Detecting charger voltage and switching batteries

Discussion in 'The Projects Forum' started by nickorossa, Aug 18, 2010.

  1. nickorossa

    Thread Starter Active Member

    Jun 28, 2010
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    Hi

    I have an electric jeep for my daughter, which I recently added a "boost" button to bump the voltage a little :eek:

    The jeep now consists of a 12v and a 6v battery. I have a charger which has a switch on it to change from 6v to 12v for charging each battery.
    http://www.thebatteryshop.co.uk/durite-612-volt-27-amp-lead-acid-battery-charger-485-p.asp

    What I'd like to do is have a single charging port on the vehicle (like a lighter socket/plug) to plug the charger into and have a circuit to detect if its currently set to 12v or 6v and switch the charging to the appropriate battery.:confused:

    Attached is a circuit diagram of the wiring in the jeep in its current form.
    B2489 Barbie Jeep Wiring.jpg

    Any thoughts on how best to do this?

    Thanks for your help.

    Nick.
     
  2. nickorossa

    Thread Starter Active Member

    Jun 28, 2010
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    0
    Hi

    I have two thoughts on how this may be doable.

    The first being a cheap and nasty solution to use a relay that won't switch unless the voltage is above that of the 6v charger output. That would be somewhat abusing the relay I guess and probably not doing the charger much good.

    The second would be to use some sort of comparator circuit and compare the voltage at the charger to that of the 12 volt battery and then control a relay to redirect the charging. Any ideas what suitable comparator could be used; would a LM2903P be suitable?

    Thanks.

    Nick.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    First, why don't you tell us what the AH capacity is of each battery?

    Better yet, why don't you provide the manufacturer and part numbers for the batteries? That way we can locate the manufacturer's datasheet to determine the correct charging procedures to use.

    For starters, have a look at this post in the "Tips 'n' Tricks" thread:
    http://forum.allaboutcircuits.com/showpost.php?p=262143&postcount=38

    Then have a look at this thread:
    http://forum.allaboutcircuits.com/showthread.php?t=41757

    And then realize that it's possible to build a reasonably simple charger/float maintainer that will take care of both batteries. Just plug the thing in the wall when not in use.

    Of course, there will need to be additions to the basic design to make it 6v/12v compatible.
     
  4. nickorossa

    Thread Starter Active Member

    Jun 28, 2010
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  5. SgtWookie

    Expert

    Jul 17, 2007
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    OK, a datasheet for the 1st battery in English and German is here:
    http://www.yuasa-battery.de/deutsch/download/NP_D_Engl.pdf



    You can cut to the 2nd page of the thread. You won't get all of the explanations if you do that, but it's worth a read-through.

    If you want the longest battery life, you'll charge them no faster than 1/5 the AH rating (1/8 or less is better) and never let them get below 70% of a full charge. Using temperature compensation as shown in the last schematic I posted will help a great deal towards maximizing battery life. The little charger you're thinking of using does not have temperature compensation.
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    That's going to be the hardest part of the solution, it's my guess children would drive these things until they almost had to be pushed home.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Something to reduce that likelihood could be rigged up using an LM3914 as a "gas gauge"; when the batteries get low, the circuit just disconnects them.

    It would need to have a fairly long RC time constant, so that surges from sudden starts wouldn't cause it to trip early.
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I hope they never take that IC away from us and, matter of fact, I'm surprised someone hasn't already integrated it into a small package that includes the LEDs in the most commonly sought after choices.

    They are becoming hard to find in DIP format.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Last edited: Aug 24, 2010
  10. nickorossa

    Thread Starter Active Member

    Jun 28, 2010
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    0
    Hi

    Well that provides me with the details on a future project I had planned. I'm assuming this is an equivalent chip from bgmicro
    http://www.bgmicro.com/ICSLM3914.aspx

    However I'm still not sold on building my own charging circuits and buying a suitable power supply; when I already have the charger and am just looking for a way to detect the output voltage and redirect the charging to the appropriate battery. Essentially just to stop the 6V battery being charged at 12v as that can't be good for it!


    Nick.
     
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Knew about Jameco, just worried about their stock level. Didn't realize Goldmine has them and I order from them on occasion as well. I need a couple for an upcoming project, guess I'll order plenty of extra to stuff in the parts bin. Also glad to hear about the N-1 version, guess they haven't forgotten us after all.

    Mouser only provides an NTE cross and Digi-Key apparently has no stock.

    I've seen the surface mount alternatives but I hate working with SMD unless I have to, guess if I had all the proper irons and supplies it wouldn't be as bad.

    Just one SMD project to build, probably this weekend if I can get the time to lay the board out. It's an RF preamp that runs at a pretty high gain and very low noise so I have little choice in the matter.
     
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