onan generator does not output a true 240V split-phase supply. Instead, sub-10kW Onan Marine/RV generators are usually dual-in-phase 120V generators. They feature two distinct 120V windings (Leg 1 and Leg 2) that are perfectly in-phase with one another to keep the neutral wire from overloading.
Conversely, the MidNite Solar Rosie is built natively as a true split-phase (120V/240V) system. To achieve its full continuous 7,000-watt output and maximum 120-amp charging capacity, it expects 240V across Leg 1 and Leg 2—meaning the two legs must be 180° out-of-phase.
When you feed the Rosie dual-in-phase 120V lines from an Onan:
1. Phasing Conflict: The Rosie reads 0V across L1 and L2 because they are in phase. It determines it is not receiving valid 240V split-phase power.
2. Fallback Behavior: The Rosie drops back to single-leg mode, qualifying only one 120V leg (Line to Neutral) for charging.
3. Half-Capacity Charging: Because it only draws from one leg, it is restricted to half of its charging capacity, resulting in extended generator run-times, excess diesel consumption, and noise.
Is it a hardware or a firmware limitation?
The user's buddy is correct that the internal layout resembles two combined 3,500W modules. However, a firmware update alone cannot easily resolve this.
• The Hardware Problem: If the internal charging bridges are hardwired across a single 240V rail or strictly tied to 180° phase-separated inputs, firmware cannot magically bridge the two independent, in-phase lines without triggering a direct short-circuit or overloading internal parallel components.
• The "Victron Kludge" Reality: This is exactly why users looking for high-amperage mobile setups default to Victron. In a Victron ecosystem, you can utilize a 120V/240V autotransformer to convert parallel 120V lines into proper split-phase power, or network multiple independent MultiPlus units to handle un-synchronized inputs dynamically.
Current Workarounds
Until MidNite Solar adds structural firmware changes or internal bypass hardware to support dual-in-phase 120V inputs, users with this exact configuration generally fix it outside the inverter:
• The Autotransformer Fix: Install an external 120V to 120/240V step-up autotransformer between the Onan generator and the Rosie's AC input. This mechanically shifts the parallel 120V lines into a genuine 240V split-phase supply, allowing the Rosie to charge at its maximum 120A rate.
• Dedicated Standalone Chargers: Run a secondary industrial battery charger (like a dedicated AC-to-DC chargeverter) directly off the generator's second 120V leg straight into the 48V battery bank, bypassing the Rosie's internal charger altogether
Ruan had mentioned he would discuss with team on how they could tweak the firmware to allow for 60A charging to be selected when running at 120V input. I don't think there's a technical limit as such.
Brucey, does Ruan work for Midnite? I am not familiar with him. I hadn't thought about autotransformer, but I went with Midnight so I didn't need to do something like that in the first place. In all my discussions with the folks at Midnite no one mentioned my Onan 120/120 would have This issue, we did have discussions about the Onan not working at all, but they assured me that was fixed, and it is working, but only at 1/2 capacity.
Now with Diesel at gold pricing running the genny twice as long for the same charge seems like punishment for not going with the Victron Kludge.. -Bill
Sorry typo there, Ryan from Midnite.
If they can do that I would think the Big Rosie is Ready for Primetime! And I could start recommending it to all the folks with my type of Onan Genny. I only know a few folks with the 240v Onan's, but I know hundreds with the 6-8kw Onans and their crappy 120/120v systems. I guess it was a lot cheaper to do it that way. Not sure how, but it had to save at least a Nickle..
There are other reasons besides cost why the RV industry uses those 120/120 generators.
Most RVs these days don't have 240V loads and everything runs on 120V. So there is no need to put in a heavier, more costly 240V generator. The 120/120 design can more easily do load sharing between the two sides since they are both on the same phase. If you put a heavy load on one side, it can just pull fluidly from the generator's single power pool without overloading a specific leg like a 240V generator would.
Since it is a physical phase issue. If you want to use all the power your generator has to offer with the Rosie, you have two hardware options:
1. Add a Victron Autotransformer: This will take your two in-phase 120V lines and physically convert them into the true 120/240V split-phase power the Rosie expects.
2. Use an External DC Charger: You can plug a dedicated charger (like an EG4 Chargeverter) into the generator's second leg and run it directly to your 48V battery bank. This bypasses the Rosie's AC input limits entirely and lets you charge at full throttle.
( if it was me I would probably go with charger option).
No one can change the past decisions or design choices so it probably isn't healthy to dwell on that and instead formulate a plan to move on to achieve your ultimate goal. But yeah it does sort of suck a bit to have to spend another 500 or $600 to do that.
Years ago I bought three 100 ah 48v lithium batteries when they were in high demand. Turns out they didn't use the best quality cells and had some physical design that wasnt' good. They still work but pale in comparison to the new 314 ah battery with excellent cells , a vastly better bms, and so much better breaker and fuse design.And the new battery was almost a third the price of the other three. But that is the way it is and I moved on. If I had only waited 3 or 4 years I could have saved thousands of dollars for better product , right ? But that is a good problem that technology got better for less $$ .
Larry
Quote from: ClassicCrazy on October 14, 2026, 11:10:21 AMNo one can change the past decisions or design choices so it probably isn't healthy to dwell on that and instead formulate a plan to move on to achieve your ultimate goal. But yeah it does sort of suck a bit to have to spend another 500 or $600 to do that. Larry
You may want to go back and review Brucey's comment from Ryan.
Try setting breaker size to 50A, topology to 120/120, parallel both AC in L1 and L2 and AC out.
I hope you are allowed to parallel the 120's from the generator.
This will give you maximum charging.
I run this setup in my RV with breaker size set to 32A as I'm on 30A shore power.
My generator is Onan 3600 120V
Remember, loads effect power available to the charger section, loads are powered first and what is left gets routed to the charger. Rosie limits the AC in current to 75% of breaker size. Your 7500 probably tops out at 6000 watts due to a .8 power factor.
Quote from: Wizbandit on October 14, 2026, 09:40:36 PMTry setting breaker size to 50A, topology to 120/120, parallel both AC in L1 and L2 and AC out. I hope you are allowed to parallel the 120's from the generator.
This will give you maximum charging. I run this setup in my RV with breaker size set to 32A as I'm on 30A shore power.
My generator is Onan 3600 120V Remember, loads effect power available to the charger section, loads are powered first and what is left gets routed to the charger. Rosie limits the AC in current to 75% of breaker size. Your 7500 probably tops out at 6000 watts due to a .8 power factor.
I tried all that over the summer. But I used 60a for the breaker size. The Midnite gets both 120v lines, one on L1 the other on L2. Still only get 1/2 the charging capacity, no matter the load being almost nothing to a microwave running. Each leg maxes out around 4500 watts in prior testing. Plenty to run both AC's and the microwave but not much else unless maybe on the other leg than the Micro. -Bill
It almost certainly something in the firmware if there isn't a physical hardware problem.
Quote from: bigbillsd on October 14, 2026, 11:07:36 PMBut I used 60a for the breaker size. The Midnite gets both 120v lines, one on L1 the other on L2. Still only get 1/2 the charging capacity, no matter the load being almost nothing to a microwave running. Each leg maxes out around 4500 watts in prior testing. Plenty to run both AC's and the microwave but not much else unless maybe on the other leg than the Micro. -Bill
It almost certainly something in the firmware if there isn't a physical hardware problem.
No, if you do not connect L1 and L2 together with wire at the L1L2 terminal block it may not work unless L1 and L2 are physically connected at the generator output. If 2 legs of 120V come off separate windings of the generator and they can't be shorted together that will cause Rosie to use only L1 and pass L2.
I have a 40A 120V feed on #8 Awg wire feeding my RV 30 amp Receptacle and I set breaker size to 32 and I get 39A DC charging and if I run breaker size up to 60AAC and with about 800W of loads I see 55-56ADC charging. I don't leave it very long so just to test tonight or it will trip the 40AAC breaker.
SO if you connect AC IN L1 and L2 together at Rosie and the generator does not barf you should see higher DC charging. You must also connect AC OUT at Rosie.
Quote from: Wizbandit on October 14, 2026, 11:36:58 PMSO if you connect AC IN L1 and L2 together at Rosie and the generator does not barf you should see higher DC charging. You must also connect AC OUT at Rosie.
So you think I should attempt to wire both my QD7500 outputs together? I'm not an electrician, but that sounds like it might let the Magic Smoke out.. I worked with Ryan over the summer and he never suggested doing that. That genny is Very expensive and I cannot afford to make is Smoke. Is this something Midnitesolar says is ok?)
Check with Onan / Cummins to see if they can be put in parallel. I don't know, but you may only get 50% of 60A if not. Generally AC does not parallel well... They may be 120VAC each but slightly out of phase.
Quote from: bigbillsd on October 15, 2026, 12:39:12 PMQuote from: Wizbandit on October 14, 2026, 11:36:58 PMSO if you connect AC IN L1 and L2 together at Rosie and the generator does not barf you should see higher DC charging. You must also connect AC OUT at Rosie.
So you think I should attempt to wire both my QD7500 outputs together? I'm not an electrician, but that sounds like it might let the Magic Smoke out.. I worked with Ryan over the summer and he never suggested doing that. That genny is Very expensive and I cannot afford to make is Smoke. Is this something Midnitesolar says is ok?)
He's saying to feed both 120V outputs from the gen to the Rosie (keeping them seperate). Not to connect them together before the Rosie.
Not to combine the 120V outputs
Quote from: Brucey on October 15, 2026, 02:31:22 PMHe's saying to feed both 120V outputs from the gen to the Rosie (keeping them seperate). Not to connect them together before the Rosie.
Not to combine the 120V outputs
That is how its been setup since Day 1. L1 & L2 into the Rosie are connected from the QD7500's two 120v outputs. (not 240v) -Bill
Did you try setting topology to 120/120 and see what it does? I have never tried that without at least paralleling AC ins. It probably won't work but try it. It probably needs a new section in firmware to allow it but wouldn't expect to see any firmware work this year or even early next year.
Now two little Rosies working independently would work fine, they can't be stacked so they don't care about phase.
Quote from: Wizbandit on October 15, 2026, 05:21:24 PMDid you try setting topology to 120/120 and see what it does? I have never tried that without at least paralleling AC ins. It probably won't work but try it. It probably needs a new section in firmware to allow it but wouldn't expect to see any firmware work this year or even early next year. Now two little Rosies working independently would work fine, they can't be stacked so they don't care about phase.
Yeah, Ryan had me try that. It didn't make any change at all so after a few charges I moved it back to 120/240. -Bill
Quote from: Wizbandit on October 15, 2026, 05:21:24 PMDid you try setting topology to 120/120 and see what it does? I have never tried that without at least paralleling AC ins. It probably won't work but try it. It probably needs a new section in firmware to allow it but wouldn't expect to see any firmware work this year or even early next year.
Now two little Rosies working independently would work fine, they can't be stacked so they don't care about phase.
My understanding of the issue is that the Rosie firmware assumes 120/240V operation, and therefore it halves the max breaker size value, even when set to 120/120V mode.
Quote from: Brucey on October 16, 2026, 11:42:17 AMMy understanding of the issue is that the Rosie firmware assumes 120/240V operation, and therefore it halves the max breaker size value, even when set to 120/120V mode.
I will keep my fingers crossed that they can change that assumption with a firmware fix.
Should I call them to officially request this fix? That one fix would make the Big Rosie a real world device for the majority of the diesel pusher world! The only other thing that is odd is the grid support but maybe other folks would think that is normal as I don't have any experience with other implementations of grid support.