Lifepo4 boost voltage max/recommended time?

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Fabian
Fabian Registered Users Posts: 131 ✭✭
in Off Grid Solar & Battery Systems #1
What is the max and recommended boost time i should use to charge my Lifepo4 battery. I currently have a 24V 100ah Redodo Lifepo4 battery running on my solar system. At first my mppt is charging in bulk/mppt mode after which it switches to boost mode when it reaches a certain voltage.
I currently have it set to 20mins for boost mode. I'm hoping i can push it to at least 1hr or 2hr. I have reduce the boost voltage to 27.4v because i don't want to charge up to 100%. I want to make 90% my max charge at all times to achieve a lot more cycles out of the battery.
Even thou i set the boost to 27.4v on my controller, it reach to 27.2v and then begins to display boost and basically stuck there and won't reach up to 27.4v so i am guessing is because the boost time is too low because after the 20mins it changes from boost to float and still remains at the 27.2v and i have no option of turning off the float mode. So i am assuming if i increase the boost time to a much longer time then it will evenutally reach up to 27.4v and remain at that voltage.

Please advised me on the best move forward. I am currently using the Renogy Rover 100amp mppt Li charge controller and i have it on the user mode settings where i can adjust the parameters.


Best regards,
Fabian

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  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    #2
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    You didn't say what your bulk voltage is on your MPPT. I'll just assume that your charge profile is: bulk 27.4V, boost/absorption 27.4V, and float 26.8V (those would probably be reasonable to use for LiFePO4, although your bulk/boost voltage could be a little higher, like between 27.6V and 27.8V.)

    My guess is that there is enough current going into the battery that you are seeing a 0.2V drop between the MPPT and the battery. So the MPPT thinks it has reached 27.4V but the battery is only seeing 27.2V.  For an LiFePO4 battery, the current will drop pretty quickly in boost mode once the battery approaches 100% SoC. That would cause the voltage drop between the MPPT and the battery to decrease, so the battery would get to 27.4V.  It would be ideal if your MPPT had a "tail current" setting, where it would switch from boost to float. However, looking quickly at the manual for your MPPT it looks like it doesn't have a tail current setting, so it will just hold the boost voltage for the amount of time set.

    You can probably safely increase your boost time by a bit. Try setting it to 40 minutes. If you have a way to measure it, see what the current going into the battery is while in boost. You'd like it to be down to 2A or so before you go into float. 

    If your MPPT allows you to set the bulk voltage separately from the boost voltage, try setting the bulk up to 27.6V while leaving boost at 27.4V. That will allow the MPPT to get a little further up the voltage curve before it switches into boost, which will in turn get your boost stage close to done.
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
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  • Graham Parkinson
    Graham Parkinson Registered Users Posts: 160 ✭✭✭
    #3
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    Isn't the temporarily higher voltage of "Boost Mode" intended for stirring the electrolyte in lead acid batteries to avoid stratification?

    Worth stepping back a minute and thinking about what is limiting the charge current in your LiFePO4 charge cycle.  Sounds like it may actually be the resistance of your battery charging circuit creating a voltage drop that limits charging current.  Wouldn't it be better to have lower resistance wiring instead that allows charging without having to overcome a transient voltage drop, a voltage drop that can suddenly become unwanted high voltage as battery acceptance tapers off when nearly charged?

    Using "Boost Mode" with LiFePO4 seems to me potentially dangerous.  Imaging you have an almost fully charged battery that kicks into Boost Mode and the time window is set long enough to drive charging voltage up over the upper limit voltage knee?

    Maybe there are others who know better but Boost Mode doesn't seem anything but a not really appropriate "kludge type fix" in this case.  It may be useful, but isn't really the right fix?

    Offgrid in cloudy PNW

    MacGyver'ed museum collection of panels, castoff batteries and generators - ready for state of art system install .... parade of surviving and dead generators: H650, Ryobi 900, Briggs and Scrap Iron 2000, H2200, H3000, Kubota 3500, Kubota 4500, Onan 7500

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  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    #4
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    The Renogy manual is pretty clear that what they are calling the "boost" stage is what most call "absorption". I've seen that with a few other SCC manufacturers. So substitute absorption in the OP's post, as well as mine.
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
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