Voltage mismatch

electrictrauma
electrictrauma Registered Users Posts: 10 ✭✭
in Advanced Solar Electric Technical Forum #1
I have an inverter that will only charge up to 60 volts. My battery config is 8s so max voltage is around 68. What is a work around to get the most out of my batteries? 
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #2
    make and model of your charger ?
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • electrictrauma
    electrictrauma Registered Users Posts: 10 ✭✭
    #3
    Its an outback skybox
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #4
    What type of batteries (sounds like 6 volt * 8 batteries = 48 volts nominal).

    For most Lead Acid chemistries, 62-64 volts is fairly aggressive charging (or equalization charge).

    I am guessing you are in central California(?) and have warm summers and mild winters (unless your system is in the mountains).

    For our friends in the great white north, running a battery bank well below freezing can require >62-64 VDC for nominal charging voltages:
    • -0.005 volts per cell per C (26C - -16C) * 24 cells = +5.04 volt charging voltage offset for a -16C (3F) 24 cell (48 volt FLA) battery
    • 59 volt charging (equivalent to 14.75 volts @ 12 volt) + 5.04 volts 64.04 volts Absorb Setpoint charging
    There are some industrial models of FLA batteries that may need more than 62-64 VDC to charge or float charge. And cold weather for our Canadian friends has been an issue for cold weather charging... Many typical (good quality) AC inverters have a high battery cutoff voltage of 15.0 volts @ 12 volts (or 60 volts for a 48 volt battery bank). And they have had to shutdown their inverters when charging >15.0 volts.

    If the Skybox inverter-charger module is limited to 60 VDC... That is very sad. We have tripped over the 60VDC limit in cold weather (and with some industrial batteries needing 62-64 volts for corrective charging at room temperature) many times here... I would have hoped a new system design from a major manufacturer would have done better.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • electrictrauma
    electrictrauma Registered Users Posts: 10 ✭✭
    #5
    No they came from an industrial install, each rack has two sets of 8 lithium ion in series, so 68 volts and 68 amp per rack. I’m in the bay area too. I admit I was shocked, just assumed it would be versatile since it was new. I can try to get them to mod the software but not holding my breath. Barring that there must be a clever way to do it. Anyone?
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  • electrictrauma
    electrictrauma Registered Users Posts: 10 ✭✭
    #6
    I had a thought about running it 48 volt with 4 in series and then putting a fifth in series an connect the five batteries to the lithiums and charge the lithiums with a voltage regulator converter from ac, not sure the current would transfer fast enough.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #7
    7 modules in series? Gives you 59.5 VDC (and a reduction in bank capacity by 1/8th).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • electrictrauma
    electrictrauma Registered Users Posts: 10 ✭✭
    #8
    Might work Bill, haven't really gotten into the racks other than to see there are two wired in series. Would be a pain to take them apart and they have 8s bms but its a thought.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    edited August 2020 #9
    What type of Li-Ion battery has 7.5v cell voltage?  14s or 16s would be more reasonable. There are likely several hardware reasons the inverter cannot go over 60vdc.

    There is an additive to Li-Co that takes their max charge to 4.4 v/cell.  For 16s that would take max charge to 70.4v and 14s to 61.4v.
    More common are 4.2v and 4.3v / cell max charge. LFP are 3.65v/cell.  They don't have to go that high unless you are trying to get the fastest recharge time.

    You should find out exactly what type you are dealing with.  I personally would not have high AH's of LiCo batteries in my house.

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  • electrictrauma
    electrictrauma Registered Users Posts: 10 ✭✭
    #10
    They are racks of two 8s connected in series. Took one out and seems to be working fine at the 60v max of the inverter.
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