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Thread: dual charge controllers to one battery?

  1. #1
    depger4 Guest

    Default dual charge controllers to one battery?

    I am starting a small solar system. I want to run a few electronics in my network, the stuff that runs 24/7. I have a smaller 17AH battery and have a 20W solar panel, and a 7amp charge controller.

    Since this system is not worthy of an expensive charge control system, I wanted to know the viability of putting two charge controllers in parallel, or just connect them to one battery. One controller would have the solar panel. The other would be an ac/dc power supply to maintain power. Would this work, or would it not work correctly?

  2. #2
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    Default Re: dual charge controllers to one battery?

    Yes, you can parallel multiple charge controllers... You need to make sure they both see accurate battery voltage--so typically you tie them both to the battery lugs (or bus bar).

    However, which ever controller has the higher voltage setting--it is the one that will do the work and/or do the final charging on the battery (say one is a 3 stage charger and the other is a single stage charger--the single stage charger will probably eventually over charge the battery--all things else being equal).

    In your system, it is possible that the AC charger will do close to 100% of the charging and the solar charger may do nothing much unless the power is out (or at best, the solar will charge ~5 hours per day and the AC will charge the other 19 hours).

    In any case, even if the solar charge controller supplies some energy, it will not really save much of your AC power (standby losses are still present on the AC charger when plugged into the wall).

    Lastly, you need to know the amount of current your load will be taking... The 20 watt solar panels will only be able to output somewhere around 2-4 AmpHours per day in good sun--if your load is much larger than that, the solar panel will not be doing much for you...

    And regarding your AC charger--they tend to over charge batteries instead of going into lower voltage for "float" charging (~13.8 volts or a bit less) to keep the battery from boiling dry.

    One very simple method I have used is to hook a lamp timer and have the AC charger run only a few hours per day (based on load seen by the battery--you can have a 1amp AC charger on 2 hours in the morning and 2 hours in the evening--and get about 4 amps*hours of charge)--Have saved many car batteries on trickle chargers from boiling dry.

    -Bill

  3. #3
    Wayne from NS Cana Guest

    Default Re: dual charge controllers to one battery?

    Yes you can parallel them, BUT only on the output side, going to the battery bank, NOT on the input side. Each controller would have to have it's own, separate source feeding into it, then all the controllers would feed the same battery pack.

  4. #4

    Default Re: dual charge controllers to one battery?

    Quote Originally Posted by BB. View Post
    Yes, you can parallel multiple charge controllers... You need to make sure they both see accurate battery voltage--so typically you tie them both to the battery lugs (or bus bar).

    However, which ever controller has the higher voltage setting--it is the one that will do the work and/or do the final charging on the battery (say one is a 3 stage charger and the other is a single stage charger--the single stage charger will probably eventually over charge the battery--all things else being equal).

    -Bill
    I have been doing something similar to this for a few years in my little "play" 12V system. I have ~50W of thin-film panels using a simple single stage controller along with a 50W multicrystalline panel connected to a quality 3-stage Morningstar controller. Both connect to the same 80AH AGM battery. I wonder if have been harming my battery by having the single stage controller? The battery still seems to be doing OK after 3 years.

    Very early on I connected the thin-film panels to the 3-stage controller as well but was told that it would not be good to mix the panel types.

    Note, I am about to add a 135W panel to this system to the 3-stage controller. I think I will likely remove the thin-film panels as I should have plenty of energy for my loading.

  5. #5
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    Default Re: dual charge controllers to one battery?

    The 50watt and single stage controller is probably not causing you problems... Between the low current (~3 amps) and the fact that it only runs 3-5 hours per day (9-15 amphours)--you are almost the equivalent of a trickle charger for your 80 amp*hour battery.

    Add to the fact you are loading/discharging the battery (probably daily)--the one panel and single stage charger is just fine.

    If the battery was sitting unloaded for weeks/months at a time during the sunny season--then it would probably be better for you to disconnect this charger and let the other 3 stage charger manage the storage charging for your battery (plus, you don't need the extra 50 watts during storage anyway).

    -Bill

  6. #6
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    Default Re: dual charge controllers to one battery?

    i will say that you could possibly overcharge the battery. the single stage controller i will assume it to be just a voltage regulator so technically that is 2 stage as it will allow full bulk of the pv until it hits the absorb setpoint. if the voltage stays high and never is allowed to go to a float stage after long periods of being fully charged this will slowly cook the battery. being an agm if any venting occurs you will not be able to replace it unlike a standard battery.
    btw i charged my old wally world deep cycle batteries firstly by a sunsaver which is basically like your cheaper controller in that it held the voltage point and caused the current to diminish. maintenance was high but doable. later the controller blew out and i decided to charge them straight to the pvs as it was 128w of pvs into over 400ah of batteries. they fried. that's about the point i decided upon my agm (a concorde pvx1040) and a bluesky sb50 with bts. no problems, but i feel you will have problems if you go straight to the batteries from the pvs and a slower death with one that stays at full charge for long periods with a controller of that type that just holds the voltage high. your agm is sensitive to damage from gassing moreso than a standard battery and they are friable with lower power levels. i will agree the current may be low enough, but not when holding the voltage high and doing this for sustained long periods. add to that it is an agm and you are taking a bigger chance as you can't water it if it does gas.
    NIEL

  7. #7

    Default Re: dual charge controllers to one battery?

    The simple charge controller I have is a cheapo yellow 7 amp ICP model. I don't know if you call that 1 or 2 stage...I think it is just either charging or not charging depending on battery voltage.

    A slightly different subject...is there a good accurate way to determine where I am regarding battery life? Maybe getting to 50% depleted voltage (12.06V according to faq) occurs more quickly than it should? For example, I should have to use 40AH to get there with my 80AH battery, but maybe it happens after only 20AH? Is there a curve somewhere showing how a measurable battery parameter declines?

    Actually, it does seem to me that my battery is running down quite a bit more quickly than it should. Maybe my battery is pretty far on its way down, not too surprising I guess for a ~3 year old battery. Most of the cycles have been pretty shallow though.

  8. #8
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    Default Re: dual charge controllers to one battery?

    If it is a wet cell battery, do you use a hydrometer once in a while to ensure that you are fully charging the battery and that all the cells are balanced (equalization one in a while).? Using distilled water to refill?

    And, are your loads light (1/20 to 1/10 of the battery AH or 4-8 ampst maximum average current for an 80 AH battery) capacity? And is your charging current at least 4 amps?

    Is this a true deep cycle storage battery or some RV/Marine battery?

    Do you cycle the battery daily, or does it sit (or used for very light loads) for weeks, or longer, at a time and use quite a bit of water during non-use? A charger with a float setting would help in this usage.

    If your battery is getting fully charged, equalized at least once per month, clean distilled water used, and the connections are clean--there is probably nothing more you can do other than replace the battery (possibly with a better brand next time).

    -Bill
    Last edited by BB.; May 14th, 2008 at 7:41 PDT.

  9. #9

    Default Re: dual charge controllers to one battery?

    AGM battery...don't do any maintenance. It is a true deep cycle battery..bought at Bass Pro Shops. I believe these are made by East Penn which makes Deka batteries.

    I have my parallel charge controllers w/ two 50W panels so I suppose I can sometimes get combined ~5 amps for charging. Usually though, I think I am below 3 amps as often only the multicrystalline panel is getting full semidirect sun. This should get a big boost when I add my 135W panel. (Although now I am leary of even including my old 50W mutlicrystalline panel in parallel with it as the 50W panel is ICP (junk?) that has a listed "operating voltage" of 15V...much less than 17.3V of my new panel at max power. I will measure to see if ICP panel really is 15V or is actually higher..don't trust the specs)

    Of course on rainly/cloudy days I get almost no charging and the battery discharges more and more. Every once and a while the LVD of my charge controller kicks in which means I guess I am below 20% left on battery.

    Regular loads are usually light...usually 2 to 3A or less (I often just have a few low wattage CFL-bulb lights and phone chargers on this system) that run usually for several hours most evenings.
    Last edited by wxh3; May 14th, 2008 at 9:04 PDT.

  10. #10
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    Default Re: dual charge controllers to one battery?

    Sounds like there is nothing much more you can to save/monitor the battery (without using a relatively expensive battery monitor).

    LVD's generally use the battery voltage as a disconnect point--and they are very inaccurate as the battery voltage is very dependent on load. So, usually they are set very low and the battery is pretty much dead at that point.

    You might as well try paralleling the two panels together if the Morning Star controller you have a is PWM (not MPPT type). The worst, that I can think of, that would happen is when the battery is cold, and very near fully charged, which ever is the lower voltage panel, will simply stop supplying current and your array output current will drop by ~1/2.

    But, you want this to happen anyway because the battery is near fully charged and you don't want to over charge it (especially an AGM which cannot have water added back in after it has been overcharged).

    Or, at least, purchase a cheap charge controller for the 50 watt thin film panel (assuming the panel is still working OK)... 3 years sounds a bit short of life for the AGM battery...

    -Bill

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