Pls help! How to choose solar Panel Watts for LiFePo4 Battery with 12.8V, 18Ah?

Hello Guys
I Really don't know how much solar pannel wattage is required for my LiFePo4 battery with 12.8V,18Ah capacity.
PFA for my battery datasheet

Please guys help me step by step to choose best Solar Panel Wattage for my Battery

I am basically going for off grid solar system for my college project
I am using LiFePo4 battery for energy storage and will be using ready made MPPT Solar Charger Controller
In my area I get sunlight for 8hours and
I will run my system for 9 hours on battery

I am going to use solar panel for charging the battery .
Guys I want to learn and know like how to safely charge my battery without damaging it from Solar Panel using MPPT Solar charge controller
like what's the formula?

Please anybody help me I am newbie to this field
I really appreciate it
Thank you

A whole bunch of assumptions, but try this.

18ah / 9 hours = 2ah/h device consumption

2ah × 8hours = 16ah to run the device while charging the battery

16ah + 18ah = 34ah total required power from solar

34ah / 8hours = 4.25ah/h

4.25ah × 12v = 51watts

51w + some efficiency overhead for your location = maybe 75w panel?

@aryan
Ok, pretty simple. Much of solar follows the standard electrical P/I*E "pie charts" if you've seen those.

P = Watts
I = amperage
E = volts.

Lets find out how much a "nominal" 12v solar panel is capable of under BEST conditions. How about we do a napkin-calc on a 30-Watt panel:

30w / 18v = 1.66 amps

The 18v reference is the "standard" voltage used when making calculations on nominal 12v panels, not 12! There are more details, but this should be good enough to make a decision. It also happens to be the reason we need a charge-controller, since a battery if allowed to reach 18v is not good.

The specifications you show are for a 6ah battery, but the key is that with the cells they are made of, they don't want to see any more than 0.5C input charge current. I'm assuming your 18ah version is the same.

Here "C" stands for the rated capacity. Sooooo... with your 18ah battery, the most current they designed to accept would be .5C, (18 * 0.5 = 9) or half the total rated capacity. In this case, no more than 9 amps of continuous charge current.

Huh, ok, back to the panels! Knowing the P/I*E example given above about how to figure out panel specs, lets work it a different way so we don't exceed 9A:

9A * 18V = 162 WATTS

(Again, 18v is the napkin-calc standard for panel voltage - don't confuse with your 18ah battery - that is just coincidence). So with your 18ah battery specifying that you can put no more than 9A charge into it, the max would be 162 watts. Nobody makes a panel wattage that specific, so you could drop that back to an easier to find 120w panel for a little less charge input.

How much is that? Do the calculation: 120w / 18v = ????

Of course it depends on your budget too, but this was just the way to figure out what the max panel wattage vs what the manufacturer says is the max charge current is.

Usage: you only have 18ah of storage to play with a fully charged battery. That is now up to you to measure your devices - during the timeframe you plan to use them - if they will exceed that. That means using either an ammeter on your multimeter to measure current draw (multiplied by your time usage), OR perhaps simpler, an inexpensive dc-clamp ammeter that clamps around a wire.

Recharge: Without getting bogged down into details, on a small starter project like this, you take advantage of LiFeP04's ability to handle larger input currents than most lead can. Here, the manufacturer states that you can charge at 0.5C, and when calculated out, shoot for the largest panel closest to our 162 watt calculation. Weather and other factors will dictate if you actually make it or not in a daily routine.

This should get you started.

A 100W panel would provide headroom in your stated plan and be very easy to expand down the road.

Make sure your mppt controller is actually an mppt and not one of the many $12-$50 PWMs with an mppt decal on it...
A decent actual mppt in a small capacity would usually start at about $60 for one like an epever that might last a while btw. A hundred bucks would let you also have the junk you need to monitor solar output and such performance.