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battery isolator vs relay

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  • battery isolator vs relay

    I've got a question about using a relay (cheap) vs a battery isolator/diodes (expensive!)

    Before I go any further, I know about the "reason" why you should never use a relay, namely once the relay closes the drained battery will draw 47 billion amps from the charged battery, damaging both batteries and melting wires and causing large fires in the process.

    But instead, what if the relay was attached to close only about 5-10s after the engine was started?

    Then the power will be provided by the alternator instead of draining the primary battery, this should work, no?

    Alternatively, can I just build a battery isolator using a couple of diodes? Hell, even those diodes are much more expensive than a relay. You'd need what, probably 100-120A diodes, or 2x60? At like $10 each you'll need what, 4 of them? Still not a very cheap venture.

    [Edit: I'm on crack, you can get MBR6045's for like $3 each... $12 for diodes + wires + heatsink isn't that expensive. And yeah you can buy them for $40 or so, the place I was looking at was grossly overpriced.]

    I tried searching for topics like this here and didn't find any.
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  • #2
    Is a battery isolator really all that expensive? I seem to remember seeing a low amp one GI Jo's for about $30 and a high amp one for about $50.


    • #3
      Here is a good site that illustrates three different ways of connecting two batteries to one alternator.

      Direct Page Link

      It shows a) Diode Type Isolators - same as the ones you buy b) Solenoid Type Isolators - manually switched isolator and c) Dual batteries without an isolator.

      Piles of good info at this site, here is their Main Page
      Shuttle ST62K, 80Gb, MediaEngine, Irman, Wireless NIC, Win2K, Blah Blah Blah...


      • #4
        The isolators that I saw were like $100 (CDN) sorry, I was looking at

        As I keep searching though, the only decent ones that I see are > $50USD, which in my books constitutes "expensive" for what I want, especially since I could build one for like $8USD (using 2 or 3 45V 60A diodes in parallel for each direction)

        And now that I think about it, maybe I'll just build one.

        I'm trying to figure out other neat ways of doing this... I was thinking maybe using solar cells to augment the power supply. Basically you need about 250mA at 5V (so 1.25W, or 1.8W with switching regulator, or 3W with a linear). So with a 5W solar cell, you should be able to power the computer in STR during the day and generate enough power to charge the battery to recover the loss drained during the night. You can get a 5-watt solar charger for $50USD over here at Canadian Tire (probably cheaper elsewhere). Could be interesting.
        IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.


        • #5
          Lets see where this goes...I am very interested...did you ever think about trying it in a friends junker

          I am thinking about just saying F-it and running dual alternators...very expensive



          • #6
            I don't see how dual alternators would really help anything, unless the problem is you're drawing too much power for the poor alternator to handle.

            The issue here is a dual battery system and how to isolate them.

            I think either method would work, but using diodes (since you can actually make one yourself for ~$20 all-said-and-done) seems like the best solution.
            IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.


            • #7
              well, i have so much crap in my car now, i am worried about the pull on the yes, the diode method seems like it may be good in my situation also....hmm, i would like to see how this an update?



              • #8
                no, in the last 15 minutes nothing has changed.

                Ask me after tuesday (last exam). Then I'll be playing with all my stuff
                IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.


                • #9
                  LOL, no Telek...the 'got an update' part was intended for the person who started this forum...I am not an, that was funny...

                  jmm, did you get the post about the Krocera vga screens on ebay...I posted to you in the one we have going, "Mailbox is full, Question for Telek"....



                  • #10
                    well i don't know just how young you guys are , but i've been buying stuff from jc whitney for a real long time. i got my battery isolator from them. bought it around 1980 for a van-never used it. put it in my car in '86. its never let me down yet. sometimes you can get pretty good stuff there for a good price--just dont buy their tools!
                    99.9% complete.



                    • #11
                      So is this setup correct. I know nothing about power, but I have this problem.

                      First of all, my batteries are connected using a relay that switches on only when the car is running. If the car is off, the relay is open. That part I think is correct. Is that the case?

                      The problem is that the batteries are not holding their power very well. I can actually see the power drop as I stop at a stoplight, especially at night.

                      Does this mean that the alternator is just not producing enough juice to run the setup?

                      Would upgrading the alternator solve the problem or is there a bigger issue with the batteries and the way they are connected. One of them is a brand-new Optima yellow-top and I don;t want to mess it up!

                      What brand of alternator should I consider for a '95 chevy tahoe?

                      Thanks in advance for your replies!


                      • #12
                        I used to have a relay setup, where the relay connected the two batteries together, only when the engine was running (ignition relay) and the second battery was only connected to the stereo amp. Didnt have any problem with it, worked for years. I have an isolator on the second battery, that connected the two batterys together, incase the vehical batery was flat. Got me out of a flat battery a few times, ( get in car, flat battery, doh, get out, close isolator, start, drive away)
                        Ford XR6T blueprint 4.0 L twin cam turbo.
                        Xenarc 7" touchscreen M10000 Audigy 2 NX
                        ITX 120W PS 80gig HD, 256MB Ram, USB wireless internet/LAN
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                        600watt 6chan USA audio AMP


                        • #13
                          So would I be better off with an isolator an not the relay? I found a great one with very little resistance (diode effect).

                          Kind of expensive though.


                          (UPDATE) This device is actually a combiner/isolator and can be configured for a secondary battery to be the aux startup battery if your main battery is handling the load or as an aux battery controller if your loads are connected to the second battery.


                          • #14
                            here is a good place to find a guide for alternators, etc.



                            • #15
                              I also had the dual battery through the relay (actually a ford starter solenoid) in my old Blazer. Same deal, if the ignition was on, the relay was closed and both batteries got charged, with the switch off...(or even during the start cycle I think) the batteries were isolated.

                              So I could run my stereo with the engine off, and still make it out of the woods.

                              The isolator has no moving parts, and always keeps the batteries isolated, with the relay the batteries are tied together after the engine starts.

                              There are different isolator sizes depending on the batteries...
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