questions

Discussion in 'The Projects Forum' started by NY10, Sep 21, 2010.

  1. NY10

    Thread Starter Member

    Jul 30, 2010
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    0
    Hi,

    I am trying to come up with some kind of formula that will effectively or optimize the battery usage.

    I have two batteries, primary 24v and secondary 12v.

    I am using these two batteries simultaneously. For example, 12v from primary and 6v from secondary.

    Does anyone know how to come up with effective or optimizing formulas for this type of situation ?

    Thanks.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Missing lots of details. Size of battery, type of battery, current required by primary and secondary loads.

    ???
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    1. Figure out what you want to optimize.
    2. Determine what factors affect #1
    3. Adjust #2's to make incremental increases in #1

    Without a LOT more detail, that's all one can say.
     
  4. NY10

    Thread Starter Member

    Jul 30, 2010
    47
    0
    I hope this give you enough information to point me to right direction or help me out.

    Primary battery
    http://www.all-battery.com/rectangularnimh24v4200mahbatteryfore-bikesscootersandrobots-1.aspx

    Secondary battery
    http://www.all-battery.com/onenimh24v2000mahbatterywithbareleadsfore-bikesscootersandrobots.aspx

    variable definitions:
    splitper = percentage of primary battery used that cycle. 1-
    splitper is secondary battery percentage
    dutyCycle = period
    thorttlepos = throttle position
    hightime_p = time primary battery is high for that period
    hightime_s = time secondary battery is low for that period
    lowtime_p = time primary battery is low for that period
    lowtime_s = time secondary battery is low for that period

    how variable are currently used:
    hightime_p = dutycycle*throttle pos*splitper
    hightime_s = dutycycle*throttle pos*(1-splitper)

    lowtime_p = (1-dutycycle)*thottlepos * splitper
    lowtime_s = (1-dutycycle)*throttlepos * (1-splitper)

    I need to find formula for split percentage. That depends on throttle
    pos (position) and secondary battery
    It needs to be such that the average output voltage is the same for
    the case where there is only 1 battery being use and if there is both
    batteries being used
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    Not even close! That may say more about me than your project, but it's unclear what you're trying to accomplish. Some questions:
    1. Why are there 2 batteries in the first place (instead of one larger one, or two identical ones)?
    2. Why would you bother to switch from one to the other, to get higher average current than one can safely supply?
    3. What exactly is the goal - you mentioned average voltage, but is this time-averaged voltage at some specified current draw?

    Batteries are complex - they drop voltage depending on charge state, respond to temperature, have internal resistances that vary with age, and on and on.

    BTW, those are both 24v batteries, not 24 and 12v as noted in your first post.
     
  6. NY10

    Thread Starter Member

    Jul 30, 2010
    47
    0
    The project that I am working on is a scooter project that have a better energy efficiency therefore the scooter travels longer distance. (Kinetic energy recovery system)

    1. Why are there 2 batteries in the first place (instead of one larger one, or two identical ones)?
    => 2 batteries are used. One is primary and the other is secondary. The primary battery is used for supplying voltage and secondary battery may used for supplying voltage while charging as well.

    2. Why would you bother to switch from one to the other, to get higher average current than one can safely supply?
    => I am applying a PWM (pulse width modulation) to make my scooter a longer lasting.


    3. What exactly is the goal - you mentioned average voltage, but is this time-averaged voltage at some specified current draw?
    => The ultimate goal of this battery thing is that I want to make my scooter travel longer distance.


    Batteries are complex - they drop voltage depending on charge state, respond to temperature, have internal resistances that vary with age, and on and on.

    BTW, those are both 24v batteries, not 24 and 12v as noted in your first post.
    => Yes, I mistyped the first post. Two batteries are 24v with different Capacity rating of battery.

    I hope this clears thing up little bit better. I want to get at least some advice on the subject as well as getting some direction.

    Thanks
     
  7. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,789
    945
    PWM will do nothing for making a battery last longer in your application. It can only help with something like making the motor go slower, while also providing a higher average torque output.

    Not something that matters where the amount of power needed to go from point A to point B in a set amount of time remains the same. Nothing changes that fact. A motor uses X amount of power to go a set rate of speed on level terrain with you on board the scooter. Changing the way that power is applied will do nothing for HOW MUCH energy it takes. Unless you go slower to save energy, or push with your feet going up hill, it will always take the same amount of power(Unless the wind is at your back :) )

    So your problem is one of your own making on that point. As for having two batteries instead of one, that WILL increase your range in miles by the amount available in the second battery. I assume these are Lead acid batteries, since most others are too expensive for the average person to utilize in this way. Lead acid batteries can be driven pretty hard as far as current draw is concerned, but the amount of power available from them vs. the current removed from them is not a linear amount. 100 amp hours of battery will only deliver that amount at a stated steady delivery over 8 hours or 20 hours(depends on the rating method). If you remove the power quicker then the actual amount you will get will be less than the rated power. If you remove the power slower then the capacity will actually increase. Meaning you might get the equivalent of 110 amp hours worth of power if you take out less than the amount used to rate the battery.

    8 hours at 12.5 amps will be the rating for a 100 amp hour battery.

    If you draw 25 amps you will not get that amount for four hours DESPITE the 100 amp hour rating. at that rate of discharge(larger than the rating amps) you will get maybe 3 or 3 and 1/2 hours.

    If you draw current from the battery at the rate of 10 amps you will get more than 10 hours of discharge time (10hr x 10 amp = 100 amp hours)

    Knowing this you should see the course of action is clear. Running the batteries together, discharging at the same time will lower the average rate of discharge for each battery, thereby giving you a larger reserve than the stated discharge rated at 8 hours and X amount of amps.

    Any of this clear?

    If not keep asking questions on what ever you don't understand and we will get you straight on it.
     
  8. NY10

    Thread Starter Member

    Jul 30, 2010
    47
    0
    I am not running two batteries at the same time.

    I was thinking, running primary battery for certain amount of time, and then running secondary battery for certain amount of time, and then go back to running primary battery...back and forth.

    So the whole point of PWM is to make motor go slower or faster.

    How can I apply this PWM to what I want or need. Like I mentioned, I am using kinetic energy recovery system application that when I ride a scooter, it will use the battery. If I brake, it will storage energy to secondary because there will be kinetic energy.

    The whole point of this project is to use kinetic energy mechanism to drive a scooter for a longer range.

    If so, is there any better way of implementing it ? I've already have two battery (primary and secondary), IC driver, and other small parts.

    Please let me know what you think of this. I would really like to hear your opinion on the subject.

    Thank you very much.
     
  9. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,908
    2,169
    Sounds like a cool project. If you look at just battery usage/recharge management it's better to let the battery that needs to be recharged drop below a 80% SOC because the charge efficiency factor (energy in/stored) is so much better for lead-acid batteries below this point. You still have to give them a full charge regularly to keep full capacity.


    http://photovoltaics.sandia.gov/docs/PDF/batpapsteve.pdf
     
    Last edited: Sep 24, 2010
  10. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    This is true for the NiMH batteries the OP referenced as well. Near full charge, most incoming energy will show up as wasted heat. I believe peak efficiency (admittance, the percent of input energy that gets stored) is actually around 50% charge. Sadly that means your battery should be ~2X as big as you might otherwise think, so that it can operate in that range and still give the power you need.

    I'm still a little confused why you would ever want two batteries instead of one bigger one. Like a hybrid car, you're either charging or drawing off. I don't think you'd ever want to be drawing power off one battery while charging the other. I can see where the electronics might be simpler if you have the charging system and the power system separated, and only occasionally switch the roles of the two batteries. But surely you could use a single battery and just switch state, eg. go into recharge mode when the brake is applied, otherwise you're in discharge.
     
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