Voltage dropping too fast

roadrunnerjoe Registered Users Posts: 2
edited December 2018 in Solar Beginners Corner #1
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.


  • BB.
    BB. Super Moderators, Administrators Posts: 33,442 admin
    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:

    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)

    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.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    > charge to 13.8V daily

    Quite low for lead acid battery while charging and quite high for after charging.  Look into having your batteries tested.

    I am available for custom hardware/firmware development

  • roadrunnerjoe
    roadrunnerjoe Registered Users Posts: 2
    This is great info Bill. The inverter is a 1500W MSW I got with the panel from Windy City. It has three outlets with individual on/off toggles and has a display for either current system voltage or wattage output. As I plan to use it mainly for outages, I don’t currently have output expectations but that is definitely something I will need to do. The batteries have been on the panel since they were purchased so they are charged full everyday, I have a feeling this may be my weak link. I tried charging a portable cell phone charger yesterday using a USB port so the inverter wasn’t even on and the voltage dropped to 10.4 in just under an hour. Once I unplugged the device the voltage steadily climbed back to 12.5. As jonr suggests, I think I’ll test the batteries. I guess I shouldn’t have expected much from Harbor Freight batteries. The numbers you provided based on location are probably pretty close using Philly as I’m up in New Hampshire and not getting much sun these days, just a couple hours above the trees. Thanks for all the info you provided, as always I’m open to any and all comments and I’ll let you all know about the batteries’ status. 
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    You need to plan  this system out!
    Do you want emergency lighting or to replace the grid in an outage?
      Virtually all LED lights are 12 v at the heart of the light, so get rid of the inverter if it is an emergency light..
    If you want grid replacement, you really need larger storage and a smaller inverter m the size to be determined by the sum of all the items you want on at the same time.  Look at higher quality inverters, like Cotek, that give you specifications on power efficiency etc
    i.e. a good inverter will have a surge rating that is ~ 2 times the continuously rated output.. like this 

      you might be able to get away with a 200 W  unit... http://www.cotek.com.tw/product_detail.php?prod_id=JCU5IyE=

    hope this helps.

    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • BB.
    BB. Super Moderators, Administrators Posts: 33,442 admin
    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:


    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).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset