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Hello
I would like to use Node Red to change the absorption voltage of the MPPT depending on the SOC.
So
SOC less than 50% , Absorption Voltage= 13.64
SOC greater than 50% , Absorption Voltage= 13.37
To keep my 12V lithium iron battery in a state of charge around 50%, because that is best according to the data sheet, is that even true?
Node Red is already running on my Cerbo GX.
Which code do I have to enter now?
Thanks for your help
Sam
If you are storing your battery then yes choose those voltages. But if not charge them properly.
The answer as to the voltages chose, is going to rather depend on where the BMS keeps the cell balanced.
If balancing does not occur at the voltage you set then it is no good.
And then why? Use it or loose it.
Lifepo4 calendar age and age in cycling. So use them as the BMS allows. Use and charge it how the manufacturer recommends. Keep the warranty.
Hello
Thanks for the quick reply.
The battery has a Daly BMS.
And is freely programmable.
The BMS balance from 3.2 volts per cell, so 12.8 volts.
And a difference between the cells of 0.02 volts.
The cells are so good that the difference has never exceeded 0.02 volts after 6 months of use. So the BMS has never had to balance.
I have only noticed when I charge the battery with over 13.5 volts. (The battery has 12V/280Ah.) It is at over 95% SOC. A cell difference of 0.017 volts.
However, if I charge with 13.45 volts up to 98% SOC, the difference remains below 0.01 volts.
I only need about 300 watts per day for video surveillance when I'm not there. And the solar panels deliver 1000 watts per day when the sun is shining.
That is, unless the weather is really bad for two weeks. The battery is sufficient when I'm not there.
That's why a charging voltage of 13.37 volts would be enough, because normally it's only a week of really bad weather.
And at 13.37 volts, the battery fluctuates between 65 and 75% SOC.
Only if the weather is unexpectedly bad for two weeks would it be good if the absorption voltage were to go to 13.64 at below 50% SOC, because then the battery charges faster.
If I only charge with 13.37 volts, the cell difference remains at less than 0.005 volts.
Of course the battery will last forever when in use, but when I am there and connect power tools, it is also very demanding.
And I don't believe that a lithium iron battery only has a service life of 20 years, but much longer, perhaps even 50 years, if you treat it so well.
Because I built an electric bike with Headway lithium iron batteries 14 years ago. And I usually drained the battery with 2C. And charged it with 14.6 volts in an hour, and after 1000 cycles and 10 years, the battery was still almost as good as new. Unfortunately, the bike was stolen. So I believe that lithium iron batteries are even better than many believe, even if you treat them badly, just why not try to get 50 years out of them. We will see.
It also took me a long time to realize that the battery is charged with 13.45 volts, almost up to 100% SOC. Just with less amps. But that's OK for me.
Because when I'm there and need a lot of power, I manually increase the absorption voltage to 13.64 volts, which is completely sufficient for my solar panel with a real maximum output of 210 watts.
Have a nice evening
Sam
I have set the Absorption voltage to 13.44 V and set a fixed absorption time of 14 hours.
Now the battery fluctuates between 80-90%.
The battery voltage fluctuates between 13.28V just before sunrise and 13.44V in the afternoon.
The cell voltage difference is always below 0.005 according to the Daly BMS.
I opted for this setting because it is supposedly no problem to charge the LiFePo battery at a low voltage up to 90%.
Because if I had let it fluctuate around 50%, as recommended by the manufacturer, I would have had to increase the absorption voltage to 13.64V when I was there for the weekend.
Which is inconvenient and perhaps more damaging.
Although I have to say that my goal is for the battery to last 50 years.
Which is perhaps possible with these settings.
I find it a pity that there is little information on the subject of the durability of LiFePo.
Because I can't prove whether these settings are really the best.
