I want to make a dual battery setup on my car. Doing some research I came across the two main techniques to do this, both based on isolate the two batteries being fed from same alternator to avoid charge issues.

Wanting to avoid the voltage drop and heat issues of diode isolators, I started to check mechanical isolation (relay).

There are a lot of ready to deploy products in this line, most of them looking like this. But I'm kind of stuck on how they work. They look like simple big fat spdt relays.

While the instructions are clear and mount the batteries to it doesn't look like a problem, I'm puzzled. Most of the instructions wire 12v directly with the relay coil.

If I'm not wrong, this will make the relay change it's state when I turn the car on, charging just one battery. Then when I turn off the power, relay should change again and charge (but no, because the alternator would be off) the other battery.

I am missing something? Is this not a relay as i know it? Or it is a solenoid? Should I add a switch to control the relay?

As an example, here is a mounting diagram. I am missing something simple af and I don't know what.

Edit for clarification:

The purpose of this setup is the usual one among the car and audio hobbyist. I want a secondary battery to make use of some gadgets without compromise the main battery.

  • \$\begingroup\$ It is not clear to me why would you need two batteries. Is for backup? Do you want to double/increase the capacity? Please clarify. \$\endgroup\$ Apr 4, 2018 at 12:55
  • 1
    \$\begingroup\$ Would also bee good to add a small delay (one or two minutes) between engine start and paralleling the batteries. After the engine start the main battery will take a lot of current to recover from the effort. The alternator suffers. If yo add the second battery just then the alternator will suffer even more. Wait a minute and the main battery will already be "calm". In modern cars the main controller will raise/lower the alternator voltage to compensate for this. Check yours with a voltmeter => 14.5 for some time after engine start then down to 13.8 \$\endgroup\$ Apr 4, 2018 at 13:51
  • \$\begingroup\$ @ToniHomedesiSaun how could I do that? Looks like a nice feature to keep my alternator happy. (Please don't say 555 ahaha) \$\endgroup\$
    – David P.
    Apr 4, 2018 at 16:55
  • \$\begingroup\$ This is common enough for vehicles with a large "house" or "hotel" load, such as RVs or yachts, where you want to be able to run anything from a ham radio to a microwave without risking draining your starting battery. Typically the house battery is a deep-cycle type which can tolerate being deeply drained without damage, which the starter battery is not. \$\endgroup\$
    – CCTO
    Sep 16, 2021 at 17:53

2 Answers 2


In a dual battery setup, all accessories required to be powered with the engine stationary would be connected to the accessory battery.

With the engine running the isolator relay would parallel the primary and accessory batteries. With the engine stationary the isolator relay would isolate the accessory battery from the primary battery system.

With the engine running, both the batteries would be charged by the alternator and would together feed all the connected loads when required.

With the engine stationary, the isolator relay would ensure that the primary battery does not get depleted by the accessories being powered.


The relay connects the second battery to the alternator only when the engine is running so it can be charged, which is why the wiring diagram shows a 12v feed WHEN the engine is running.

When the engine is not running, the second battery is isolated - to supply power to your external devices without affecting the vehicle battery - this is based on your second link...

Edit :The battery that is "top right" labelled « vehicle battery » in the link is always connected to the vehicle - the diagram does not show the standard vehicle wiring, only the extra wiring necessary. And I am assuming that the battery in the lower part of the diagram is the second battery.

  • \$\begingroup\$ Although it's not clear from the diagram, the implication is that the alternator is permanently connected to one of the batteries. The other one is isolated when the engine is off. \$\endgroup\$
    – Finbarr
    Apr 4, 2018 at 13:21
  • \$\begingroup\$ @Finbarr the battery that is labelled « vehicle battery » is always connected to the vehicle - the diagram does not show the standard vehicle wiring, only the extra wiring necessary... \$\endgroup\$
    – Solar Mike
    Apr 4, 2018 at 13:28
  • \$\begingroup\$ You must be looking at a different diagram - in the one linked in the question both batteries are labelled "Car Battery". \$\endgroup\$
    – Finbarr
    Apr 4, 2018 at 13:39
  • \$\begingroup\$ @Finbarr Ok, so I saw the label on the battery top left... But the diagram and my comment about it not showing the standard vehicle wiring still stand. Edited... \$\endgroup\$
    – Solar Mike
    Apr 4, 2018 at 15:08
  • \$\begingroup\$ I get your comments but now the diagram has even less sense for me. Is one is wired to the alternator AND the relay? Why? It has some sense to me to use the NO connection as a switch to parallel them, but why the main battery in the NC connector? \$\endgroup\$
    – David P.
    Apr 4, 2018 at 17:05

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