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kmr avatar image
kmr asked

Multiplus/48/3000 connects to Generator but doesn't charge. DVCC or tail current issue?

Hi All,

I'm running an off-grid system using MultiplusII/48/3000 with a Cerebo GX, MPPT/250/100 VE.Can rev2, SmartShunt 500A all connected with VE.Can. My battery setup is 20x 250A 12v Gel batteries in 48v with 3 battery balancers.

On the Multiplus AC input we have a 7.5Kw generator that we've configured to start when GX relay is turned on. We want to use the generator on cloudy days top up the batteries while briefly running our critical load.


When we start the generator and the Multi accepts the input, we see the system taking over 1000W from the gen to supply load and help charge the batteries alongside the MPPT250/100 as we expect. However, after less than 2 minutes, the load on the generator starts to drop in a very controlled fashion. It appears that either Multi or another device is limiting the generator input for charging, even when the MPPT is clearly still in bulk and the batteries need charging. This makes us very nervous about when bad weather will come and we'll be reliant on the generator charging the batteries.


We have not enabled DVCC yet for this install as we're not entirely confident about its impacts with our setup and running these Gel batteries. We also have issues with the Smartshunt's tail current settings not being able to set the % value below 0.5%. Given we're running over 1000Ah storage capacity, a tail current value of 0.5% is still 50A! We'd need either an Amp value or 0.05% to get to a more normal 5-7A tail current setting. I believe this results in our batteries always showing 100% as soon as the MPPT's start charging in the mornings.

We suspect it may be the tail current setting with battery SOC always being at 100%, but we are unsure this is why the Multi is not using the generator for proper sustained charging along with the MPPT. Perhaps because we're not using DVCC and this will help with global control of all the devices working together?

Attached some pictures showing setup and generator on but not charging. When MPPT is on, it give charge to battery. When MPPT is off, generator still doesnt charge.

Gen with MPPT.jpg
Gen without mppt.jpg
Smartshunt settings.jpg
Multiplus settings 2.png
Multiplus settings 3.png
Multiplus settings 4.png
Multiplus settings 5.png
Multiplus settings 6.png
physical setup.jpg

Multiplus-IIGeneratorSmartShuntDVCCcurrent limit
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1 Answer
JohnC avatar image
JohnC answered ·

Hi @KMR

Couple of quick things without delving deeply..

Your Multi has gone to Storage mode. You could turn that off with daily cyclers, and it's not the place to be when using a genset. Indeed, even Float mode doesn't offer value when running a genset.

Your 0.5% shunt Tail means 5A, not 50A. You may find on the shorter days of winter that even that may be a smidgen low to achieve with practical regularity.

Don't fear DVCC. It may even help with your tuning, as you won't need to wonder if your kit is seeing different values of V and T once things ramp up.

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JohnC avatar image JohnC ♦ commented ·
Also, your shunt's 95% Eff and 1.25 Peukert are likely both a little high, hence overestimating SOC. A longer term tuning thing, so keep in mind while mastering the basics.
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kmr avatar image kmr JohnC ♦ commented ·

Hi John. Thanks for the quick feedback!


I never noticed it going into storage mode. Dove into the history and the inverter does seem to briefly go into storage mode only for the time when the generator turns on - see attached.

When you say we need daily cyclers, what do you mean exactly?
I'll turn off storage mode in VE config and see how that goes.


As for smartshunt, you are correct, it is 5A, my mistake! I modified the 0.5% on of our systems and on our second identical system left it as 4% - huge difference and SOC seems to be behaving correctly now (pending more fine tuning). See attached. As you mentioned, we'll start playing with eff and Peukert values.

When turning on or off DVCC, im guessing not much changes/happens as far as power output goes? We're weary of any changes since the system is currently powering critical communications equipment and we dont want an outage for any reason.

Multi storage when Gen on.png
4percent vs 0.5percent on 1000Ah.png

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JohnC avatar image JohnC ♦ kmr commented ·
@KMR

Yeh, I love VRM graphs, they say so much.

By 'daily cyclers' I mean loads drawing down batts overnight, then usually topped up with solar daily. Seems you qualify, but only just. Drawing down to 95% SOC once a day and seemingly having plenty of solar to bring it back up quickly is only light cycling. And maybe your genset just gets a run occasionally as a test. All good, given your critical use situation.

DVCC is often just a 'tidy up'. Same V across all kit, same T for Temp Comp across both chargers, and the Current from the Shunt gets transferred across to the mppt so it can be used by the Tail function there to terminate Abs at a consistent time to prevent overcharging of the batts. It can do things like limit collective current too, but you have to enable these things and make multiple errors to stuff up. Don't fear it.

Re tuning the shunt. Grab the Peukert Calculator from the Victron website and determine yours from the C20 and C100 (say) specs of your batts. And even then you could err on the low side, which will ensure that SOC won't get overestimated with your low loads. The Charge Efficiency you'll need will be quite low, as Pb's are very inefficient topping off 5% each day. Even starting at 90%, you may need to work down.

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kmr avatar image kmr JohnC ♦ commented ·
Indeed, the graphs are great to work with.


Yes, our cycles are very light but mainly by design as we need long runs without power input if things go sideways with the weather. We typically do manual starts of the generators once a week for a short while using the GX relays.

We'll try DVCC - would be good to have single values across the board. Have noticed some discrepancies between the devices. Appreciate the comments on that.

Calculated between 1.25 and 1.26 from the battery manufacturer's data. I've put the new values at 1.26 and will see how it goes from there. Also changed the efficiency to 90%.

One last question, what am I looking for with changing the efficiency % value? In the sense of, what tells me to further decrease or increase the %?

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JohnC avatar image JohnC ♦ kmr commented ·

@KMR

What you're looking for with the combo of the Peukert and Charge Efficiency adjustments is to have the calculated SOC return naturally to the 100.0% at the same time you decide it should do that. And the Tail Current is where you decide. I reckon mine is close enough at 0.6% (1.35A), because I start to run out of daylight if I go lower. This is mine today (Peukert 1.10, CE 89%).. 1669706733423.pngDropped to Float at 7A (mppt setting, & DVCC) at 99.4% SOC. Synced at 99.8%, but almost 99.9%. I'm happy with that, but to retain such accuracy I might tweak CE and Tail seasonally for lower winter batt temps, shorter days, and a deeper batt drawdown.

Usually you'd settle on a Peukert and run with that, just adjusting CE. But your high P might need a further look at. It's common to see a high P at heavy discharge rates, but not so much with very light discharge like yours. The shunt will compensate SOC relevant to C20, so don't use a C5 rate in the Calculator, which commonly results in a high P. Then if you discharge up at C100 (like you) it will apply too much SOC 'credit'. I haven't explained this very well, but come back if you need and I'll have another try..

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1669706733423.png (39.4 KiB)
kmr avatar image kmr JohnC ♦ commented ·

This helps a lot, thanks John! I've created a custom chart and seems I'm getting close. Float seems to kick in around just under 4A. I've put a 15min charge detection time which accounts for the small time delay in the graph, might change that to get it closer as well.


screenshot-2022-11-29-210142.png

Will keep testing over the days to come and see how it behaves with small changes.

Much appreciate your help!

-Kevin

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