Placement Containment

Shari Prange

©1993 Shari Prange

One of the first design decisions in converting a car to electricity has to be where to put the batteries. In a typical car, there are sixteen to twenty of them. All other design decisions will depend on where the batteries are placed, and how they are oriented. The obvious consideration is available space, but other important factors include weight distribution and current path.

Battery Placement

For stability and handling, the ideal arrangement would be to have all the batteries in a single block, between the axles. This can be done in a van by sinking them into the floor, or building a false floor above them. In a pickup truck, the batteries can ride in the bed, but a better plan is to tilt the bed and install the batteries underneath it. This gives good handling and maintains cargo space. Some of the batteries may be placed under the hood as well.

In a passenger car, the batteries almost always end up split between the front and rear of the car. Weight should be kept inboard of the axles as much as possible, and balanced between the front and rear. If either end is significantly heavier, handling will be poor. The weight should also be kept as close to the ground as possible for a stable center of gravity.

It is perfectly acceptable to cut into the chassis to sink the batteries, if it is done with proper care. No structural members should be cut or weakened. The batteries should be enclosed in a sturdy welded rack that will reinforce the area where the chassis was cut. Care needs to be taken to ensure that the batteries do not cause any interference with the axles, suspension, etc., and that they do not extend down far enough to diminish the road clearance. Once the batteries are installed, use caulk or duct tape to seal openings between the rack, box, and chassis to restore weatherproofing and reduce road and wind noise.

Above: A rack for a block of eight batteries to be sunk into the chassis. The batteries will be enclosed in a box, with steel straps across the top. Photo by Shari Prange

Battery Accessibility

Accessibility is also a consideration in battery placement. You will want to be able to get to the battery fairly easily to inspect the terminals and check water levels. For this reason, you don't want to stack batteries on top of each other.

Current Path Layout

Once you have a general idea how you would like to place the batteries, make a scale drawing of the pack and label positive and negative terminals in the correct places. Now start to lay your circuit path.

The most positive cable out of the pack will connect the main contactor to the positive terminal of whichever battery you declare to be first in the string. The batteries are then connected in series, positive to negative, like a daisy chain. If the pack is split in two or more locations, the last negative terminal of one location will connect by cable to the first positive terminal of the next location. Finally, the most negative cable will come from the last negative terminal in the series and go to the speed controller.

Are any of the interconnects long and awkward? Do any of them interfere with each other, or the battery caps? (Note: check in advance what style caps your batteries will have.) Do the most negative and most positive cables come out of the circuit anywhere near the contactor and controller?

Try to keep interconnects and cable runs as short as possible, although sometimes long runs can't be helped. Avoid complicated or criss-crossing interconnects. With these thoughts in mind, reexamine your layout, and see if turning some of the batteries

180° will simplify the circuit. If not, maybe turning some of them 90° into a slightly different configurations will help.

Component Locations

Obviously, you will be deciding the locations of some of your components in the course of designing your battery layout. As mentioned earlier, you need to know where your controller and main contactor will be in order to plan the battery connections to them. It is more important for them to be close to each other and to the motor, than to be close to the cables from the battery pack.

Circuit breaker placement is also a factor. Ideally, the circuit breaker interrupts the most positive cable between the batteries and the main contactor. If this is not practical, it can be placed between any two batteries in the series. It is best to have the breaker within easy reach of the driver. If it is located away from the driver, some kind of reliable remote method of flipping the switch is needed.

In our Voltsrabbit, eight batteries fit under the hood in a strange, split-level arrangement, and eight fit in a box behind the back seat. The most positive cable ran from the main contactor under the hood, to the circuit breaker in the dash, to the rear battery box. The most negative cable from the rear box connected to a positive battery terminal in the front batteries. The most negative cable of the overall pack emerged in the front, near the controller.

When you are running cables fore and aft like this, it is advisable to run both positive and negative cables side by side. This will diminish electrical noise that could interfere with some of the components.

Battery Racks

Once you have juggled all these factors into a satisfactory layout, you are ready to start designing the racks and boxes to secure these batteries in place. All batteries should be secured adequately to stay in place, even during a collision or roll-over. Don't be fooled into thinking their weight alone will hold them in place. Department of Transportation crash test tapes of early electric cars show poorly secured batteries sailing into the air in 30 mph barrier tests. Watching in slow motion as they fly forward to crush the crash test dummy

Above: Rear Voltsrabbit battery pack installed, showing rack and box sunk into the chassis. Note the ventilation fan on the right, the exhaust vent on the left, and the round hold down blocks on the lid of the box. Photo by Shari Prange

Above: Front Voltsrabbit battery pack installed. Note the split-level arrangement, and steel hold down frames around the battery tops — all held in place by stainless steel bolts. Photo by Shari Prange against the steering wheel is a sobering experience. Other cars suffered little battery displacement.

Battery racks should be made of minimum X6" x 1X" welded steel stock and straps, or a material of equivalent strength. If the rack is large, use reinforcing straps across the bottom. If you are not trained as a welder, hire someone who is to do the work. Your battery racks are not the place for on-the-job training.

Additionally, the batteries need hold-downs across the tops. These can be bars or rigid straps bolted into place, or an angle-stock frame that encloses the tops and is bolted down. Of course, the hold-downs must be placed so they cannot accidentally short across two terminals. The following are NOT suitable hold-downs: plexiglass, nylon straps or belts, plastic shipping straps, or ready-bolt.

At the minimum, racks must be painted to prevent rust and corrosion. This can be done with spray paint. The best option, if you can afford it, is powder paint. This paint is applied as an electrically charged spray powder to an oppositely charged part. The powder will actually flow around corners to coat evenly and fill small spaces. It is then baked to a tough, ceramic-like finish which is non-conductive and corrosion-resistant.

A nice added touch is a thin sheet of plastic on the bottom of the rack (if there is no box) covered by a layer of Battery Mat. This is a felt-like material impregnated with acid neutralizers. Placing it under your batteries will help protect the rack from accumulated battery acid mist.

Battery Boxes

It is best to enclose batteries in boxes whenever possible. It is absolutely essential if batteries are inside the passenger compartment. The box will improve performance by regulating battery temperature. In an accident, it will protect both the batteries and the passengers. In operation and charging, it will keep fumes away from passengers.

For economy, the box can be made of plywood and painted. Plywood is readily available and easy to work. Recommended thickness is M", and is the absolute minimum . Plan to take the finished boxes to a packing and moving shop and have them banded with steel packing straps at two levels for reinforcement.

Like battery rack paint, there is a better but pricier option: welded polypropylene. This material is stronger than plywood, so you can use as little as X" thickness with proper rack support under and around it. It doesn't need to be painted, and is acid-proof. (In fact, it is used industrially for acid bath tanks.) It is lightweight, and makes an attractive package. You can get boxes made at a plastics fabrication shop.

Batteries should be held down inside the boxes as well. If you hit a bump at speed, they can fly up against the box lid. To prevent this, attach small blocks to the lid at the corners of the batteries. When the lid is closed and strapped down, the blocks will hold the batteries down inside the box.

Insulation, Heating, & Ventilation

In colder climates, you should insulate or heat battery boxes. Insulation can be done with X" sheets of polyurethane foam inside the box. Do not use styrofoam, as it reacts badly with battery acid. If space permits, insulation can be built into the battery box floor and lid as well. There are battery heating blankets available for use in large diesel trucks that can be adapted to a battery box and plugged in like a block heater.

Batteries enclosed in boxes need to be ventilated, especially in the passenger compartment. There is a slight danger of explosion from collected hydrogen, although this is very unlikely. It is also important to provide ventilation to remove gases that will encourage corrosion and that are unpleasant to passengers.

Ventilation can be passive since little gas is produced while driving. Openings placed in the normal airflow will flush out any gases. Since hydrogen is lighter than air and rises, ventilation holes need to be along the top part of the box. During charging, active ventilation is required. Use a non-arching fan, such as one that has been approved for marine bilge use. The most convenient and safest arrangement is to wire the fan so that it comes on automatically any time the charger is engaged.

Checking Dimensions

Once you've chosen your battery layout, rack and box options, it's time to start on a specific design. Add X6" to each dimension of each battery. The reason for this is that batteries swell with age. If they are fitted too snugly at the beginning, it will be impossible to remove them when they are worn out. I know of one pack in which the center battery's acid had to be siphoned so the battery could be cut up and removed in pieces.

Before you start fabricating racks and boxes, check your dimensions against reality. Add the thickness of the battery rack and hold-downs material, the thickness of the box material, the dimensions of the batteries themselves, and the 116" per battery allotment for swelling. The total of all these measurements should give you the maximum dimensions of your pack.

Now build a dummy pack to those dimensions out of cardboard or foamcore. Install it in the car and see if it fits. You may find surprise interferences such as hood reinforcements. If so, it's better to find out now and make the necessary adjustments.

Temporary Installation

Once your battery racks and boxes are designed and built, install them loosely, but don't install the batteries yet. The racks and boxes will be used for placing other components and wiring, but there will be times when it is easier to work on the car if they are taken out.

If done right, designing and fabricating the battery layout, racks, boxes, and hold-downs can be one of the most demanding parts of the conversion. It also offers a great deal of satisfaction when it is finished.


Author: Shari Prange, Electro Automotive, POB 1113, Felton, CA 95018 • 408-429-1989


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