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Smaller than 100Ah batteries for 48V system.

Quankl

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Apr 10, 2022
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My question is whether the recommended specs for growatt/mpp which is typically 100/200Ah of storage is essential.

Where I am is 85+% sunshine for a good 5-8+ hours depending on year in the Caribbean.

Batteries are the hardest part to obtain here. Can I settle for 4, 12v 20Ah batteries for about 1KW of storage to supplement by daytime usage in the event of cloud cover? I know this will be best to useless at night but until I can afford some server rack batteries.

This is my typical day almost 300/365 days a year. Pure sun.

So my question is, is 48v 20ah a definite no for these 48v inverters or is it just a major inconvenience to have
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Research into the phenomena called "ripple current", which is in part mitigated by battery size. I would be concerned that a battery that small would produce levels of ripple current high enough to potentially damage the electronics of your inverter?
 
I feel your pain. Nobody ever complained about scraping sunshine off their windshield, you know.

How much power do you consume during the day might be relevant to the discussion. Otherwise, I think you nailed it. Not a lot of reserve power. The inefficiency of your inverter will probably kill you as much as anything with only 20Ah of capacity. At least it is a buffer during those occasional wisp of things you call clouds. Others may have more insight.
 
For the day time usage im looking to use a AC split unit at home. Thats basically it along with a desktop, television and standard draws(Router, modem, radio). At night I would use the grid input on the inverter and use grid power at night until batteries arrive. (Once I research this ripple current thing to be sure)
 
For the day time usage im looking to use a AC split unit at home. Thats basically it along with a desktop, television and standard draws(Router, modem, radio). At night I would use the grid input on the inverter and use grid power at night until batteries arrive. (Once I research this ripple current thing to be sure)
Start out with an itemized list of what needs powering, and how many watts they will consume. A very good rule of thumb I've always used is the 2X rule. That is have 2X the amount of panels as to what your load is. Same thing applies to your inverter, and also generators.

So, let's say your air-conditioner consumes 1500W while running. To keep it stably running, you should have at least 2X that wattage in panels, or 3000W. Same with the inverter. Let's say your max itemized usage is 2000W. Get at least a 4000W inverter.

Here is what the biggest problem for you will be. Anything running on an electric motor, such as a refrigerator, pump, compressor, or air-conditioner, will have what's called "starting surge" or "inrush current draw". It typically lasts only for 500 milliseconds or so, but the starting surge can be 3-5X the running wattage. My well-pump runs on 2000W of power, but at startup pulls 9100W, measured with an inrush current meter. At's ~4.5X.

Usually, every single watt the panels are putting out, goes to supply the startup surge, but in your case, with such a tiny battery, I'm not sure the system could supply the starting surge. What you may find is that the system functions to power the lights, and the computer, but when the air-con switches on, the inverter goes into shutdown.
 
Remember that those 48v MPP and Growatt units need 1500Wh of power a day just to exist and operate. 48v @ 20Ah = 960wh - 10% battery reserve the BMS likes to hang on to = 864Wh. 864Wh / 60w = 14Hrs of battery life for the unit to exist. That just barely makes it through the night with 0 load.

If it makes life easier, look into Marine Deep Cycle lead acid batteries. Yes, they're only good for 1/2 the rated AH capacity but they're also readily available pretty much anywhere in the world. On the mainland you can get a WallyWorld DC29 120Ah battery for about $100 out the door good for 60Ah or 3x what your little LFP's are capable of. Yes, they take more space and are heavier, but the Island Tax isn't nearly as bad and shipping is usually included in the price at your local hardware/boating supply store.
 
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