Non identical amperage batteries in series.

spacebass
spacebass Registered Users Posts: 87 ✭✭
So what happens if You have say 2 x 210 ah 12 v batteries in series and You add in series 2 x 100ah batteries 12v from the same manufacturer from the same range looking like the identical technology (cells) but smaller? 

Comments

  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    Short answer is they will be destroyed, the lower capacity will overcharge the higher undercharge.
    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.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Are you making a new 48 volt bus, or just adding  AH capacity to the existing 24 volt bus?

    In series (48 volt bus), the AH capacity of the 48 volt battery bank is the capacity of the smallest AH battery or cell. 210 and 100 ah batteries all in series, the capacity of the back will be 100 AH @ 48 volts.

    If you add 210 ah 24 volt string in parallel with a 100 AH starting of otherwise identical batteries (brand, model, and chemistry), more or less you will have a 310 AH @ 24 volt battery bank.

    The above are the "first order" effects of series or paralleling battery strings.

    The second order effects are many too--And as McGivor says, those include the issue that larger cells have higher leakage current and when you put the big and small AH cells in series, the smaller cells need to be "over charged" to get enough current to properly charge the larger AH cells in series.

    Think of placing a flashlight battery in series with a car battery... The little cell will probably be cooked by charging/discharging current from the larger series cells.

    Also, different construction (like flooded cell vs AGM/GEL) behave differently... Then you get into Li Ion, which can do vary dangerous things if mixed/matched/operated incorrectly...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • spacebass
    spacebass Registered Users Posts: 87 ✭✭
    I am planning to add two more 210ah 12 v batteries for 48 volts but the question crossed my mind , what actually defines a battery unit against a number of new similar  cells ?.

    I have been to the panel controller matching tool you kindly advised the board of Bill, and it seems like most likely 2 and 3 panel stings and paralelled strings  are OK with the 150 watt 60 amp controller with my set, place etc.

    I have added a 100 amp 500 volt DC breaker between the controller and batteries is this OK ? I can find nothing about defining voltage in a DC breaker .

    Thanks again. 

  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    The "battery unit" would be the nominal 2v cells.  Packaged as a 12v battery, a 100ah 2v cell will be ~ 1/2 the size of a 200ah cell. 

    As Bill said, putting the smaller cells in series with larger will effectively make it a 100ah bank.

    The breaker voltage spec is maximum.  Lots of DC breakers are designed for RVs etc and have a max of (eg) 30v.  Some are dual use AC/DC with much higher AC voltage ratings, but can be used for DC at lower voltage.

    Maximum breaker size should be in your controller manual.  
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    Definition of a battery: A battery is a combination of like cells connected in a series arrangement to obtain a nominal voltage, the cells should be of equal capacity in order to work in conjunction with one another. The cells  may be part of a " battery" in the form of a monoblock, however all monoblock batteries used in series should be of equal capacity to ensure all cells are also of equal capacity.

    Mixing cells, or batteries containing cells of different capacity is not a good practice, additionally if they are of equal capacity  they should also be of similar or equal  age if combination is considered, especially with lead acid as older cells, or batteries, in the case of monoblock  tend to draw te the remainder to the lowest common denominator, the weakest cells  or in the case of monoblock, the weakest battery  whatever the case may be..

    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.
  • spacebass
    spacebass Registered Users Posts: 87 ✭✭
    Estragon said:
    The "battery unit" would be the nominal 2v cells.  Packaged as a 12v battery, a 100ah 2v cell will be ~ 1/2 the size of a 200ah cell. 

    As Bill said, putting the smaller cells in series with larger will effectively make it a 100ah bank.

    The breaker voltage spec is maximum.  Lots of DC breakers are designed for RVs etc and have a max of (eg) 30v.  Some are dual use AC/DC with much higher AC voltage ratings, but can be used for DC at lower voltage.

    Maximum breaker size should be in your controller manual.  

    Yes the manual specifies 100 amps but does not specify any volts for a novice thicko 500 volt breaker for 24 volt battery bank raises questions.

  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    The breaker voltage limit is to do with the dielectric strength along it's ability to quench an arc, higher voltages have the ability to sustain an arc over a greater distance. The same applies to fuses, if a lower voltage rating is used there is the risk that even when tripped or blown, current could still flow with the arc spanning the gap, heat generated by the arc can also create a fire hazard.

    When selecting either a DC breaker or fuse, the voltage rating should be equal to or greater than the maximum voltage applied in the circuit and have a DC rating, AC only rated devices rely on the voltage being at zero potential every half cycle, or every 30th of a second in the case of 60Hz which dramatically reducing the arc potential and therefore no suitable for DC useage.
    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.
  • spacebass
    spacebass Registered Users Posts: 87 ✭✭
    mcgivor said:
    The breaker voltage limit is to do with the dielectric strength along it's ability to quench an arc, higher voltages have the ability to sustain an arc over a greater distance. The same applies to fuses, if a lower voltage rating is used there is the risk that even when tripped or blown, current could still flow with the arc spanning the gap, heat generated by the arc can also create a fire hazard.

    When selecting either a DC breaker or fuse, the voltage rating should be equal to or greater than the maximum voltage applied in the circuit and have a DC rating, AC only rated devices rely on the voltage being at zero potential every half cycle, or every 30th of a second in the case of 60Hz which dramatically reducing the arc potential and therefore no suitable for DC useage.
    It is a DC breaker, thanks!