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rnwbl asked

Help understanding MPPT battery algorithm thresholds

I'm testing a small solar panel (6-7W) > charge controller > battery (12V) & load (12V - 5V USB charger) setup using a SmartSolar MPPT 100/15 but I'm not understanding the thresholds for switching from Bulk to Absorption to Float or if the current presets are suitable for my set up.

The battery I'm using is a Duracell DR7-12 12V / 7Ah battery. Everything I've read about batteries like this describe 'fully charged' as approx. 12.5-ish volts and this battery has reached that level of charge in the charge controller's Bulk mode plenty of times - even with Load connected - but so far, even with no Load and the solar panel at 7W - I've not seen the battery get past 12.7V so the charge controller has stayed in Bulk mode.

The part I don't understand is that on the 'Gel Victron deep discharge (2)' battery preset - the threshold for switching from Bulk to Absorption appears to be set at 14.40V and then Float is set at 13.80V.

So - two questions...

1) Should I really be expecting this 12V battery to be hitting a charge of 14.40V before coming out of Bulk charge mode or should I be creating a preset more suited to this battery?

2) If Float is the stage after Absorption - why is it set at a lower voltage?

Thanks.



MPPT Controllersbattery charging
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kevgermany avatar image kevgermany ♦♦ commented ·

Three stage charging:

Bulk: max current, voltage rises during this period, battery gets to about 80%

Absorption: charger switches to this stage once set absorption voltage reached (normally 14.4V for AGM, lead acid etc.) and maintains the voltage. Current drops due to battery resistance increases as it fills.

Float: battery considered fully charged once current drops below the set tail current value (e.g. 4% of battery capacity). Charger drops voltage to float voltage (usually about 12.7V) to keep battery fully charged.

As it's Solar, clouds can mess you around. So it's a good idea to increase charge detection time to suit local conditions and reduce the tail current setting as well. This can take a few days or weeks to get right.

After charging, a fully charged 12V battery should, after resting and with no load, be about 12.7V. This will slowly drop due to internal self discharge.



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mvas avatar image
mvas answered ·

12.7 Volts at the end of Bulk Mode is not 100% charged - maybe only 80% SOC?
Yes, you need more than 12.7 volts when charging.
Yes, Float voltage is lower, to stop charging - ie to offset self-discharge
The charging voltage will always be higher than the resting voltage
Somewhere between 12.7v - 12.9v is a good resting voltage for 100% SOC AGM, depends.
Does Duracell supply a datasheet and charging instructions?

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rnwbl avatar image rnwbl commented ·

Just to clarify - it's the battery that isn't getting past 12.7 volts - even when the solar panel has been outputting 20-25V for hours.

I don't understand why I'm not seeing anything anywhere that says a 12V battery is not fully charged until it's reading 14+ volts.

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pwfarnell answered ·

The MPPT that you have is very large compared to the panel size, at 12V it can handle 180W. This may mean that it is not very efficient at low power because it is over specified so you charging current to the battery may be lower than expected. You may also be using more power than you get from the solar, hence it never fully recharges.

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rnwbl avatar image rnwbl commented ·
Yes - I realise this charge controller is over-specced for the panels and the battery I'm using it with but this is meant to be a 'proof of concept' system so I can get my head round the different parts before spending more money on bigger panels and more batteries.


Original question still stands though - is it correct that the charger algorithm is looking for this battery to be charged to >14V before switching out of Bulk mode?

Thanks.

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mvas avatar image
mvas answered ·

The voltage on your battery dropped from 12.62 volts, down to 7.47 volts? YIKES !
The battery is severely under-charged.
You have an 84 Watt-Hour battery, at 100% discharged,
then you only recharged with 20 Watt-Hours?
This is bad ...
You need way more Solar charging vs way less Load discharging.
Yes, 14.4 volts is a good voltage to stop Bulk Charging and start Absorb Charging,
which you have never done?

Also, that small battery must power the Smart MPPT, when the sun is down.

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rnwbl avatar image
rnwbl answered ·

This is the question I'm trying to answer:

Is it correct that the charger algorithm 'Gel Victron deep discharge (2)' is expecting this 12V battery to be charging to +14V before coming out of Bulk charge mode?

I would have expected 'fully charged' to be closer to 12.5-127V.

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mvas avatar image mvas commented ·

I said ... Yes, 14.4 volts is correct, to stop Bulk mode and then enter Absorb mode.

I asked you for ... the Datasheet and the Charging instructions from Duracell.
Where is it?

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rnwbl avatar image rnwbl mvas commented ·

"I asked you for ... the Datasheet and the Charging instructions from Duracell. Where is it?"

I couldn't find anything on the Duracell site. I've contacted support via email to ask them.

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pwfarnell avatar image pwfarnell commented ·
Yes, when charging lead acid type batteries you need to get to 14.4V with a reasonable current to have them fully charged. The voltage of a full battery depending on type is 12.7V to 12.8V BUT to charge it you need a higher voltage to force the charging current back into the battery. If you had only say 13.2V for charging it would take many many days to recharge because the recharge current would be too low. To recharge in 8 hours needs a higher charge voltage. This is why Bictron uses these values by default, they understand battery charging.
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rnwbl avatar image rnwbl commented ·

I'm so sorry to have to keep asking this - I feel like an idiot but I can't put money into a bigger system until I understand fundamentals.

The >14V voltage being talked about here e.g. the 14.4V Absorption threshold - is that the voltage that the battery needs to be charged to i.e. the voltage I would get if I measured across the battery terminals with a multimeter or is it the voltage that needs to be going into the battery from the charge controller?

Again - I apologise for what is probably a very basic question for people who know the answer.

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pwfarnell avatar image pwfarnell rnwbl commented ·
The charger needs to be able to develop 14.4V. In bulk mode when the battery is significantly discharged and the charger is providing max current the charger output voltage and battery voltage will be below 14.4V. As the battery fills up, the voltage will rise to 14.4V at the output of the charger and the battery. The battery may still not be fully charged at this point, maybe only 80 to 90% charged so the voltage will remain at 14.4V for another 2 hours as the charge current tapers off ( lead acid batteries). When the charge current is less than 1% of the battery capacity (below 0.07A for your 7Ah battery) at 14.4V then the battery is charged.


Before you go spending money on systems I believe that you need a much better understanding. Have you downloaded and read Energy Unlimited per the other link as this covers all the basics. There is plenty of info on line about charging, have a good read.

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pwfarnell avatar image
pwfarnell answered ·

I suggest that you read the Victron publication "Energy Unlimited" because you need to understand battery use and charging before implementing a large system.

See

https://www.victronenergy.com/upload/documents/Book-Energy-Unlimited-EN.pdf

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