Switching from 24 Vdc battery to 24 Vdc grid power

Discussion in 'The Projects Forum' started by nfalls, Mar 29, 2016.

  1. nfalls

    Thread Starter New Member

    Mar 29, 2016
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    I just started on a research team where the system is powered by two 12 V batteries that are charged using a solar regulator, but need the circuit to be powered by grid power when the batteries drop below 23 V. I have the 24 Vdc grid power, but need something to switch between the two when the battery voltage drops below 23 V or charges back to 24 V. I also cannot have any loss of power during the switching process. Any help on what sort of device I could use would be greatly appreciated.
     
  2. Marcus2012

    Member

    Feb 22, 2015
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    Hi :)
    Have a look at "Prioritized Power-Path Controllers" and see if that's the sort of thing you're looking for. If it's a non-overlapping hot-swap you're after.
     
  3. tcmtech

    Well-Known Member

    Nov 4, 2013
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    If the 24 volt power supply has an adjustable output turn it down to 23 volts.

    If not put a diode or two in series with the 24 volt grid source so it only starts supplying power when the voltage in the batteries has dropped below that point.

    Either way when the load exceeds the solar panel output and the battery voltage has dropped the grid power will stabilize it and keep it from dropping any further.

    BTW if you are using common lead acid type batteries and didn't already know the full charge point should be around ~28.8 volts not 24. If you did then never mind. ;)
     
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  4. nfalls

    Thread Starter New Member

    Mar 29, 2016
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    Thanks, that appears to be the sort of thing i was looking for.
    I talked to a professor today though and he suggested i just make my own out of mosfets and a capacitor to hold it over during the switch. Any thoughts on this?
     
  5. Marcus2012

    Member

    Feb 22, 2015
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    Not too sure what he meant but I think it's very similar to what tcmtech mentioned earlier but using a P-channel as a switchover. I'm not sure but I'm thinking something like this but with inverted inputs and a capacitor between each to minimize ripple on switchover..
    [​IMG]
     
  6. tcmtech

    Well-Known Member

    Nov 4, 2013
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    I don't see any reason to over complicate the system being it is using DC power and properly set power supply voltages will automatically dictate when what power source takes over as the system voltage goes up and down.

    If the grid power supply is set at 23 volts then it will try and keep the system and battery above 23 volts but should the solar panel feed supply more power than the load is taking it will automatically start charging the battery and raising the system voltage to a point where the grid power is no longer carrying any load again.
     
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  7. Marcus2012

    Member

    Feb 22, 2015
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    I'd have to agree I can't understand how to completely saturate a MOSFET for a hard-quick switch without additional components (comparator etc.).
     
  8. dannyf

    Well-Known Member

    Sep 13, 2015
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    That's very hard to do.

    What about using relays?
     
  9. Marcus2012

    Member

    Feb 22, 2015
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    It can just be done as tcmtech described earlier with no lose of power at all but this topology might cause problems with the lead acid batteries not reaching maximum potential before switching back on from mains power. If you want a reliable switching with a set overvoltage and undervoltage window for prioritized power rails I would use the LTC4417 (I couldn't find a 2 pathway one). It's entirely possible I was wide of the mark regarding what your professor said so see if he wouldn't mind clarifying what he meant.
     
    Last edited: Mar 30, 2016
  10. dannyf

    Well-Known Member

    Sep 13, 2015
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    "with no lose of power at all"

    It can be done with low loss of power but it cannot be done with no loss of power. That's simply not possible.
     
  11. tcmtech

    Well-Known Member

    Nov 4, 2013
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    [/QUOTE]

    The way I am suggesting would keep the batteries from over-discharging but would still rely 100% on the solar part of the system to do the actual battery charging once the load drops off far enough to allow for it.

    There's no real other way that I can think of if the solar side of the system is to be the primary power and battery charging source.

    If you have to disconnect the battery and solar power parts to recharge the battery and depend 100% on the grid for the power it rather makes the whole solar and battery part of the system rather pointless being they are obviously not large enough to keep up with the overall demands.
    If so you would be better off to just ramp the grid power side up to 29 volts via dedicated manually activated independent charger so it supplements the solar panels to recharge the battery rather than disconnecting them from the system.

    It would be the most efficient use of both the solar panel power plus grid power and it would not require anything to be disconnected.
     
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  12. Marcus2012

    Member

    Feb 22, 2015
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    Ahh semantics :p You are right I suppose :)
     
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