Batteries and Battery Racks

Posted by admin November 30th, 2008

One of the hardest tasks when designing an electric car is to decide on the battery pack. The batteries you choose are going to be the heart and blood of your car. Which ones you choose and how many of them you use will determine how fast the car will be and how far it will go.

There are a number of technologies available. Each has it’s advantages and disadvantages.

Lets start by saying what is required is what is called a “deep cycle” battery. These are batteries which can be discharged to within 25% of their power without damaging them. A typical car starter battery is not designed to do this. Deep cycle batteries are used for marine applications like trolling motors or golf cart batteries.

There are 3 types of batteries I considered for my project.

1. Wet Lead Acid – This is your standard car battery or golf cart battery. They are made of plates of lead immersed in an acid. They are cheap, easy to obtain, are reliable, and not easily damaged with charging. The down side is they are heavy, require adding water to the cells every now and then, and they must be kept upright so they don’t spill acid.

2. AGM – Absorbed Glass Mat batteries also use lead and acid, but the the acid is absorbed into a glass fabric so the acid doesn’t spill. These batteries are also typically sealed and do not require maintenance. Because the acid in embedded in the glass mat, they can not spill acid and can be used on thier side or even upside down. The disadvantage is that you must be very carefull not to over charge them. Doing so will damage the battery.

3. Lithium-ion polymer batteries – These batteries are an evolution of the standard laptop battery. They can be quickly charged, are light weight, and hold a lot of energy. The major downside to these is that they are still very expensive. Pricing out a battery pack to run the car was going to be between $20-30k. This is the technology I eventually want to run my car on, but it has to wait till the price comes down.

Weighing in price, weight, and size, I have narrowed my choice to either Exide Orbital 34DC 12 volt deep cycle or the Optima D34/78. These batteries have a 10 inch x 7 inch foot print and both about 55ah.

Based on these batteries I have begun designing the battery racks. The frame was modeled up in 3D Studio Max and used as a model for calculating out the battery positions and designing the rack. The rack is designed to not only old the batteries but also keep them from sliding forward in a hard stop or accident.

Frame loaded with 24 batteries

Frame loaded with 24 batteries

Battery Holders are in red.  Note the addition of a roll bar as part of the system

Battery Holders are in red. Note the addition of a roll bar as part of the system

Battery holder Side View

Battery holder Side View

Top View

Top View

Version 2 - Holds up to 24 batteries

Version 2 - Holds up to 24 batteries

Version 2 Racks

Version 2 Racks

Side View

Side View

Notes:

The electric motor is massed out in purple in these renderings. The transmission is in dark grey. The existing frame is in blue.

The frame is fabricated with 3 types of members, the roll bar is 2″ x .095″ tubular steel, the racks are fabricated from 2″ x .25 angle and 1″ x .125″ angle. Bolt plates are made from .25″ steel plate. The frames are all MIG welded.

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