Are there any significant upsides or downsides to moving the battery to the the trunk? My impression is that it's just a preference thing that doesn't really affect anything at all in any real way. Here is what I have - want to know what I am missing:


  • May affect manufacturer warranty if your car is covered
  • May affect insurance pay out in accident? (really doubt this, but who knows)
  • Effort / parts cost to install
  • Shock / fire risk: Single highest-power-in-car wire going through cabin (presumably through firewall, into cabin, into trunk through passenger-side floor)
  • Mechanics may be reluctant to work on it (doubtful if they understand the basics)
  • Wasted trunk space - need battery bracket, suddenly not so easy to fit luggage
  • More rust - Drill trunk for ground wire location
  • Fire risk from battery gasses? (Seems like a dated risk, but it's still a battery in the trunk with felt / cloth, rather than behind a firewall)


  • More room in engine bay! (My sole reason for considering this - I have to take half the car off to work on things)
  • Better weight distribution on most cars (so negligible that it doesn't matter)

General question, but the car I am thinking of doing this is a manual 2009 Civic Si. If I get rid of the battery, get a small intake, get rid of the ABS, half size the radiator, I may actually be able to get to things like the starter without spending 2 days taking the car apart.

  • If you are really only using the battery as a "starting" energy source, then perhaps a smaller batteryis what you want. I got one to replace a car battery that died. It was about 1/3 the size and weight of the original battery. Besides the extra price for the battery, the only additional cost for me was a couple of wood blocks to act as spacers so I could mount it the same spot as the orginal battery.
    – gns100
    Nov 28, 2022 at 16:43

2 Answers 2


I think you have most of the stuff covered as far as pros/cons. The "main" reason people do this is weight distribution, as you stated. This is for the performance to place more weight in rear of the vehicle. Some may want to move it to help clean up the engine bay from a "show" type perspective.

There's a few things you need to think about when doing this:

You mention the gas emissions from a regular battery inside the interior of the vehicle. This is a real concern. The main mitigation for this is one of two ways. First is to encapsulate the battery inside a capsule which is then vented to the outside of the cabin. Secondarily is to get a battery which does not vent, such as an AGM or Lithium. AGM is a little more expensive than a regular lead acid. Lithium is a LOT more expensive, but has some other trade offs as far as it being lighter.

When installing a battery in the rear of the vehicle, you have to ensure the cables you use will work for your application. The longer the run (from front to rear) the large the gauge of wire is needed. The larger the amperage draw, the larger the gauge of wire is needed. When you've got both factors to consider, you need to run even larger gauge wire. The type of wire you use is important as well. While cheaper and may work, copper coated aluminum (CCA) is something I'd avoid. Pure copper is by far a better choice, but is a little more expensive. It will handle the electricity load a lot better for a lot longer than will CCA.

When running the cable, run it as far as you can outside of the cabin, then when you punch it up through and into the cabin, you can do this one of two ways. You can punch a hole through the body, then put a grommet to prevent cable chafing.

The second way you can do this is to purchase remote battery cable firewall bulkhead connectors, which you cut the cable and put wire ends on them, then bolt the ends onto the bulkhead connector posts, one inside and one outside of the cabin. This seals the cabin so you don't have moisture ingress. They look something like this:

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Anytime you drill any holes through your body shell for running the wires or affixing the battery, ensure you paint these areas. Getting a rust encapsulation type paint (such as POR15, though there are many on the market), will ensure no rust starts forming. You can also use regular paint as well, but the rust encapsulator works best from my experience. It provides a coating on the metal which will prevent rust a lot better than paint over time.

  • Appreciate the answer as usual Paulster!
    – VSO
    Nov 27, 2022 at 16:47

Another point is voltage drop - we already use a thick positive cable between the starter and battery. That cable will be roughly 4x longer causing voltage drop so you need a thicker wire (lower gauge number)

And of course adding more and thicker wire offsets the weight balance improvement, while increasing the overall weight.

Related - you may have to run a dedicated negative lead from the battery to the engine block as well - a traditional ladder chassis is great for this, but you have a unibody so a second run of wire may be necessary.

It may be easier to upgrade your battery terminal clamps with quick release and the battery holddown with another QR. That lets you remove the battery easier anytime you need to get into it.

Also it sounds like you're going in from above, on some tasks that can better be done from below the vehicle. Life might get easier if you have a pair of ramps and chocks and a creeper.

  • 2
    I said "easier" not "cleaner" :-)
    – Criggie
    Nov 28, 2022 at 3:38
  • 1
    Good point about adding wire weight - that's something I didn't think about it at all. It doesn't matter on this car, but definitely a good point I don't hear discussed.
    – VSO
    Nov 29, 2022 at 13:59

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