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Heating and cooling: running a DC fan off of a battery charger ?

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    • CommentAuthorgyrogear
    • CommentTimeAug 26th 2016
     
    I have two battery chargers - one is "automatic" (to avoid over-charging): per booklet, the absorbed power = 90W.
    Booklet also says : "do not use this device for anything other than charging a battery". fair enough.

    My second charger is old and I have lost the booklet, but it works!
    It charges at 3.5 A, off 240V AC.
    I have used it to run an automobile fan - 90 W, 12 V, it runs at around half-speed, which is OK for my application.
    I am (1) curious to calculate the absorbed power and (2) curious to know whether I should not use it in this application ?

    (I don't want any fires !).

    Thanks for any assistance !

    gg
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 26th 2016
     
    • CommentAuthorEd Davies
    • CommentTimeAug 26th 2016
     
    To get a reasonable idea of the power it would be best to measure the voltage and current that's coming out of the charger. Any typical multimeter should be able to do that: usually measuring on the 10 amp range in series with the fan and on the 20 volt range across the fan connections in turn - be very careful to replug the meter leads correctly between the current and voltage connectors otherwise your meter might go bang.

    Sorry if this is too well known to be worth saying, but if it is 12 V and 3.5 A then the power being absorbed would just be the product of the two: 42 watts. In reality, I expect the charger would current limit at about 3.5 amps but what actual voltage this would happen at is harder to say: could easily be anything from 10 to 13 volts or even further afield.

    Running a motor at half speed like that could in theory be a problem but I'd expect an automotive fan to be robust enough not to worry. It's probably designed to be run slowly anyway. Are you just experimenting (air leak testing?) or planning to run it for long periods? I doubt battery chargers are all that efficient so it's probably not an ideal set up for long-term operation.
    • CommentAuthorgyrogear
    • CommentTimeAug 26th 2016 edited
     
    Thank you, gents !

    ST, I d/l your manual - it looks a bit over my head, but might make a great winter read provided I can get my head round their English ! At 1st glance, however, they don't seem to make a distinction between AC and DC motors - which I find rather surprising...

    Even if they did, putting it all into practice on a Dusty Old Item purchased from a scrap yard might in fact prove to be an intellectually challenging task to this writer :shocked:

    =========
    Thanks, Ed, I gathered that the draw would be at least what you said - V x A. plus a bit for heat etc.
    I read somewhere on the web that automotive radiator motors are not built for "continuous service" which seems to be defined as more than 60 minutes continuous use, or five ON/OFF cycles in a one-hour period.

    I therefore assumed that running it at half the rated amps (3.5 instead of full amps, off of a car battery...) would suit my purposes.

    Guessing it pushes 1600 CFM at full power, therefore, I should be getting 800 CFM off the charger.
    I just wondered, as an uninitiate, what was the issue with running a DC motor off a charger.

    (Compounded by the fact that I "don't quite trust" the 12-volt power supply that I swapped off the neighbour for my antenna dish...). In fact, I guess my solution is to buy a tester, learn how to use it, then check out the PS...).

    (Funny how explaining one's problems throws up possible answers !)

    As you (inevitably) :bigsmile: guessed, this is an experiment to drag bigtime air for lowest cost ! from my (in-progress) aspirated solar wall.

    gg
    • CommentAuthorCWatters
    • CommentTimeAug 26th 2016
     
    If the 12V fan is going at half speed then that suggests it's not getting the full 12V from the charger. That implies the charger is operating in "current limiting mode".

    On some power supplies current limiting mode is intended as a form of safety protection not a long term operating mode as it can overheat. However I would hope that a battery charger can operate in current limiting mode for long periods, especially if it's intended to be used as a float charger.
    • CommentAuthorEd Davies
    • CommentTimeAug 26th 2016
     
    Posted By: CWattersHowever I would hope that a battery charger can operate in current limiting mode for long periods, especially if it's intended to be used as a float charger.
    Current limiting would apply when it's charging a very flat battery (or trying but failing to charge a dead one) rather than when it's floating in which case it would be in voltage-limiting mode.
    • CommentAuthorgyrogear
    • CommentTimeAug 26th 2016 edited
     
    Since the 90-watt fan would normally take 90/12 = 7.5 Amps, and since the charger can only provide 3.5 Amps, then would it be OK to assume that the charger is operating at its nominal rating ?

    The arithmetic is important to me, as I want to compare the price per CFM obtained: at the moment I use 2 x 29-watt AC fans = 58 watt-hours to give 2 x 175 = 350 CFM.

    Running the car fan effectively doubles the CFM (to 800) and if it does not effectively double the power draw, then I am saving on energy...

    Therefore the aim of the game is to be able to state how much the car fan is actually using, in terms of watts.
    Assuming that the rating of the charger is 12 x 3.5 = 42 watts, "plus a bit for noise and heat", then adding on some rounding, assuming we get to 58 watt-hours, which equals the same power draw as the two AC fans, but for twice the CFM.
    Which looks like a good deal from my own (admittedly non-technical but economic) viewpoint...

    Basically, for perspective, the fan will run 12/24, for 365 days a year.
    Which will cost me around 45 Euros p.a.

    gg
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 26th 2016
     
    Posted By: gyrogearthis is an experiment to drag bigtime air for lowest cost
    That is interesting, I did something similar for my BSc.

    You could go back to first principles and work out the mass of air moved, at what speed and by unit time, then see what the wattage of the fan is, that will give you an overall efficiency figure.
    • CommentAuthorEd Davies
    • CommentTimeAug 26th 2016
     
    Best thing to do would be to get a plug-in energy meter and measure the power going into the battery charger. That likely won't be very reliable when the charger is lightly loaded but my guess would be that it's reasonably accurate when it's loaded. Still, a plug-in meter which tells you the power factor would be reassuringly less inaccurate.

    Posted By: gyrogearsince the charger can only provide 3.5 Amps, then would it be OK to assume that the charger is operating at its nominal rating ?
    It's reasonable to assume it's at its nominal current but likely the voltage will be dropped significantly.

    Pedantic asides:

    >the price per CFM
    Price per CF (cubic foot) presumably.

    2 x 29-watt AC fans = 58 watt-hour
    58 watts. “-hour” is spurious here.

    Posted By: SteamyTeaYou could go back to first principles and work out the mass of air moved, at what speed and by unit time, then see what the wattage of the fan is, that will give you an overall efficiency figure.
    You need the pressure difference rather than the speed to work out the power you're getting out.
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 26th 2016 edited
     
    Posted By: Ed DaviesYou need the pressure difference rather than the speed to work out the power you're getting out.
    Yes, may be easier to do with a home made manometer than a home made mass flow meter.
    • CommentAuthorgyrogear
    • CommentTimeAug 27th 2016
     
    Thanks again both - I found this...

    http://www.nyb.com/Catalog/Letters/EL-07.pdf

    I rather like the idea of a pitot tube, actually !

    However, I will also check out the plug-in energy meter as it sounds like I ought to have one anyhow !

    gg

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