Minimum solar panel capacity

tabbycat
tabbycat Solar Expert Posts: 55 ✭✭✭
in Off Grid Solar & Battery Systems #1
Crown batteries recommends a finish charge rate of 3% to 4% for their batteries based on the 20 hour AH battery rating (deep cycle). Since the last 10% to 20% of battery charging (the finish charge) takes as long as the bulk charging phase I feel comfortable sizing my solar panel output based on this recommendation for my situation where battery use is infrequent (once a week or less) and vary rarely going below 50% DOD. Does this seem like a reasonable assumption. Thanks. Steve S.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Minimum solar panel capacity

    Avoid having the battery sit for days/weeks or longer below ~75% state of charge, that will reduce sulfation of the battery bank.

    That is one reason that having too small of solar array (or other charging source) can be a problem--taking too long to get the battery recharged from deeper discharges (the other being mixing of the electrolyte--most vendors of flooded cell recommend 5% minimum rate of charge--and some recommend 10%).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • tabbycat
    tabbycat Solar Expert Posts: 55 ✭✭✭
    #3
    Re: Minimum solar panel capacity

    Even with the minimal solar capacity (3% to 4% of the 20 hr AH rating) the panels are idle most of the time. The charge controller verifies the state of charge. Adding more panels would seem to be pointless. The Florida sun may have the same effect as adding more panels though. Thanks. Steve S.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Minimum solar panel capacity

    It isn't a matter of the time the panels sit idle; it's a matter of having enough panel to properly recharge the batteries at a sufficient rate when they need it. For occasional-use applications this is a problem because people understandably don't want to spend money on panels that they only fully utilize once a week. But if the batteries aren't charged properly, you buy batteries a lot more often than you should.

    Instead of charging from the panel, get a generator and charger and do the Bulk and some of the Absorb cycle with that. Charge before you leave. Then you only need enough panel to stay ahead of the self-discharge rate when you're gone. Much cheaper than buying hundreds of Watts worth of panels. You can take the generator with you when you go.

    The 3%-4% rate Crown mentions is likely best interpreted as the Absorb end Amps, and would not include any additional power requirements for concurrent loads.

    So for examples (not precise calculations or recommendations) on a 220 Amp hour 12 Volt battery bank:
    Full charge panels: 22 Amps * 14.4 Volts = 316 Watts less derating = 411 Watt array.
    Maintenance charge panels: 6.6 Amps * 13.8 Volts = 91 Watts less derating = 118 Watt array. You might get away with less, but sometimes the sun doesn't shine so bright. At any rate you'd be looking at 1/4 the size of array.

    The Florida sun may help you with the length of time it shines, but it also shines hot which can knock panel power down considerably.
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  • tabbycat
    tabbycat Solar Expert Posts: 55 ✭✭✭
    #5
    Re: Minimum solar panel capacity

    Batteries are as good as their warranty. You can buy an AGM for two or three times the price of a flooded battery and they both have the same warranty - one or two years. Replacing batteries frequently is good practice whether or not their capacity is reduced. WalMart and Sams make this financially possible. Thanks. Steve S.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Minimum solar panel capacity

    Terribly sorry; I thought you had posted looking for advice.
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  • SCharles
    SCharles Solar Expert Posts: 123 ✭✭
    #7
    Re: Minimum solar panel capacity
    tabbycat wrote: »
    Replacing batteries frequently is good practice whether or not their capacity is reduced.


    I'm missing something, perhaps. I don't get why one would do well to replace his batteries frequently. I'm not being flip, I just somehow got a bit lost as to the topic of this particular forum string.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: Minimum solar panel capacity
    SCharles wrote: »
    I'm missing something, perhaps. I don't get why one would do well to replace his batteries frequently. I'm not being flip, I just somehow got a bit lost as to the topic of this particular forum string.

    You're not missing anything. To replace batteries before their usable capacity falls below your system requirements is a needless waste of time, effort, and money. So is a plan of chronically under-charging batteries and replacing them when they drop dead prematurely, which is one we see a lot around here - albeit mostly unintentionally.
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  • tabbycat
    tabbycat Solar Expert Posts: 55 ✭✭✭
    #9
    Re: Minimum solar panel capacity

    I apologize for being a cheapskate but buying a generator just to extend battery life seems awfully expensive. The Crown website says the average life expectancy of their batteries is three years. If you are using your batteries enough to require routine charging then a generator would make sense but infrequent use doesn't justify the expense and the maintenance of a generator. Whether you have a generator or not, once your batteries start showing a decline in capacity, trying to extend their life is more trouble than just replacing them. Thanks. Steve S.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #10
    Re: Minimum solar panel capacity

    tabbycat,
    is it your opinion then that one can undercharge and abuse batteries and that they will last 3 years? good luck on that one as i think your way is more expensive.
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  • tabbycat
    tabbycat Solar Expert Posts: 55 ✭✭✭
    #11
    Re: Minimum solar panel capacity

    Let me try one more scenario (maybe two) and see what I can get away with. Let's assume you have a 24 volt 100 AH battery bank (2400 watt-hours). One day a week you use about one HP-hr (700 to 800 watt-hours or 30 AH). You have a 35 volt 4 amp solar panel. In Florida it will give you 20 AH/day. In two days it will have replaced your one HP-hr. Now the panel is idle for five days. Let's not let the panel off so easy. Let's install another 100 AH 24 volt battery bank. And then another. Now the panel is working six days a week. We've tripled our battery capacity and we only need one panel. We'll let the panel have Sundays off. This is the ultimate in solar frugality. But the battery cost is adding up. Thanks. Steve S.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #12
    Re: Minimum solar panel capacity

    Steve S, yep that makes sense - it's a very common practice around here where people have remote weekend houses and size the battery to last for 2 days and then use the panel to recharge during the remaining 5.

    If it's sized right then best case scenario is that the batteries don't even get used much during weekend and stay about 80-90% SoC; Then once in a while worst case arrives and the batteries are discharged to 50% SoC by Monday morning, and they'll take a few days to reach 100% again. It's not ideal that they sit below 80% SoC for a few days, but it's not the end of the world either.
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  • machineman
    machineman Solar Expert Posts: 129 ✭✭✭
    #13
    Re: Minimum solar panel capacity
    stephendv wrote: »
    Steve S, yep that makes sense - it's a very common practice around here where people have remote weekend houses and size the battery to last for 2 days and then use the panel to recharge during the remaining 5.

    If it's sized right then best case scenario is that the batteries don't even get used much during weekend and stay about 80-90% SoC; Then once in a while worst case arrives and the batteries are discharged to 50% SoC by Monday morning, and they'll take a few days to reach 100% again. It's not ideal that they sit below 80% SoC for a few days, but it's not the end of the world either.

    Yep, thats my set up. Works great in the summer. 820w of panels and 440ah (24v) of Sams club (costco) golf cart batterys. After a 3 day weekend with TV, lights, fridge, and some power tools I'm down to 75% when I leave. Cloudy/snow winter time is a different story. Within the 3 days I need to run the Gen at least once and when I leave its about 50% with Low-Cut-Voltage set at 22V.

    Off Grid Cabin, 24V 440ah 6V GC battery bank, Xantrex MPPT60-150 CC, Magnum MS4024 inverter-charger, >1200w Solar bank

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  • tabbycat
    tabbycat Solar Expert Posts: 55 ✭✭✭
    #14
    Re: Minimum solar panel capacity

    Now I need to find a switch that transfers the load from one battery bank to the next while simultaneously directing the output of the solar panel charge controller to the battery bank that is in use. Thanks. Steve S.
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  • tabbycat
    tabbycat Solar Expert Posts: 55 ✭✭✭
    #15
    Re: Minimum solar panel capacity

    Switching from one battery bank to the next manually is satisfactory because you are there when there is a load on the batteries. However you are not there when the batteries are being charged so it would be nice if the charge controller could sense the SOC of the battery banks automatically and direct the charging from one bank to the next. When you come back the next weekend the charging would be taken care of and you now have plenty of battery capacity for the weekend. Thanks. Steve S.
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  • tabbycat
    tabbycat Solar Expert Posts: 55 ✭✭✭
    #16
    Re: Minimum solar panel capacity and Noco Genius battery chargers.

    I've been looking at the Noco Genius battery chargers and noticed that their output is 3 1/2 amps for battery capacities up to 120 AH. They consider this output to be sufficient to bring the batteries up to full charge. If the 3% to 4% output of the Genius chargers re the battery capacity is considered sufficient then shouldn't this output be adequate for solar panels ignoring the time element. Thanks.
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  • tabbycat
    tabbycat Solar Expert Posts: 55 ✭✭✭
    #17
    Re: Minimum solar panel capacity

    Sometimes the answers are simple. The Echo Charger from Xantrex does this. It's a B2B (battery to battery) charger that will switch the output of the panels to the auxiliary bank as long as the voltage of the primary bank is more than 13 volts (26 volts in a 24 volt system).
    tabbycat wrote: »
    Switching from one battery bank to the next manually is satisfactory because you are there when there is a load on the batteries. However you are not there when the batteries are being charged so it would be nice if the charge controller could sense the SOC of the battery banks automatically and direct the charging from one bank to the next. When you come back the next weekend the charging would be taken care of and you now have plenty of battery capacity for the weekend. Thanks. Steve S.
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