Any way to simultaneously draw 12V and 24V from a pair of SLA batteries?

Discussion in 'General Electronics Chat' started by jamiphar, Jun 5, 2013.

  1. jamiphar

    Thread Starter New Member

    Dec 9, 2010
    10
    0
    Can anyone think of a way to connect a pair of 12V sealed lead acid batteries so I can simultaneously draw both 12V (parallel) and 24V (series)?

    I tried sketching it out using several diodes to isolate the parallel connections from the series connections, but I couldn't find any way that would work. I'd think there has to be some way, maybe with the use of an IC or by inverting one voltage relative to the other.
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    There is no way to achieve both at the same time. You can only draw the 12v from one battery and 24V from both, makin the first one discharge much faster than the other one.
     
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Why not describe your project in a little more detail ??
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,387
    1,605
    Put 2x 12VDC batteries in series.

    Power 24V load directly.

    Power 12V load thru step-down switcher.

    This guarantees an equal load to each battery.
     
  5. paulktreg

    Distinguished Member

    Jun 2, 2008
    611
    120
    Make one of your 12V batteries twice the Ah rating of the other presuming equal loading?
     
  6. jamiphar

    Thread Starter New Member

    Dec 9, 2010
    10
    0
    Yeah, I would definitely want to have a parallel 12V draw to keep the battery levels even. Then I can charge them in parallel.

    I'm planning to make a universal battery pack using two 12V/12Ah SLA batteries. I would have a few buck-boost converters hooked up to various outputs. I would have one converter set at 5V and connected to a bank of USB ports for charging phones and what have you. I would have other adjustable converters hooked up to binding posts or generic DC locking jacks and I could make adapters to power anything from 3-30V.

    I can do all this with the batteries in series or in parallel, but I'd like to have the option of switching the voltage to make certain conversions more efficient. I'd like to be able to have different extremes of voltages being drawn and still have the better efficiency of having the source voltage as close as possible to the output voltage.
     
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    How much current do you want at the highest (30 volt ?) output.
     
  8. jamiphar

    Thread Starter New Member

    Dec 9, 2010
    10
    0
    I didn't have any specific current requirement in mind, but the batteries can handle a 2A or so maximum discharge, so I suppose somewhere around .8A at 30V.
     
  9. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,795
    If you are using switchers that can handle an input voltage of between about 8V and 28V, then functionally it doesn't matter if you have them in series or parallel. If you want to be able to easily reconfigure them to use whichever connect is most efficient, then just use a DPDT switch to manually reconfigure the second battery.

    You can then have a single output that is either 12V or 24V at the flick of a switch.
     
  10. jamiphar

    Thread Starter New Member

    Dec 9, 2010
    10
    0
    Yeah, that could work. I figured if I went that route, I would use a relay or transistor to switch it automatically when the output voltage is above or below a given threshold. For example, anything above 18 or 20 volts could switch to the 24V configuration.

    I just wanted to see if there was a way to have both source voltages simultaneously since I hope to have more than one variable voltage output.
     
  11. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,795
    Or, you could try Googling something like, say: PIC QEI module

    The first thing that comes up is a PDF file from Microchip titled, "Measuring Speed and Position with the QEI Module"

    Seems like that might be worth looking at.

    Wonderful thing, this Google. Perhaps it will catch on someday.
     
  12. jamiphar

    Thread Starter New Member

    Dec 9, 2010
    10
    0
    Was I supposed to know to Google "PIC QEI module?"

    I'm looking at the PDF. What exactly would that module accomplish with this setup?
     
  13. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,795
    Oooopssss!! :eek: My apologies -- that was a response to a completely different thread. Don't know how that happened. Sorry!
     
  14. jamiphar

    Thread Starter New Member

    Dec 9, 2010
    10
    0
    No problem!:p I was trying to figure out if I missed something obvious in one of the previous posts. It's all good.
     
  15. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,795
    So, to answer your questions:

    Sure. Isn't everyone a telepath?

    Not one damn thing!

    :D
     
  16. jamiphar

    Thread Starter New Member

    Dec 9, 2010
    10
    0
    Good to know, thanks for the clarification!

    One thing I'm trying to make is a low voltage cutoff for these batteries so I don't drain them below whatever their healthy minimum is. I think it's somewhere around 10.8V. Any ideas on this?
     
  17. jamiphar

    Thread Starter New Member

    Dec 9, 2010
    10
    0
    I don't think I caught what you meant at first, but now I think I do. You mean have the batteries in series for 24V, and then use the batteries individually for 12V but have something like a comparator switching the load from one battery to another as they drain? It seems like that could work just fine as long as there's no dead time during the switching.
     
  18. eKretz

    New Member

    Apr 21, 2013
    17
    3
    Couldn't you just run the wiring so the 12V load is connected to both batteries in parallel so it draws from both equally and the 24V load is connected in series? Or am I missing something simple?
     
  19. jamiphar

    Thread Starter New Member

    Dec 9, 2010
    10
    0
    Well, if you were drawing a 12V load from the parallel config, and then changed to a series config for a 24V load, there wouldn't be any problem. The issue come in when you have both configs connected at once and end up with a short across one of the batteries.

    For the series connection, you would have the negative of Batt#1 hooked to ground, positive of Batt#1 hooked to the negative of Batt#2, and the positive of Batt#2 hooked up as your 24V+ output.

    For the parallel connection, you would have the negative of Batt#1 hooked to the negative of Batt#2, and the positive of Batt#1 hooked to the positive of Batt#2.

    If you had these hooked up simultaneously, you would end up with a short across Batt#1 because the negative of Batt#2 (with the positive of Batt#1 already connected to it from the series config) would be connected to the negative of Batt#1. Make sense?

    You can possibly get around the short by putting a diode between the negatives of Batts #1 and #2 when you connect them in parallel, but you would still end up with both sources getting both 12V and 24V. That's kinda where I got stuck and started the thread.
     
  20. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,795
    No, what he is suggesting is to use a 24V battery (it just happens to be made up of two 12V batteries in series). Take this 24V battery and use it to power multiple 24V circuits. One of those 24V circuits can be a switch-mode power supply that is powered by 24V but that outputs 12V (or whatever you want). The conversion efficiencies on switchers can be quite high these days and they are very cheap.
     
Loading...