Battery management system

Discussion in 'The Projects Forum' started by tntero, Jun 5, 2014.

  1. tntero

    Thread Starter New Member

    Dec 5, 2013
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    Hello there guys, It's been a long time since my last visit here! :p

    So, for this time, i was wondering if anyone could give me a few tips on building a battery indicator and charge controller, and by charge controllor, i just mean that it disconnects the batteries from power when they are fully charged and reconnects them when they fall below a set voltage.

    This is for a school project in addition to being something my boss asked me to do.

    Currently I'm studying using LM3915, an LED bargraph driver to indicate battery status and possibly connect a relay to one of its outputs, probably the last LED so when this LED is powered, the relay would be powered too and disconnect the battery charging cables. when the value falls below, the relay would be unpowered, reconnecting again the charger. Possibly there could be another relay so that it keeps the first relay connected for a while after the battery goes down in voltage, this way, the charger would never be in that point where it connects and disconnects regularly because on a histeresis.

    This is still on an early phase of development, so i can accept any tips, suggestions or ideas. Although it is not final, i have the idea nailed down, i just need some help with interfacing and making everything work well together

    EDIT: The batteries to use are Sealed Lead-Acid 1.3Ah 12V batteries, 2 in series to make the 24V

    Thank you for your answers,
    Antero Duarte
     
    Last edited: Jun 5, 2014
  2. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,237
    How you charge a battery and when to stop the charge depends upon the type of battery. A NiMH battery is different from a lead-acid, is different from a Lithium-ion. So what type of battery do you want to charge.
     
  3. tntero

    Thread Starter New Member

    Dec 5, 2013
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    You're right, i was to include battery type and voltage but i totally forgot :p I'm editing to include this, thank you anyway
     
  4. crutschow

    Expert

    Mar 14, 2008
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    3,237
    Normally a lead-acid battery is just left on a trickle-charge to maintain the charge after reaching full-charge. Is there a reason you need to disconnect the battery?
     
    #12 likes this.
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,542
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    Another way to create hysteresis is to follow the 3915 with a set-reset flip flop. Have level 6 set the FF to connect the charger, level 9 reset the FF to disconect, and level 10 light an overchagre warning light.

    ak
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,292
    1,255
    Lets see how complicated this can get. :rolleyes:

    Will the battery be running something while it is charging and discharging? If so is the load constant or can it vary? What is the charger like? (link?)
     
  7. #12

    Expert

    Nov 30, 2010
    16,321
    6,818
    I'm with crutschow on this one. I built a float charger for an SLA battery and watched the charge current go from 60 ma to fluttering between 1 and 2 ma in about 30 minutes. That battery has been on float charge between uses (starting the lawn mower) for over a year and shows no signs of weakness.

    It just isn't all that complicated.
     
  8. tntero

    Thread Starter New Member

    Dec 5, 2013
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    No special reason, I just wanted to disconnect the battery to reduce the standby current, but since #12 is saying the standby current on this will be as low as 1mA, i think there is no need for that... On the other hand, i wouldn't want the project to be just that, you know what i mean? I'm doing an intership in the company i work for because that would be the only way to finish the course, since i'm working and studying at the same time, so i didnt have much time to think and to dedicate to an internship project like they call it. I wouldnt want a grade just to be passed but i also dont have much time on my hands...

    The company works with home automation, more specifically, our stronghand is sectional garage doors automation, which led me to develop a system for when your network power is disconnected and you want to open your garage door. It really is not a necessity, but rather, an extra confort option.
    It is nothing new and nothing outstanding or even technically complicated, the easiest option would be to just run the batteries in parallel with the power source and connect them after the mains power has been rectified. Then the search for something a little more complicated came along, and i wanted a battery level indicator, so i found a circuit with the LM3915, which due to the lack of time hasn't been tested...
    But i tested the batteries, and powering the motor with the batteries, but it seemed to lack power, both when starting the motor and when running, so i thought of super-capacitors and bought 10 and made 2 banks of 5, making for a 27.5V 1,4F bank, but it was like nothing was there, making me think it might be the way the board was developed (i had nothing to do with that, so i don't really understand what could be causing this, since i didnt work to the company at the time).

    I'm just stuck with this idea, i cant change now because i told the school the project would be this one, i dont want just to connect the batteries, i dont have the time to dedicate myself 100% to this task, as the company needs me for my actual job...

    Thank you for your help guys, and for ronv, who was asking for a link (the rest of the questions i think i answered in the above paragraphs), this is the circuit im basing on to implement the system
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If you want to justify your project, one advantage to your system over a simple trickle charger is that your's will work with any battery chemistry. At the concept level, separate the control system from the charger. Sure if you're going to produce thousands you would combine the two into a reduced and optimized design. But for this project, the independence of the two functions can be seen as an advantage.

    ak
     
  10. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,237
    Some battery chemistries, such a Li ion or NiMH, can not be reliably charged with just a simple voltage cutoff.
     
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