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Absorption-float settings for Growatt 3000

apctjb

Solar Enthusiast
Joined
Jun 16, 2020
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Finally have my solar installed and playing with PV charging settings.

The Growatts have 3 step charging; bulk , absorption, float. You can set the absorption voltage and float voltage, (but not current or time.)

I have a 2P16S battery (48V) 560AH with Chargery BMS. I started by setting absorption voltage to 56.4 and float to 54.4. My PV array can supply 120A of charge current and I found that prior to reaching absorption voltage (56.4), which is >99% SOC, I had one cell come up to 3.65 triggering the BMS high voltage cutoff.

So I reduced the absorption voltage to 55.4 and same thing eventually happened No doubt I would benefit from another top balance but regardless, pushing 100A into a +99% fully charged battery and inevitably one cell is going to fill up first.

No doubt I am confused but it seems like with LiFePO4 batteries it would be better to set the absorption setting below float ( the setpoint where bulk (CC) charging stops and CV begins and current tapers down ) and allow float to top off the battery at much lower C rate. In other words set absorption at 54 and float at 56. That would avoid pushing full current into an almost fully charged battery and let the float voltage finish it off a ever decreasing C rates.

(Note that the Growatts automatically try to power loads from the array, even if you are in float with little current going to the battery. So you can be pulling 100A for loads while putting in a few amps of float)

So has anyone tried this? Does it work or am I missing something ...



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One thing I want to point out is that the chart in your post is for a lead acid batteries not LiFePO4. While those settings will work for a LiFePO4 battery, they are not optimized and can damage LiFePO4 batteries. Float charging was invented to keep lead acid batteries charged because they have such a high self-discharge rate. LiFePO4 batteries do not suffer that problem so you don't really want to use Float charging for this chemistry, or use it with a very good understanding of how the chemistry works. For instance float charging LiFePO4 batteries at the top of their charging curve can ruin them (at or over 3.65v). Any good battery charger will adjust the current based on the battery chemistry, the charger can tell when the battery is in bulk or absorption.

I would recommend you set your float voltage lower than the absorption voltage so it never comes on. LiFePO4 batteries need to be top balanced well, then used in the flat part of their charge-discharge curve. I will be using 3.5v as the max per cell shutoff (56v for the 16s pack) and 3.0v per cell for the discharge cutoff (48v for the 16s pack).
 
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