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Battery Backup time


waqar85

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Greetings!

I have a sample question if anyone can help kindly reply.

Let say my battery bank Wh is 7200wh (12x150Ahx2 +12x150Ahx2)(2 in series and 2 in parallel) 

Installed solar panels = 1650wts (3x275 + 3x275)(3 panels in series in each row and 2 rows in parallel)

sun hours = 4

charge controller 50Amp(MPPT)

so what will be my battery bank charging time?

Is it simply dividing battery Wh/Panels wts?

and if i want to decrease battery charging time than what i need to increase solar panels only? 

 

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Welcome to the forum. 

4 hours ago, waqar85 said:

Is it simply dividing battery Wh/Panels wts?

No, its not that simple. It will depend on more factors than those two alone. 

  1. How much did you use during the previous night. (For interest sake, how deep do you allow your batteries to discharge?)
  2. You can only expect about 80% of that 1650 whp to reach the batteries after all the losses. 
  3. The charging current (Depending on your manufacturers recommendation) 
  4. Lets forget about charging efficiency and temperature ext for now. 
4 hours ago, waqar85 said:

if i want to decrease battery charging time than what i need to increase solar panels only? 

Adding panels will help to increase the charging current, but only until you reach the max charging current allowed for your type of battery, after that increasing the charging current more, will damage the batteries.

Can you please give us more info  on the batteries so that we can give more accurate feedback. 

  1. Type and Model
  2. some usage history might help. 
  3. bulk and absorb voltage settings. 

 

Do you have some kind of battery monitor that can tell you the exact state of charge? 

 

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5 hours ago, waqar85 said:

so what will be my battery bank charging time?

As a rule of thumb a lead acid battery should be charged at no more than C/5 (its capacity divided by 5), and that is a VERY stiff charge current that I will not advise for sealed batteries. A C/10 charge rate is better.  In addition, once the battery reaches around 85% SoC (absorption voltage) it will begin to accept less charge and the recharge process will slow down. So much so that whatever time you need to 85%, you will need almost the same amount of time to get from there to 100%.

The moral of this story is that if you completely empty a lead-acid battery, you can't get it fully recharged in the same day. Not enough sunlight hours.

So what @Jaco de Jongh says is true. It depends on a lot of things. How deeply you discharge them, how quickly you are allowed to recharge them, how efficient they absorb charge once you reach the absorption level, and so on.

If you're using Lithium cells... well then you can usually charge as high as C/2 (empty to full in two hours), and then the math becomes very close to just dividing the battery capacity into the rating of the PV module and putting up enough modules 🙂

 

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