Charge rates and voltage

cptdondo
cptdondo Solar Expert Posts: 105 ✭✭✭
in Solar Beginners Corner #1
I have a very newbie question....

I have a 270W array feeding two Trojan T135 batteries via a Morningstar Duo (PWM) controller. I also have a fridge that puts a variable load of 0-5A on the batteries. It's been overcast and the panels have been putting out around 1 to 6A, and the controller is showing a voltage of around 12.4V (this varies a bit as the fridge draws more or less power).

In the morning, the batteries under load show 11.8V (this is normal - they bounce up to 12.1 as soon as I take the load off). As the day gets brighter, the voltage increases and right now, pretty close to solar noon, I get almost 7A from the panels.

So my question is this: Are the batteries getting charged?

I know the panels are providing just a bit more power than the fridge needs, but 12.4 just doesn't seem like a high-enough charge voltage. Can someone help me out here? Is the solar system actually putting AH back into the battery, or is it just running the fridge?

Thanks!
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
    Re: Charge rates and voltage

    it sounds like pvs are running the frig and when the sun goes behind the clouds the battery is providing the power so it is not even breaking even here. i suggest getting a good charge into the batteries somehow even if you need to bring them up with either a generator or utility ac and a charger. taking loads off may quicken the charge, but you can only go so far with that as the frig needs to run sooner or later. in the long run, maybe more pv and an mppt cc would help your situation.
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  • cptdondo
    cptdondo Solar Expert Posts: 105 ✭✭✭
    #3
    Re: Charge rates and voltage

    I'm OK with not breaking even; the firdge uses about 20 AH in this weather and yesterday the solar panels produced about 13 AH. But I'm OK with losing a bit every day - as long as I can get through about 4-5 days I'm fine.

    I'm trying to figure out if the batteries actually charge or if the voltage / current has to be above some threshold to actually put AH back into the batteries.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Charge rates and voltage

    Charge rate is net of power from controller less power to loads.
    So 7 Amps - 5 Amps = 2 Amps to the battery when that is the output from the charge controller and the load is on.
    Technically, that is a charge. But it is not enough to properly recharge the batteries.
    You should not be okay with "not breaking even". 20 Amp hours use less 13 Amp hours production equals 7 Amp hours loss every day. If this is not corrected soon your batteries will be spending too much time at too low a state of charge resulting in sulphation, which means reduced capacity and shortened battery life.

    This is the #1 mistake made in off-grid systems.
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  • spanel
    spanel Solar Expert Posts: 36 ✭✭
    #5
    Re: Charge rates and voltage

    shouldnt he be seeing somewhere around 17amps as well at full power?

    Chris

    p.s. still learning.. dont yell at me to much! LOL
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Charge rates and voltage
    spanel wrote: »
    shouldnt he be seeing somewhere around 17amps as well at full power?

    Chris

    p.s. still learning.. dont yell at me to much! LOL

    In full sun he should see around 14 Amps:
    270 Watts @ 77% efficiency / 14.4 Volts charging = 14 Amps

    But he's dealing with cloudy weather so panel output is significantly reduced.
    Yet another reason why you should try for more panel rather than less; the sun doesn't always shine so bright! :roll:
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,654 admin
    #7
    Re: Charge rates and voltage

    More or less--AH in less than AH out from battery bank is slowly discharging your battery.

    Discharging a battery below 75% state of charge for days/weeks/months will sulfate the battery bank and cause early death.

    Discharging a battery bank often below 50% state of charge will limit its cycle life (typically to 500-1,000 cycles maximum for a "typical" deep cycle battery).

    Discharging a battery bank below ~20% will run the risk of permanently damaging the battery bank.

    For "emergency use"--deficit charging the battery bank may be acceptable.

    However, the longer you do this, the less life the battery bank will have.

    Not sure what size a Tojan T-135 battery is... But, it would be a good idea for you to have a battery charger (AC/Genset) that can charge at a rate of 5-13% or so (higher is OK, but expensive for solar, many Trojan Batteries recommend a 10% rate of charge)...

    So, if you have a pair of 225 AH @ 6 volt batteries in series for 12 volts--that would be a recommended ~22.5 Amps of charging.

    13 Amps is >5% rate of charge--but, it will only last you so long (as you have seen).

    Many folks would get a small AC genset (like a Honda eu1000i/eu2000i and a 12 volt battery charger)--Recharge in the mornings (or every other morning) to keep the batteries from getting too low--and let the solar assist during the rest of the day.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • cptdondo
    cptdondo Solar Expert Posts: 105 ✭✭✭
    #8
    Re: Charge rates and voltage

    To clarify:

    The batteries are charged either by:

    Engine alternator
    Generator, which drives a 45A 3 stage charger
    Grid, which drives the same 45A 3 stage charger
    And now, solar panels

    Typically, the RV will get about 3-4 hours/month runtime on the generator, some 10 to 20 hours/month drive time, and anywhere from 0 to 24 hours/month on the grid.

    The fridge only stays on maybe 4 to 10 days/month, and never (or almost never) longer than 5 days at a time.

    So, during the times when the van is not connected to the grid, not driven, and where we cannot run the generator, I'm OK with a slow discharge as the batteries will be fully charged by one of the other methods as soon as practical (within a few days). :D

    From what I gather, as long as I make sure the batteries get charged up regularly and soon after discharge, I should be OK with the sort of "deficit" charging I'm getting on cloudy days from solar.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Charge rates and voltage

    That time schedule should work. What you don't want to see is so many days of deficit charging (the 7 Amp hour loss per day) that the batteries get below 75% SOC without being recharged right away. That will shorten battery life. Below 50% SOC without next-day charging and you'll probably be looking for new batteries much sooner than you'd like. Possibly the same day you discover they refuse to charge back up. :cry:
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #10
    Re: Charge rates and voltage

    Your fridge, when starting up, will run for 12 hours to cool itself down, which will be a big drain on the batteries each outing. Maybe that will happen while driving and you can recharge via the vehicle alternator / inverter/ 120VAC charger.

    i didn't catch what size your batteries are?

    Do you have a battery isolator planned so the engine starter battery does not die if the house batteries go low ?
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • cptdondo
    cptdondo Solar Expert Posts: 105 ✭✭✭
    #11
    Re: Charge rates and voltage
    Cariboocoot wrote: »
    Below 50% SOC without next-day charging and you'll probably be looking for new batteries much sooner than you'd like. Possibly the same day you discover they refuse to charge back up. :cry:

    Been there, that's why I got solar. :-) I want to limit the depth of the discharge.

    Thanks for the reassurance.
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  • cptdondo
    cptdondo Solar Expert Posts: 105 ✭✭✭
    #12
    Re: Charge rates and voltage
    mike90045 wrote: »
    Your fridge, when starting up, will run for 12 hours to cool itself down, which will be a big drain on the batteries each outing. Maybe that will happen while driving and you can recharge via the vehicle alternator / inverter/ 120VAC charger.

    i didn't catch what size your batteries are?

    Do you have a battery isolator planned so the engine starter battery does not die if the house batteries go low ?

    The whole system is a bit more complicated, but yes, there is an ACR (automatic charge relay) which monitors the various voltages and only connects the house batteries to the engine battery when all is well.

    The fridge itself has a smart controller which limits compressor use when on batteries. If it seems charge voltage it kicks into high gear; once on battery power it slows down. It will also shut itself off if the batteries drop below some very low threshold (11 volts or something like that.)

    So in an ideal world we plug the van in and turn the fridge on the night before we leave, insuring full charge and a cold fridge. But sometimes it's not practical, and sometimes we use the van as a base camp (sitting in one place) and without the ability to run the generator because of fire or noise restrictions.

    The van has two battery banks; one pair of Trojan T-145 (260A @ 20 hr) and one "normal" (ie cheap discount store) 12V deep cycle battery, with a switch between them. The Duo charges both (and is the main reason I went for it, instead of a more efficient MPPT controller).

    I set it that way so that when the main bank starts running low, I can switch to the single battery and I have enough juice to a) spend one more night, and b) have enough power to start the generator without waiting for the alternator to charge the house batteries.
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