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mallo avatar image
mallo asked

Onan 4kw and Multiplus-II

Hello, I have a 1120Ah LiFePO4 12V battery being charged by a MultiPlus-12V 3000VA 230V.

I have connected the Multiplus in-line of the feed to the 240V AC distribution panel and using the feed through facility to run the 1KW immersion and some other small domestic loads.

I also have a 4KW generator for when the "shore supply" isn't available, when the shore supply is connected the Multiplus will charge the battery fine and will also heat the hot water.

However running the generator is a different story, when I do so the Multiplus will pass through the 1KW heating load and if I turn the charging on the Multiplus will start charging, if I turn the charging up every now and again the charging stops, the Multiplus switches over to inverting and the heating load is now being picked up by the inverter and the batteries, after a minute or so the Multiplus switches back to pass through the heating load (onto the generator) and starts ramping up the charging again, it will then run sometimes for a while or sometimes more or less straight away it will trip again.

I am pulling around 3Kw which I would of thought the 4Kw generator should be able to cope with without too much of an issue.

I have set the current input limit to 16A, changed the setting in the input from "shore power" to "generator" and that may have stopped it tripping as much, but not completely.

If I cut the charger back in the DVCC it helps as does turning the immersion heater off, but the whole point of running the generator is to load it up, as this is the best for them.

HELP anyone with any ideas???

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3 Answers
Mike Dorsett avatar image
Mike Dorsett answered ·

Typically a diesel generator is rated for a load power factor of 0.8. This effectively means that the generator will cope with 4kVA, but the engine can only produce 3.2kW at the output of the generator - or 80%. You are probably getting under voltage or under frequency dropouts from the multi, which can be checked by looking at the PE log. (connect with Mk3 and VE configure to see this.)

This also means that you will need to set your AC limit to 14A (3200/230) while using the generator.

Loads connected to the AC out 2 are only powered from AC in, not from the inverter, so loads connected to this won't drain the battery.

Typically a diesel generator should be loaded to between 25% and 80% of its rated power to run hot enough to prevent glazing the bore, with 25-30% average load being the minimum. Most efficient operation is achieved between 60% and 75% of rated power. This means that for your set, the optimum load point would be around 2.4kW, or 180A @ 12V i if no other loads are on. If you run the water heater, then the charging load should be ~1.4kW, or about 100A. (I used the water cooling circuit of my generator to heat the water through an immersed coil).


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mallo avatar image mallo commented ·

Hi Mike

Thanks for your reply, I thought there was some logging with the Multiplus, but I couldn’t find it, I will have another look when I can get down on-board again.

When I was running it again yesterday I was getting the same result, if I cut back the charging rate it was better which is line with what you are saying with the loading of the generator. I can fairly easily cut the charge rate back with the restrict the input.

Interestingly my previous charger which was an older “heart interface” also a 120A unit, would run at full capacity (well I think it did), with the immersion heater on, but that was charging lead acid and probably wasn’t working the charger as hard as LiFePO4.

I understand what you are saying about the reduction on the generator output and about loading the generator (I always like to load it as much as possible), and thought the Multiplus would work well, but perhaps I am now slightly limited to the output of the generator, but I can heat the water with the diesel heater and perhaps I am better using this and using the generator mainly for battery charging???? Need to play around with it a bit more to find the optimal settings/setup.

Also I need to find the PE logging…..

Thanks for your help…..

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derrick thomas avatar image
derrick thomas answered ·

If you have "weak ac input" enabled that can cause your issue.

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mallo avatar image mallo commented ·
Thanks, I did try that but it cut the charge rate back, and when I have the shore power on it works fine and I would be looking to work the charger harder…..


It’s a shame that the default setting profiles for the “shore power” can’t be set up differently to the “generator”??? Or can they be??

I don’t want to be breaking out the laptop each time to “tick” the “weak ac input” on……

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JohnC avatar image
JohnC answered ·

Hi @Mallo

You'll need to experiment. All gensets are different, and I've found they often have quirks that are hard to understand. Like I have a 6kVA Yamaha that will run fine charging up to ~3.2kW. If I allow the Multi to charge more than that it will reject the genset. Yet add a load (Multi passthrough) and the genset can provide almost 5kW without rejection. This is easily accommodated with the relevant limits set, so if yours is doing something like that you'll need to find your own limits.

It seems (for me) like the Multi under heavy charge is actually 'distorting' the genset power somehow, whereas grid has a big buffer behind it and may not be affected in the same way. (I don't have grid to test that).

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mallo avatar image mallo commented ·
Hi John,

Thanks for this, there is certainly something funny going on and I will find the sweet spot for the generator, the sales pitch for the Onan was that it was the best output for its class, which I think is the case to a certain degree, but I agree it does seem strange that it runs the charger fine for a while and then all of a suddenly goes out of sync/unstable.

I can take the immersion heater load off the generator and and heat the water using a diesel water heater and then max out the charging, as I said in one of the earlier posts it would be nice if the default setting could be changed when you select the "generator" or "shore power".

It was just that the old "heart interphase" charger would do the charging no problem with the immersion heater on too, or at least it seemed to do so, but that was with lead acid batteries and not LiFePO4 cells that probably have a lower resistance and taking as much energy you can throw at them.

Thanks for your help/observations........

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