Voltage below 12 volts ok to charge with?

SolaryellowSolaryellow Registered Users Posts: 10
My small scale wind project is going to use mall pm dc steppers

No charge controller

Just a diode and 2 12 volt deep cycles parallel.

If the output of the motor only generates 6 or 8 volts at times, is that still going to charge the battery?

I can easily get it to output 12 + volts, but lots of times it will probably be less.
Is this ok?

Secondly, does a charge controller only switch on to send power to a battery when it exceeds 11 or so volts?

What happens when direct connecting to a 12 volt bank of batteries And your motor starts to put out 16 or 18 volts?

The motor in question only states 24vdc 1.8 amps, but drill test and 300 rpms is indicating about 10-12 volts.
Not sure of current forgot to check.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,004 admin
    Re: Voltage below 12 volts ok to charge with?

    SY--Welcome to the forum!

    Regarding your question... Electrical Flow is a lot like water flow.

    If you do not have enough voltage to exceed the battery voltage, no "forward" current will flow.

    And, even if the voltage is high enough, if current is near zero, then you are not moving any energy...

    P=V*I (power = voltage * current)

    There are various electronic ways of "bumping" up voltage (boost mode power supplies is one). However, it is generally not done with a variable voltage/current source as it gets pretty complex pretty quickly to design and build such a power suppy.

    Normally, you match the "generator" to the load... In your case, if you are using "12 volt flooded cell batteries"--you need to generate substantial amounts of power (current) at roughly 14.2-14.5 volts. Anything less than ~13.6 volts will not really even charge your battery bank.

    Specifically, you asked about other solar charge controllers---they typically require an input voltage of Vbatt+2volts to move current into the battery. Many charge controllers use the input voltage to run their internal electronics (MPPT type)--so they can "wake up" at lower voltages, but they will not output any charging current until Vsolar>Vbat+1-2volts

    Even if you just add a blocking diode (instead of a charge controller), you still will have ~0.2-1.0 volts of drop accross the diode (depending on diode and current)--So Vgen>Vbatt+Vdiode-drop

    While we try to help everyone here--there is a hardcore do it yourself wind site at Otherpower.com (including a forum).

    www.otherpower.com

    Your tests may or may not be accurate as to the output voltage (different DC Motors have differing requirements for wiring and such)--but choosing 300 RPM max is probably not a bad place to start for a direct drive unit.

    But, unless the DC Motor is designed to take the dynamic loads of a prop (and trust loads too)--you would probably need to build an offset belt drive -- and can choose a ratio that better drives the motor.

    If you need a good quality, off the shelf, smaller DC output wind turbine for 12 volts--take a look at the "TLG" Wind Turbine. You can see what it takes to design and build a turbine that will withstand the elements.

    www.tlgwindpower.com

    I cannot give you any first hand information on how well it works--but it seems to have a popular following out there.

    Lastly, I am not a big booster of Wind Power--But (from what I have seen) most wind systems do not generate near as much power as the owners hope (or believe).

    TLG 500 Wind Generator (thread)
    Small windpower a scam ? Survey says SO (thread)

    FAQ type stuff
    Wind Power Basics (Home Power Mag)
    Is Wind The Answer? Read This First (Wind-Works.org)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RandomJoeRandomJoe Solar Expert Posts: 472 ✭✭✭
    Re: Voltage below 12 volts ok to charge with?

    If you direct-connect to the batteries, you have to get above the battery voltage (plus any other drops - your diode will drop 0.7V so that has to be added as well) to push *anything* into the batteries. And in order to charge them in any kind of timely manner, you need to get fairly well above the battery voltage. It is possible to "float charge" a battery, but it takes an exceedingly long time to get the last bit of charge into it. And the "float" charge would be in the neighborhood of 13.5-13.8V. Most chargers go to 14.1-14.5V (what is called "absorption" phase) to get the battery charged in a timely manner.

    It is theoretically possible to use a "boost" power supply that would take your lower voltage and raise it above 12V, but in reality it is highly unlikely to be of use. There are losses involved, and you will have very little power available anyway at those slower speeds, so you may burn most or all of the power available during those times in the power supply itself.

    I have two different charge controllers. One is a more basic PWM, the other is a fancy MPPT model. The PWM one will "switch on" (basically connecting the panels to the battery) any time it senses voltage on the panels. So, even just at dawn it indicates it is "on" but in reality there is no current flow yet. The MPPT one will try to generate power whenever the input voltage has risen so many volts, and if it can't generate a minimum amount of current shut off back off. It too will start to try not long after dawn, but doesn't actually stay on doing anything for several hours.

    Of course, both of those are connected to solar panels and are not suitable for wind turbines.

    I'm not a wind turbine expert, I cheated and bought one! ;) Mine (an Air Breeze) has a built-in regulator - it won't put out anything until it's over the battery voltage, and shuts itself down automatically if the voltage exceeds an adjustable maximum to prevent overcharging.

    But for a homebrew turbine, the usual control method isn't a charge controller, but instead a dump load. A circuit watches the battery voltage, and whenever it exceeds a preset value the dump load is turned on to use excess power. That can be almost anything, commonly a bank of resistors, or water heater elements. The most basic dump load control just turns on the output when the battery exceeds "X" volts, then turns if off when the voltage falls below "Y". Fancier models can PWM the dump load, for finer control.

    There is a company designing/testing an MPPT wind turbine controller, but AFAIK it isn't in production yet.

    For more than you ever wanted to know about charging batteries, try here:
    http://www.batteryuniversity.com/

    And for all kinds of DIY wind turbine information, you can try here:
    http://www.fieldlines.com/

    Edit: And I see BB beat me to it... :cool: The 'fieldlines' link above is the forum side of the Otherpower site he mentions.
  • BB.BB. Super Moderators, Administrators Posts: 32,004 admin
    Re: Voltage below 12 volts ok to charge with?
    RandomJoe wrote: »
    Edit: And I see BB beat me to it... :cool:

    It was down to the wire--2 minutes and it could have gone the other way. :D

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SolaryellowSolaryellow Registered Users Posts: 10
    Re: Voltage below 12 volts ok to charge with?

    Thanks, everything was answered in those posts.

    I am only doing this for fun

    Personally I like the solar grid tie systems better, and had my eye on a 6k system

    Would have that going but CT state rebate program benefits the installer not the consumer.

    I just wanted to build something in the meantime.
    All those links and sites are nice to have, and i couldnt find the forum on the otherpower website.




    Forgot to add

    So it is ok to throw 20 or more volts into a battery to charge it?

    I was originally going to use a dump load, but the question about using higher voltage to charge had me wondering.
  • BB.BB. Super Moderators, Administrators Posts: 32,004 admin
    Re: Voltage below 12 volts ok to charge with?

    It is the www.fieldlines.com llink that RandomJoe pointed to...

    It is a bit confusing as a forum... The Opening Post is called a "Full Story"--Just click on the link to read the "thread".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RandomJoeRandomJoe Solar Expert Posts: 472 ✭✭✭
    Re: Voltage below 12 volts ok to charge with?
    Forgot to add

    So it is ok to throw 20 or more volts into a battery to charge it?

    I was originally going to use a dump load, but the question about using higher voltage to charge had me wondering.

    No, 20V would be too high. However, unless your charging source is seriously oversized what will happen is the voltage gets held down to the battery voltage and the amps go up. That's what you need the dump load (or charge controller if solar) for - once the battery has charged, the terminal voltage will elevate quickly, and if you get much above 14.5V the battery will start to gas excessively. If your battery is a sealed type, you will quickly kill it!
  • BB.BB. Super Moderators, Administrators Posts: 32,004 admin
    Re: Voltage below 12 volts ok to charge with?
    Forgot to add

    So it is ok to throw 20 or more volts into a battery to charge it?

    I was originally going to use a dump load, but the question about using higher voltage to charge had me wondering.

    The battery really sets/regulates the voltage--unless you are dumping so much current into it that you are overwhelming the chemical processes in the battery.

    When you try to match power to load... You try to match the I*V characteristics between the source and the load.

    For example, I could use two 12 volt batteries in series to charge a 12 volt batter... But the the "voltage drop" from 24 to 12 is generally wasted as heat because if the internal resistances of the batteries and the wiring.

    That is what MPPT controllers are for. They are energy conversion devices that efficiently (typically >90%) converter power from one voltage to another.

    For example. You want to charge a 12 volt battery at 20 amps using converter... Roughly (ignoring losses):

    P=I*V=12 volts * 20 amps = 24 volts * 10 amps = 240 watts

    Think of the MPPT type controller as the rough equivalent to an AC Variable Transformer.

    So--while you could charge your battery with a DC generator outputting 20 volts at 10 amp... The battery may see 14 volts at 10 amps:

    P=I*V=10a*20v=200 watts (optimum output from DC Generator)
    P=I*V=10a*14v=140 watts (actual charging into battery)

    Many devices (such as solar panels and generators) are current limited in their output--so when you drop their output voltage (heavier load), their output current remains relatively constant. So--an MPPT controller can optimize the Pmax=Isrc*Vsrc from the source (generator/solar panel/etc.), and convert that into the optimum Pmax=Ibatt*Vbatt of the destination (battery).

    You can read a nice summary about MPPT vs PWM controllers at the MPPT FAQ from our host's webstore.

    There are a few companies out there that are working on MPPT controllers that match the I*V curve of a DC energy source to that of the battery bank (Outback may be shipping and Midnite Solar is working on getting one out "soon"?). However, I would bet that these are for larger wind turbines than you are working on right now.

    Note--for the most part (as far as I know--which is not a lot)--Solar Charge Controllers are designed to only "drop" voltage--not to "Increase" or "drop/increase" voltage. It is not that it is terribly difficult to do--it is just usually more expensive (more components, less efficent, more copper, etc.).

    By the way--the reason most wind turbines use a "dump" load is because the typical Horizontal Axis Wind Turbine will overspeed in high winds with no load. So, the charging system is (typically) designed to always put a constant load on the wind turbine to keep it from overspeeding--and "dump" excess charging current into a DC electric heating element.

    -Bill

    PS: +1 for RandomJoe ;)
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SolaryellowSolaryellow Registered Users Posts: 10
    Re: Voltage below 12 volts ok to charge with?

    Thanks all...

    I understand the dump load part, if I ever do build something bigger, I would use that..I was thinking electric heater element too, or some light bulbs.

    Because I was using a very small DC motor, I didn't think I would be generating that much power enough to worry about over charging, it won't generate more than maybe 1 or 2 amps tops..at full speed..
    I was thinking trickle charge for my deep cycles, but now I see how I have to not only overcome the battery voltage 13.5 to 15 volt range, but also the diodes .7 drop as well...(The water flow analogy paints a clear picture)
    Good thing I am well versed in that department...:)

    The concern was if I do generate more than 15 volts or so...
    Because the drill spin test has got it up to 15 or 16 volts and probably could go a little more..
    BUt if I only have 18 volts say, and low low amperage, 1-2 amps, I was hoping that would at least charge it...

    My AC battery charger has a 12 volt 2 amp slow charge..this is what I am comparing it too...

    A dump load circuit could still be used just in case I assume, Or I can downsize to a 6 volt battery bank..
    I just want to tinker with cheap night lighting in the house. 6 volt could be ok too.


    I also took the motor apart again, and did manage to see the wiring inside..

    Some weird stuff I have never seen.

    8 coils, of which the 6 wires only go to 6 of them...
    the other 2 coils windings are independant..

    3 wires are ganged up on 3 right side coils
    another 3 wires are ganged up on 4 left side coils...

    needless to say, this one is beyond me..

    I will stick to the simple 2 wire motor I have..
  • BB.BB. Super Moderators, Administrators Posts: 32,004 admin
    Re: Voltage below 12 volts ok to charge with?

    "Small" is always relative...

    Small charging current (for a lead acid battery in good shape) <1% of Amp*Hour rating. Probably don't need a charge controller.

    Minimum Rate at which to charge a LA battery--~5%

    Maximum Rate at which to charge a LA storage battery--~13%

    The "average" car sized storage battery is around 80-100 Amp*Hours (at 20 hour rate).

    And, if you don't watch the water in the battery--even a 1 Amp "Trickle" Charger will "boil" the battery enough to expose the plates in ~6 months.

    For a 1 amp trickle charger (supposed to be designed as a "maintenance charger")--I found that I have better luck if I put them on simple lamp timers... And set them for ~1-4 hours of "on time" per day... My in-laws have a newer car with a bunch of computers, GPS, Lojack, and used to have OnStar (analog). I found that I needed to run the 1 amp charger about 4 hours per day to make up for the constant drain of the "accessories). "Plain vanilla cars"--about 1 hour per day seems to work well.

    -Bill

    PS: Solar panels are probably a better/more reliable method of keeping your batteries charged.

    Either bigger panels and a charge controller if you have a lot of batteries--or just get the old VW ~5 watt crystalline solar panels for $5-$20 each from EBay for individual batteries (search for vw solar charger).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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