I have purchased a brand new Toyota Innova. I need to install a a mac mini, 2 laptops and make this car a mobile office.

I need to know if I can install another car battery in parallel, and will there be any negative side effects ?

I know that a battery in parallel should work, as both will charge and discharge together. And I will get enough power to charge phones while driving (or when engine is set to idle).

What will it take to get this working ? Or is it even possible ?

6 Answers 6


Yes it can be easily done, but based on your scenario it's not necessary unless you want to sit for extended periods without running the engine.

If you are going to leave the vehicle running for Air Conditioning for instance then you don't need to do anything it should work fine as equipped from the factory.

Here are some things to consider If you still want to install a second battery:

  • Mounting Location If there is room under the hood that's the best place, if you have to mount it inside you need to make sure you use a vented (to the outside) battery case. Batteries can put off explosive vapors when they charge and discharge so you don't want it venting inside the vehicle. Location will also limit the physical size of the battery.
  • Battery Type I would recommend a deep cycle battery over a car battery as they are designed to be repeatedly charged and discharged. They are the least expensive options. You can also look at Gel Cell, Nickel Metal Hydride (may not be suited for your application), or Lithium-ion. The more electrical capacity of the battery you choose the longer you can run without starting the vehicle.
  • Battery Isolator This will allow you to charge both batteries automatically but will not let you run down your starting battery. Keep in mind if you are using dome lights or anything else that's factory installed it will run the cranking battery down. You don't have to do this as simply connecting them in parallel will work, you just risk running down the cranking battery.

Here are some examples of battery isolators

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High Performance Battery Isolator 140 Amp

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Auto Isolator I, Model #: 091-139-2-12

The factory equipped alternator should be sufficient for your application

  • If you go with LI-ion, you'll also need an appropriate charger, as those don't have the same simple charging method (constant 14-something volts) that lead-acid batteries have. I believe this also applies to NiMH. Gel I'm unsure of.
    – Mathieu K.
    Nov 1, 2016 at 2:21

Following on from Hillsons' answer, your best bet is to get a proper split-charge system with a leisure (aka traction) battery as the second battery. These are especially designed for powering equipment for long periods of time, with a different discharge pattern to suit prolonged low-level use, rather than a normal car battery which is designed for the short, heavy current draw required for starting an engine.

The split-charge system ensures that the starter battery always has enough charge to start the engine, then charges the second battery. Modern ones use clever electronics to avoid interfering with the car's own electrical systems.

  • Will this do the job ebay.in/itm/320641471174#ht_1775wt_1141
    – Siddharth
    May 15, 2013 at 15:45
  • 1
    Oh yes, speaking of boats and RVs, you should use a boat/rv deep cycle battery for the second battery, they're designed specifically to power your equipment and cycle through full charges. Your regular car battery will suffer an early death if you constantly drain and recharge it. May 15, 2013 at 18:44
  • 1
    @Siddharth That will not work for your application it only rated at 30 amps. I have linked to some isolators in my answer that you can use. May 15, 2013 at 19:13

Yes, absolutely it can be done. Consider that this is common practice for boats and RVs. I myself added a second battery to my boat just to ensure I had enough juice for the stereo ( granted, if you kill the only boat battery you have, you can't just leave it parked in your driveway, you're stranded in a river or the ocean, a potentially dangerous situation ).

It's worth noting though that the Toyota Innova doesn't have a large engine, and you might need to upgrade your alternator in order to power 3 computers. You could either upgrade your alternator when you add the second battery or play it by ear and just buy a nice alternator if you find that yours won't cut it ( the link is just an example ). You might also consider what's called a battery isolator in order to charge multiple batteries but only allow one battery to be run dead.

  • 1
    Alternators normally output A LOT more power than it takes to run 3 computers. May 15, 2013 at 15:06
  • Thats what I thought, since drained batteries normally charge and function well after a 10 km drive. Do I really need to worry about a new alternator or just worry about switching to charge the 2nd battery after its done with the first ?
    – Siddharth
    May 15, 2013 at 15:40
  • 1
    That depends on the computers. A mac mini and two laptops are probably like 200w at max maybe like 50-80w avg/idle which is not much additional load for an alternator.
    – Mike Saull
    May 15, 2013 at 22:09

I use an ArkPak (made in Australia) that will house any battery up to 130amp/hrs. Can be charged in car/or via 240v adaptor or solar panels. Can be hard wired into car. Has inbuilt 150w inverter/pos. and neg. terminals/2x cigarette plugs type 12v ports/pure sine wave so you can run laptop/charge phone or camera. Decided advantage is that if not hard wired you can use it around the house during power outages. Great for running portable fridges too. Retails for $399. Check out sales@arkcorporation.com


It's possible that you will have a problem with your car's wiring not supporting enough amps for all of those machines running concurrently. 2 laptops and a mini running all at the same time could be over the typical 10 or 15 amps (== 120-180 Watts at 12 Volts, minus the inefficiency of inversion from DC to AC and some other factors) that DC outlet circuits in most car electrical systems are rated for.

The gauge (thickness) of the wiring used determines the maximum load it can safely convey; based on this, manufacturers provide fuses or circuit breakers that ensure that the load will not exceed that level. Putting too much current through a wire will cause it to heat up, like a light bulb filament or a stove burner, to the point of being a fire hazard.

Some cars have DC outlets on more than one circuit; if this is the case with the Innova, you may be able to avoid such problems by hooking the different machines up to multiple DC-AC inverters plugged into separate circuits.

Otherwise, you may need to connect your second battery up to a new circuit and make sure that you use wires that can easily take the load you're planning to put on it (and use fuses or breakers to avoid fire hazards in the case of overload).


Not all Battery Isolators are Created Equal

To expound on the answer from Move More Comments Link To Top, there are some differences in charge isolators that should be made clear. Some operate with diodes to split the charge current to each battery, while not allowing back-feed from one battery to the other.

In principle, this works well enough, but in practice, there is a ≈0.7V drop across all diodes which prevents either battery from seeing a charging current strong enough to de-sulfate the lead plates in the battery. This will lead to early battery failure and is the main reason many people believe that 'boats eat batteries."

If, instead, one were to have a relay that would selectively connect the batteries during charging and disconnect them during use, that would present the ideal solution. As it happens, some other engineer thought the same thing and developed a product around that concept!

The one I have personal experience and recommend with is from Blue Water Systems and can be found here. The principle of operation is pretty simple: if the device detects voltage above a threshold, it connects the batteries with a direct connection via a solenoid. If the voltage drops below a different threshold, the device disconnects the batteries.

There are a couple options available as to whether you might want to provide charge system current to the aux battery for extended use, or would prefer to isolate the starter battery during starting to protect delicate devices on the 'house' system.

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