question

nelsonpk avatar image

Why Float so early

My 75/15 solar charger seems to go to Float too early, the BMV is telling me 80% SOC.

Could the battery be fully charged already & the BMV is not correct?

I should add, the battery is 300AH LifeP04 & charger settings to suit.

Screenshot_20210911-122303.pngScreenshot_20210911-121726.png

mppt charging
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3 Answers
Paul B avatar image
Paul B answered ·

The system does NOT use SOC to change charge states, all chargers use voltage and thus the MPPT also use's voltage and from your pictures you are reaching 14.23 volts and the mppt unit will then switch from bulk to absorbtion for 1 hr and hold that voltage then go to float (depending on battery selection settings), If you are unsing a BMV battery monitor then under battery settings in that unit change the charged setting to 14.1 volts and then the SOC will reset to 100% when ever the voltage reaches 14.1 volts

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So are you saying the battery is probably fully charged?


The BMV was set to 14.0v, now 14.1v.

Voltage shown on BMV is only 13.15v & 83%.

Never seems to reach 100%.

on the screen you sent it shows that the MPPT is reaching 14.23 volts so there for the batteries should also be at that voltage. 14 volts would also be fine for that battery setting.

so check why there is a difference between the battery connection on the mppt and the voltage at the battery.

maybe small wires , bad connections etc etc


I will check that, thanks
13.14v @ battery terminals
13.54 @ MMPT battery connections.

13.12v @ BMV700

Your battery is getting to 14.23 volts as per the Smartsolar, so you need to work out how. also why was there a hi of 15.20, Maybe your batteries BMS is turning off, and voltages are then spiking, there is nothing more I can suggest but you need to do the voltage test when the battery is at 14.2 volts

you are also draining the batteries very deeply as well as they are going down to 11.98 volts at times. I am presuming you are using lifepo4 . so also if the battery cells are unbalanced and the bms is turning off because one cell is over voltage, then this will trigger a hi voltage as well.

So check the system when its up at 14 odd volts and see why

1631344978810.png

1631344978810.png (83.5 KiB)

Again today the charger has gone into Float as it reached 14.6V at some point (battery setting).

The Battery at the terminals is only 13V, same at BMV & only 73% SOC.

Screenshot_20210913-122801.png

Maybe the wires from the MPPT to the battery are to thin/long and due to that there is a high voltage drop during charging.
nelsonpk avatar image nelsonpk Matthias Lange - DE ♦ ·
I actually checked that today & found a 1v drop.

The cable is about 6m long.

I was thinking maybe the settings need to be higher to compensate for the loss.

It has gone back into "Bulk" late in the day but too late for any boost, battery only at 78%

nelsonpk avatar image
nelsonpk answered ·

After watching the charger go through all stages of charging today I think I have the answer & solution.

The charger runs in "Bulk" untill the charger gets to the 14.6v Absorption setting & continues at that rate for 2 hours thereabouts.

Unfortunately due to the voltage drop through the 6 metre cable from the charger to the battery, the charge rate is only 13.11v so not really getting a good charge.

After the Absorption phase the charger goes to Float & runs at 13.6v which is only 13.1v at the battery so does pretty much nothing else all day.

This is the final voltage for the day & only gets the battery back to around 80%.

Solution:

Instead of changing the settings to compensate for the losses which could overcharge the battery, I will move the charger from the solar panel & position it next to the battery with minimal length cable.

This way the charger will deliver what it should & extract more from the solar panel, at present the power from the panel is being throttled back as it's seen to be not needed.



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Philip Barclay avatar image
Philip Barclay answered ·

An alternative would be to install a Smart Battery Sense on the battery. This will ensure the charge controller is seeing the true battery voltage rather than at its own terminals.

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Good option, I didn't know about this gadget