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Voltage sensing split charge design

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  • Voltage sensing split charge design

    Hi,

    I've been thinking about how to charge my leisure battery for a while and have been trying to design a "smart" circuit to check the starter battery voltage and once this is past a set level (and therefore charged) to swap the alternator output to the leisure battery. I've seen lots of variants on the big relay connecting the two batteries in parallel and controlled by the alternator no charge circuit but I want to have two completely isolated systems (common ground though as I don't want to have to wire a negative through to everything off the leisure battery) so that I don't have to worry about the leisure battery draining the starter when they;re connected or voltage drops over diodes in isolating them. Saying this if my idea is useless I'll probably go with this as it seems to work, I'm just paranoid about not being able to start the car!

    Anyhow after fiddling around with op-amp comparators and FET designs I came across an interesting bit of information, range rover classics (and possibly other models) have a voltage sensitive switch (VSS) inline with the rear demister to protect from over voltage. Could I use this to check the voltage of the starter and then control a relay switching between the two batteries with the alternator as the common wire. As the VSS is off above a set voltage (11V I think, will need to play with it and use a divider to compensate for needing a higher cut off) I'm thinking of having the starter as NO and the leisure as NC. This box http://www.power-store.com/view-item...d=1138&id=197&from merlin equipment seems to achieve what I'm after (although connecting both batteries in parallel again), though at £90 it's quite steep. The page might be a better explanation than my rambling!

    Could someone electrically minded have a look at the attached circuit and let me know if it should work or if it's complete rubbish? Electronic design has always been a bit of a dark art to me and beyond very simple stuff I get easily confused so don't trust myself not to have designed a fire waiting to happen or a box that will sit there and do nothing!

    Thanks

    P.S. apologies for the black background, if it's hard to read will try to make a .doc version from the .eps file (which has a nice easy read black circuit on a white background but can't upload that)
    Attached Files

  • #2
    I wired my leisure battery like this....
    It's isolated from the starting battery when the engine is off, and gets charged when the engine is running.

    Last edited by BUSHWHACKER; 4 August 2008, 21:39.

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    • #3
      To be honest I'd cheat and get this

      http://www.towsure.com/product/1828-...bination_Relay
      Gone from 4x4 to 1x2

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      • #4
        Mine works on a relay with a switch on the dashboard. Couldn't be bothered with these fancy set ups.
        Hold my beer and watch this

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        • #5
          Bushwhacker - that's the setup I've seen described a fair bit, my only concern was linking a depleted leisure battery to the starter battery (especially if I'm only running my engine for a short time) can the leisure draw current from the starter once they're charging or does it not affect it?

          To be honest I'd cheat and get this

          http://www.towsure.com/product/1828-...bination_Relay
          Yeah I'd seen that but I couldn't find any info on how it works apart from the brief description on that site. Have you got one and if so does it work well? For a tenner it'd be great if it does work!

          Mine works on a relay with a switch on the dashboard. Couldn't be bothered with these fancy set ups.
          I considered that but I have the worst memory in the world so don't trust myself with a manual switch! Otherwise it'd be a lot easier.


          Thanks for the suggestions guys, those systems do seem a lot simpler and hopefully my concerns will be unfounded. Cheers

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          • #6
            I used it to repair the towing electrics on my sisters truck (mine puts out +12ve on all pins all the time, hers wasn't putting out +12ve on the switched terminals at all). Fitting was easy just ran a cable from the battery to the relay, and then a negative to a good earth point. Basically it only supplies power when the batteries are above a certain voltage. When you start the engine it takes a couple of seconds before you get +12ve from the outputs, as one of the outputs is for charging the leisure battery in the trailer I'd have thought it would do what your after.
            Gone from 4x4 to 1x2

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            • #7
              Cheers Maverick! Sounds like it's what I'm after and much simpler and cheaper than my ideas, will order one tomorrow.

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              • #8
                I have heard of the ones from towsure sticking on. I have never had any trouble and I'm on my second (changed car and didn't bother to rob the relay out of it)
                At tick over they do tend to click in and out if there is a lot of demand. I also have a length of cable with a 12S plug on one end and crocodile clips on the other end to charge a battery.

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