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Secondary Battery Installation

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  • Secondary Battery Installation

    My car is supposed to shut the headlights off when the car turns off. On occasion, it doesnt and my battery can't start my car. A jumper pack would alleviate this and I could take care of the situation myself. However, they are $60-150 and id rather invest that money into something multifunctional.

    Install a secondary battery in the trunk with an isolator/relay installed between the two batteries. The battery in my trunk would be used to keep my computer in sleep while the car is off (Intel Atom, SSD, 2 memory sticks). If my car decides to not shut off my headlights again, I would then be able to disconnect everything from the secondary battery and use jumper cables to jump my main battery. I would do it with jumper cables because the main lines I have between the batteries are not large enough to send the cranking amps without the potential of melting something/starting a fire.

    Am I missing anything?
    What type of battery should I use? (SLA/AGM/Deep Cycle/Starter/# Of AH)
    How do I calculate the amperage drawn while my computer is sleeping? (Or would getting something to measure it be easier, needed for AH)
    Car PC parts Purchased [90%]
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  • #2
    Personally I would fix the problem with the lights or the switch or the sensor. If it drains the battery, it will only take 2-5 times of complete draining to kill that battery for good. For your computer I would get a deep cycle for the trunk, keep in mind that a deep cycle is not a starting battery. It would get you out of trouble, but too many jump starts and you will be replacing the deep cycle eventually. Without fixing the original problem you will get stuck in a vicious cycle of buying batteries. Just my 2 cents SNO


    • #3
      Sno's covered it well. He's spot on about killing your main battery.

      And I agree about NOT getting a jumper pack. They usually only have a 12V AGM of about 15-18AH (early packs might use 7AH) and it's much cheaper getting just the battery - whether an isolated second battery or a standalone.

      And good to see you understand the limitation about manually connecting the isolator for a jump start (which only requires a switch and 2 diodes for the UIBI or charge-light controlled relay/isolator, and also for many voltage sensing isolators - ie, where you can intercept its control wire to the relay/solenoid).
      However... If you use a 30A isolating relay & 30A wire with 30A self resetting breakers at each end, then IMO there is no harm using it to try to "jump" start - unless the main battery is really flat. (This is more for flatteries that need just a bit more juice.)
      If it doesn't work, you can manually jump start. The breakers should reset within minutes if that.
      If you are using a larger relay and cable, it's even better. My set up usually uses a 60A relay, some very high rated cable, and 50A self resetting breakers and I used to manually switch on the UIBI/relay when my aged cranker had problems. But my (reduction) starter motor is only 140A compared to the OEM starter of ~240A.
      Some might argue jumping puts stain on the relay, but a 30A is cheap. My 60A relay is only ~$10.

      And unless your interlink fuses are of quite high rating, I'd strongly suggest self resetting breakers. Despite at most an 8A load, my 30A fuses used to blow as a result of the 2nd battery's initial inrush current if it was slightly discharged.

      Of course that's only if you fit a 2nd battery after fixing the headlight problem. But a 2nd battery for PCs and independence of the main cranking battery is real nice. Plus it can be a good emergency battery (eg, 15AH or larger, though 7AH may suffice depending on the vehicle etc).
      Last edited by OldSpark; 07-27-2013, 10:16 AM.


      • #4
        Sno's suggestion would have been my suggestion too... Fix the problem or it will cause you other grief down the road.

        Every time you fully discharge a lead acid battery you half its life. So Sno is accurate with his number and that is for a brand new battery.

        You could get a device something like this:
        I have seen something that would work well for you called "Battery Buddy" but I can only find pictures of it on the net. It is a similar device but the idea is that if your battery drops to a dangerously low voltage the unit will disconnect the battery saving you from having to replace it as well as allowing you to start it. This or any other option should be used as an "insurance" policy to save your vehicles battery and to give you the ability to always start. The battery buddy requires you to hit a reset button to start your car. The price should be in the neighborhood of $50-$80 but it is piece of mind if you have issues like this.

        You CAN add a second battery but if you don't need it don't add it. Unless you have a carPC or stereo system you play a lot with the vehicle off you really don't need a second battery. Since CarPC automotive supplies have shut downs based on timers or battery voltage you should make use of them instead of a second battery. You could always as a back up put a small UPS style battery in front of your CarPC. You can get chargers for these batteries that hard wire into your car and then you run the PC off the battery. Just make sure you run a wire from the ground of this second battery to ground to make sure your computer works when connected to the rest of the vehicle if you are hooking to any components powered by the normal vehicle power. I would make sure the battery is sized big enough to be able to run your PC until the timer times out which is totally dependent on how you have it setup.

        Beyond that the battery buddy or similar device might be helpful in your situation but realistically you need to get your issue repaired regardless. Also as far as a carputer is concerned most automotive PC supplies should have a shut off on them to kill your computer before it kills the battery. Most do it by timers.

        If you DO decide to add a second battery you should go to a respectable RV dealer. They will have lots of experience with multiple battery setups and know what works with what. They can provide you with pro's and con's to what is currently available. There are difference of opinions between people on this system of what the best setup is so I strongly suggest an RV dealer for the current scoop regardless of what is said here. I would not trust a car audio dealer necessarily because their experience will be much more limited and they may tend to steer you towards more expensive products or ones that simply don't work. RV dealers are a totally different animal and in the long term you are more likely to be happy and spend less money on something that works.

        For the simplest dual battery setup you want a constant duty solenoid. Standard relays tend to not be as reliable and can't handle the momentary surges you are likely to come in contact with. A constant duty solenoid is a special purpose solenoid built similar to a starter solenoid but where as a starter solenoid is designed for high amperage, low duty (short time on), a constant duty solenoid is designed to run fully with the car. Plus if you want to jump the car you can normally just place a very short small jumper from the good battery terminal to the "on" terminal on the relay and you are "jumped". I will end this by stating in a 2 battery system you need batteries that are matched. All this means is you want batteries designed to run together. The closer the match the better life you can expect from the batteries. if they are not matched you could end up with a reduced life on one of the batteries or it may never be fully charged. There are other types of battery isolators but they each have their pros and cons.
        Last edited by redheadedrod; 07-27-2013, 10:33 AM.


        • #5
          That prioritystart is a discontinued product (I wonder why?). It also has an 8-12mA drain.

          Two batteries connected with a relay are seen as one battery by the alternator.
          Even if there were path resistances, that has no appreciable effect unless the voltage drop is permanent and below battery requirements. And that only happens for improperly sized interlink cabling (which is common is Australia so that companies can sell $200 dc-dc converters to the ignorant).

          Basic theory - and hopefully common sense - dictates that 2 loads connected in parallel see the same voltage.
          If a battery drops below 13.8V due to an initially high recharge current thru its cabling resistance, that current gradually drops and then the normal load (and battery recharge else float current) means a return to its proper voltage. (Again, assuming proper design, and of course a typical modern alternator with 14.2-14.4V output.)

          And yes, I am the said "other member". The above theory comes from basic texts (Ohm's Law and basic circuit/network theory (Kirchhoff, Norton, etc)) and a host of sites like the12volt, bcae1, etc, as well as (in my case) secondary education. The battery stuff comes from professional experience and is supported text and sites like batteryFAQ, IEEE, theBatteryUniversity, etc.
          But feel free to discuss it with redheadedrod's school teachers. Just make sure you specify that this is when charging the batteries involved.

          And redheadedrod - all alternator regulators are voltage sensing.


          • #6
            Not going to fully bite OS... But the differences you and I have are based more on opinion than fact since I have also used many of the same sources for my information and research done talking to engineers in companies that make those products. And I will leave it at that.

            And one note about batteries in parallel. A voltage sensing alternator is going to modulate its output voltage based on the voltage of the battery it sees. With batteries in parallel the voltage seen will be of the highest voltage battery. With this in mind if your alternator shuts down the output voltage because the most charged battery is charged then the lesser charged battery will never get a full charge if the alternator drops below the "charge" voltage. Thus the second battery never gets fully charged. This is why batteries need to be matched even in parallel. If you are able to otherwise sense the batteries voltage and modulate the output to each battery depending on load required then you can properly charge both batteries without over or under charging either. However as pointed out in the past devices that do this are hard to come by and when you do find one they are generally expensive. In general usage however choosing similar batteries should be good enough. As an example Just be aware that if you commonly discharge a deep cycle battery and put it in line with a normal battery that stays mostly charged that this deep cycle battery may never fully be charged.

            All alternators on cars built today and probably last 20 years are voltage sensing but not older cars. Such as my '72 or 73 olds... They put out a constant voltage regardless. Those style systems would work much nicer with a multi battery system because they would ALWAYS charge the batteries fully. Manufacturers only went to the voltage sensing alternators to save gas since the alternator wouldn't be running full bore all the time. Some race cars have clutches on their alternators that disengage the alternators for high speed drag runs too.

            (I did say I wasn't going to bite right? )


            • #7
              Oh please Red - how can two batteries connected in parallel have different voltages?

              I'll put alternators into simple terms. They maintain a regulated voltage. They (try to) maintain up to 14.4V, typically 14.2V. In ye olde days that figure was 13.8V.
              The only other difference is that some alternators extend their voltage sensing to (f.ex) the battery whilst others (like D+ and older alternator/regulators) sense the alternator output voltage.
              But they are all "voltage sensing" in that if that "sensed" voltage drops, they increase rotor current to compensate, and vice versa.

              And you are not biting. I wouldn't even call it a bark.

              Now please, for once - stop and listen - and THINK!.
              Once you realise that paralleled circuits have THE SAME voltage all I've said should finally make sense.

              In case you still don't get it....
              IS THERE ANYONE HERE THAT AGREES with Red that paralleled batteries will have different voltages?
              Is there anyone that agrees with me?
              Is it time we ran a poll?


              • #8
                if you connect 2 batteries with different charge levels in parallel, higher voltage battery will start charging the lower voltage battery. the current from the higher voltage battery to the lower voltage battery will continue to flow until the voltages equalize. of course, when they are connected in parallel, the potential difference is same between the poles of individual batteries but as soon as they are disconnected from each other (before the current flow goes to zero), the voltages of the batteries will not be same anymore.
                looking from the alternator point of view, it will monitor the voltage of its output, which means actually the voltage of the battery that its charging. same story, if connected parallel (alternator and the battery(ies)), then the voltages will be the same, excluding the difference caused by current and cable thickness and length, in other words voltage drop.
                a fully charged battery (14.4V) can NEVER maintain the same voltage when its connected to a less charged battery in parallel. which in this case alternator will try to increase the voltage at its output to 14.4V. meaning, it will charge both of the batteries until the voltage of parallel connected battery group's voltage rises up to 14.4V(only possible when both of the batteries are fully charged).
                ps. fully charged battery is considered as 14.4V in my text, but different applications consider different battery voltage levels as fully charged.

                coming to the main question,
                I would try to solve the main problem instead of trying to cover the symptoms with different remedies.
                Last edited by canstb; 07-27-2013, 03:36 PM. Reason: typo
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                • #9
                  Thank you all for your inputs! I do plan on fixing the headlight issue, but thats another can of worms for another forum. I have been planning on doing a second battery installation for quite a while now. In fact, I had all of the parts I needed except the battery this morning. I picked up a battery this afternoon and reran all of my cables. So far so good, I just need to find something to tap into for my remote. It needs to read 12V+ when the car is on, and 0 when the car is in accessory & off. Any ideas?
                  Car PC parts Purchased [90%]
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                  • #10
                    IGN is +12V when the car is "on" and cranking.

                    The headlight is probably a bad connection or motor problem. (Rotate fuses; disconnect & reconnect all connectors; maybe spray the motor/gear/hinge area.)
           may be good for that issue.

                    And with apologies for maintaining this hijack, but a correction....

                    canstb - a fully charged 12V lead-acid battery is ~12.7V, not 14.xV. 14V is voltage that the alternator or charger raises the battery terminals to.
                    What you say about paralleling of batteries is totally correct. And because both are getting above 13V, it is impossible for one battery to discharge into another (surface charge excluded).
                    Thanks for your replay (and support).


                    • #11
                      Originally posted by OldSpark View Post
                      And with apologies for maintaining this hijack, but a correction....

                      canstb - a fully charged 12V lead-acid battery is ~12.7V, not 14.xV. 14V is voltage that the alternator or charger raises the battery terminals to.
                      What you say about paralleling of batteries is totally correct. And because both are getting above 13V, it is impossible for one battery to discharge into another (surface charge excluded).
                      Thanks for your replay (and support).
                      right, I was not clear;
                      the voltage level of 14.4V I took as an example in my previous post as "a battery fully charged(14.4V)" is the voltage during charging and charging current almost 0amp. Which btw, 14.4V is actually the highest safe charging voltage for 12V SLA battery. thats the reason I added a note at the end, saying different applications consider different voltage levels as fully charged. (eg. constant voltage charge or constant current charge, float charge or boost charge)
                      and yes, after a while the fully charged battery is disconnected from the charger, without any load connected to it, its voltage drops below 14V and after 2-3 hours should settle and keep at 12.7V if the battery is healty and in good condition.

                      having done this explanation, I beleive I was again not clear about the parallel connected batteries charged by one alternator;
                      let me explain with a scenario; -I will keep the same voltage level as referance.
                      if a second, less charged battery is connected parallel with the alternator and the first battery(fully charged, almost 0amp current from alternator to battery and 14.4V), the voltage will drop because less charged battery will act like a load to the charged battery. and alternator will regulate its output up to 14.4V again in seconds maybe shorter time. the charged battery's voltage was 14.4 already, so there will be no/almost 0amp current flow through that battery. but the less charged battery had lower voltage before it was connected, so the alternator will start feeding/charging the less charged battery. so both of the batteries will be fully charged after adequate time.
                      If the charging is stopped after both of the parallel connected batteries are fully charged, without any load connected to batteries, both of them should keep their charge level of 12.7 V IF internal resistances of the batteries are same. (same type batteries' internal resistance may vary according to their health and condition)

                      I didt mean to support or disagree anyone, I feel quite uncomfortable when forum members start arguing and I seem like to be a side to any of them. I just wanted to share my knowledge and experience, hoping to help finding an answer to the topmost question.

                      To keep both of the batteries in the car and safely charge them, I would build a voltage comparator circuit with an op-amp and feed a relay when the primary battery's voltage is above 12.8-13V. 13V is only possible if alternator is charging. and with this relay I would connect both batteries' plus poles to each other (minus poles are already connected to chasis of the car) through a high amp diode so that current only flows to second battery from the alternator, not the other way to the primary battery. this will ensure that the alternator will feed current to secondary battery, but secondary battery will not feed the primary battery in some circumstances. if the alternator is not charging/engine is not running, the voltage will drop below 13V for sure, then this relay would isolate both of the batteries from each other, keeping the main battery charged for the next engine cranking.
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                      • #12
                        Alas you support what I have been saying all along...

                        Any load change causes a voltage drop or increase that the alternator response to. Modern alternators can respond fast enough to make headlight dimming unnoticeable, but even old alternators usually respond well with one second.

                        Yes - 14.4V (2.4V/cell) is the highest charging voltage for lead acid batteries (wet or SLA sealed). Above that, noticeable gassing starts etc. Above 14.4V is used for battery maintenance - eg equalisation & recovery.
                        The voltage required to "fully charge a battery" is 13.8V - ie, 2.3V/cell. (But that is insufficient to remove (soft) sulfites.)

                        A fully charged battery takes its float current, not 0A (zero Amps). Float currents are typically 100mA - 2A for automotive batteries,.

                        Though a (cheap?) battery charger may "stop" charging batteries, an alternator does not.
                        Whilst an alternator is operating, it is charging the main battery (etc). Vehicles do not disconnect the alternator from the battery when the battery is full.

                        Whether batteries maintain charge or whether one discharges into another parallel battery is independent of their internal resistance. That latter is solely dependent on whether the battery's "internal voltages" match (ie, their OC terminal voltage at any given time).

                        Be aware that the above merely adds to your info - it does/should not contradict it.
                        I knew what you meant by "fully charged" (whether 14.4V or 12.7V) but some would read your first reply as saying that a fully charged 12V lead-acid battery is 14.4V. (Whereas it should be 12.67V at STP - ie, 25C.)
                        AFAIAConcerned, in no way do you disagree what I have been saying for years.

                        As to your last paragraph, that's what the UIBI/charge-light controlled and voltage controlled isolator do - they only interconnect batteries when charging - hence no "high discharge currents" from battery(s) to other battery(s).
                        However diode isolators are a different and independent issue. But see Simbalage22's problem and other Simbalage discussions for ramblings about that.

                        But I see no need to continue this side debate unless there is a supporter for Red.
                        Should Red or anyone else wish to continue this paralleling discussion, then start a NEW thread specifically for that purpose.
                        This constant hijacking of threads is pathetic.


                        • #13
                          Today I was able to correctly setup my remote line. The second the relay connected the two batteries, they share the same voltage (well at least as fast as I can read both volt meters). I couldnt be happier with the setup. I feel like i have a bit of insurance for the next time my car battery dies. Thanks for everyone's input. Now its time to fix the headlight problem...
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                          • #14
                            Great to hear! Like I said, the batteries will have the same voltage (as the alternator - which will be charging when the batteries are interconnected assuming a UIBI (charge-light controlled) or "smart" (LOL; voltage sensing) isolator).
                            And yes, I like the "redundancy" or back-up offered by a secondary battery.

                            Let us know how you go with the headlight fix. It's your thread so you could continue any issue here, or start a new thread.


                            • #15
                              The headlight issue has been taken care of as well! I took your advice OldSpark and removed/reconnected while cleaning all of the connections. Well I say it has been taken care of, since it seemed to be random before I can not confirm this for sure. But I have left my headlights in the "on" position everyday for over a month and haven't seen them left on (I now consciously check it before I walk away from my car). Thanks again for everyone's input!
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                              Car PC software Setup [80%]
                              Car Audio setup [60%]