Modelling batteries  


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 Lithium and VRLA batteries requie different models. How does Homer manage these difderences

VRLA battery has three stages of the  recharge; bulk charge, absorption charge and float. During bulk charge intellegent rectifiers are in constant current mode. As absorption starts  rectifiers enter constant power mode until recharge current is near zero. Once this point is met float occurs. Absorption phase can be 3-4 hrs in a 4000Amphr battery giving little time for a pure solar site  to recharge  the battery after a 50-60% discharge and then to complete the absorption stage at which time the battery is fully charged. 

How is the absorption phase modelled in Homer?

  • Lithium batteries do not have an absorption stage so how do we run  and compare two models each using different battery  chemistries

 

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Hi Kevin - great question. 

HOMER has several different battery models to be able to accurately represent the different chemistry / function. 

The "idealized model" is used for Li-Ion batteries, and assumes a flat capacity curve.

The "kinetic model" is used for Lead Acid batteries, because it models the battery as if it were two storage tanks - an 'available' and a 'bound' storage tank, which are connected by a small connection 'pipe'.  This effectively reduces the capacity at high charge/discharge rates. Like you said, after a battery has been discharged, it takes some time before the battery is functionally fully recharged. Let me know if that doesn't make sense.

Additionally, the most accurate/complicated "modified kinetic model" can be used for either type of battery and can include rate dependent losses, temperature dependence on capacity, cycle lifetime estimation, and temperature effects on calendar life. 

And, just so you know, there are two other storage models: one for flywheels and one for a storage system where the cell stack and electrolyte can be sized and replaced independently. 

Hope that helps,

Steffi

 

 

 

 
  
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