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billknny avatar image
billknny asked

Looking for a Float to Bulk Solution

I know this has been a long discussed topic, and from this: https://community.victronenergy.com/questions/175/how-do-quattro-and-multiplus-decide-to-switch-from.html

I think I understand the behavior that is hard wired into the Multiplus. But it is giving me trouble, and I am hoping for advice for practical solutions.

System is a Multiplus 3000 connected to a Cerbo. There is also an MPPT connected but this does not seem to be an issue, it normally starts each solar day in Bulk and runs appropriately.

I have a 24V 465 A-hr bank of Firefly AGM batteries of 4 parallel strings of 2. The programmed float voltage is 26.4V, therefore I expect the automatic transition to Bulk to happen when the voltage drops below 23.8V (26.4 - 2.6). The problem I have is that even with a 50 Amp load applied my battery bank does not drop below 23.8V until the SOC is around 40%. Obviously, I can not allow the SOC to drop that low as a routine matter.

I get around this manually by going trying on Equalization, and then immediately cancelling it, which gives me the option of either Absorption or Float. If I pick Absorption, that gives me the performance I need.

Is there automated way of doing this or narrowing the voltage difference? Maybe I need to go into Node Red and customize something based on SOC? Maybe somebody else has done that and has advice on where to start?

multiplus ve.busfloat
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3 Answers
Alexandra avatar image
Alexandra answered ·

@billknny

Is there a GX and a shunt there?

Are you using ESS? Or mobile so not at all?

Have you tried setting the repeat Absorption trigger interval shorter? I think the standard setting is one week interval. (Not the duration setting) have you unchecked storage?

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JohnC avatar image
JohnC answered ·

Hi @billknny

The underlying issue is that your 26.4V is down in 'nowhere' territory for Pb batteries. And by setting that you're actually telling the Multi you're happy with it maintaining that level. It's too low.

Where to set it depends on what you're doing. From a previous post, you have a genset. So do I, and my Float is set way higher. It doesn't matter because my genset isn't worth running in Float, so I never let it get there. But when I start it, it always goes straight to Bulk.

You need to tell us more..

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kevgermany avatar image kevgermany ♦♦ commented ·
@billknny

As well you can look at reducing rebulk offset.

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billknny avatar image billknny kevgermany ♦♦ commented ·
Thanks for that helpful comment... It's one direction I an going to try the next time I am plugged into shore power.
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billknny avatar image billknny commented ·

I am totally confused by your comment. The manufacturer specified float voltage for these batteries is 26.4 Volts. So what do you mean "it is too low"? What do YOU think is appropriate for a float voltage? When plugged into shore power I am not going to let the batteries float at much higher voltage.

My generator is a DC genset, and charges the batteries directly so it does not go through the Multiplus at all, and is not relevant to the issue. This only comes up when plugged into shore power.

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JohnC avatar image JohnC ♦ billknny commented ·

@billknny

Ok, so you're not using a genset with the Multi. Shore power with a recommended 13.2V Float and holding. Now you want to go back to Absorb for some reason, against the recommendation.

And that's fine, your choice. But the Multi needs to be told. Try setting it's Repeated Absorption Interval to whatever you choose, and it will do it for you once a day (or whatever).

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geomz avatar image geomz billknny commented ·
If your batteries are stable enough to maintain a voltage above your set float voltage, even under a high-ish discharge rate, then the system has no way to know when is a good time to go back into bulk —so as to avoid excessive bulk.


I had the same issue with my lithium batteries—since lithium is able to maintain a flat voltage curve over considerable discharge rates. I ended up setting my float voltage (a bit) higher to make sure the charger went back into bulk when the voltage dropped to a “tolerable” level for rebulking.

All of that is to say: I would charge your system to 100%, put a good load on it for a few minutes and see what voltage it drops to, look for what you would consider tolerable to renter bulk, and set your float voltage to somewhere around that.

26.4v also seems really low to me too. It might be what your vendor says is float, but it will cause these kinds of issues, if the battery voltage is that stable. Most (dumber) 12v lead acid battery chargers maintain a float voltage of ~13.7. Doubling that for your 24v system puts us at 27.4.

You could just try that and see what happens. Without knowing more about your specifics, that should be plenty safe for a lead-acid battery to float at.

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billknny avatar image
billknny answered ·

I am not going to float my AGM batteries at a full Volt higher than recommended. That would be a disaster, they would gas and be destroyed.

I know the voltage/SOC curve for my batteries. All the values are below the float voltage, I am not sure why you think it would be otherwise.

Under a steady 50 Amp load (~ C10), the curve looked like this on the last capacity test:

100% 25.31V

95% 25.29V

90% 25.18V

85% 24.98V

80% 24.84V

75% 24.65V

70% 24.54V

65% 24.37V

60% 24.21V

55% 24.04V

50% 23.95V

45% 23.81V

40% 23.59V

You can see that even with a steady 50 Amp load (WAY higher than the normal draw, the Multiplus would not begin a Bulk charge on its own until the SOC was between 40% and 45%--far lower than I let it go during normal operation.

If the answer is "just always let it float," why do I have a smart battery charger? I could have the same result from a dumb power supply just set to the float voltage.

At this point, I am looking into writing a Node red script that does the following: If (shore power) AND (SOC < 80%) THEN begin a Bulk charge.

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geomz avatar image geomz commented ·

That’s fair. Your batteries, your decision. All anyone is doing is trying to help you solve your problem. But from the way I understand the system works, it’s all working as designed.


You have a float value set for 26.4v. Rebulk triggers at FloatV minus 1.3v per 12 volts. Meaning, in a 24v system, the differential is 2.6v. So, 26.4v - 2.6v = 23.8v. It needs to be there for about a minute, steady. And as you provided above, that seems to be right where your system renters bulk.

It’s just math.


I seriously doubt floating at 27.4 would “off gas your battery” or what have you. Lead-acid is lead acid, and it’s just chemistry. That value is well within float for lead. There’s likely little difference between your brand and dozens of others. You can call them and ask, I suppose.


But, if you don’t want to change the float value, and the rebulk differential is pretty much firmware locked (as far as I’m aware), you could try checking the box for Lithium Battery in veconfig. That will reduce the rebulk differential to .2v / 12v —so in your case .4v. That 26.4v float minus .4v would have you reentering bulk any time voltage drops below 26v, which according to the chart above, is always. You *could* then reduce your float (rather than raise it) and get closer with a 25.4v float. That would have you reentering bulk at about 80%.


But, if you do write that script and it does the job, great! I’m sure others on here would like it as well, if they’re in the same boat :)


Good luck.

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