20Ah lithium pack

It does really depend on your needs and where the system will be located... Li Ion batteries are "nicer" than lead acid because there is no minimum charge current for Lithium, and those batteries can take very high charging currents vs Lead Acid.

But, the old 5% to 13% rate of charge rule gives you a sufficient charging current/energy to charge during the day and use much of the battery capacity at night... 5%--If you discharge the bank from 90% to 20% state of charge--That is 70% of capacity, 5% would take about "14" hours of full sun to recharge (most places get around 3-5 hours of "full sun" per day (except during winter).

70%/13% rate of charge = 5.4 hours of full sun to recharge (one very sunny summer day).

But your energy needs are, will affect the size of the array (charge during the day, use power at night... Or charge/use power during day, etc.). Also, if this a a portable operation, you don't want to all around large "glass windows" on a hike...

The math would be:

• 20 AH * 14.5 volts charging (or whatever voltage your bank takes) * 0.70 capacity * 1/0.77 panel+controller derating * 0.05 rate of charge = 13 Watt panel minimum
• 20 AH * 14.5 volts charging * 0.70 capacity * 1/0.77 panel+controller derating * 0.10 rate of charge = 26 Watt panel nominal
• 20 AH * 14.5 volts charging * 0.70 capacity * 1/0.77 panel+controller derating * 0.13 rate of charge = 34 Watt panel "typical" cost effective maximum

And then there is sizing the system based on your location and weather conditions. For example, say of fixed panel pointing south at an angle for good average solar harvest:

http://www.solarelectricityhandbook.com/solar-irradiance.html

Bengaluru, central India
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 77° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
6.41	6.79	6.61	5.87	5.82	4.85
Jul	Aug	Sep	Oct	Nov	Dec
4.34	4.27	4.73	4.79	5.13	5.74

Say you want to use 70% of capacity of the battery bank over night and recharge it the next day in January...

• 20 AH * 14.5 volts discharge * 1/0.61 DC Li Ion system eff * 1/5.74 hours of sun (Dec Average) = 82 Watt panel minimum

And if you want to charge the battery bank in two hours of sun...

• 20 AH * 14.5 volts charging * 0.70 capacity * 1/2 hours charging rate * 1/0.77 panel+controller losses = 102 Watt panel

With solar power systems, the details matter (battery charging voltage, AC inverter or DC direct energy usage, location, weather, your energy needs). The above gives you some typical ranges of how "average people" may use the solar power + Li Ion battery bank.

While Li-Ion batteries are very light and can store 2x more power in the same space as a lead acid battery--They are also very sensitive to over charging and over discharging... Depending on the chemistry, you can ruin the battery to having a battery catch fire/explode.

Most good quality Li Ion battery banks have some sort of cell or battery monitoring system (BMS) that is designed to protect the cells from damage or entering unsafe state of charge/voltages.

There are a lot of different Li Ion battery chemistries out there... They have different charging and operational voltages, different levels of maximum current flow, and most do not like to operate below freezing temperatures. LiFePO4 (lithium iron phosphate) Lithium batteries are generally considered the "safer" type of batteries... But what you have, I do not know.

And the above panel sizing is based on "break even" calculations (average sun recharging 100% of your use in X number of hours/days of sun). Some days you get more sun, some days you get less... If you "need" XX AH @ 12 volts every day, you should almost double the array size (to allow for a bit of hazy/cloudy weather, etc.). Even just some haze of overcast, you can cut your predicted harvest by 1/2.

Hope this helps. and good luck!

-Bill

Being that you are (probably) in India, what you can get local/online is probably different than what we would get here.

In general, PWM controllers are simpler and smaller. And for smaller systems, a PWM controller + ~80 Watts of panels is probably going to be much less expensive.

MPPT controllers tend to be larger and have a lot more options/functions (memory of energy harvest, computer interface, some even have Internet interfaces).

An 80 watt panels will be (80 Watts / 18.0 Volts Vmp) = 4.4 amps Imp (Voltage maximum power; Current maximum power)... So, you do not need a very big controller, which means not very many MPPT controllers out there.

I will suggest that you look at what controllers are available to you and see what features fit your needs. Also, most (all?) LFP batteries do not use temperature compensation for charging voltage--Zero temperature compensation (lead acid batteries need a lower charging voltage as they get hot, so the charge controllers typically have a --0.005volts per degree C per Cell reduction over 25C).

You should find a charge controller that lets you set T-comp to Zero or can have T-comp disabled.

Also, some manufactures will "conformal coat" (pour/dip liquid plastic over the electronics) to seal out dust/moisture/bugs (and give better vibration resistance) (this does make it more difficult to repair the electronics, if ever needed).

You can look at a good selection of controllers we have in the US from Northern Arizona Wind & Sun. They have good quality hardware, plus service and support. But they are not local to you:

https://www.solar-electric.com/residential/charge-controllers.html

Look at MorningStar controllers... They have a very wide range of produce from smaller PWM&MPPT controllers to high end MPPT controllers. Many of their mid to high end products do support computer interfacing and you may find a smaller controller that supports programming of Vbatt and T-Comp settings.

Midnite Solar would be another to look at... They have a couple smaller controllers in PWM and MPPT design.

For example, here is a very nice PWM controller from Midnite--But does not have the ability to turn -T-Comp off. and only a couple of charging profiles optimized for Lead Acid batteries:

https://www.solar-electric.com/lib/wind-sun/brat_manual.pdf

You probably will find a Chinese or locally available controller that will support your LFP battery... And you can go back to your battery supplier to see if they have any suggestions.

Again, you have to make your decisions based on your needs... Small/lightweight portable system. A permanently installed system with solar panel(s) > 3-10 meters away from the charge controller... Etc.

Making sure you have some basic tools too will help. I highly suggest (at the very least) an AC/DC Current Clamp DMM. There are many AC only current clamp meters, and I suggest the DC capable clamp meter--These are very handy to understand what is going on with the charging/discharging of your system and helpful if any debugging is needed down the road.

To give you a couple of ideas of the type of meter I am suggesting (pick the vendor and meter that meets your needs):

https://www.amazon.com/gp/product/B07546L9RT (inexpensive but good enough for our needs)
https://www.amazon.com/gp/product/B019CY4FB4 (mid-range meter)

There are lots of panel mount meters out there these days that do Voltage/Current/Power/Energy on very nice displays if you wish something like that.

-Bill