Adding batteries to existing battery bank.

yrtrnc
yrtrnc Solar Expert Posts: 54 ✭✭
in Off Grid Solar & Battery Systems #1
Hi guys. 

I have 12 x Victron 100 Ah 12.8 V lifepo4 batteries wired as 51.2v 300 ah battery bank. They have been in use for about 6 months. I have around 90 charge cycles on them.

DO you think it would be OK to add 4 more batteries to existing battery bank to make it 51.2 v 400 ah? If yes how do you suggest I go about it? 

Regards 
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    I don't think it would be an issue... HOWEVER, make sure that the old/new battery bank voltages are just about identical. If one was at 50 volts and the other resting at 52 volts, there is a good chance that you would get a very high charge/discharge current between the two battery sets.

    Charging each battery set separately to the "same" resting voltage, then connecting should be fine (l.e., probably even 0.010 volt difference between the two banks could be an issue).

    There are other ways to "skin this cat" (discharge one bank to match the other, use a lightweight length of cable between the two banks (to limit charging/discharging currents through, for example--not verified--20 feet of 14 AWG cable) until they match voltage) or other means are possible.

    The one thing I know nothing about it the BMS system (if present) and what its limitations may be (bank AH capacity, how to make connections, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • yrtrnc
    yrtrnc Solar Expert Posts: 54 ✭✭
    #3
    Thanks Bill

    Sorry forgot to mention that there is a BMS. It's limited to 500 ah bank for the 100 Ah batteries. So that should be fine for 400 ah bank. 

    Not sure how I'm going to manage balancing all batteries as I am off grid and reliant on existing batteries. I wouldn't be able to take it offline for long. My idea was to charge the new batteries one by one then add them when the bank is fully charged.

    Regards 
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    edited December 2019 #4
    If you are bolting single cells into each parallel cell group, then matching by cell is important.

    If you are paralleling at the 51 volt series level, then only the overall voltage match is important (for paralleling connections).

    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • softdown
    softdown Solar Expert Posts: 3,885 ✭✭✭✭
    #5
    Piece of cake.
    First Bank:16 180 watt Grape Solar with  FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries
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  • yrtrnc
    yrtrnc Solar Expert Posts: 54 ✭✭
    #6
    Hey Bill

    OK so I'll switch off inverter charge each set separately through the charge cont. and once all are same connect them together to form the bank.

    How long should they rest before I can take a reliable reading? 

    Regards 
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #7
    I don't know, but I believe that Li Ion batteries generally do not have a "surface" charge like you can see on Lead Acid... Once the cells are close enough in voltage (if cells, probably on the order of a few 0.001s volts apart, they should be good to go.

    If I recall correctly, if you are busing cells, if they are >0.100 volts apart, that is too much spread and can cause high current flow as they "balance"...

    But verify--I am not a battery engineer/expert by any means.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited December 2019 #8
    A new LFP battery will not be fully charged, they are stored at partial state of charge usually ~40%, adding it to a fully charged bank is not a good idea, the cascading current into the partially charged one may cause permanent damage. 

    The voltage of all the batteries need to be equal before connection, preferably within 100mV or 0.01V
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • yrtrnc
    yrtrnc Solar Expert Posts: 54 ✭✭
    #9
    mcgivor said:
    A new LFP battery will not be fully charged, they are stored at partial state of charge usually ~40%, adding it to a fully charged bank is not a good idea, the cascading current into the partially charged one may cause permanent damage. 

    The voltage of all the batteries need to be equal before connection, preferably within 100mV or 0.01V
    Thank you for pointing this out. I will fully charge the batteries individually to balance cells before adding them to the bank. 
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  • tuchmybuttercups
    tuchmybuttercups Registered Users Posts: 1 ✭
    #10
    Following up on this as I'm interested in adding batteries later on.  How did things turn out?
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