Why the low voltage mppt

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Andy Todd asked Jul 5, '23

Why the low voltage mppt

What's going on? Should I worry about the 11.something voltage? What could I do?

On the HISTORY screenshot the MIN BATTERY voltage goes down to around 11.90V to 12V at some time during the night. I’ve attached a TREND screenshot showing the profile repeating on typical sunny days. The batteries never deplete to less than 80% SOC and recharge to 100% by early afternoon. There's 420Ah of 12V lead acid and 460w of solar split between two Victron 100/20 mppts. The only things running at night are a fridge and an anchor light.

It’s now coming to the end of a sunny day and the HISTORY screen is showing charging at 12.99V BATTERY voltage and 1.6A. At the same time a separate battery monitor (with shunt) is showing 13.05V and discharging at 4.6A. Why the difference and what is this telling me about the system?

According to a rough power audit we use 840Wh overnight. And 840Wh (actually 856Wh) is around 17% of the 420Ah capacity, which is what we've averaged over June. This has been the same profile since the batteries we installed a year ago and it was the same profile with the previous batteries. I can't believe the batteries are bad otherwise we'd have long since not have been able to get through the night. I think I've got my sums right. Am I missing something?

MPPT SmartSolar

3-days.jpg (80.2 KiB)

bar-chart-2.jpg (124.1 KiB)

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Andy Todd answered · Jul 08, 2023 at 12:35 PM

Sorted. Loose terminal screws on both mppts. Now seeing 14.4V. Thanks for the help.

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Answer 1 · 2023-07-05T18:23:54Z

mvas answered · Jul 05, 2023 at 06:23 PM

How long has it been since this battery bank was at 100% SOC ?
I do not see 14.4 volts on any day.
What is the voltage setting for Absorb mode?
Flooded or AGM ?
How old are these batteries?
These batteries appear to be chronically under-charged.

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Answer 2 · 2023-07-05T20:01:35Z

Andy Todd answered · Jul 05, 2023 at 08:01 PM

They've been a 100% by 1300h every day for the past 30 days at least. They stay at 100% until the sun gets low in the sky in the evening here in Greece.

It never gets to 14.4vas indicated on the charger, but I think that's because of a voltage drop in the cables from the batteries to the charger measuring the battery voltage. There's a Matervolt shunt type monitor that always shows a higher voltage.

Absorb mode 14.4V. Pic of setting attached.

Flooded

They're about a year old and have always exhibited the same charge/discharge profile for these given Med-sunny-day conditions.

Undercharged... How come with the settings on the chargers? It's dark now the near-resting voltage is 12.7V with a few LEDs drawing 0.4A and the monitor saying 98%

battery-set-up.jpg (68.9 KiB)

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mvas commented · Jul 06, 2023 at 04:11 PM

No these batteries have not been at 100% SOC.

Look at the 1st screen shot "History" that you posted...
Day #1, max = 14.31v
Day #2, max = 14.27v
Day #3, max = 14.21v
Day #4, max = 14.19v

Do you see 14.4 volts?
I do not.
You need to get your cables "fixed".
You need 14.4 volts at the battery bank.

In your Trends Graph ...
Where do you see Absorb Mode holding the voltage steady @ 14.4 volts for an hour?
The Battery Voltage ( Blue Line ) peaks 14.1 volts.
As soon as your battery battery bank reaches approx 14.1 volts,
the Solar Watts (charging amps) immediately starts dropping towards zero.
Is this because the sun is starting to "set" on your PV array or
is the Charge Controller doing that tapering?

BTW ... 12.0 volts is approx 50% SOC.
Yet, you claim the battery bank is never discharged below 80% SOC.

If you use 840 Watt-Hours every night,
then you need to recharge with 933 - 1,050 Watt-Hours every day.
You have never done this.

Everything you posted says ...
You are chronically under-charging these batteries.

Also a "Bosch L5 075 flooded LA 140Ah" looks more like an AGM to me, not "flooded"?

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Answer 3 · 2023-07-06T04:49:50Z

kevgermany answered · Jul 06, 2023 at 04:49 AM

Please post a shot of the battery settings in the shunt.

I guess you're getting similar figures from the other mppt. Are the batteries connected as a single bank?

The shunt appears to be monitoring one mppt not the whole battery bank.

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Answer 4 · 2023-07-06T14:50:00Z

Andy Todd answered · Jul 06, 2023 at 02:50 PM

The battery monitor is an old Mastervolt CSC on a Combi Dakar. So, no app and screen shot, but BULK is 12.0v to 14.4V rising, ABSORB is 14.4V constant, FLOAT is 13.5V constant (battery current less than 2% of capacity). Peukert Exp 1.27P, C.E.F. 90%, Number Cycles 15.

Yes. Two 100/20’s in parallel showing very similar data, but usually slightly different.

Yes. Single bank of 3 Bosch L5 075 flooded LA 140Ah.

Does this help shed any light… At the beginning of today, before the mppt’s started charging and at a typical lowest daily SOC the mppt’s showed battery at 11.96V 0A. At the same time the monitor said 12.04V discharging at 4.2A.

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pwfarnell commented · Jul 06, 2023 at 04:24 PM

If they are flooded lead acid batteries have you checked the water level and topped up if required.

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Answer 5 · 2023-07-07T02:30:17Z

kevgermany answered · Jul 07, 2023 at 02:30 AM

On the charging. The MPPT has to see a battery voltage of 14.4V. At this point it switches to absorption mode, maintains 14.4V until the current drops to the tail current setting. Usually 4% of battery capacity. At the start of the 14.4V absorption phase the battery is roughly 85% charged. Once tail current setting is reached, MPPT switches to constant float voltage.

Your graphs and the MPPT trend shows this isn't happening.

Attached graph from my system showing how it should look. Note my 24V voltage settings converted to 12V are lower than yours due to different battery type. Note also the increasing SOC when the constant voltage absorption phase is reached.

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kevgermany ♦♦ commented · Jul 07, 2023 at 02:31 AM

The graph

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screenshot-2023-07-07-03-23-26-576-nlvictronenergy.jpg (395.0 KiB)

Answer 6 · 2023-07-08T09:53:10Z

Andy Todd answered · Jul 08, 2023 at 09:53 AM

Keygermany, that’s very useful.

My history screen shows bulk, absorption and float for each day. How is this happening if the mppts never see the 14.40V initiation voltage?

Now the batteries are in absorption. The mppts each say 14.20V and 11.90A. While the battery monitor says 14.17V, 21.6A and 95% full. Does this suggest anything?

What should I start looking at to sort this out? Any ideas will be most welcome.

Nevertheless, I appear to have full batteries at the end of each day and adequate power to run the boat. And the daily profile has been the same since the batteries were installed a year ago. If I can’t find an answer is there any way this could be damaging/shortening the life of the batteries?

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nickdb ♦♦ commented · Jul 08, 2023 at 11:27 AM

If you have temperature compensation on, that will also have an effect.

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Answer 7 · 2023-07-10T12:15:22Z

Andy Todd answered · Jul 10, 2023 at 12:15 PM

nickdb (and all), See pic. I turned all off, but the anchor light overnight. The batteries bottomed out at 12.76V. Consequently, they reached absorptions early in the day at the set 14.4V, before it gets ambient hot here in Greece. It then when into float at the set 13.8V. After a few hours it dropped to 13.5V, still in float. I read your comment and turned the -16.5mV/degree compensation of and it went back up to 13.8V. The temperature sensor is in the charger (not a remote sensor on the battery). The casing of the charger is currently sitting at 33C. The batteries are at 27C.

On previous days, after a full overnight load. I think the bulk phase took long enough that the chargers temperature compensation kick in during bulk and the batteries never saw 14/4V. Does this sound feasible?

Do I need to have temperature compensation on?

temp-comp.jpg (106.5 KiB)

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nickdb ♦♦ commented · Jul 10, 2023 at 12:22 PM

For conventional batteries and warm climates, temp compensation is a good idea but I would check the value and confirm it with the battery manufacturer's spec.

I would also get a proper sensor and attach it to the battery terminal, internal estimation is a bit inaccurate.

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