question

spicedreams avatar image

Add more battery or more panels?

My system is ~3 months old and has been working well through our NZ summer, but, on cloudy days the battery voltage drops into the <50% SoC range. With winter coming up, I am trying to prioritise between adding battery capacity or adding panels. My sense is that it's 'now or never' for battery because I hear that new batteries adopt the 'age' of existing batteries.

My question is, what information produced by the system should lead me to decide on more batteries or more panels? Hoping someone can help...

My off grid system comprises:

  • 6x 320 W Trina brand panels (roof mounted so point more east than optimal)
  • 3 kW Victron EasySolar II GX (seems to consist of:
    • SmartSolar MPPT 250/70 -Tr charge controller
    • Multiplus-ll] 48/3000/35-32 inverter/charger
    • connected to VRM about 1 month after install )
  • BMV 700 battery monitor (no VE.Direct connection- should there be?)
  • 4x 12V 260 Ah AGM batteries in series

Our power draw averages around 250 W 24x7 plus 30 W for the EasySolar II GX itself. VRM says over last 30 days, consumption has been 171 kWh and production 192 kWh. I haven't found anywhere that VRM (or the remote console) tells me "but production would be higher by X if you had more storage".

Hope someone with more experience can help.

Thanks, Graham


offgridSolar Panelbattery capacity
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Hi Graham, 171kWh consumption and 192kWh production is just impossible with your current setup. Based on 8 hours with full sun, you will get around 15kWh produced by your PVs. Your batteries can theoretically provide around 12kWh, but in order to keep them longer don't get under 50% charge, which means they can provide around 6kWh.

You have to consider a 3 days period without sun, in order to calculate your battery bank size, and to adapt your PV production to recharge them in 1 days.

Basically, your Easysolar consumption is 250W/day, + 6kW/day , which means in 1 day you have reached the 50% of your battery bank. => all your installation is undersized.

Start adding PVs, as I mentionned in a previous message, and search for the sun orientation in the afternoon, so you can get much more power.

Michel

BMV 700 battery monitor (no VE.Direct connection- should there be?)

Yes a VE-direct link with your GX will give useful data like my summer snip. You can see there was not any particular bad solar weather. The blue section is the battery Soc, derived from the BMV. The upper line is the max Soc for the day, the lower line is the minimum Soc for the day, the solid blue middle line is the average battery Soc for the day

summer-solar-harvest-power-consumption.jpg

My winter snip shows a different PV array performance, and resulting battery Soc graphs..

winter-solar-harvest-consumption.jpg

If I was to recommend a path, Your batteries are undersized, upgrade.

Solar panels are cheap (relative to batteries) upgrade.

Winter is coming...

seb71 avatar image seb71 klim8skeptic ·

And enable DVCC.

5 Answers
JohnC avatar image
JohnC answered ·

Hi @spicedreams

You have roughly enough battery for the average day (summer), trouble is all days aren't average. A generator, say 3-4 kVA, would get you over the bad ones. If you want to last x days, then you'll need more battery, maybe much more.

You're light on panels when the days get shorter. To have enough for winter average days you'd need to roughly double what you have already to see similar performance to what you see now in summer.

There's a System Planner: https://www.victronenergy.com/markets/off-grid

It's interactive, so move the sliders about to see what the end result is. It's close to what you seek in support for 'what to do', and is actually pretty good. The 'Autonomy' thing might convince you of the merits of a genny vs spending on batts. Note it uses Li batts to do the calcs, so compensate the kWh value for pb's.

Bear in mind too that not all your consumption goes through the batts, so best to run heavy loads in daytime, ideally after Float is reached.

Your system/usage is actually surprisingly similar to mine. I have a generator (2x), and only need more panels.. one day.. (~Mildura, Aust)

I only run my floodeds down to a min of 75%, and my 'autonomy' is 24 hrs. You may decide to do differently, your choice.

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Thanks @JohnC, that System Planner is a great find I knew nothing about.

I have a generator but it's a cheap petrol thing and I have no confidence it produces a pure sine wave, I only bought it to run power tools. Do you know whether the EasySolar II GX can accept "grid" input from such a thing?

> Note it uses Li batts to do the calcs, so compensate the kWh value for pb's.

Oh for a spreadsheet so I could actually understand the workings! Using the Planner with Pbs, you have to guess their assumptions about Li batteries (nominal rating, allowable depth of discharge) before you can "convert" to your own assumptions about your batteries.

@spicedreams

Yes, your Easy can take a genny feed on the AC-IN terminals. It'll be picky about the quality of the power it accepts, so no need to guess about that. (If it rejects it won't tell you why, so then investigate further).

If a small genny, set the input limit in the GX low and work up. 3.7A (~850W) is the lowest setting you can use there without switching off Power Assist. Then there's https://www.victronenergy.com/live/multiplus_faq

The time to drag it out and set it up is about now. You're gonna need it without more panels, but having it there will give you better sleep at night. Your batts will thank you as well.

With the Planner it's best to treat it as a guide, and try to relate it's output to your own experiences. Like if you did double your panels, does winter become like last summer on the graph?

Like the others have mentioned before, get that BMV up and connected, DVCC too. You have the best kit on the market, so explore it, optimize it, and enjoy it. And if necessary, ask about it.



wkirby avatar image
wkirby answered ·

In the winter time I always find that I need more PV.
If your battery doesn't get full at least once a week on average, then you need more PV. Adding more battery will not make it get full quicker.
If your battery is full by mid day then you need more battery, adding more PV will make it full earlier.

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Eduard avatar image
Eduard answered ·

Lots of good opinions, but we have to make a decision.

Battery capacity is too small, at least 12 of them.

Solar must be double the size.

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spicedreams avatar image
spicedreams answered ·

It is summer and my battery gets full most days, but worryingly low (from a life cycle perspective) when it is less than sunny. Clearly there is a massive difference between a sunny day and overcast. But I am not seeing enough detail in VRM or Remote Console to know when it reaches capacity on average (I can see today, yesterday, day before etc- but over a week or a month?)

Is there a way to download records to analyse with a spreadsheet or similar?

Or better, is there a summary metric in VRM or Remote Console that says "you would benefit from more battery" or "you would benefit from more solar"? I can't believe I am the first to ask this question...

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I found the answer for myself: in VRM under Advanced there is a tiny icon (cloud with down arrow, near top left) which lets you download data for the period shown in the graphs on that page.

Now all I need to do is interpret all that data ;-)

Is there a reference somewhere to the meanings of all the fields? Some are not obvious to me, for example VE.Bus charge power is always a negative number, whether the battery state is charging or discharging.

A field showing "wasted solar" would be really useful... showing how much more power the MPPT controller would send to the battery if there was more battery to soak it up.

michelg avatar image
michelg answered ·

Hi Spicedreams, I have designed my PV installation with an angle of 60 degrees as in winter, in my area, the optimum angle is 67 degrees, but difficult to setup. 4 arrays are south oriented providing me power from 8AM / 02PM, and then decreasing power, so I also added 2 arrays west orientated in order to get more power during the afternoon, as the sun do turn itself from south to west.

You should consider such kind of option, as it will certainly resolve your issues. BTW, you should probably double your current 1800Wp installation, in order to get your batteries charged. It is clearly not enough for your consumption.

Michel


img-20180507-112940.jpg


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Thanks @michelg. Your arrays facing in different directions... are they on different MPPT controllers? My distributor has said that all arrays on the same controller should face the same way, because the controller will charge based on the lowest voltage from any array. Hence I have two arrays both pointing the same azimuth (dictated by roof space, not perfect north).