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Using a table fan to recharge batteries : fact or fiction?

Discussion in 'Power Electronics' started by jeru, Feb 10, 2013.

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  1. jeru


    Feb 10, 2013
    I read somewhere that if you put a table fan that runs on batteries outside in the wind, it will charge the batteries in stead of draining them. I can see the logics in it, as an electro motor can also be used as a generator (?), but is this true?

    If so, is it possible to over-charge the batteries? And will this be a somewhat effective way to charge batteries?
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    It's a bit more complex that how you've described it, but yes, you can do something like this, and yes, it is very possible to overcharge the batteries.

    The effectiveness depends on the amount of wind and the power that the fan can generate. The blades on a fan are not very efficient for generating power -- they will tend to stall as the wind speed increases (at the very least they will generate a lot of drag).
  3. jeru


    Feb 10, 2013
    Thanks steve for your reply. Is there a simple way to prevent over-charging?
    For instance use an existing battery-charger?

    As you might have noticed, I am an electric noob. But the idea of using solar of wind power to charge batteries is making me want to learn more.
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Firstly, you want to prevent the battery from discharging via the motor... To do that you place a diode in series with the battery and the motor (all in one loop) so that power can only flow out of the motor (generator) to the battery and not vice versa.

    Preventing overcharge can be anywhere between simple and really hard depending on the type of battery.

    It's easiest for lead acid and lithium batteries, hardest for NiMH and NiCad.

    But we would need to know what sort of battery you intend to charge.
  5. ElectrocutedBaboon


    Feb 2, 2013
    I am curious, Steve, how does a battery become overcharged?

    I understand that the voltage source pumps electric energy into the battery. Let's take an AA battery, as an example, with 1.5 volts made available when it's 100% charged.

    After using the battery some, the voltage drops to 1.4v. Let's ignore the calculations used to predict how much energy is still available in the battery because that will depend too much on its chemistry--I'm to understand--and varies from one type of AA battery to another.

    At this point, the only thing we're concerned with is that the battery is NOT full, and that its maximum voltage (open circuit) is precisely 1.4v.

    To (re)charge it to 1.5v, the battery charger needs to deliver a sufficient voltage potential. Meaning, if the battery charger provides only 1.0v, the AA battery will NOT be recharged (instead, the battery would try to "charge" the battery charger, if such thing were possible).

    What will happen if the charger provides EXACTLY 1.5v, at let's assume more than adequate amperage? Will the AA battery EVER be overcharged in this situation? Or will there finally be reached a state of equilibrium at 1.5v where the battery is charged no further because its charges sufficiently "push back against" the charges of the battery charger?

    I'm guessing battery chargers are designed to deliver more voltage than the battery is ready to accept, but only because as the state of equilibrium was approaching, the rate at which the battery charges would slow to a crawl. So you'd end up having batteries that are charged to 95% instead of the full +100% (of rated capacity). Unless you're willing to wait, literally, an infinite amount of time. Please correct me if I'm wrong on the infinity part.

    Also, a side question that might help me and those reading this to understand what's going on:

    If I connect two AA batteries together, which are SIMILAR in voltage but NOT IDENTICAL (say 1.45v and 1.52v), positive-to-positive, negative-to-negative, and leave them in that configuration for a few weeks, I expect that the batteries will end up having identical voltages. Furthermore, I expect that this will NOT overheat the batteries, and that this configuration can be left intact infinitely, the batteries surviving into their shelf life expectancies.

    Am I correct in my assumptions made in the above paragraph?

    Thank you. :)
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Easily. It happens when you try to charge them beyond the point at which they can store any more charge.

    An analogous question is "How does a water tank get too full?"

    That's a reasonable analogy

    Firstly, when you 1.5V battery, I hope you are talking about a rechargeable (say NiMH or similar) battery, NOT a standard non-rechargeable AA battery.

    In fact, depending on the battery, it may have 1.6V to 1.7V across it when completely charged (for a non-rechargeable battery). Rechargeable batteries will have between 1.2 and 1.6V depending on the chemistry (NiCd are 1.2V)

    OK, I'll assume that.

    That's correct so far...

    Well, it depends on the chemistry, the temperature, etc. Some rechargeable batteries WILL be damaged is charged this way. Others may not get charged fully.

    In some battery type (lead acid is a good example) float charging works like this. Most battery technologies will withstand this for some time without a lot of damage, but it is not recommended for any of them (aside from Lead acid).

    Even with Lead acid batteries, the charge voltage is lower for float charging than for a periodic charging.

    Li-Ion batteries are closest to Lead Acid in that they can be charged from a current limited voltage source, however the recommendation is to terminate the charge when the current delivered at the voltage limit falls below a certain value.

    In a manner of speaking this is correct. Whilst you are charging the battery, the internal resistance causes a voltage drop which helps limit the current. This means the terminal voltage during charge is actually higher than the voltage effectively being used to charge the cell (which must always be in equilibrium).

    In theory (but not in practice) there will be an infinite wait. In practice, internal heating of the cell can reduce its voltage, allowing the same current to charge the battery more. This can lead to the "correct" voltage resulting in massive overcharge.

    This is especially important in cells where, following a full charge, additional energy is expended as heat, and where this causes the cell voltage to drop (NiMH are an example of this). Lead-acid overcharging results in electrolysis of water and that expends energy in another way that results in less heat (but still damage -- especially if the battery is sealed).

    Well, the voltage at the terminals will become *exactly* the same as soon as you connect them,

    No, this configuration can result in high currents flowing between the batteries, lots of heat, damage to batteries, and even fires.

    What actually occurs depends on many, many factors.

    As long as the batteries survive the equalization process, yes, a parallel connection can be left indefinitely.

    But note that both (all) the batteries must be the same type, and be at the same temperature.
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