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  • advice on simple alternator excitation circuit?

    here's my problem:

    i have a 3 battery 160amp isolator under my hood such that my three batteries do not draw off each other (1 for car, 1 for amps, 1 for 'puter/etc)

    the alternator has this wire coming off of it that reads the voltage of the battery and then decides how much voltage to spit out, called te exciter wire i suppose

    it seems that when i connect a small wire from my main car battery to this wire, that battery becomes fully charged and causes the alternator to quit charging, but one of the other batteries may NOT be fully charged

    so i suppose i need some circuit that will input 3 voltages, and output the lowest of those values out the other end... this sounds pretty simple, but the few EE classes i've had (and gotten c's in) arnt much help at this point (twas a while ago)

    this will give that exciter wire the lowest voltage, and ensure all of my batteries are sufficiently charged

    can anyone offer some design advice/insights?

  • #2
    Diodes?
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    • #3
      look at op amps, something simple around some 741's should work. Problem I see here is that you could overcharge the already charged battery.
      Get more complex and build one circuit with feedback into the alternator to keep it putting out say a constant 14v, then three separate 12v charging circuits powered from that which charge their batteries independently. I believe you can buy fancy charge controllers like that from RV shops for a $$.

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      • #4
        I never thought about it really. Does this mean that the sure power isolator that I bought, will tell the alternator to quit charging when the main battery is full?
        2.4 celeron
        150 watt OPUS
        micro atx
        xp pro
        tv/radio tuner card
        audigy
        Dual monitor Radeon
        802.11G Mimo
        xm pcr
        dvd
        gps
        8" Lilliput touch screen
        IGuidance
        dual optimas
        battery isolator
        15" screen
        My Truck

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        • #5
          The surepower isolator doesn't tell the battery anything, the alternator stops charging when the regulator inside it or the vehicle engine computer tells it to.

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          • #6
            Originally posted by n8scstm
            I never thought about it really. Does this mean that the sure power isolator that I bought, will tell the alternator to quit charging when the main battery is full?

            Well. . . the thing about putting batteries in parallel (you isolator should be doing this) is that the capacity of each battery is additive to a total capacity. When the first battery is getting charged, it should also be charging the other two batteries at the same time. If by chance one battery has a higher charge than the other the two batteries closest battery to it will should charge it. The first battery should give you an acurate voltage to give the "exciter wire". If your batteries are going dead, you may have hooked up the isolator wrong. .
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            • #7
              how about some comparators, zeners, transistors, 3-input OR gate.

              each battery feeds into a comparator input which compares that voltage to that of a 12V zener diode. If the battery voltage is greater than the 12V (From zener) the comparator is to output low - a logic 0 (this depends on the comparator u buy and how u set it up).

              So, you'd have three outputs from comparators ....if one were to go below 12V that particular output would be logic 1 (high). These three are then fed into a 3 input OR gate. The OR gate will sense that if any battery is below the 12V it's ouput should be high (logic 1). This high value can be inverted so to trigger a PNP transistor to feed a reference 12V (say from another zener) into your alternator feed. The alternator will think the battery (any of them) is at 12V and start outputting it's charge.

              Or you can be simplier as to take the comparator outputs into diodes then into the PNP transistor ....but I wouldn't do that. Hope what I said makes sense. It's the first thing that came to my mind so it might necessarily be the best.

              The question is - does your isolator know where to put the charge? [I've never been a fan of isolators - the cheap ones usually just use schottky diodes that don't know what's going on...just simply isolate]
              Check out my SQ Competition carputer install

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              • #8
                Originally posted by eugenen
                The surepower isolator doesn't tell the battery anything, the alternator stops charging when the regulator inside it or the vehicle engine computer tells it to.
                Then what is the sense wire and the acc. wire for? My understanding is that the acc. wire tells the isloator that the battery is full and then in turn tells the alternator to quit charging if the voltage is high enough. But this only reads voltage from the main battery. That's the whole point of this thread.
                2.4 celeron
                150 watt OPUS
                micro atx
                xp pro
                tv/radio tuner card
                audigy
                Dual monitor Radeon
                802.11G Mimo
                xm pcr
                dvd
                gps
                8" Lilliput touch screen
                IGuidance
                dual optimas
                battery isolator
                15" screen
                My Truck

                Comment


                • #9
                  The acc or E wire controls the isolator circuitry, the sense wire is where the vehicle regulator gets it reference from, but yes it is a compromise since the second battery is isolated there isn't any way to know if its charged or not so you have the potential to under charge it. That is why you have the fancy charge controllers in the big RV's which have multiple batteries, they have a sense/regulation circuit for each.

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                  • #10
                    Do it right with battery isolators. If the batteries are all connected in parallel, They will draw through the system and charge properly. The isolator is just for making sure that the main battery is only connected to the important systems of the car so you can be sure it will start up every time. I've had banks of batteries, they all charge properly. They all maintain almost the exact same level (variations in cell quality, I'll assume). BTW, batteries wont charge in order like that, they will all draw up to their capacity, without the first battery overcharging.

                    Also, you have 3 batteries under the hood? Methinks you are overdoing it. Your computer doesn't need much power, You didn't list the details of your stereo, so I'll assume it isn't huge. I only have 2 batteries, and that runs the car, computer, multiple monitors and a 5000 watt set of 4 amplifiers.

                    Dave

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                    • #11
                      ISOLATOR question about Charging

                      Originally posted by therussman2002 View Post
                      here's my problem:

                      i have a 3 battery 160amp isolator under my hood such that my three batteries do not draw off each other (1 for car, 1 for amps, 1 for 'puter/etc)

                      the alternator has this wire coming off of it that reads the voltage of the battery and then decides how much voltage to spit out, called te exciter wire i suppose

                      it seems that when i connect a small wire from my main car battery to this wire, that battery becomes fully charged and causes the alternator to quit charging, but one of the other batteries may NOT be fully charged

                      so i suppose i need some circuit that will input 3 voltages, and output the lowest of those values out the other end... this sounds pretty simple, but the few EE classes i've had (and gotten c's in) arnt much help at this point (twas a while ago)

                      this will give that exciter wire the lowest voltage, and ensure all of my batteries are sufficiently charged

                      can anyone offer some design advice/insights?

                      This is what I am after... My alternator has this sense wire (Ford Aspire) I have a 3 leg Vanner isolator installed. The big lug is connected to the center pole, and bat 1 and 2 respectivly.. My problem is what therussman2002 describes. As the alternator has the regulator built in and controls when its on between the S & L Wire... how would you wire it up to see the voltage of both batteries? I have it the way it is stock.. and at one point had JUST the starter on the front battery, and EVERYTHING else on the back battery.. But because the front battery is only drained for a second starting.. The alternator is only sensing that battery and ignorning the state of the second battery..

                      I too though diodes, but wouldnt that create a 24V current to the sense? if not, still would sense that 1 battery is 100% thus ignoring the second one and shutting down (IE: still say hmmmm i see 14.4v kick off)

                      Someone said there is some smart switch that can do this, but I've yet to find it.

                      I got this isolator and the reece lavelle alternator from an ambulance.. But the alt is toooo big for my little ford... So if anyone knows how to properly install this so it will correctly charge both batterys without connecting them ++ and -- because i want them separated (IE ISOLATED) from each other!

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                      • #12
                        Alas this is a long dead thread - long before my time.


                        Alas the solution is to reduce the inter-battery resistance. Isolators should only add milli-Ohms to the interconnection.


                        For the OP and other suggestors, and readers - the alternator does not stop charging, it continues to put out its 13.8V to 14.4V set voltage. The main battery merely stops absorbing current (except for its "float" current - typically up to an Amp or two).
                        The other batteries are still being supplied with that same (say) 14.2V, but their is voltage drop along the distribution because if the IR losses (current I through the interconnection resistance R, where the current I is the sum of the battery's recharge current plus its load current.
                        It is not a mere case if averaging or sampling voltages - that is inappropriate without some form of regulator between the alternator and the "higher voltage" battery(s) (eg - a PWM chopper).


                        Summary: Improve your inter-battery distribution. Ensure there are minimal voltage drops to the battery +12V and along the ground path.



                        PS - Simbalage, do you have a charge light?
                        Last edited by OldSpark; 08-16-2011, 10:39 PM. Reason: PS...

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                        • #13
                          Yes I have a charge light, as well as a sense wire connected to a 3 pole switch as well as a digital voltage display.. This way in the even one battery is drained flat, flipping the switch will not only display the charge in each battery but will trip the sense wire to acuratly measure the weaker battery so it doesnt stop on account that the other battery is full. Thats the only flaw with an isolator but you are correct to an extent.. the isolator will give the charged battery a float and concentrate on the battery with most resistance.. only factor is most alternators "Do" have a sense wire that tells it to kick back charging when the battery wired to it is full.. SO large wire goes to alt term on the isolator and my switch has 6 legs.. so the top 3 work the led digital display, and the other 3 at the bottom direct which battery will feed the sense wire. Havent had any problems with overcharging/undercharging of these 2 yellow top optimas.. Likewise relays are better when you want to increase the overall capacity of a system, but arent really meant to isolate batterys as people use them... because imagine a dead battery being thrown at a completly charged one.. that good battery had to work twices as hard to feed that dead battery.. would be like starting your car everytime and throwing a dead battery at the alternator every time... like jump starting someone dead car everytime.. Isolators just even out the load and charging and do what they were ment to... Separate hence why they cost more then solid state relays .... Just my 2 cents on the matter

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                          • #14
                            Sorry, but you do not understand how a car's electrical system works.
                            There is no "kick in", the sensing is simply to regulate the systems voltage to (say) 14.2V.

                            12V battery systems are akin to a "constant voltage" circuit.

                            If the alternator stopped charging, the system would drop immediately, eventually going under the ~12.7V of a fully charged battery (with surface charge dissipated).


                            Of course all alternators have a sense wire - how else do they regulate? With D+ types, the sense is internal (hence their traditional SS isolator problems).


                            Alas we don't have any problems. We don't seem to have surge problems.
                            We charge both and all batteries with the same source voltage, automatically, and without any switching.

                            I think I have sufficiently addressed the failed battery issue. And it has no effect on cranking the car!

                            I'm posting free.

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                            • #15
                              Originally posted by OldSpark View Post
                              Sorry, but you do not understand how a car's electrical system works.
                              There is no "kick in", the sensing is simply to regulate the systems voltage to (say) 14.2V.

                              12V battery systems are akin to a "constant voltage" circuit.

                              If the alternator stopped charging, the system would drop immediately, eventually going under the ~12.7V of a fully charged battery (with surface charge dissipated).


                              Of course all alternators have a sense wire - how else do they regulate? With D+ types, the sense is internal (hence their traditional SS isolator problems).


                              Alas we don't have any problems. We don't seem to have surge problems.
                              We charge both and all batteries with the same source voltage, automatically, and without any switching.

                              I think I have sufficiently addressed the failed battery issue. And it has no effect on cranking the car!

                              I'm posting free.
                              Sense wire hence my way of explaining.. I know about 12v.The sense wire also tells the alternator to stop charging in the event the battery is charged, until the system places a demand on the alternator, Likewise its a matter of preference. You wont see your setup on any large 260amp or higher application unless of course battery separation isnt the goal.. Putting batteries together sure your relay will do just fine.. but they both have flaws.. Relays have moving parts hence fail.. for the 0.2v loss my diodes cause i aint complaining. Least I know my battery's life isnt shorted using the type of setup u are talking about
                              Last edited by Simbalage22; 08-17-2011, 09:13 AM.

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