Jeep Cherokee EV Conversion - Part 1

I’m on a green energy kick  to do my part to prevent us all from being horribly killed by global warming. I’ve been looking into EVs, but the options are pretty limited and pretty expensive, as well as being filled with fancy features I don’t want or need. Instead, I decided to convert the Jeep XJ I had sitting around.

EV conversions have been around for quite a while, but they are starting to become even more viable, as batteries are becoming cheaper and cheaper. It turned out, my co-worker was also considering converting an old VW beetle, so we decided to pair up and help each other.

Vehicle

1989 Jeep Cherokee (XJ)

Batteries

While you can buy batteries, they are quite expensive, so we decided to look at salvaged batteries. It is quite affordable to get a Nissan Leaf or other EV that has been wrecked. The batteries are usually undamaged as they are safely in the middle of the car. Also EVs tend to be written off pretty quickly for some reason.

George Andres, my co-worker, was able to round up a salvaged Nissan Leaf battery pack from a 2014 Leaf. It is 24Kwh, which we split so we can do initial testing and building, albeit with very short range. I would like to find a second pack to get up to about 36 kWh of capacity, but finding one reasonably close has proven somewhat elusive.

Motor

The first choice you need to make is AC vs DC in a motor. DC motors are cheaper and used to be more common, but AC has started to take over. I is also the primary type for commercial EVs. DC motors are cheaper but they don’t allow for regenerative braking and they freewheel, where are AC motors give more of an engine brake feel. I went with an AC-50 at 96 volts, with the hopes of possibly running two in the future, but for now we’ll see how one works.

If I do another build, I might go for a salvaged motor from a wrecked EV instead.

Design

On a traditional EV, there is usually a very limited drive train, often a single gear ration. Most EVs run roughly an 8:1 gear ratio, although it varies slightly as well as some EVs run a 2 gear system.

With conversions, it is common to keep the full clutch and transmission and use a couple gears to get the right power/speed ratios you need. These often involve a large adapter plate that connects to the bell housing and the electric motor mates to the flywheel.

The large adapter plate option involves a lot of extra weight, especially rotational weight, since it keeps the whole clutch and flywheel setup. I thought I could do without all that extra weight. A rule of thumb seems to be that for cars, rotational weight is about a 3:1 comparison to curb weight, so every kg of weight saved in rotational is like 3kgs saved. This of course is very rough and depends on a lot more variables, but I did some rough math to try and figure it out. Keep in mind I am a programmer and not an engineer, so these numbers might be all wrong.

A flywheel basically stores energy in it, so that your motor keeps going round and all that. Basically, as you're accelerating, you're putting energy into that flywheel that you don't get back until you reduce rpm.

The farther away from the center the weight is, the more energy it takes to move, or the more energy it stores once it is moving. The purpose of a flywheel in a regular ICE (Internal Combustion Engine) car is to smooth out the rough “up/down” jerky energy that pistons provide and keep the drive shaft spinning nicely between pushes. It is deliberately a large heavy disk. On the jeep it is 12kg, which is quite a bit

Here are some sample calculations, with how much we figured the fly wheel could be shaved down, as well as the resulting power loss.

26.5lbs x 13.75in = 33hp seconds @ 5000 RPM
19.5lbs x 12.25in = 19.5hp seconds @ 5000 RPM
9lbs x 12.25in = 9hp seconds @ 5000 RPM

These are horse-power seconds, not hp PER second, so you have to spread it out over the acceleration time.

0-5000rpm in 10sec:
3.3hp lost
1.95hp lost
0.9hp lost

0-5000rpm in 5sec:
6.6hp lost
3.9hp lost
1.8hp lost

That rotational weight actually sucks up a TON of energy. I can see how you save energy not having the crankshaft and other spinny bits in the system and electric vehicles feel snappier, not just from the instant torque. Also, I *think* that this would apply most dramatically at the lowest end, since hp=torque x rpm so you'd be zapping a lot of torque there, but less as you sped up, since it is spread out more and the torque curve is flat. It depends on a lot of things I think, and this is where I got confused. With that, I decided to go with a design similar to this

Transmission to Motor Connection

My father built a custom connection to mate the AC-50 electric motor to the jeeps transmission. It uses the remnents of the clutch as a mounting plate because it has the proper splines and was easier than finding a replacement setup. It also looks a bit longer than needed, but that is because we have to make up the space where the bell housing used to be and I didn't want to cut down the shaft on the transmission, in case I want to change designs later.

What's Next

I need to get the transmission and adapter put into the Jeep, right now I'm waiting on a replacement transmission mount, since the original one is cracked and broken. Then it is a motor mount and battery boxes.