# First solar project - using 20Ah battery to charge laptop and a few iPhones and GPS

Registered Users Posts: 16 ✭✭
I started reading this forums a few weeks ago and I think to really get a handle on it I should jump in and get my feet wet with a project.

I want to keep it charged with a solar panel and use it to keep my laptop, iPhones and gps charged.

The laptop uses a 110v 18W charger
iphone and gps can be charged using USB. I was thinking about using a usb car charger wired to the battery. would that work?

I have a 12V 20Ah battery pictured below that I can use for this project.

for panels I'm thinking using one of these. Which panel would keep the battery charged? I am in southern California sun.

80 Watt - 129.00
60 Watt - 79.90
40 Watt - 55.00

please recommend a good controller that has some room to grow and an inverter that will work for my needs.

this should be fun to do.

Re: First solar project - using 20Ah battery to charge laptop and a few iPhones and G

Welcome to the forum Smoothisfast,

Using typical rules of thumb for charging of 5% to 13% with a 20 AH 12 volt battery, assuming 0.77 system derating for charging and 17.2 Vmp for your panels:
• 20 AH * 1/0.77 derating * 17.2 Vmp * 0.05 rate of charge = 22 watts minimum
• 20 AH * 1/0.77 derating * 17.2 Vmp * 0.10 rate of charge = 44 watts nominal
• 20 AH * 1/0.77 derating * 17.2 Vmp * 0.13 rate of charge = 58 watts max cost effective
You could go with a larger panel like the 80 watt too... It will pretty quickly recharge the battery.

Also, make sure you have a battery charger with a sealed battery charging setting. Or else you will boil the battery dry pretty quickly.

Say you use the 60 watt panel, for a typical 5 hour sunny summer day with fixed array tilted at latitude, you will collect around:
• 60 watts * 0.52 end to end system derating * 5 hours = 156 Watt*Hours a day (min-average)
Inverter wise, for a longer battery life, I would suggest around C/8 rate of discharge which works out to an inverter of:
• 20 AH * 12 volts * 0.85 efficient inverter * 1/8 rate of discharge = 25 watt nominal output
Yep, not a very large inverter.

If that was an AGM battery, it may support a much higher rate of continuous discharge (some AGM's are rated to discharge completely in 15 minutes such as a computer Uninterruptable Power Supply). However, they do not last very long in these types of applications (1-2 years before needing replacement).

If you expect your loads to be mostly during the middle of the day under bright sun, you could use a larger inverter (solar panel supplies a large part of the current--the battery is, more or less, a big capacitor)--But even then, you are still only looking at a 60-80 watt inverter.

Note on "growing" a solar system... Usually, they are not very easy to grow by adding a bit here or a piece over there. You are much better off deciding your power needs and designing the system to support those needs.

More power means heavier wiring and larger AH battery/battery bank. Larger battery bank means more solar panels to keep it charged. More power eventually means a larger/new inverter running at higher voltage, which needs more batteries for a 24 or 48 battery bank, and more solar panels to charge the bank, and possibly a new MPPT charge controller to better manage the power from the array.

For a small, starting charge controller, MorningStar makes some good quality PWM solar charge controllers.

Is this what you are looking for?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 3,009 ✭✭✭✭
Re: First solar project - using 20Ah battery to charge laptop and a few iPhones and G

Welcome to the form "smoothisfast".
Your battery: two things - - - First, it looks like it might well be of the gelled/sealed electrolyte type. If so, it will require a controller capable of taking care of it, not some cheap thing available at many retail stores that could allow the battery to charge to the point where gas bubbles would build up between the gel and the plates, thus separating the two, or "boiling" the battery dry and ruining the battery. Second, the amp hours required to charge your lap top, especially when considering the extra to make up for normal ineffiniencies involved with inverters, chargers and related circuitry, could strain the 20 AH source battery, and discharge it below it's comfort level.
Beyond that, your idea looks doable and a good learning project for getting your feet wet. The USB car charger should work fine for the GPS and iPhone.
As to later upgrading this starter/learning project, that's often a difficult thing to do, so many changes to be made it's often better to start over with everything geared to the bigger system.
Good luck and may this project lead to bigger and better things
• Registered Users Posts: 16 ✭✭
Re: First solar project - using 20Ah battery to charge laptop and a few iPhones and G

your insights on the battery are interesting. How do we calculate the power strain and comfort level of a battery?

I looked at the charger and here are the specs

input 100 - 240v - 1.5A
output 16.5 - 18.5v - 4.5A Max
• Registered Users Posts: 16 ✭✭
Re: First solar project - using 20Ah battery to charge laptop and a few iPhones and G

Bill, that is exactly what i was looking for. Now I can play with the numbers to better understand it. Many thanks for your time in writing it up.
Re: First solar project - using 20Ah battery to charge laptop and a few iPhones and G
your insights on the battery are interesting. How do we calculate the power strain and comfort level of a battery?
You can search for manufacturer information on your battery or similar sealed lead acid batteries.

We use rules of thumb to initially size a solar power system to meet the general usage requirements of charging during the day and using some number of hours of constant power during the evening.

And hopefully, you will get the power out of your system that you expected and a good life from the system without over-stressing the battery.

Small batteries like these usually are not going to last that long or supply that much power anyways. But you can certainly experiment and see how everything works (and make inexpensive mistakes).

Deep Cycle Battery FAQ
www.batteryfaq.org
I looked at the charger and here are the specs

input 100 - 240v - 1.5A
output 16.5 - 18.5v - 4.5A Max

That does not sound correct for a properly regulated lead acid battery charger that you can leave plugged in for days/months at a time without damaging the battery.

Batteries are very sensitive to charging voltage... Short term a sealed lead acid battery would need around 14.2 volts for a few hours to charge, then around 13.2 to 13.6 to "float" the battery without boiling it dry.

Also, batteries are very temperature sensitive and a permanently connected battery charger would also have temperature correction for the output voltage (around -5mV per degree C per cell or ~-30 mVolts per C for a 12 volt battery). Better/higher end battery chargers even have a remote temperature sensor to keep track of battery temperature.

The two major ways people kill a battery bank--Overcharging a battery bank and undercharging/discharging to dead. Doing either for too long or too much can kill a battery in hours or days.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 16 ✭✭
Re: First solar project - using 20Ah battery to charge laptop and a few iPhones and G

the charger specs is for my Laptop charger.

input 100 - 240v - 1.5A
output 16.5 - 18.5v - 4.5A Max
• Registered Users Posts: 23 ✭✭
Re: First solar project - using 20Ah battery to charge laptop and a few iPhones and G

Greetings smoothfast,

I have a similar small setup, and instead of an inverter use a car (12vdc) charger for the laptop. It works and the cost was less than inverter. One consideration would be the size/capacity of your laptop battery vs your 20ah battery.

Mike