Dual Battery Switcher

Update(2010-10-17) I’ve made some changes to the PCB design. There is now a RC filter on the sense input and a polarity protection diode on the power supply.  I’ve also corrected some parts values.  The changes are available from the GIT repository.


My parents are very keen on 4×4 and getting away into the bush. They have two lead acid batteries in the Land Rover, the main one and a second battery for running the fridge, lighting etc. What they wanted was, a small device that could detect when the vehicles engine was running and after a set time-out start charging the second battery. The design presented here detects the charging voltage of the alternator and will switch a low current relay after a set time.  You can use the relay to control a high current contactor or other solid state switch.

Theory of operation

Looking at the schematics you can see there is not much to this design. It only consists of 4 ICs and a couple of caps and resistors, but I’ll try explain how the system works.

When your car’s engine is off the battery voltage is around 12v (if you have a 12V lead acid battery), but once the engine is running the alternator starts charging the battery and the voltage rises to 13.5V – 14V. This rise in voltage is detected by the comparator IC1. Once the Sense voltage rises above the set point the comparator resistor R6 pulls the CONTROL line high and triggers the monostable 555 timer IC3A. The output of  IC3A is active low, so the TIMER line is held high while the 555 timing. The TIMER line powers the timer circuit of the Astable configured IC3B, this blinks LED2 to show the user that the timer is actively counting down, it blinks at about 2HZ.

The comparator set point can be adjusted with R3, the IC3A time-out via pot R8, and the rate LED2 blinks with R10,R11 or C8.

The 74HC00 Quad NAND gate controls the logic behind LED2 and activating the relay K1. The following truth table describes the states of the system.

Click for PDF

Inputs Outputs
0 * * 0 0
1 0 0 1 1
1 0 1 0 1
1 1 0 1 0
1 1 1 0 0

Remember that LED2 is active low and the Relay is active high.

PCB design

The PCB was laid out as a single sided board. The wide track widths makes this board very easy to etch at home.

I used Eagle3D to generate the 3D renderings of the board. All the parts are there except the relay, I couldn’t find a suitable model for it. I hand edit the POVRay file to make sure all the components are in the correct orientation and position. You can download the POVRay file from the git repository (see bottom of this page).

Component side rendering of the PCB

Etching and building the prototype

I used a piece of  CIF presensitised PCB. The board has a UV sensitive coating on it,  you expose the board to a positive image of the design you want to etch. Where ever the coating is exposed it washes off during development. Once the board has been developed you can etch it.

Assembled prototype

As you can see in the image below, there was a small mistake with the layout of the PCB. The power supply pins of the comparator (IC3) where reversed on the schematic and I only noticed it after etching. I guess that’s why you build prototypes!. The fault has been corrected in the schematics and the PCB layouts.

Track side of prototype



You can download the schematics, board layouts and POVRay files from the git repository.

git clone git://stewartallen.org/dual_battery.git

or browse the repository here.


Creative Commons License
Dual Battery Switcher by Stewart Allen is licensed under a Creative Commons Attribution-Non-Commercial-Share Alike 2.5 South Africa License.
Based on a work at git.stewartallen.org.

Posted at February 8th, 2010.

14 Responses to “Dual Battery Switcher”

  1. Robin says:

    Hi Stewart,

    Try a TL431 voltage/zener ic and a 4060 timer as a variation.
    Interesting ideas you have.

    • Stewart says:

      Thanks for the comment.

      There are many ways to do the timing, this was a case of “What do I have in the lab” 🙂

      One thing I would change though is to put a low pass filter on the comparator input and or add some hysteresis. If the set point is just at the right voltage and the relay pulls in it causes a volt drop big enough to reset the timer. It’s easy to fix (just adjust the set point higher), but it still should be dealt with properly.

      But I can report that the system has been working for about 3 months now with out any issues.

  2. alen says:

    Hi, sorry for my bad English but I’m Italian,
    I have a camper and I want use this switcher to switch every 22 minute the charging of alternator in two AGM battery.
    I can adapt this circuit for this work?
    Also I don’t understand where I have connect “sense x1-2”

    • Stewart says:

      Yea I’m sure you can adapt this quite easily, you’ll probably just need to turn the timeout 555 from monostable to astable mode and maybe also adjust the R8 and C7 values to give you 22 minutes.

      The “sense X1-2” connection is only if you want to monitor a voltage different to the supply voltage. You can just add a jumper between “supply X1-1” and “sense X1-2” if the board is run from a single supply.

      Good luck with your project. Let me know how it goes.

      • alen says:

        i’ve add jumper, but when I turn on the switcher the red led is on and the green led remains off although I move the trimmer.

        • Stewart says:

          I’d start by checking power supplies to the comparator IC1 and supply to the 556. The check that the output (CONTROL) of IC1 goes high while you adjust the potentiometer R3. If still nothing then start probing the 556 timer and check the out puts of the two stages. You can inject test signals if needed on the CONTROL and TIMER lines.

          Good luck

  3. alen says:

    I’ve resolve the problem, it was a false contact.
    another question: has the switcher the timer cyclic?
    because now the process occurs only once instead of go on forever

    [22′ ON]-[22′ OFF] -[22’ON]-[22’OFF] … ect.

  4. David Hayen says:

    I am looking at making this Dual battery timer relay for a tafe project. Is it possible to get the updated circuit board drawings to do the etching.

  5. Alexander says:

    I`m Trying to put switcher similar to this one. But the effect I`m searching is when the first battery go low like 2 volt. then will be swiched to the secound and charge the first.
    I hope to get help.

    • Stewart says:

      What type of batteries are you trying to use, at 2V a lead acid battery will be severalty flat. At 2V the relay will not even switch. Can you give me some more details about your project, maybe I can point you in the right direction.

      • Alexander says:

        So I`m using 12 V. Battery from a car. The point is like I use the full Battery to do my stuff and in the prossec to charge the Empty one. But When the first battery got to 10v. I have to switch To the secound battery and charge the first one. this is the point of my work.
        Hope I was clear:).

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