Will this system work?

I want to set up an alternate power system to power a few small lights and a security system for a building that can be unoccupied for 3 or 4 days at a time. I think I can get by with 150 to 200 amp hours of batteries and I think I already have most of the equipment I need. However, I have a few question about putting things together and I would also like to know if there are any concerns or fatal flaws with what I am proposing. I’m certainly no expert with any of this so would appreciate any help or any comments you might have.

Here’s the equipment I have:

Two 100 AH marine batteries.
Iota 15 AH battery charger with the IQ4 controller (3 stage charge).
1000W msw power converter.
Shell 55W solar panel with a 3 amp, 13.7 V voltage regulator.
Two Thin-Lite 8W outdoor lights

I was originally just going to just use the charger to keep the batteries up, but I had the solar panel, and was not using it, so figured I would include it in the system. My questions:

The IOTA owner’s manual says that when connecting the charger to a battery, install a breaker within 18" of the battery. What size wire and what type/size breaker should I use here?

The solar panel and voltage regulator were salvaged, but are almost like new, so I would like to use them if they will be of value. But with the charger hooked up continuously, will the solar panel do anything besides serve as a backup to float charge the batteries if the power to the building goes out? (The float charge on the IOTA is 13.56 V) Should I put the charger on a timer so it only comes on for few hours in the late afternoon to finish what the panel doesn't do? Should I get a small charge controller for the panel instead of the voltage regulator? What else should I be concerned about with this system?

Thanks for your help!


  • BB.
    BB. Super Moderators, Administrators Posts: 33,408 admin
    Re: Will this system work?

    Usually, you need to know your power requirements--and work at getting those as low as possible.

    And, if your system requirements are low--the 1,000 watt MSW inverter is probably way overkill... You would be much better off with a much smaller TSW inverter--both for less overall power used ("wasted" in the idling 1,000 watt inverter) and for a better AC waveform at your loads (MSW can cause problems for some devices/appliances).

    For a small system, the Morning Star 300 watts 12 VDC - 120 VAC TSW inverter is hard to beat.

    If you have lighting needs--using LED's with reflectors and/or motion detector based lighting will save lots of power too... Plus, motion detectors (12 volt model @ 10 amps here) can "alarm" trespassers as they don't know if somebody will notice them as the lights go on (or if somebody saw them and turned on the lights).

    Next, you have to decide how long you want the system to be able to operate without AC power... Hours, Days, etc.?

    Say you want your 2x100AH 12 volt batteries to run the loads for 3 days without AC power and 50% maximum battery discharge (for long battery life):

    2 x 100 AH / (3 days * 1/0.50) = 33 amp*hours per day

    33 amp*hours / 24 hours = 1.375 amps (average) of load for 3 days

    Or, power/watt wise:

    1.375 amps * 12 volts = 16.5 watts...

    Of course, if you run the lights only at night (or on a timer/motion detector), and the "security hardware" does not draw much power--you can juggle the day/night loads--such that they add up as recommended...

    If, your loads only need to run a few hours (or overnight) until an electrician can get there to repair the AC power--then you can change the assumptions above.

    But, you can see, if my numbers are anywhere close to accurate, a 16 watt load is nowhere near the needs of a 1,000 watt inverter.

    Regarding your breaker/fuse rating for the charger... a 15 amp charger would require a 15amp/0.80=18.75amp or 20 amp service (with a 20 amp DC rated breaker or fuse, and ~12 awg minimum wire size--for a short run from the battery to the charge controller).

    Make sure that you use fuses/breakers for all of your battery connections... Lead Acid batteries can output a lot of current and will fry most any wiring to your loads if there is a short somewhere/sometime.

    Your 200 AH of batteries would need a ~10-20 amp charger--so you are OK there.

    Solar panels--sort of depends on if they will "do any good" for your situation...

    55 watts of panels / ~17 volts Vmp = ~3 amps of output. Typical summer/winter sun (and battery losses, inverter losses, etc.) would give you around 6-12 amp*hours of useful charge per day (depending on your system's location, amount of sun, weather, etc.).

    Depending on your loads--that may be a significant help, or not... ???

    The float charge is OK for your batteries. Make sure the charger is installed in the same area as the batteries--so they have roughly the same temperature (especially important if you have wide temperature swings at your site).

    Eventually, you will want to replace the Marine batteries with either true deep cycle batteries (longer life) or with AGM or some other sealed type batteries (for less maintenance; checking water, cleaning battery tops, etc.)

    Your IOTA charger is a very nice one... And has a "high/low" float voltage switch... If the system operates in "standby" 99.9% of the time, use the Low Float setting to help prevent boiling your batteries dry (reduces the amount of water needed to add every month). If this system cycles daily/weekly--then set to high float to keep your batteries well charged for daily/weekly use.

    Does this address your basic questions?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: Will this system work?

    Bill, thanks for all the good info!

    I didn't think much about the 1000W converter. I was proposing to use it mostly because, like the rest of the equipment, I already have it and it doesn't cost me anything. I'll have to look into getting a more appropriate converter.

    The security system and lights will be, as much as possible, motion control activated and/or night only. I was a little more optimistic in figuring my available power than your calculations were, so I'll have to check my power consumption again. I eventually want to get better batteries, and a few more amp-hours, but the batteries I have are in such good shape I hate to waste them. So I'll try things for a while with these batteries and see if they work. If not, that's a good excuse for some better batteries. (It occurs to me that I wouldn't be wasting the marine batteries if I bought a boat. And my wife might not kill me when she sees the boat....)

    Thanks again.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,408 admin
    Re: Will this system work?

    Your batteries will work OK... If they are cycled a lot (such as daily), they may last a few years vs 5-7 years for "golf cart" style storage battery (just wild guesses).

    Powerwise--accurate measurements (get a kill-a-watt meter for AC loads--and use it around your home to measure loads/appliances and see where your $$$$ is going).

    And for solar/battery systems, conservation is much better than throwing money at the load/problem.

    And lastly, the major ways batteries are killed... Over charging, running out of water, under charging, and running too much load and running the batteries dead.

    An automated system really requires a battery monitor so that you can, at a glance, tell the how much charge is on the batteries--and if this is a critical system (or used by others), setting up an automatic shutoff (Trimetric for cost, the Xantrex LinkLite or LinkPro if you want a programmable output to turn off your loads or set an alarm)... Problem is you really cannot just "measure" the battery voltage is the battery is under load or under charge--needs several hours of rest for an accurate voltage reading (or use a hydrometer and measure specific gravity--does not work on sealed batteries).

    Depending on your needs--it is easy to blow the budget trying to build a bullet proof system.

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