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EV Battery Racks

Motor and rear battery rack finished and installed.
Motor and rear battery rack finished and installed.
Front battery racks, done and installed
Front battery racks, done and installed

This page outlines the process of designing, building and installing the battery racks into my Toyota MR2 EV project car. This process involved a lot of measuring and fitting, mockups, welding and some Chassis Modifications.


What is a battery rack?

... And why do you need one in an EV? Well this is pretty obvious. Batteries (especially lead acid) are big and heavy. They need to be properly contained and supported in order to function properly, and also for safety reasons. Ideally a battery rack will also insulate them from extremes in temperature or other environmental conditions. If a battery rack is located within the passenger compartment of the vehicle, it must also cover and seal the batteries and provide ventilation to avoid the possiblilty of fumes entering the passenger compartment and to keep anything from touching the battery terminals. In all cases, battery racks should be placed such that battery weight is as evenly distributed on the car chassis as possible. Another consideration is batteries should be placed such that there is adequate protection and crush space so that in an accident they are not crushed, which would probably cause a mess of acid and probably a lot of smoke and melted metal and plastic (but, probably not much fire, as nothing in a lead acid battery is very flammable).

Typical battery racks in an EV conversion are made out of an angle iron frame, and may have plastic bases and sides. In most car conversions, there are two battery racks, one up front, and one in the back. This is done to spread out weight and because there usually isn't enough space for all the batteries you need in one place anyway. Pickup trucks and purpose-built EV chassis usually can put the entire battery pack in one place, as low as possible and as close to the center of the vehicle as possible.

For my Toyota MR2 EV, I plan a 10-battery rack in the engine compartment that will sit above the motor and transmission. A second rack will hold 7 batteries in the front trunk compartment. The rear trunk will be kept free for cargo. The front compartment will be fairly well sealed up so I will incorporate a ventilation fan. The engine compartment is open to the elements, but I will install the batteries high enough up that they should not get too much exposure to road dirt and such. Due to the fact that I live in a moderate climate, I am not going to worry about insulation issues. None of the batteries will be in the passenger compartment, which is a good thing. I will have to modify the chassis in both the front and back to get sufficient clearance to fit the batteries. In the back, this will be minimal, however modifications will be substantial in the front compartment.


Rear Battery Rack

My rear battery rack design is the result of many hours of measuring and design time, all with the goal of fitting as many batteries into the compartment as possible. I started with cardboard battery mockups, and then a cardboard battery rack mockup, before building the real thing. The rack will be well protected in the event of an accident as it is sandwiched between two sturdy bulkheads, and is located between the rear wheels of the car. There is plenty of crush space to the rear (the trunk) and lots of space to either side as well.

Cardboard Mockups

Cardboard mockup of rear battery rack
Cardboard mockup of rear battery rack
Cardboard mockup next to real Trojan battery
Cardboard mockup next to real Trojan battery

One of the very first things I did after getting ahold of the MR2, was to make a couple of cardboard battery mockups. I made them for the purpose of trying to figure out how to cram as many batteries as possible into the car, and to decide which form factor of battery among those I was considering would fit the best. A cardboard mockup is much easier to handle than the real thing for obvious reasons. Though I did not do it, you could even take one when looking at cars you are considering for a conversion (See my EV Chassis Selection page) though the seller may wonder what the hell you are doing, trying to fit a cardboard box into the engine compartment. Later on, I made another mockup of the rear battery rack itself, to ensure that the proposed battery layout would fit into the rear engine compartment, and to ensure I would be able to install the rack itself after making it.

Battery deck

Battery Rack raw materials
Battery Rack raw materials
Early test fit of battery deck into car
Early test fit of battery deck into car
passenger side battery rack mounts welded to the frame.
passenger side battery rack mounts welded to the frame.
Rear battery rack deck sitting in car with 3 batteries, for final clearance check.
Rear battery rack deck sitting in car with 3 batteries, for final clearance check.

Like most EV conversions, my battery rack is made of angle iron. There are three 2" and one 1.5" angle iron cross pieces. Overbuilding this is not a consideration as this rack needs to support 700 pounds of batteries (ten 70 pound batteries), plus have enough strength to retain them in the event of a collision or rollover event. Eight mounting points (one at each end of four cross pieces) will attach it to the car. The rack will support three rows of batteries, two rows of three, and one row of four, all with short sides adjoining. This arrangment was the best fit for this engine compartment. I originally planned on two rows; one of 5 and one of 4 (long sides adjoining) but after getting all the old brackets and supports cleaned out of the engine compartment, I decided that I could fit one additional battery by changing the layout at the cost of some minor chassis modifications.

Chassis mounting brackets

I welded up eight mounting points to support the rear battery rack; two for each of the four cross pieces. All of these will be welded to the MR2's frame rails. Each one must support about 100 pounds of static load, so like the battery deck, they must be sturdy. The supports will be made from 1.5" steel tubing or angle iron, cut at angles and lengths such that once welded to the car frame, they provide a flat, level mounting. I started by welding the two mounts that are closest to the middle of the battery deck. I then attached the rack deck temporarily using only these two mounting points, and then used the deck itself as a template and jig to make the remaining supports and hold them in place while I welded them into position.

The two frontmost supports on either side are made from 1.5" steel tubing with a washer welded over the top, and a nut welded onto the other end of that as a place to run a bolt down into to hold the rack. The rearmost two supports are made from angle iron. The furthest rearward supports were problematic to weld in due to the way the car frame was constructed. Had I known this would be the case I would have positioned them elsewhere, but what is done is done. Those welds are ugly but they should be OK.


Battery rack deck conflicting with rear bulkhead
Battery rack deck conflicting with rear bulkhead

Chassis modification

As is obvious from the picture, I needed to make some minor Chassis Modifications to allow the battery deck to fit into the engine compartment in the position I want.


Front Battery Racks

These will consist of one 5-battery rack in the main front section, and a 2-battery rack, also holding the smaller 12v accessory battery, located where the radiator and cooling fans used to be. The reason there are two is to go around some structural portions of the chassis that I cannot modify or remove. Installing these front racks required several Chassis Modifications to the front of the car.

5-Battery Rack

Front 5-Battery Rack, test fit
Front 5-Battery Rack, test fit
mounting point for Front 5-battery rack welded to car frame
mounting point for Front 5-battery rack welded to car frame
Front five-battery rack, with brake vacuum reservoir test fit.
Front five-battery rack, with brake vacuum reservoir test fit.
Front five-battery rack, done and installed.  Brake reservior is attached underneath.
Front five-battery rack, done and installed. Brake reservior is attached underneath.

This battery rack resides in the main front compartment of the car. This area used to carry the spare tire and some other systems of the car. This rack presented the tightest fit of the three, and required a fair amount of Chassis Modifications to make enough room for the rack and its five batteries. In several places there is nearly no clearance between the rack and the car chassis, and the fifth battery will nearly touch the end of the brake master cylinder. Nonetheless, it all fits. This rack will also hold the vacuum reservoir for the EV Braking System. The reservoir will hang off the bottom of the rack to take advantage of the large but relatively inaccessible tub area below the battery rack.

This rack hangs off the frame rails of the car from four mounting legs that I welded onto it. I had to make a pair of recessed, threaded mounting points (a washer and nut welded together) and weld those onto the frame rails (taking advantage of some preexisting holes in the frame) for the rear mount points since the inside of the frame tube was unreachable. for the front mounting points, I just drilled through and fished a bolt through from the inside by taping it to the endof a stick and pushing it in from the front. The Vacuum Reservoir just hangs off the bottom of the rack from a pair of brackets I welded to the bottom. It is held onto the brackets with a pair of hose clamps.

2-Battery Rack

Front 2-Battery Rack, mostly done
Front 2-Battery Rack, mostly done
Front 2-Battery Rack, test fit
Front 2-Battery Rack, test fit
Front 2-battery Rack, complete and painted.  Holds 2 batteries plus 12v accessory battery.
Front 2-battery Rack, complete and painted. Holds 2 batteries plus 12v accessory battery.
Front 2-Battery Rack, installed.
Front 2-Battery Rack, installed.

The 2-battery rack, also holding the smaller 12v accessory battery, will be located where the radiator and cooling fans used to be. This rack mounts in front of a structural portions of the chassis (front suspension crossmember) that I cannot modify or remove. The two-battery rack will need some additional protection from the elements. I will attach some 1/4" lexan (or some other strong but light plastic) sheets to the front and bottom of this rack to protect the batteries from wind, road spray and rock hits. Fitting this rack involved Chassis Modifications of removing the old radiator support, and modification to the front bumper cover support so that I can remove batteries from this rack without taking the whole bumper off.

Originally I was going to carry the Accessory Battery (necessary for the 12v components in the car) on the 5-battery rack, but as it turns out there is better clearance to put it on the 2-battery rack. I decided to use a common garden equipment 12v battery (basically a very small starting battery) for this purpose. Of course its dimensions were slightly too big for my already-made rack, so I widened the section slightly where this battery will be carried.


Trunk Battery Rack

Trunk battery rack mostly done.
Trunk battery rack mostly done.
Trunk battery rack uses space underneath the trunk formerly occupied by exhaust system.
Trunk battery rack uses space underneath the trunk formerly occupied by exhaust system.
Battery rack to carry four batteries in trunk.
Battery rack to carry four batteries in trunk.
Battery rack installed in trunk.
Battery rack installed in trunk.

As detailed on my Range and Performance Upgrade page, I decided to add four additional batteries to the car after about 1500 miles of driving. I did this to correct for a goof in my planning that led to my running a lower voltage battery pack than I originally intended. The only way to add more batteries to the car while preserving any cargo space, was to make Chassis Modifications to the trunk of the car to allow a sunken battery rack to be located in the floor of the trunk. This battery rack bolts to the rearmost frame rails of the car (using two preexisting holes in the frame, and two new ones drilled for the purpose) and carries four batteries. There is still about 6 inches of total crush space between the rear bumper and the back of the battery rack, so in at least a moderate rear end collision the batteries should survive. Getting this assembled battery rack into the trunk was a bit of a challenge as it is too big for the opening of the trunk to allow it to drop in vertically. I had to put it into the trunk rotated 90 degrees, and then once inside the trunk, with the hole cut, I was able to rotate into position in the trunk and drop it in the hole that was cut in the floor.

I will build a lightweight lid out of a piece of a pickup truck bed liner to fit over the top of this battery rack, so that I can place cargo across the top of the batteries.


Battery tie downs

Batteries need to be held down to their battery racks. This is not because there is any danger of them blowing away, but vibration would inevitably cause them to shift around if they were just sitting there. Also, hitting a bump or getting in an accident could cause them to fly about if they were not properly retained. My design will be several small frames that fit around each row of batteries, and bolt to the battery deck to hold the batteries in place. I will build these later, after the batteries are sitting in place in the car.