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Thread: Solar Charging questions

  1. #1
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    Default Solar Charging questions

    Now that I've joined the Solar cult and spent some money I have a few questions/assumptions...
    First,
    Everything I read about battery charging says that it takes 12 to 15 hours to fully charge a battery http://batteryuniversity.com/learn/a...d_acid_battery
    Since the sun doesn't provide 12 hours of light most days, is it safe to assume that you won't charge your battery back to 100% on solar in most cases?

    Next, I've got a cheapy amazon solar controller. Battery charging is full recharge until the voltage hits a programmable cutoff ( I've set it at 14.2), then it just maintains it at 14.2 ( 2.35v/cell). When I get home I plug it in with my fancy converter and it tops off the battery ( and does some anti sulfanation stuff) Does this seem reasonable?

    I know I can get an MPPT that has the three stage charging, but when will you ever get to stage 3?

    Now, I'm running a walmart marine/deep cycle 85AH battery. The last one lasted 5 years. ( about 2 months past the warranty). I've been reading a lot about depth of charge and cycle impact. If I assume that I'm camping every weekend for 5 years then I would have 2 cycles per weekend (520 cycle lifetime). Given the limited life of the battery, I shouldn't impact the life of the battery if I discharge it to 75 or 80%.


    Did I get any of this right?
    Thanks
    Tom
    2007 JKU with Homebuilt 5x8 Tear Drop.
    Previous Rigs:
    1969 CJ5 with 225 V6 /3sp and OD
    1984 Jeep Cherokee with 4cyl/5speed ;-(
    1967 Bronco with 5.0HO and 4R70W on 35s
    1978 Ford E250 with 1971 429 overlanding and I didn't even know it!
    1978 Georgieboy 22 ft RV Dodge 440 Blew this one up!
    1984 Class C RV with 460
    1996 Jeep Cherokee with 4.0HO

  2. #2
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    That site has a lot of misinformation, check other sources too.

    If your power( wattage ) is enough, 5-7 hours can get to 100% Full from empty (50%)

  3. #3
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    > Since the sun doesn't provide 12 hours of light most days, is it safe to assume that you won't charge your battery back to 100% on solar in most cases?

    > I know I can get an MPPT that has the three stage charging, but when will you ever get to stage 3?

    As long as you consume less than you put back no problem.


    > Next, I've got a cheapy amazon solar controller. Battery charging is full recharge until the voltage hits a programmable cutoff ( I've set it at 14.2), then it just maintains it at 14.2 ( 2.35v/cell). When I get home I plug it in with my fancy converter and it tops off the battery ( and does some anti sulfanation stuff) Does this seem reasonable?

    As long as either you only go away for 2-3 days.

    or replace your batteries mire frequently.

  4. #4
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    > Now, I'm running a walmart marine/deep cycle 85AH battery. The last one lasted 5 years. ( about 2 months past the warranty). I've been reading a lot about depth of charge and cycle impact. If I assume that I'm camping every weekend for 5 years then I would have 2 cycles per weekend (520 cycle lifetime). Given the limited life of the battery, I shouldn't impact the life of the battery if I discharge it to 75 or 80%.

    No that will murder it much faster than keeping to 50%.

    From Sam's Club or Batteries+, get a pair of flooded 6V Duracell GCs, maybe $180

    Will last much longer, cheapest possible actual deep cycling per AH.

  5. #5
    Kinda a long read but prob the best place to learn about prober solar charging of batteries.

    https://handybobsolar.wordpress.com

    You can charge you batteries 100% one one day I do it all the time. Just depends on how low you drain them. after 80% they charge much slower so need more time than say 50-60%.

    You have to ditch that cheap amazon controller to get proper charging.
    2002 4Runner 4wd. Dual Yellotops, Yaesu ft7900, ARB 50qt, 32" LED bar, 24" LED bar, 1000w sine wave inverter, superflow 12v compressor, Wilson 4g cellphone booster, 80w Roof Mounted Solar Panel, Viar 400p compressor.

    My build

  6. #6
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    Thanks for the feedback. I'll probably just run with what I have until this battery gives up the ghost in 4 more years. Then it will make sense to upgrade to MPPT and the better batteries.
    Tom
    2007 JKU with Homebuilt 5x8 Tear Drop.
    Previous Rigs:
    1969 CJ5 with 225 V6 /3sp and OD
    1984 Jeep Cherokee with 4cyl/5speed ;-(
    1967 Bronco with 5.0HO and 4R70W on 35s
    1978 Ford E250 with 1971 429 overlanding and I didn't even know it!
    1978 Georgieboy 22 ft RV Dodge 440 Blew this one up!
    1984 Class C RV with 460
    1996 Jeep Cherokee with 4.0HO

  7. #7
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    23
    Quote Originally Posted by SoCal Tom View Post
    Everything I read about battery charging says that it takes 12 to 15 hours to fully charge a battery http://batteryuniversity.com/learn/a...d_acid_battery
    Since the sun doesn't provide 12 hours of light most days, is it safe to assume that you won't charge your battery back to 100% on solar in most cases?
    A quick, back-of-the-envelope calculation: If you run your 85Ah battery down to 50% each day, you'll theoretically need 42.5 Ah of solar charging per day. Factor in charging system inefficiencies, and call it 55Ah. If you have 5 sun hours (not hours of sunlight) per day, that means you'll need 11A out of your solar panels. 11A * 18V= 198W of solar. So, 2x18V/100W panels would charge your battery each day. You have to factor in that the charging current will taper off, but if you do you bulk charging early in the day, the solar panels should have enough time at reduced output (late afternoon/evening) to top off your batteries before sundown.

  8. #8
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    Quote Originally Posted by Step-Hen View Post
    A quick, back-of-the-envelope calculation: If you run your 85Ah battery down to 50% each day, you'll theoretically need 42.5 Ah of solar charging per day. Factor in charging system inefficiencies, and call it 55Ah. If you have 5 sun hours (not hours of sunlight) per day, that means you'll need 11A out of your solar panels. 11A * 18V= 198W of solar. So, 2x18V/100W panels would charge your battery each day. You have to factor in that the charging current will taper off, but if you do you bulk charging early in the day, the solar panels should have enough time at reduced output (late afternoon/evening) to top off your batteries before sundown.
    My math is a little different. Assuming I need the 42.5Ah at 12.5V, then what I need is 531Watts. Assuming the 5 hours of "Sun" hours , I would need 106 watts of charging per hour, add in the 30% efficiency losses now I need about 140 watts worth of solar panels.

    My question isn't so much about whether I can push back the required amps into the battery, but more about whether the chemical reaction in the battery can happen effectively in that time frame. Smart battery chargers hold the voltage for many hours to get the battery to 100%, so why would they do that when they could accomplish that in just a few hours?
    2007 JKU with Homebuilt 5x8 Tear Drop.
    Previous Rigs:
    1969 CJ5 with 225 V6 /3sp and OD
    1984 Jeep Cherokee with 4cyl/5speed ;-(
    1967 Bronco with 5.0HO and 4R70W on 35s
    1978 Ford E250 with 1971 429 overlanding and I didn't even know it!
    1978 Georgieboy 22 ft RV Dodge 440 Blew this one up!
    1984 Class C RV with 460
    1996 Jeep Cherokee with 4.0HO

  9. #9
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    Quote Originally Posted by SoCal Tom View Post
    My question isn't so much about whether I can push back the required amps into the battery, but more about whether the chemical reaction in the battery can happen effectively in that time frame. Smart battery chargers hold the voltage for many hours to get the battery to 100%, so why would they do that when they could accomplish that in just a few hours?
    I've heard the same thing. Lead acid batteries require several hours of low amperage charge to get from 80 to 100%. 50-80% you can do quickly. And if you don't get them to 100% on a regular basis they wear out.

    One reason I'm going with LiFePO4.
    2016 Toyota Tundra SR 4x4 Double Cab Long Bed 5.7L..... DIY camper on the way...

  10. #10
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    The "2x18v" is a somewhat dubious assumption.

    With a PWM controller, the operating voltage of the solar would be battery voltage, which will be quite far below the solar panel's Vmp.

    With an MPPT controller, the solar would be operating at Vmp, but the output to the battery will be at battery voltage.

    So...

    85ah battery down to 50% SoC - 42.5ah. Batteries are not 100% efficient, so to recharge 42.5ah, you'll need 42.5 x 1.2 = 51ah. Divided by 5 hours good sun, around 10a/hr into the battery.

    At battery voltage.

    Which will change over time as the battery absorbs energy and builds up a surface charge on the plates.

    So say 12.3v to 14.6v. 2.3v difference, let's split the difference and use 13.5v for estimating... 13.5v x 10a = 135w. So with an MPPT controller, 150w of solar would get it done. With a PWM controller, the solar Vmp might be 18v, but the solar is actually operating at 13.5v (average over time), which is 30% below Vmp, so let's say 135 x 1.3 = 175w to get it done.

    (Actually, that's wrong, but I used it to show a common mistake... yes 13v is around 30% lower thsn 18v - BUT - 18v is around 150% of 13v. So 135w x 1.5 = 202w is correct.)


    But that doesn't answer the original question about absorb time...

    The answer is: it depends on the voltages, which are variables.

    If you bulk stage the battery to 14.4v, the bulk stage will be shorter than it would be if you bulked to a higher voltage like 14.8v. But at 14.4v, the battery is less full than it would be at 14.8v, so the absorb stage is going to take longer if you switch from bulk to absorb at 14.4v than if you switch from bulk to absorb at 14.8v.

    It's not a 1:1 correlation. Bulking to a higher voltage might add an hour or two to the bulk stage, while shaving 3-6 hours off the absorb stage.

    Different batteries have different voltage limits, specified by the manufacturer. My pair of 200ah Chinese AGMs specify (printed on the side of the battery) 14.7v bulk, 14.7v absorb.

    Basically, any lead-acid battery can be taken up to 14.6v-14.8v AS LONG AS you don't let it get hotter than about 125 degrees Fahrenheit.

    Flooded lead-acid batteries will of course use more water when taken to higher voltages, but that's a small price to pay for shorter charge times and increased battery lifespan.

    Sealed lead-acid batteries, such as AGMs just need to be kept from momentarily opening the popoff valve and losing irreplaceable water.

    My Victron MPPT is set to 14.7v bulk and 14.7v absorb. When it detects life from the solar panel (sun is coming up), it measures the battery voltage and uses that to set a max time limit on the absorb stage. Once it enters absorb stage, it will continue until the amps flowing drops below two amps, or the timer elapses, whichever comes first.

    But no matter what, absorb stage will end when the sun goes down.


    Shore power chargers are usually conservative. Lower voltages, which takes more time, but time doesn't much matter - shore power, unlike solar, doesn't have to get the job done in 8 hours or less.

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