Dumb questions to (hopefully) help me understand

prometheus1351prometheus1351 Registered Users Posts: 1
Hi all... I joined up with the idea of making this post in hopes that it pushes my brain into the realm of starting to understand - so any help is appreciated. I've tried to research and study how electricity, batteries, and solar works, but I've realized it's just something my brain refuses to grasp. I'm hoping the answers to a few of my dumb questions below can help me: 

- One thing I'm trying to understand is how long it would take to charge a battery from say, 75% to 100% using a solar panel (using a charge controller). If this over-simplification is valid - let's say my 100 AH battery has 75 AH charge left. If I use a multimeter and see that my panel is producing 1A, does that mean it would theoretically take 25 hours to recharge with that particular amount of sun? What role does voltage play in this scenario and just in general?

- I have 4 solar panels that say it's rated for 18v and 25W - which also means 1.4A ea, even though not listed (?). Okay, I'll just admit it - it's the harbor freight bundle of 4 panels... don't make fun. Anyway, I'm interested in using these with my RV batteries - 2ea 6v 230 AH duracell batteries in series  (https://www.batteriesplus.com/product-details/golf-~-scrubber/battery/duracell-ultra/sligc115). My idea is just to add a little extra juice to the batteries over the course of a trip, not to live off of. Is this a complete waste of time? If my above question is accurate I'd only be gaining 1.4A x 4 panels = 5.6A back every hour of perfect sunshine (and ignoring inefficiencies and losses). Worst case, I was considering toting along another 100AH 12v battery to run a separate inverter so I don't need to run a generator to get AC power. 

After writing this all out, I think some input on the above information might be enough to get the hamster wheel turning. I'm realizing it really just boils down to why voltage matters and does metered amps over the course of an hour translate to hours it takes to regenerate a battery. 

Thanks in advance to anyone able to offer some help. 


  • RCinFLARCinFLA Solar Expert Posts: 1,332 ✭✭✭
    edited October 21 #2
    The upper 25% recharging of lead acid has poor charging efficiency.  Between 65% to 75% efficiency.  For 230AH battery, last 25% would be 58AH/70% effic., needing about 80 AH.  Battery voltage will average about 14v during this period.

    If using a PWM charge controller, which just puts panels across battery, and you get full sun you get about 5.6 amps so would take 80AH/ 5.6A = 14.3 hours.  You likely won't get full output from PV most of the time.  Usually get 5-6 hours of equivalent full sun per day depending on your location and season.

    Little less then 3 days if not too cloudy.

    LiFePO4 battery has much better discharge and charging efficiency, in 90+% range overall.
  • PhotowhitPhotowhit Solar Expert Posts: 5,413 ✭✭✭✭✭
    It's hard to wrap around efficiencies of charging.
    But you most likely have lead acid, flooded or AGM. Flooded will run around 75-80% and AGM 90-95%.

    Also there is a basic correction for solar panels between their rated value which is done as a 'flash test' not allowing the panel to heat up and reality when the solar panel gets hot while being in the sun, this drops normal output to around 75% of panel rating for Mono and Poly panels, and I THINK amorphous panels like your Harbor Freight.

    The basics of charging are that the charge controller regulates the voltage at the early stages of lead acid battery charging the battery will limit how high the voltage rises, once the battery is around 80-85% full the voltage will rise high enough that they battery could over heat, so the charge controller steps in to limit how high the voltage goes.

    If you are running an active system, the 100 watt HF array will help, if properly angled toward the sun, by contributing 4-5 amps until the battery can't use that much. Some of that has to do with the battery type as above and size. 

    Understand that mounted flat on an RV, it's unlikely they will be well angled toward the sun, in addition any shadow can reduce their output substantially.  FWIW - I think Amorphous are a bit better in this respect, but think 1/3 to 1/4 of the production with a small shadow...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • BB.BB. Super Moderators, Administrators Posts: 31,156 admin
    There are a lot of details--But a rough first approximation:
    • AH to charge / amps charging + 2-6 hours Absorb (25% discharged 2 hour absorb, 50% or more discharged, 6 hours)
    • 25 AH / 1 ampd = 25 hours
    • 25 hours + 2 hours absorb (for lead acid batteries) = 27 hours of "full sun"
    And that brings us to location, season, and tilt of panels... A quick website to look up hours of solar per day is:


    Say Los Angeles, panels tilted, the hours of sun per day (this is noon-time 1,000 Watts per sq meter value--Or total energy harvested from sun rise to sun set):

    Los Angeles
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 56° angle from vertical:
    (For best year-round performance)

    So, if you had a 100 Watt panel in October:
    • 100 Watts * 0.52 off grid AC solar eff * 5.85 hours of sun per day (Oct) = 304 WH per day (assuming AC inverter and FLA batteries)
    • 100 Watts * 0.61 off grid DC solar eff * 5.85 hours of sun per day (Oct) = 357 WH per day (assuming DC loads and FLA batteries)
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Sign In or Register to comment.