question

clueless avatar image

Decreasing state of charge despite able solar

We have a 210w solar panel with a MPPT 75/15 solar controller connected to 2 AGM 6v batteries in series for a total of 400Ah. When plugged in the batteries are at 100% state of charge and are in float phase. When we take our camper out on a trip, each day the Smart Shunt readings show the state of charge decreasing by about 4-6% each day. When I go to the history page for the Smart Charger it shows between 210-300Wh of power generated each day from our solar panel. It also shows that we consume between 10-30 Wh per day. With these figures I would assume that we generate more than enough power from our solar panel to fully recharge our batteries. Would you please explain why this does not occur and why we see a steady decline of our state of charge?

Thanks

MPPT Controllers
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4 Answers
snoobler avatar image
snoobler answered ·

Peukert for lead-acid should be 1.25.


To be clear:

"solar controller registered our daily consumption at 10-20Wh"

The solar controller "consumption" only reports what is being pulled from the load ports of the MPPT here:


1642545519332.png


If you have ANY loads that don't use these ports, then the solar controller won't see them, and that number is meaningless.


Your smart shunt reports daily usage in Ah, and this should be everything that comes out of the battery at its current state of charge:

1642545851820.png


On the History tab, it shows the last discharge:


1642546025135.png


Concerning Solar: This is barely better than the worst time of year for solar in the Northern Hemisphere. Assuming NO clouds/panel flat on roof, at your latitude, you can likely only harvest about 30-40% of what you can get during the summer. Inclement weather impacts that even more. As such, your 200-300Wh/day sounds about right for the conditions you describe.


You are using a bad reference for your consumption unless ALL of your 12V loads are going through the load port I circled above (almost impossible you are due to the 15A limit on the load port). You are using far more than 10-20Wh/day.


Note that a single LED light will use 10-20Wh/day by being on only 2-4 hours. In short, if you are using more than a single LED light for 2-4 hours/day, you are using more than 10-20Wh/day.


You are using a junk number for consumption, so you think something is wrong when it's not. Your consumption is properly reported by the smartshunt assuming it's connected properly. The reason you see a daily drop in % charge is because you have insufficient solar to meet your daily consumption.


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6 comments
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You are amazing, such helpful and understandable information


I can't thank you enough
Happy to help... I was clearly repeating myself trying to get it right... :)
I'm having a similar problem of decreasing SOC with my 12v 250/100 MPPT-Tr VE.Can supplying 8 Lithium battery string. It is not a "amp hour use that is greater than recharge capacity available". It is that the Mppt supplies 80 amps @ 13.5v(bulk) until around noon and then switches to 14.2v (absorption) with only 1-8 amps and 3 or more hours of bright sun (80amp) available. Accordingly, everyday It gets farther from 100percent SOC. All loads and the battery go through the smart shunt. I am using the default Victron battery preset Lithium settings for charging. I'm stumped and appreciate any and all suggestion to solve this!

-Steve

LFP does not charge like lead. The bulk of the charge is taken on in the 13.4-13.8V range with a short spike at the end to the "absorption" voltage.


"Bright" sun is meaningless unless it's perfectly perpendicular to the panel.


A LFP battery is fully charged at 3.55-3.65V/cell and 0.05C end current. You are likely getting very nearly fully charge, but are not properly triggering the "charged" criteria on the shunt.


I recommend the following:


MPPT:

Absorption 14.4

Float 13.6

Tail current 0.05 * your amp hours (expert settings)

Absorption time FIXED at 30 minutes. (expert settings)

Low temp charge protection 5°



Shunt:

Charged 14.1V

Tail current 6%

Peukert 1.05



Make sure the smartshunt has a temp sensor and is VE.Smart networked with the MPPT to provide voltage/temp data. A smart battery sense is also acceptable.

Much thanks to you snoobler for taking the time to help me and solve my SOC discharging problem. I made the setting changes you recommended and my SOC immediately changed from 80% to 100%. I then let it run a full cycle of constant load discharge (over night) and recharge (with the same load) today back to 100% by noon. It then held that at 99.7% until the sun went down.

I had read all of Victron manuals and tried every possible permutation over the last four months without success. You should be writing the set up manuals for Victron!

-kc6wrd

Worth checking Youtube for Victron videos - one of which is about optimizing the sync with the shunt:

https://www.youtube.com/watch?v=4ReljfO3k6M


snoobler avatar image
snoobler answered ·

Battery capacity only adds when put in parallel. Do you actually have 2X 400Ah 6V batteries in series, or do you have 2X 200A batteries in series? If 2X 200Ah, you only have 200Ah @ 12V.

The charge controller only knows what is being consumed at the load port. If you have devices/loads being used that do not use the load port, then the MPPT has no clue about them and does not report them.

210-300Wh of daily harvest from a 210W panel is pretty pitiful. Assuming you get very poor sun.


You are likely using in excess of what your panel can produce daily.


Please include your MPPT and SmartShunt configurations.

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Please note my response below.

Thanks

kevgermany avatar image
kevgermany answered ·

It may also be that there are loads bypassing the smart shunt. Are there any connections to the batteries that bypass the shunt?

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Please note my response posted below.

Thanks much

clueless avatar image
clueless answered ·

My thanks to Snoobler and Kevgermany. Below is info for further clarification.

I do not believe there are any loads that bypass the Smart Shunt.

Our batteries are 6v Lifeline AGM's, 400Ah each in series, therefore 12v, 400Ah and as I understand it, in most circumstances one should not routinely go below 50% discharge, therefore approximately 200Ah of useable battery.

The 220-300Wh of daily yield from our 210W solar panel was achieved a week ago, 3 weeks after the winter solstice in central Idaho with a mix of sun and clouds. Again the solar controller registered our daily consumption at 10-20Wh. We basically only use the batteries to run LED lights, run the pump for the heater which is propane fired, water pump, radio and a couple USB charging stations. The frig is run with propane, therefore no large draws on the batteries

The MPPT controller is set with Absorption at 14.4v, Float 13.3v, Equalization 15.2, Max Abs Time 2hrs and tail current at 2amp.

Smart ShuntCharge Voltage 14.1v, Peukert 1.1, Charge Efficiency 97%

The above figures are recommended settings from the manufacturer of our batteries.

I am not very knowledgable when it comes to electronics, solar panels, etc. While I suspect that our setup provides sufficient battery capability for our needs I still don't understand why the solar panel generates 220-300Wh per day, we consume 10-20Wh per day, though there is a daily decline in our batteries' state of charge of 4-6%, approximately 80% after 4 days relying solely on solar.


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