Any need to isolate two chargers in parallel with diodes?

techntrek
techntrek Solar Expert Posts: 1,372 ✭✭✭
in Off Grid Solar & Battery Systems #1
On another forum members are arguing that two chargers in parallel (AC and solar) should be isolated from each other with diodes to prevent "backfeeding". Thoughts?
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  • jaggedben
    jaggedben Solar Expert Posts: 230 ✭✭
    #2
    Re: Any need to isolate two chargers in parallel with diodes?

    If they have different ampacities then they should have fuses or breakers installed so that a fault in the smaller one cannot cause the device to be set on fire by the current from the bigger one. (The same is true for charger vs. batteries.) From a code standpoint they should have separate overcurrent protection regardless. Breakers are also convenient as disconnects for servicing one or the other.

    I don't think any diodes are necessary or would do much of anything.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Any need to isolate two chargers in parallel with diodes?

    So they want to add two semiconductors to prevent one semiconductor device from back-feeding another semiconductor device? Can anyone not see the utter uselessness of adding two components that will only succeed in dropping Voltage on the line?

    Lots of folks use two solar charge controllers in parallel on the same bank without any such 'blocking diodes' and they work fine together, even when different controllers. You can also run your solar charge controller and battery charger to the same bank at the same time without any worries.

    Whoever is promoting this idea either has a lot of diodes to sell or failed electronics 101.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #4
    Re: Any need to isolate two chargers in parallel with diodes?

    Use my argument that the Battery is the "heart" of your system...

    Charger A to Battery--Diode needed? (battery stays at ~12-15 volts 24x7). No--none is needed.
    Charger B to Battery--Diode needed? (battery stays at ~12-15 volts 24x7). No--none is needed.

    So, two chargers both charging one battery--why would diodes be used.

    Now--In N+1 redundant power supplies, we use diodes as a way to isolate a failing power supply (shorted output, over voltage "crowbar tripped", etc...

    But when we do this in low voltage systems where a diode drop is "too much" to ignore (~0.2 to 1.0 volts, depending on type/size of diode), then we use "remote voltage" sensors so that the supply will add +diode drop to its output voltage to get the regulated voltage desired on the common bus.

    So--If you wanted two AC power supplies in a redundant (fail safe) installation--Then you can put a diode on each output and "take care of" the diode drop by adjusting the power supply output voltage or use remote voltage sense.

    Normally, instead we would simply use the fuses instead... If one of the supplies becomes shorted to ground (or way too high voltage), the battery would source/sink extra current and blow the protective fuse/breaker to the "failing" supply).

    But in solar, we normally are not worried about N+1 redundancy, we are worried about separate power sources charging a common battery bank. Each power supply should be 100% "safe and reliable" when connected to the battery bank (behind the proper protective fuse/breaker for the wiring+device limitations).

    Note that when you add isolating diodes, diodes themselves can fail shorted or open (plus large diodes are metal cases that may have to be insulated from the surrounding grounded metal (sheet metal boxes, etc.). So what happens next?

    Also, with computer power supplies, they have several fault protection schemes. A "crowbar" that shorts the output if the output >> regulated voltage (grounds power output to protect loads)--Very exciting if connected to large AH battery bank without fusing (a DC computer power supply can safely feed a dead short--not so for a battery bank).

    They also will fold back on current output if Current>>rated current (keep from overheating power supply). For a battery charger, you want the current to hit a peak value and hold until the "absorb" voltage is reached, then the battery charger holds constant voltage until float voltage is called for.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #5
    Re: Any need to isolate two chargers in parallel with diodes?

    The charge controllers only see the battery voltage as has been explained. My system has been running for years with two controllers connected directly (through fuses of course) to one battery bank. One controller from Solar, one from hydro. Unless you disconnect the batteries from the two controllers, and leave the controllers in parallel, there is no way they could "back-feed" to each other, and even then they wouldn't, as any properly operating CC will not allow it's output voltage to exceed whatever the battery voltage should be, so it would be no different than being connected to a fully charged battery. As was mentioned, connecting isolation diodes would drop the output voltages and waste watts. Some people just want to make things a lot more complicated than they need to be. Good thing you came to this form to get the truth.
    If however the "AC" charger mentioned is a cheap POS (piece of sh) it's unloaded voltage could nearly double, which would put a proper charge controller into "battery over-voltage" alarm if the batteries were disconnected, but the alarm would, or could be reset once the batteries were reconnected. But why would anyone in their right mind disconnect batteries from multiple controllers and / or AC battery chargers and leave them powered up?
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  • unicornio
    unicornio Solar Expert Posts: 217 ✭✭
    #6
    Re: Any need to isolate two chargers in parallel with diodes?

    I also I have two controllers in parallel (FlexMax60) charging my battery and both work sync (using the FlexNet DC) doing their job right. charge controllers already have a system that prevents the current return on their output circuits, so it is not only not necessary, but it is counterproductive to put diodes here and there is enough adequate protections to prevent any faulty item may cause any problem ...

    hope this help!...;-)
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #7
    Re: Any need to isolate two chargers in parallel with diodes?

    I should also add that many solar charge controllers will not function unless they are directly connected to the battery bank. If you use a blocking diode, the charge controllers may not even turn on.

    I also have a Battery Tender brand of AC Charger, and it will not function until there is battery voltage present on its output too (it resets to default if there is no battery power).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
    #8
    Re: Any need to isolate two chargers in parallel with diodes?

    The short answer is no - BAD IDEA - I have never seen a charge controller that could be "back fed".

    The diodes will add a .6 to 1 volt voltage drop, which means that the charge controllers will not see the correct battery voltage.
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #9
    Re: Any need to isolate two chargers in parallel with diodes?

    If the quality of the solar or AC charger is that poor that they need protection from "backfeeding" they would be very suspect in everything else they do, I think the garbage bin would be a good place for them.

    I use an AC 25a charger at same time as solar charger is in operation and also when its not. I have the AC charger automatically switched on if the battery voltage drops below preset,and also its turned on when using video projector or computers.
    Video projector get its hours confused if it is not switched off by the remote ,And we get many blackouts at unpredictable times.
    I think the idea of even using blocking diodes is beyond belief and for sure they are going to create problems for the chargers estimating the battery voltage,as many chargers cant be adjusted for a diode voltage drop.
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  • unicornio
    unicornio Solar Expert Posts: 217 ✭✭
    #10
    Re: Any need to isolate two chargers in parallel with diodes?

    yes!...is true, I have not contemplated that also I have a charger (OB VFX3024E) with which I can charge my battery using a AC genset even if both FM60 are working ... certainly if we have quality equipment need not worry about the strange phenomena that may occur with "careless" systems manufacturers...
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  • boB
    boB Solar Expert Posts: 1,032 ✭✭✭✭
    #11
    Re: Any need to isolate two chargers in parallel with diodes?

    I don't think that using a diode will even work.

    A diode between the charge controller's positive terminal and the battery positive terminal will prevent the
    typical charge controller (CC's used around here) from even turning on in the first place if the cathode side
    is connected to the battery positive.

    This is because these typical CC's (Outback, MidNite, etc) need the battery just to get booted up.
    Any ones that are this way won't work at all with that diode in there, unless you turn the diode around...

    If you turn the diode around, then the charge controller will be powered but then it won't be able to charge
    the battery because the CC's battery terminal will be more positive than the battery but the diode will
    be reverse biased.

    boB
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #12
    Re: Any need to isolate two chargers in parallel with diodes?

    Right on the money boB! You explain exactly what would happen with diodes in series with any reasonably designed controller. Nothing! Nothing at all. :D
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #13
    Re: Any need to isolate two chargers in parallel with diodes?
    boB wrote: »
    If you turn the diode around, then the charge controller will be powered but then it won't be able to charge
    the battery because the CC's battery terminal will be more positive than the battery but the diode will
    be reverse biased.

    You could put two diodes paralleled in opposite directions. That way the charger would work just fine.

    (Won't do much of anything and will just waste power and make setpoints inaccurate, but if people really want diodes . . . )
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #14
    Re: Any need to isolate two chargers in parallel with diodes?
    bill von novak wrote: »
    You could put two diodes paralleled in opposite directions.
    (Will just waste power and make setpoints inaccurate, but if people really want diodes . . . )
    Hahahaha Good point Bill.
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  • unicornio
    unicornio Solar Expert Posts: 217 ✭✭
    #15
    Re: Any need to isolate two chargers in parallel with diodes?
    bill von novak wrote: »
    You could put two diodes paralleled in opposite directions. That way the charger would work just fine.

    (Won't do much of anything and will just waste power and make setpoints inaccurate, but if people really want diodes . . . )

    yeah! ... this is the best way to put something to do nothing! ... fantastico! ...hehehe
    people want diodes! ... lol
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #16
    Re: Any need to isolate two chargers in parallel with diodes?
    unicornio wrote: »
    yeah! ... this is the best way to put something to do nothing! ... fantastico! ...hehehe
    people want diodes! ... lol

    And for those who want complete safety and isolation between the systems, use two back-to-back diodes in series. Now nothing bad (or good) can happen.
    SMA SB 3000, old BP panels.
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #17
    Re: Any need to isolate two chargers in parallel with diodes?

    Hahaha This is getting better by the post. Hahahaha
    Just too bad some of the self-proclaimed "experts" who are promoting these diodes wouldn't come read some of these posts :D
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  • unicornio
    unicornio Solar Expert Posts: 217 ✭✭
    #18
    Re: Any need to isolate two chargers in parallel with diodes?
    waynefromnscanada wrote: »
    Hahaha This is getting better by the post. Hahahaha
    Just too bad some of the self-proclaimed "experts" who are promoting these diodes wouldn't come read some of these posts :D

    sometimes technicians jokes can be a really exciting and funny!... ;-)
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  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    #19
    Re: Any need to isolate two chargers in parallel with diodes?

    Excellent arguments, everyone. ;) I will pass these on, they wouldn't listen to me but maybe they'll listen to you...
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  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    #20
    Re: Any need to isolate two chargers in parallel with diodes?

    Well, I posted all of the arguments and the response has been....

    nothing. LOL.
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  • unicornio
    unicornio Solar Expert Posts: 217 ✭✭
    #21
    Re: Any need to isolate two chargers in parallel with diodes?
    techntrek wrote: »
    Excellent arguments, everyone. ;) I will pass these on, they wouldn't listen to me but maybe they'll listen to you...

    where you post the "arguments"?....;-)
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #22
    Re: Any need to isolate two chargers in parallel with diodes?
    techntrek wrote: »
    Well, I posted all of the arguments and the response has been....nothing. LOL.
    So it seems that when faced with the truth, they drift off, and vanish in the forest.:D
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  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    #23
    Re: Any need to isolate two chargers in parallel with diodes?

    The thread in question is here: http://www.zillerelectric.com/forums/showthread.php?t=2246

    (its a generator forum, no competition with Wind Sun)
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