Battery Box Design

October 2004

Requirements

Battery boxes need to be:

• Light
• Strong
• Chemically resistant
• Corrosion resistant
• Electrically resistant
• Be able to contain the batteries and their chemicals in a crash
• Withstand flexing
• Easy to assemble and integrate into donor car
• Relatively low cost

Materials

Battery boxes can be constructed from a wide variety of material

Material	Strength (MPa)	Density (g/cm3)	Yield	Corrosion Resistance	Electrical Resistance	Assembly	Cost
Fiberglass	145	1.529	Poor - can crack		Good	Can be done at home	$
Aluminum	400	2.7	Good	Can corrode if next to dissimilar metals	Poor	Requires aluminum welding	$
Steel	250	7.85	Good	Rusts and can corrode	Poor	Easy to weld	$
Polyethylene			Good	Good	Good	Specialty welding	$
Rigid PVC	34.4	1.38	Good	Good	Good	Sheets can be glued	$
Titanium							$$$

Material Notes

Fiberglass and other Composites

Use a woven fiber for best strength

Carbon fiber is strong, but fiberglass can be made almost as strong, and is much cheaper.

Kevlar is quite strong, but is expensive, requires curing in an oven and is difficult to work with once cured.

For large sheets to build in stiffness, use a foam core. This has the added advantage of insulating the batteries

Steel/Aluminum can be used as a stiffner and attachment plates for fiberglass or Kevlar. Only titanium can be used for carbon.

Fiberglass density is 2.6 g/cm³; resin density is 1.3 g/cm³; composite density is 1.529 g/cm³

Glass content, by weight is typically 30% www.experts.com

5/32" strength is 23,000 psi

1 MPA = 145.033 PSI

Aluminum

Use 2024 or 7075 for parts which will bear weight

Higher impact resistance than fiberglass

Aluminum is as stiff as steel and much stiffer than fiberglass

High strength to weight ratio over fiberglass

Construction Notes

Important notes to consider:

• If the box is a structural part of the car, then it is best to use the same material and thickness of the car (i.e. steel)
• If the batteries are lead acid, then it must be vented to the outside, otherwise these gasses will cause corrosion to steel and other metals.
• Electrically insulate the battery boxes, as leakage current can cause corrosion
• If operating in colder climates, insulate the battery boxes

Battery Box Sizes

The Batteries

The first batteries for the New Beetle will most likely be less expensive Optimas or Hawker batteries. Here are their dimensions:

Model	Amp*Hours	Length (mm)	Width	Height	Weight (kg)
Hawker G42EP	42	198	166	171	14.9
Optima D51	41	237	129	227	11.8
Optima D35	48	254	175	200	16.6
Excide Orbital 34XCD	50	250	175	200	18.6
Deka Intimidator (AGM)	49	260	173	225	22.3
Deka Dominator 8G24M Gel	74	260	173	225	24.3
Deka Dominator 8G34 Gel (new Oct 2004)	60	260	170	178	19.5
Max dimensions for above	n.a.	254	175	227	16.6

Battery Box Placement

There are three possible areas for the batteries:

• Behind the rear axle, in the hatchback
• In front of the rear axle, where the back seat use to be
• In front of the engine

NOTE: when calculating space for the batteries, there must be allowances for fans, heating apparatus and plastic box liners

Box Placement	Length (mm)	Width	Depth	# cells
Behind Axle	690 (730)	800 (840)	280	12
In front of Axle	460	930	280	8
In front of Engine	220	830	550	6

Engine Battery Box Layout:

Note: The batteries shown here are the new Deka Dominator 8G34 Gel Cells (appropriate only for a high voltage system). These batteries are placed on end to be able to fit into the New Beetle. Two boxes will be used, the front engine battery box will contain 8 batteries, the rear engine battery box will contain 4 batteries.

Rear Seat Battery Box Layout:

Hatchback Battery Box Layout:

Material Selection for the New Beetle Project

The rear battery boxes require cutting into the body. The removed steel supplies some rigidity to the body which must be restored. Therefore the rear battery boxes will be constructed of steel, possibly the same gauge as the body steel. In order to isolate and provide corrosion resistance, the boxes will be painted with acid resistant paint.

The front battery box will be made of the same material: gauge steel painted with acid proof coating. Although plastics or aluminum could easily be used, steel and PVC is easier and cheaper to construct and modify.

Similar to Al Godfrey's Porsche 928 the battery boxes will be insulated with 1/2" thick PE Ethafoam.

Temperature controlled AC battery heaters will line the battery boxes, keeping the batteries at optimum temperature during charing and before driving.

References

EV Structures - has information about various metals which can be used for EVs.

Plastic Materials