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JeepEV: Electric Vehicle Conversion Project
Batteries


     An Electric Vehicle's batteries are the sorce of power used to drive its motor. These batteries as a whole are often referred to as the "Traction battery pack" which usually consists of anywhere between 12 and 24 batteries (20 in my case). The batteries are most commonly 6, 8, or 12 volts each for DC applications such as this. It is important to note that there is still a single 12-volt battery which remains independant from the traction pack batteries. This battery is used to provide power for all the low-voltage devices in the vehicle (such as the radio, lighting, fans, etc.)


Traction Battery Pack Selection:

     The two types of batteries most commonly used for home-built EV traction battery packs are flooded Lead Acid or sealed (AGM) Lead Acid batteries. It is arguable which type would be the best to use, because there are pros and cons for each. For example, AGM Lead Acids (such as the Optima Yellow Top and Hawker Odyssey batteries) tend to give significantly better performance and operate much cleaner then flooded Lead Acids. However, flooded Lead Acids tend to have a higher energy density, lower cost, and longer cycle life then AGMs. Thus AGM Lead Acids are most commonly used in applications where high power and performance are most important, and flooded Lead Acids are most commonly used in applications where low cost and squeezing out as much range as possible are most important.

     However, Lead Acid batteries are not the only type of batteries found in Electric Vehicles. Nickel Cadmium (NiCd), Nickel Metal Hydride (NiMH), Lithium Ion (Li-ion), and Lithium Polymer (Li-Poly) are also utilized in EVs. Perhaps the most important attributes of any battery considered for EV use are the amount of energy the battery can store per unit of weight (gravimetric energy density) or amount of energy stored per unit of volume (volumetric energy density). Though many EVers are more concerned with gravimetric energy density because in any given vehicle free space is usually more abundant then weight carrying capacity. In order from worst to best, the gravimetric energy density of popular battery chemistries used in EVs is: Lead Acid (30-35 Wh/kg), NiCd (40-60 Wh/kg), NiMH (50-80 Wh/kg), Li-Ion (160 Wh/kg), and Li-Poly (130-200 Wh/kg).

So why aren't more people using these advanced chemistries? Well, although NiCd batteries store a bit more energy and do better in cold climates then Lead Acids, they are expensive and harder to find in large formats, and are increasingly being banned from use in traction applications due to the purported toxicity of the chemistry. Nickel Metal Hydride and Lithium Ion batteries can provide much greater range per charge then Lead Acid batteries, and offer better performance to boot. Their biggest downfall is lack of wide availability in large formats, safety concerns when operated improperly (and in hot climates), and their very expensive price tag. Thus, until these issues are overcome they aren't, and likely won't be, very commonly used in non-production (home built) EVs.

     So, for the Jeep conversion, I've decided to start off using flooded Lead Acids for my first pack simply because they are inexpensive and more tolerant of abuse then other battery chemistries. The main trade-off compared to AGMs (my second most likely choice) is that they won't provide the Jeep with race car performance or top speed. Also, they'll need to be cleaned and refilled frequently to ensure they have a long life. A fairly common brand of flooded lead-acid battery used in EVs is Trojan Batteries. This is the brand I've chosen to use as well. I'll be using the T-875 battery which has four cells for an 8 volt battery. However, this is no small battery, with dimensions of 10-3/8" x 7-1/8" x 10-7/8" and a weight of 63lbs! The goal is to have twenty of these for a total system voltage of 160 volts DC.

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All twenty (20) Trojan T-875 8-volt Lead Acid batteries were
delivered to my garage. That is 1,260 lbs. of batteries!



Traction Battery Pack Installation (Front):

     There are eight traction pack batteries in the front of the Jeep. Four sit in the lower battery rack (tray #2), and four sit above the motor in the upper battery rack (tray #1). All four batteries in tray #1 sit so that they face forwards (electrical terminals are by the side facing the firewall). In tray #2, the batteries are positioned in the same manner with the exception of the battery at the rear passenger side of the tray which sits so it faces towards the firewall (with it's electrical terminals on the side closer to the front of the Jeep). This was done simply for greater wiring convenience.

     The front four batteries are held to the tray with a rectangular top bolted to the bottom of the rack using threaded rod. The top clamps the batteries down around the edges of the case. The plan was to also use a similar top to complete the upper battery rack, but the throttle switch ended up being in the way. So I used two ratchet straps instead to hold the upper four batteries in place. They work surprisingly well, but they will be replaced by a solid top of some sort in the near future.

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Eight of the batteries are installed in the
front battery racks and clamped down.



Traction Battery Pack Installation (Mid):

     The Mid (under the rear seat) battery boxes are filled by two of the traction pack batteries, one in each box. Both batteries face forwards (their electrical terminals are closest to the side facing the rear of the seat). When the rear seat is down in its normal position, it is impossible to tell that there are batteries hiding under it, which is just great!

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Two of the batteries are installed in
the two boxes under the rear seat.



Traction Battery Pack Installation (Rear):

     The rear battery box (which is under the trunk floor) holds the largest quantity of the traction pack batteries. I had been hoping all along to fit 10 batteries in the rear box. However, the box ended up being the weird shape that it is and I had to determine the best way to fit them in. This led to me playing "battery tetris" several times to determine the best way to fit as many batteries as possible in the box. After trying lots of different arrangements, I realized that I'd only be able to fit nine batteries in the rear box. Oh well, that happens.

     Once I determined the correct fit, I had to make sure that I still had room for the negative contactor and liquid tight conduit fittings to mount to the inside of the box. Luckily I did have the needed room, so I went ahead and put all the batteries in and began wiring them up.

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Nine of the batteries are installed in the
Rear battery Box after I played "Battery Tetris".



Traction Battery Pack Installation (Rear - Box #4):

     The last battery to be installed was put into the small battery box in the trunk (box #4). This last battery box is bolted to the trunk floor, and has a lid to finish off the box. Once that battery was in place, I reinstalled the padding and carpet in the Jeep's trunk. I cut a hole out of the carpeting which is the shape of the large rear battery box to allow access to it. More discussion about the lid for the rear battery box (which is also the trunk floor cover) can be found at the Battery Boxes page. Also, information about the traction pack wiring is located at the 160 volt wiring page.

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The last battery is installed in Box #4 and
the carpeting is reinstalled in the trunk.



12-volt flooded Battery selection/installation (OLD):

     The Jeep originally used a standard-sized 12 volt car battery for starting. There would have been nothing wrong with my reusing that regular car battery to power the 12-volt system, except that it was too large. The front-most battery rack (front battery rack #2) spans too far to the passenger side of the engine compartment and would interfer with the 12-volt battery had I attempted to re-install it in it's original location. So I instead bought a smaller 12-volt battery, one which would normally be used for a lawn-mower or such. The reason why using a smaller 12-volt battery is acceptable in this application is because the DC/DC Converter is always "on" and it powers up to 30-amps of the 12-volt system (and keeps the battery charged when the Jeep is not in use). It should be noted that in some EV conversions, a 12-volt battery is not used at all and the DC/DC converter is the only component powering the 12-volt system. I had considered doing this, but it seemed to risky to me. Since I have some power hungry things like the power steering pump in my 12-volt system which others don't have, I want to be sure I have a battery to aid in heavy load situations. Plus, If the DC/DC converter were to fail, I'd still be able to drive the Jeep home (since the main contactors MUST have 12-volts to trigger -- without the low voltage system the high voltage system would never get switched on.)

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The smaller 12-volt battery used to power the low-voltage system.

     I made a mount for the new 12-volt battery out of "L" channel steel, which I cut and welded into a rectangle to form a tray. I then welded two additional sections of "L" channel to the bottom of the new tray. In these two pieces of "L" channel I drilled holes to match the existing bolt studs which the original 12-volt battery mounted to. I finished the mount by grinding and painting it. For now I'm using a strap to hold the battery in the tray, since it is very lightweight.

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The tray is made and the 12-volt battery is installed at the front
passenger side of the engine compartment.



« January 3rd, 2005 »

A New 12-volt Battery

      The Jeep's 12-volt system has been a bit bothersome because of the voltage sag which occurs due to the high loads on the system (i.e. the Power Steering pump). The result of the voltage sag is most apparent in the morning or at night when I have the headlights on, and they dim anytime I make a turn (when the P/S pump is pulling more power). I've already added a cooling fan to the DC/DC converter which helped a little. However, I've come to find that the voltage sag is also due my choosing to use a flooded 12 volt battery. The U1 Everstart I've been using has a lot of voltage sag even with small loads attached.

      I had been told that the small sealed lead acid AGM batteries made by several manufacturers were much better under load (they experience less voltage sag). After shopping around, I ended up getting a Hawker Odyssey PC625 battery for the Jeep. This is a small, 16 amp-hour battery. I never could find ANY amp-hour specifications for the U1 Everstart I had, so I can't directly compare the two based on capacity (though the U1 was likely 18-20 Ah). However, even if the Odyssey PC625 has a less capacity than the U1, its voltage will remain at a reasonable level for a longer duration, and not immediately experience a lot of voltage sag like the U1 did.

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The new Hawker Odyssey PC625 battery compared
to the old U1 Everstart used in the Jeep.


Check out the Battery Boxes Page for information and pictures
about the battery boxes/racks.


Remember, More photos are in the Photo Gallery!