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How to hook up extra battery?

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  • How to hook up extra battery?

    I was thinking about buying a capacitor to help with some voltage drops that I am having, but after reading some threads on here, it seems like that is a no-no. I have seen recommendations to buy a small AGM 12v battery. That sounds like a good idea but my question is can I hook it up so that it stays charged constantly like my car battery? It seems like it would be a pain if you had to charge it everytime it went dead. I would be running my carputer, mono sub amp, and 4 channel amp off of it, so what size should I get?

  • #2
    I hooked my second battery up in parallel after the car's relay. I have doubled my PC's runtime with the engine off.


    • #3
      second battery

      I have been reading a lot of threads regarding installing a second battery to power the carputer and other high power accessories with the vehicle off without running down the main start battery. Although most of the ideas are pretty good, a number of points have been missed:
      1. The alternator
      If you have a 'normal' car or light truck with a standard alternator, it will quickly die when the second battery is installed. The alternator's voltage regulator was designed by the electrical engineers for your car with normal loads. Adding something like a second battery will quickly exceed the capacity of the alternator if that battery is discharged.

      2. Battery isolation
      Although you can use a simple relay to connect the two batteries, a battery isolator is the best way to go. This will not only keep them separate when the engine is off, but provide some additional over voltage protection on both sides of the isolator.

      3. Mixing the type of batteries
      This is a big mistake that will cost plenty in the long run. Most people use a sealed battery (example: AGM) so it can be placed in the passenger compartment or trunk. An AGM battery is still a wet cell battery and it will vent if over charged. If it begins to vent, it will vent hydrogen and sulferic acid mist into the compartment where it is stored. The best place to put these batteries is still in a vented compartment.

      4. Connecting battereis of different sizes and ages
      This is another big mistake. When the alternator is sending out charge current, it doesn't care where it is going. All the regulator sees is a load. That load is below 12 volts and the resistance 'looks' like a battery, so it pumps out power. If you have two batteries of different sizes and/or ages, then the weaker/smaller one will take most of the charge current. The other issue is if one of the batteries has a weak cell, then it will pull on the good battery even during the charge cycle. Using a true isolator will help balance this load out.

      Before you flame me to bad, I deal with this every day repairing damage caused by customers who wire batteries and chargers incorrectly in their RVs. 12 volt systems are not complicated if you follow a few simple steps.

      1. Know your supply and loads
      Before installing any high drain system, make sure your electrical system is up to the task. If your alternator is more than 3 years old, then it is probably time for a new one before you start this upgrade. Same holds true for the battery(ies). If you are upgrading power, then upgrade to a larger alternator and replace the main start battery at the same time. At this time, install your second battery, isolator and high quality cables to handle the power.

      2. Plan all electrical runs before getting out the wire cutters.
      Know where everything electrical is in your vehicle and how it is connected. This will help prevent problems with power (and help with those ground loops).

      3. Test Test Test
      Install each component one at a time to make sure it doesn't disrupt the electrical system of your vehicle.

      4. Buy a good VOM, clamp on amp meter and bettery tester
      These tools are great. Remember you not only need to know the voltage, but the amp draw. Example: If you have a 700 ah (amp hour) battery, and your draw under full load is 100 amps, then you could (in theory) run the system for 7 hours before depleting the battery. This also opens the opportunity to design a system to shut the system down after so many amps have been pulled through. You will also need to understand ohm's law, watts law and the inverse amp law.

      I love my truckputer and really enjoy the challenge on making it work. I'm not very good with fabrication, but I like 'whats under the hood' much better.

      Good luck to you all and have fun,


      • #4
        here is what i have been putting to gether in my head the past week or so.
        install second battery somewhere. place a battery seperator in between both so that it will regulate power. charge them while the car is running / seperate them as need be for overcharging / and i believe some seperators link upon starting if too much power is lost in the primary. install a switch to link them for specific short instances such as using a winch... etc...

        then i want to hook up amps( 4 channel and mono), car pc, lcd, and any other electronics i decide like cb, ac/dc inverter... you get the idea... well hook them up to the battery and then hook up the items that require an ignition on wire, hook them up to that so they will shut on and off via the ignition key and be powered by the aux. battery.

        the tricky thing is that i want a second battery primarily to preserve the main/original battery for starting purposes. but... what if i want to run the pc and amps to listen to music with the key out because i don't want to drain th emain battery with all the other things that start running when the key is in...

        i will have to make an alternate "ignition switch" with a regular on/off switch. the switch will be in the line of the specific item's ignition detection wire and the aux. battery itself. so i could technically flip the switch and the devices turn on as they would with the key in....

        THERE IS A PROBLEM WITH THIS IDEA THOUGH... If i flip the switch on with the key in, the ignition detection wire from the vehicle is off the main battery and if i flip the switch on, it will link th e2 batteries.... i don't know how to get around this... would only happen if the switch is flipped while the key is in though.

        I just want to have the option to run some things with the key out but then they will run with the vehicle still running and have all the features of off with the key on with the key.

        would only happen if the switch is flipped while the key is in though if i wanted the devices to keep running after i take the key out instead of waiting for them to shutdown and then come back up.


        • #5
          Two batteries

          You are on the right track.

          I'm going to draw a picture and post it. Is easy in a drawing, but hard to describe.


          • #6
            quick drawing without the extra switch i was talking about. this would allow everything to be powered by the aux. battery but will turn on with the key in the ignition and shut off whe you take the key out. i still gotta find a way to allow "JUST" the things to turn on or keep running when the key is taken out.



            • #7
              I don't mean to thread crap - but I'm in a similar situation with a dual battery setup. My car is a pain in the arse though. It's a 2004 Maxima, and the alternator apparently "doesn't work with battery isolators". The plug on the alternator is simply 2 wires, black and white or yellow.

              Which leaves the option of using a relay connected to the ign-on wire.

              Not sure if your car falls into that category or not, but it's something to look into before you buy equipment.


              • #8
                If you have a typical alternator's charge light circuit, it's EASY!

                Connect an SPST (on-off) relay with its solenoid (terminal #86) to the L or D+ from the alternator (D+ for single wire alternators; L for 2 or more wired alternators - same thing; different labels).
                Ground the other #85 end.

                Connect the main battery through a fuse (or better still - a circuit breaker) to the relay (#30) and the relay output (#87) to the second battery via a fuse (or CB). The fuses/breakers should be as close as practicable to each battery.

                Only after charging begins does the relay energise and close the contact to connect the batteries.

                You may already have a similar relay circuit - older carby vehicles with electric fuel pumps often used a similar circuit, and some used the same to control electric chokes, fuel cut-offs etc.
                If you have such a relay, you could power the battery relay from that (though it might come on during cranking as part of the carby-priming etc to enable starting).

                Most alternators should be able to power a typical automotive relay of down to ~60 Ohms relay-solenoid/coil resistance (ie, ~200mA). The D+ or L circuit will typically sink 1A of current (ie, ground various dash bulbs etc) but that doesn't mean it can provide 1A at 12V....

                If your battery relay requires a large actuating current, it'd be best to have the D+/L turn on a smaller relay which can then power the larger relay.

                The size of the relay, fuses & cable depends on what you expect the 2nd battery to draw. I use a 70A (or is it 140A?) relay with 50A circuit breakers and ample wires size.

                See my posts under 2nd car battery and Smart Battery Isolator.

                The circuit is....

                It won't wreck your alternator unless you have a bad one. (They are more likely to blow after jumpstarting if the jumper battery is pulled off to fast - as I found out during my recent Bosch experience!)

                And since the batteries are not paralleled when idle, there are no matching issues - just the usual periodical battery condition checks (Read: Get a dash voltmeter!)

                And a low-voltage cut-out will preserve the 2nd battery.

                Relay(s) might cost ~$10-$20. A low-volt cutout ~$20. Much cheaper than voltage-sensing relays, and stupid diode isolators.
                And you can probably ignore most claims regarding current sharing - it's not an issue (unless you have a collapsed battery - hence the periodical checks, or a temp sensor) - a vehicle's battery system is a constant voltage system by design - the currents look after themselves. (And if someone says they regulate the current to each battery, ask them how & let me know.)


                • #9
                  any ideas on how to make a master override for the equipment in the blue boxes? a switchthat icould flip before i take the key out and the equipment in blue boxes would stay running unless i flip the switch off.


                  • #10
                    You can make a self latching relay using a normal relay and 1 or 2 switches.

                    The first switch is a momentary-on power-on that latches the relay (or keeps it energised).
                    To turn it off, either break the main power, or a second normally-closed switch can be used/pressed to turn it off.
                    I recommend adding 2 diodes depending on the power being switched (IN4004 etc = 1A etc; ~20c each).

                    So a push button(s) plus diodes plus $5 - $10 for the relay (for up to say a 60A load; maybe $20 for 200A etc).

                    A commercial low voltage cutout can also be added (~$20). (I guess I could make one for $1-$2 in parts, but international commercial products are universal & repeatable.)

                    If interested, reply else PM.


                    • #11
                      Originally posted by OldSpark View Post
                      You can make a self latching relay using a normal relay and 1 or 2 switches.

                      The first switch is a momentary-on power-on that latches the relay (or keeps it energised).
                      To turn it off, either break the main power, or a second normally-closed switch can be used/pressed to turn it off.
                      I recommend adding 2 diodes depending on the power being switched (IN4004 etc = 1A etc; ~20c each).

                      So a push button(s) plus diodes plus $5 - $10 for the relay (for up to say a 60A load; maybe $20 for 200A etc).

                      A commercial low voltage cutout can also be added (~$20). (I guess I could make one for $1-$2 in parts, but international commercial products are universal & repeatable.)

                      If interested, reply else PM.
                      I'm interested.
                      After a 2 year break, I'm back! There are so many new and familiar faces here!
                      New vehicle, new ideas!

                      Chryssi - The Changeling CarPC Project


                      • #12
                        Me and my rhetorical questions! Thanks again to Izu for his diagram & ImageShack.

                        The above was quick diagram for a pump, but the pump is just the load.

                        The 0V with an abbreviated ground symbol (the inverted T) is zero Volts and means battery negative, ground, chassis, earth etc (whatever your jargon; 0 Volts is universal).
                        The relay's solenoid (aka #85) is grounded (as is the load, battery, alternator etc).

                        The top line is the heavy +12V power from battery (fused) to the load.

                        The LHS +12V is the same or whatever +12V source you want to initiate the latch (power to load) by pressing the normally off switch.

                        The switch powers the relay coil/solenoid (aka #86) hence closing its heavy contacts (aka #30 & #87).
                        The RHS link back to the solenoid (via a diode) keeps the relay energised.

                        As shown, the main heavy supply voltage must drop sufficiently to drop out the relay and hence cut power to itself and the load. It stays off until the switch is re-closed.

                        Otherwise to turn it off, the feedback link needs to be broken. Hence insert a normally closed switch. (This is NOT shown in the fig.)

                        For low-voltage disconnect, the same has to be done - ie, break the feedback link.
                        Using something like the MW728 "battery protector" (an in-line 10A supply that plugs into a cig-socket and cuts out below 11.2V), its +12V output is used instead of connecting the feedback from the relay output. (The MW728 itself is powered by the relay output.)
                        A manual power off switch (in the feedback line - described above but not shown in the fig) can be retained/inserted in the LVCO's connection before the RHS diode. LVCO is the Low Voltage Cut-Out (or LVD = Disconnect) - eg, MW728 etc.

                        FYI re MW728:
                        The MW728 (and many similar) can be tricked into cutting off at higher voltages by adding series diodes to its input. Each diode drops from ~0.2V to typically 0.6V or 0.7V. Hence with two 0.6V dropping diodes, it its supply is 12.4V, then the MW728 sees [12.4 -(2x0.6) = 12.4-1.2=] 11.2V and turns off the load.
                        BUT make sure the added voltage drop is not so high that the LVCO doesn't switch on. (EG - MW728 turns on at 12.5V. 12.7V is an unloaded fully charged battery. Whilst charging, the supply should increase to 13.6-14.4V.
                        EG - in the 1.2V drop example above (ie - turn off at 12.4V instead of 11.2V), the MW728 turns on at 12.5+1.2V = 13.7V which is ok. [Charging systems should put out a minimum of 13.8V which is the float voltage of lead-acid batteries (ie - after the battery is fully charged). Less than this (long-term) leads to premature battery failure.]
                        A simple workaround is a switch across the diodes to short them out - ie, no voltage drop to the LVCO.

                        Alternatively, in this application, a Zenor diode could be used (they are available in various voltage dropping voltages, though usually above a a couple of volts... use common diodes instead (cheaper, and no power dissipation considerations).
                        Another possible alternative (for THIS application where the MW728's load is low & not very variable) is to use a resistive voltage divider - maybe with a pot (variable resistor) to adjust the drop-out voltage.

                        Though irrelevant in this case, one issue with the MW728 is that although it turns on @12.5V and then drops out below 11.2V, there is no timing involved. Good LVCOs have delays - eg, maybe 30 seconds till it actually switches on.
                        But that is not a problem here because one that relay turns off, no power is supplied to the LVCO until the manual on button is pressed (hence why the LVCO is powered by the output of the relay).
                        [FYI - Last week I had my MW728 cycling when powering my fridge (about 5A). EG - Battery above 12.5V; fridge on, the voltage dipped below 11.2V so hence switches off. Within 10 seconds the battery self recovered to 12.5V, so on again, then off after ~1 second and the cycle repeats. The MW728 may handle that, but not many loads! Oddly enough, it was a fully charged battery (but only an 8A charger). But it did not repeat that after running the car a while - the alternator probably supplied sufficient current to blow off the cobwebs etc (sulphation etc - typically requires above 20A charge current, else a succession of hi-voltage squirts).]
                        / end FYI re MW728

                        And there ends LatchingRelaysWithLVCO's-101.
                        More info than needed - and a fine way to make a simple set up look complex - but as with many systems, it's based on a simple/basic layout. But there are so many ways to adapt a simple system.... (It's all just building blocks really...)
                        And it helps if readers skip my (added info) parenthises, or value-add (??) FYI's, and understand the ad-hoc {[(..)]} hierarchy.
                        It'd be an even greater help if I planned before typing. But like thinking (and the web), that went out with the advent of wordprocessors (even though the two functions are independent!) - I just retain problem solution methods (and leave to doco to others). Upon re-read, I can reduce verbiage by typically 30% with much better flow/expression.

                        If required, I'll see if Izu can modify the diagram to include the above mods (with manual off and maybe maybe basic LVCO).
                        If Izu does, I'll substitute & rewrite the above.


                        • #13
                          that looks confusing. the reason i don't want to put a switch from the ignition detection wire off the amps/computer/lcd to the aux battery is because i wouldn't be able to flip the switch until the key is out... or am i wrong?

                          wouldn't flipping the switch connect the 2 batteries between the immediate aux. battery and the ignition wire from the key ignition from the primary battery?

                          would that be dangerous? or would that even work?


                          • #14
                            I haven't analysed your proposed switching setup.
                            I merely responded to your latching power requirement - ie, push a button for on, push another for off. (Or a 3-position toggle switch.)
                            And since I assumed you'd want a relay instead of a high-current switch, I described how adding two push buttons solved your problem.

                            I also assumed you would NOT be paralleling batteries for that use, though that is a separate issue anyhow.