75/15 mppt floats below the set voltage

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dan asked Mar 28, '21

75/15 mppt floats below the set voltage

Hi, i have a victron 75/15 mppt with blutooth dongle attached with no battery temp monitor. i have set up a maintenance setting (pic attached), the problem is that it its floating @13.09v and its set to 13.35v.

Ive also noticed there is intermitant data on the history and i have set absorption and float the same setting values, it seems that it reaches the 13.35voltage in bulk or absobsion then drops to 13.09v.

I also have another seperate system but with a batt monitor and no dongle but blutooth, both are set to the same config through the phone app and it keeps the correct float voltage except for the battery monitor adjustments.

Just cant work it out

Some conflicting pic data, inital data is Set float voltage 13.35v and it sits at 13.09v.

Currently the 13.15voltage is dropping back due to checking voltage setup and now its set at 13.30v so i expect it to drop lower than 13.09v, maybe 13.04v float.

MPPT Controllers

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Answer 1 · 2021-03-28T05:23:03Z

Paul B answered · Mar 28, 2021 at 05:23 AM

Hi you dont mention what type of batteries you have, but all your settings look wrong .

your absorption voltage is the same as your float voltage. Absorption should be around 14 to 14.6 volts normally depending on your battery specs and float around 13.6v

also for the mppt to start charging the voltage difference between the PV voltage and the batteries must be more that 5Vs or the MPPT will not start charging. so I suspect that your panel voltages maybe to low. so if you have more than one you should maybe change them to a series configuration to bump there voltages upto 36 volts

Just a couple of comments. for you

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dan commented · Mar 28, 2021 at 07:18 AM

The voltage is in float mode that means its charging its already gone through the bulk and absorb cycle, the voltage is not the issue, if i adjust the float up to 13.7v it will charge to 13.45v or so..

The absorb and float are set the same because the batterys are not on load during the day they are in maintenance mode for months or untill we go camping, thats why the tail is .5v so a quick bulk and absorb are achieved.

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dan commented · Mar 28, 2021 at 07:58 AM

Today

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Answer 2 · 2021-03-28T08:36:14Z

JohnC answered · Mar 28, 2021 at 08:36 AM

Hi @Dan

If you scroll down your Settings screen a little further you'll see Temperature Compensation. That will affect your target voltages.

What value is entered there?

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dan commented · Mar 28, 2021 at 09:18 AM

I dont have any temp info, there isnt any on the mppt is there?, only if i have a battery temp monitor which is on my other one, only.

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JohnC ♦ dan commented · Mar 28, 2021 at 10:05 AM

@Dan

Yes it has, but it only uses it when starting at daybreak, else any casing heat would be misrepresented.

This from my settings (48V bank, hence the high TC):

(Must be a negative value).

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dan JohnC ♦ commented · Mar 28, 2021 at 10:31 AM

Oh yes, its on tge page.

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JohnC ♦ dan commented · Mar 28, 2021 at 10:47 AM

@Dan

That's ok. but your mppt must be in a hot place to under-target the set V's at dawn. You could try setting the TC to zero, then maybe awaiting a new day.. Just to see if it follows uncorrected setpoints.

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dan JohnC ♦ commented · Mar 28, 2021 at 11:09 AM

Ok, ill reset to zero and take some temps from the battery cabinet but i dont think its hot in there, ill check it out tommorrow and get back with the results.

Thanks.

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JohnC ♦ dan commented · Mar 28, 2021 at 11:18 AM

@Dan

25C is where it starts. Higher than that it will reduce the setpoints. Lower, vice versa.

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dan JohnC ♦ commented · Mar 29, 2021 at 02:46 AM

Ok, i checked the battery cabinet this afternoon i had placed a thermometer inside this morning,it was 22degc and the voltage reading was 13.19v, i then switched of the temp compensation and the voltage increased 13.29v.

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JohnC ♦ dan commented · Mar 29, 2021 at 10:13 AM

@Dan

I guess you've got something to investigate now. Temp Comp is a good thing if the temp source is reliable and it's all working as it should.

Be careful using daily values from the history, as intermittent spikes get registered too if they occur. The Trends graphs can be useful to avoid being sidetracked by such.

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dan JohnC ♦ commented · Mar 29, 2021 at 11:00 AM

The more i try to get explanations for the data the more confusing it becomes, even without the temp compensator ive noticed the voltage drops a few mv less than the target setting, with missing daily data and a temp compensator that seems to work in the opposite direction, all leads me to belive that something is wrong/ faulty etc, i really dont know where to from here but in the mean time i know that the cabinet temp is minimal and never hot 26 c max at least for tge next 6 months of coming winter so ill just run without the temp compensator untill i find the issue.

Thanks for your effort.

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dan commented · Mar 28, 2021 at 09:30 AM

Just now

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klim8skeptic ♦ dan commented · Mar 28, 2021 at 09:34 AM

edit..........

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dan commented · Mar 28, 2021 at 09:40 AM

So this is the other unit with the temp monitor on the battery, it has the same user set and its in the driveway with the other unit in the sun, the batts would be the same temp i reckon.

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