Voltage dropping too fast

i have a very simple 12V system I put together to use mainly as an alternative to my generator during power outages. It’s a 100W panel going through a 30A controller to two 35AH batteries and finally ending at a 1500W inverter.
The other night I used the system for a single LED light. The controller was showing 13V when I turned on the light but 90 minutes later the alarm started chiming and I was now seeing 10.4V.
Clearly something isn’t right. Is the inverter too much for what I need to it to do? Have my batteries quit? They are two years old and charge to 13.8V daily.
I appreciate any advice, thanks.
The other night I used the system for a single LED light. The controller was showing 13V when I turned on the light but 90 minutes later the alarm started chiming and I was now seeing 10.4V.
Clearly something isn’t right. Is the inverter too much for what I need to it to do? Have my batteries quit? They are two years old and charge to 13.8V daily.
I appreciate any advice, thanks.
Comments
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
Measured in kWh/m2/day onto a solar panel set at a 50° angle from vertical:Average Solar Insolation figures
(For best year-round performance)
- 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
Quite low for lead acid battery while charging and quite high for after charging. Look into having your batteries tested.
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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