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My 2016 Toyota RAV4 hybrid has a strange location for the 12V battery. Usually, the battery is installed under the hood. Well, in this case, the battery is installed under the trunk. This is true although there's plenty of space under the hood. Although the car is a hybrid (and thus there's plenty of equipment under the hood), it has a really large hood, and because the car is a hybrid the 12V battery is quite small, so I believe space is not an issue here.

What is the reason for such a strange location? I understand that this is not the only car model having battery under the trunk.

I can identify several drawbacks in this arrangement:

  • The cables from the trunk to under the hood must be quite big. Most of the power is consumed under the hood (and even if the power is consumed in the rear like for the rear window heater, the fuse is still under the hood), and the current can be as big as 100A if all electric heaters of the car are on. Thick and long cables have voltage loss, weigh a lot and cost much. For non-hybrid cars, the cables must be think enough to crank the engine even when cold.
  • Storage space is wasted in the trunk. The battery compartment could be used to store some small items.
  • A separate jumper terminal is needed under the hood, because the electric rear door doesn't open if needing to jump. There is such a jumper terminal in a fuse box, but it could be eliminated if the battery was under the hood.
  • The trunk needs to be emptied in order to change the battery or to test its cold cranking amps performance. Only an open circuit voltage test can perhaps be performed without emptying the trunk.
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    The most common reason to have the battery in the trunk is to improve the weight distribution. E.g. BMW and Subaru do this for performance vehicles. I'd be surprised if that were the case here though.
    – Hobbes
    Feb 5, 2017 at 20:08
  • @ Hobbes .I think that weight distribution is the only reason .I think that you should expand your comment to an Answer.
    – Autistic
    Feb 6, 2017 at 5:59
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    Actually, googling around revealed that temperature could be an advantage in having the battery under the trunk. Batteries apparently don't like the hot temperatures near the engine. When taking into account that this car is a hybrid (low load on 12V system => doesn't require thick cables) having the battery under the trunk becomes cheaper than in conventional cars. So, now we have these: (1) space which doesn't apply here, (2) weight distribution which isn't a major factor here because the battery is small, (3) temperature, which is probably the reason here.
    – juhist
    Feb 6, 2017 at 7:26
  • @juhist I like the way you go into detail :) But most cars have the battery at the engine compartment and they work just fine without getting damaged by heat. The 12v battery is still needed for same stuff in a hybrid car such as starting the engine, so you would need thick wires still, no? My friend had an Jag S-Type which had battery in trunk and it only charged it to 13.75v. Which I believe is to avoid gassing in closed space and explosion, it was a bigger problem than little temperature in engine bay because usually batteries lasted about 3 years in that car due to undercharging.
    – Evren Yurtesen
    Feb 6, 2017 at 14:17
  • @EvrenYurtesen No, Toyota hybrids start the internal combustion engine with motor generator 1 (MG1) and the high voltage battery pack. Some counter-torque may be applied to MG2 as well to keep the permanently connected engine from causing torque in the wheels. The 12V system is there merely for lights, ignition system, and most electric accessories (AFAIK the electric air conditioning compressor however runs on the high voltage battery). There is no 12V alternator: a DC-DC converter supplies 12V power from the battery and the motor-generators. No 12V starter motor either, as MG1 does its job.
    – juhist
    Feb 11, 2017 at 14:31

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I can speak for the W12 Touareg (and V10 TDI), which employs a two-battery design. Neither battery under the engine bonnet/hood, primarily because space is at a premium (so much so that the engine and transmission have to be disconnected from the drivetrain in order to replace front arm bushings).

  • The larger battery which is used to start the engine resides under the driver's seat.

  • The smaller of the two batteries is located under the cargo hold at the back, under the space-saver wheel.

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  • Clearly, this isn't a car for me, as I have a habit of storing the jumper cables under the driver's seat. Quite handly location, because usually the driver's door is the only door that has a mechanical lock these days, and the central locking doesn't work if the car needs a jump.
    – juhist
    Feb 6, 2017 at 12:58
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There are three potential reasons why a battery could be under the trunk.

One is improved weight distribution. This is the deciding factor in high-performance cars that often are rear wheel drive or all wheel drive. Because the engine is typically under the hood, cars naturally have more weight on the front wheels than on the rear wheels, and moving the battery under the trunk is a way to combat this uneven weight distribution. However, this car already has the traction battery in the rear, and this car isn't a high-performance car, so weight distribution isn't probably the deciding factor here considering also that the 12V battery is quite small and lightweight in this hybrid car.

Another is simply space. Some cars with a really big engine or a hybrid system may simply not have enough space for the battery under the hood. However, in this case there is enough space under the hood for the relatively small 12V battery.

Third is temperature. Batteries don't like the high temperatures near the engine. Lifetime of a lead-acid battery when stored under the trunk is longer than when stored under the hood.

In this specific case, the deciding factor is probably the temperature. Also, because the internal combustion engine is started by motor-generators driven by the high-voltage traction battery, the 12V cabling from under the trunk to under the hood doesn't need to be so thick. The low current on the 12 system in this hybrid car makes it cheaper to achieve low temperatures by having the battery under the trunk. A non-hybrid would require such thick cabling that it isn't cost-effective to have the battery under the trunk.

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  1. Helps with weight distribution
  2. Keeps battery out of the heat, so it will last FAR longer
  3. Keeps battery better isolated from vibration

E.Yurtesen is wrong. In 25 years of car ownership, I have not owned a single car where having it reside under the hood was truly "okay". I just didn't know until I bought my 2007 Dodge Magnum, which has the battery under the floor in the rear hatch area. Truth is, most American mechanics don't even understand this. Maybe in a car with more room under the hood, a battery might last a little longer, but it's still a silly place for a battery due to heat. The primary reason to put it there is cost savings. Definitely more expensive to run large-gauge wire to the trunk. Also have to vent the gasses if it's not AGM or GEL. Anyway, the factory battery that came with my Magnum ran for more than 7 years, at which point I upgraded it before installing some audio gear. The old battery tested fine and I actually sold it to someone else, so there's no telling how long it actually lasted. Also, the terminals never had one spot of corrosion on them. We all know that German auto engineering is the top, and BMW & Mercedes put their batteries in the trunk. The Dodge Magnum/Chrysler 300 was the first car produced after Daimler-Benz bought Chrysler, hence the battery-in-trunk design.

On the flip-side, my wife's Acura TSX with its small engine compartment and front-mounted battery, kills regular batteries in less than 3 years, and just killed an AGM battery in 3.5 years.

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