Voltage dropping too fast

Welcome to the forum Joe.

Can you tell us a bit more about your AC inverter... It can take ~10-20 Watts itself, just being turned on and no load (tare loss).

And what are your "expectations" of power/energy (Watts and Watt*Hours) usage?

Personally, I like to design a "balanced" system. Loads drive the size of the battery bank. Battery bank and daily loads (and amount of sun for your location) define the size of the solar array.

I am guessing your batteries are near death...  Flooded Cell Lead Acid batteries need to be stored charged, and recharged every ~1 month (if not float charged).

For example, a 1,500 Watt AC inverter needs ~100 AH @ 12 volts of FLA battery size for every ~250 Watts of inverter capacity (reliable system over time, temperature, partially discharge battery bank, etc.):

• 1,500 Watts * 1/250 Watts * 100 AH = 600 AH @ 12 volt battery bank

Conversely, a 70 AH @ 12 volt battery bank would be discharged at:

• 70 amps * 1/20 hour discharge rate = 3.5 amps (for 20 hours, until battery "dead")
• 70 amps * 1/8 hour discharge rate = 8.75 amps (maximum recommended continuous draw)
• 70 amps * 1/5 hour discharge rate = 14 amps short term (minutes to ~1 hour)
• 70 amps * 1/2.5 hour discharge rate = 28 amps surge current (seconds to 10's of seconds)
• For example: 8.75 Amps * 12 volts * 0.85 AC inverter eff = 89 Watt @ 120 VAC average load for ~3-4 hours

And, ideally, you should discharge by no more than 50% for longer battery cycle life. And avoid discharging to 20% or less State of Charge ever.

Lead Acid Batteries should never be allowed to sit (stored) at less than 75% state of charge.

And to be properly recharged, you have a minimum amount of current needed. 5% for weekend/emergency usage, 10-13%+ for full time off grid usage:

• 70 AH * 14.5 volts charging * 1/0.77 solar panel+controller derating * 0.05 rate of charge = 66 Watt array minimum
• 70 AH * 14.5 volts charging * 1/0.77 solar panel+controller derating * 0.10 rate of charge = 132 Watt array nominal
• 70 AH * 14.5 volts charging * 1/0.77 solar panel+controller derating * 0.13 rate of charge = 171 Watt "cost effective maximum" array

So, your 100 Watt panel is "OK" for a backup/emergency system...

And how much energy can you harvest from your 100 Watt panel? Say fixed array near Philadelphia Pennsylvania:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Philadelphia
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 50° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.07	3.77	4.23	4.46	4.74	4.84
Jul	Aug	Sep	Oct	Nov	Dec
4.95	4.79	4.55	4.21	3.12	2.77

Here we are in December, a 100 Watt panel will produce (on average):

• 100 Watts * 0.52 end to end system efficiency * 2.77 hours of sun (Dec) = 144 Watt*Hours of 120 VAC power per day

Take your 9 watt LED light and (guess) 15 Watt Tare losses for the AC inverter = ~24 Watt load on the system:

• 144 Watt*hours per day (Dec) * 1/24 watt AC load = 6 hours per day (night)

And from the battery storage:

• 70 AH * 12 volts * 1/24 Watts * 0.50 typical max discharge = 17.5 hours to 50% state of charge on battery

Notice that you (may be) using more power for the inverter than for a simple 9 Watt AC LED Light... And it will take three days of December sun to recharge the battery bank from 50% to 100% state of charge.

Of course, I have make lots of assumptions, and skimmed over a lot of details--But the above gives you an idea of what to expect from your system for the Month of December.

I would suggest that you define what you want your system to do, and the design (on paper) a system that will meet those needs. Conservation is your friend here... I.e., a 10 Watt LED 12 volt lamp uses 1/2 the energy (or less) of a 9 Watt LED lamp on a 120 VAC AC inverter...

I will stop typing here. Ask more questions, correct my guesses, and we can go forward.

-Bill

Batteries are the "Heart" of your system. And they are the one piece of gear that if not treated "kindly", will have a very short life.

13.8 volts is usually about right for "float charging" your battery bank, but not great for fast/full recharging of a cycling battery (for flooded cell, closer to 14.8 volts for 2-8 hours or so, depending on how deeply discharged).

Here is some light reading on batteries:

http://www.windsun.com/Batteries/Battery_FAQ.htm
http://www.batteryfaq.org/
http://batteryuniversity.com/

After reading the above, you will probably be convinced that batteries do not work... 

However, they really do work. Just need to understand their needs and limitations.

A nice system for emergency backup power (not for running your refrigerator) would be something like 2x 6 volt @ ~200 AH flooded cell golf cart batteries, a ~500 Watt maximum AC inverter (there is a very nice 300 Watt MorningStar TSW 12 volt inverter and others). And somewhere around a 377 Watt solar panel/array (10% rate of charge)+ appropriate charge controller.

Add a hydrometer to measure battery specific gravity, and an DC Current Clamp DMM to monitor/repair/understand your system...

I can give you links and the math, if you are interested. Golf cart batteries because they are pretty rugged, cheap, and forgiving (just check the electrolyte levels once a month). Great for the time (night, quiet time) when you are not running the genset during the day (refrigerator, water pumping, washing machine, etc. loads).

-Bill