question

jowan avatar image

more output power needed for off-grid system with a multiplus

Hello, I am a leader of a boy scouts group in Belgium and last year I got the chance to build a portable solar energy unit which will be used for our summer camps to power fridges, lights and charge electronics.

This unit consists of solar panels, a smartsolar MPPT, a lead-gel battery bank and a victron multiplus 48/1200/13-16 inverter. I also added a raspberry pi running the venus os software to amulate a GX device. This raspberry pi is connected to the MPPT and the Multiplus using a ve.direct and ve.bus cable.

The setup works great, but there is a problem of having only limited output power and peak power (1000W and 2000Wp) because of the relatively small multiplus choice. This caused the inverter to sometimes turn off when too much power was drawn from it, for example when turning on multiple fridges at once. To remedy this, we are looking to upgrade the output side of our setup by adding an additional inverter. What I currently see as possibilities and their limitations:

- Adding another multiplus 48/1200/13-16 in parallel. This would be the cheapest option and would double the output power and peak power. However, as these small multiplus devices have only 1 ve.bus port, I would be unable to daisy chain both inverters together and to the raspberry pi, so I don't think this would be an option unless if I step away from the GX functionality.

-Adding a Phoenix inverter to the battery bank: As this would lead to a totally separate operation from the multiplus, this would lead to 2 separate AC output: one high power from the Phoenix and one lower power from the original Multiplus. This is quite confusing for people not familiar with power electronics who will be operating the mobile energy unit in the future. It also makes the original Multiplus quite redundant as the phoenix inverter will probably be capable of running the whole all the loads. This would make the Multiplus into an overqualified battery charger, only used to charge the batteries before we leave for the camp using the grid. This would also be more expensive than the other option.

-Not sure about this one, but I read about cascading inverters and I am wondering if I could cascade two multipluses together without linking them with a ve.bus cable. The first multiplus would take power from the battery bank and supply it to the second multiplus. The second multiplus would however have to be configured to not charge the batteries and only discharge them, because otherwise it would create a loop of charging and discharging the same battery bank. I'm also wondering if the output power of the second multiplus is limited to 1000W continuous, or if this 1000W is only the power it can draw from the batteries and, once supplied with input power, the total power it can deliver increases by the input power amount.


As you can see, all of these options have their downsides and I would like to know the thoughts of some people more familiar with Victron products on them, as well as maybe some other suggestions?

MultiPlus Quattro Inverter ChargerVenus OSPhoenix Invertermultiplus ve.buscascading inverters
7 comments
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You will also want to do the maths on the discharge capabilities of your gel battery pack.

Doubling the draw could lead to less capacity and a shorter life.

You would have been much better off with a smaller lithium pack and a larger inverter, cost implications aside.

There is little point in adding complexity, you will be happier with a bigger multi as suggested.

Lithium was just too high of a cost for us to consider. We took a look at all options and a Lithium battery pack with similar capacity was just far above what we could afford.

@Jowan

Did you also consider that, for example, 200Ah gel battery can only give safely 100Ah when lithium 180Ah?

Also lifespan of the gel versus lithium is pathetic.

But he already has a battery

@kevgermany

In couple years he will be looking for replacements.

I will consider Lithium once these batteries are dead, but I do hope they'll live longer than a couple of years :)

Are lead batteries really that bad? I though the specs looked ok for their price, as long as you don't overload them (which we don't intend to do). Anyways, thanks for all the feedback!

Lead batteries aren't that bad when looked after. Try avoid going below 50% and you will get a few years out of them and with seasonal use even 10 years. If they are wet batteries, make sure to maintain them if sealed then just make sure the inverters are maintaining them.

But the cycle life of lead is 500 cycles compared to 5000 of LFP batteries. So cycling everyday will get you around 500 days or 1.5 years.

2 Answers
Alexandra avatar image
Alexandra answered ·

@Jowan

Just get a 3kva it is much less of a headache on all counts. And not much more expensive, the unit will drop right in place. The other ideas take time energy and have complications not easily fixed in the field.

The cascade idea where one feeds the other is not supported and can go really wrong.

Parallel setups have their own complications especially with current sharing and these types of setups need a bit of experience.

And as you mentioned at the end of the day, people who don't know what they are doing will be using the system.

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Hello Alexandra, thank you for answering so quickly! Is it also possible to place an extra 3kva Phoenix inverter on the battery bank then, instead of another multiplus (the second option) while keeping the original multiplus only for battery charging over AC? This would be a bit cheaper and, in my eyes, offer the same functionality: charging through the multiplus and easy, non-confusing access to 3kva of output power through the Phoenix inverter.

Should work, but make sure the AC outlets/wiring are completely separate. Otherwise sooner or later someone will get it wrong.

shaneyake avatar image
shaneyake answered ·

I would just get another multiplus and run it in parallel. You don't lose any functionality, you just need to by the RJ45 splitter so you can chain and connect to PI/GX. Part Number ASS030065510

Phoenix route, I agree with all your points, parallel multiplus would be better. GX only supports 1 inverter.

Cascading inverters is not officially supported but can be done. For your setup I would parallel rather. There are systems like on Boats where Cascading makes sense but for you I would rather parallel.

Of Course the simplest option would be to sell the 48/1200 and get a multiplus-ii 48/3000 or 48/5000 but having 2 is nice in the case the one fails, which is rare but possible.

You do also want to check, like others have said that your battery bank can handle the extra discharge current.

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