What are your performance goals? I saw on your introduction post you're prioritizing range over speed.
What is your experience level? Are you comfortable with, say, assembling your own motor control board? Or just buying a motor and controller combo and wiring it in?
If you want lots of power you can get a Tesla drive unit - runs anywhere from $3000 to $8000, 220Kw to 350Kw. For a more tame experience a Leaf motor/inverter/gearbox would cost $2000 or so (in US) and give something like 80-90Kw.
DC motors are harder to source but controllers are usually easier (not that AC controllers are too difficult).
Generally, you can budget about 350 watt-hours per mile of range at ~60mph, so 35Kwh should give around 100 miles maximum.
Isaac, hi, probably fair to say I'm more comfortable with plug and play approach. Will the Tesla motor assembly be suited to the strictly RWD Merc? Given the curb weight of the Merc I'm not sure the leaf would achieve the requirements. As I said I'm a complete nube with regard to the eV world, although not afraid of fundamental car mechanics.
Right, leaf could be a little wimpy. Tesla motors are often connected directly to the rear axle, so you remove transmission and differential and just make custom half axles. A large unit won't fit in every car, a small unit is easier.
For plug and play, HSRMotors seems to have the best solution -- prices are pretty high but they have performance, reliability, and ease of use. Maybe you could look at some 3d scans and see how a Tesla unit would fit: large drive unit scanfront drive unit scan
If you want to keep the transmission (might only be possible if it's a manual) then a DC motor could be a good choice. Something like a Warp11 with a Soliton controller (cost over $5000 probably - new DC motors are incredibly expensive). DC motors give excellent torque at lower speeds, so they are well suited for a build with a transmission. However, they should only be revved to 5000 or so (depending on the motor) because brushes will wear down.
Do you have a budget? How about range requirements/preferences? It's easy to get 60 miles, 90 isn't too hard, when you want 100+ it gets a little harder to fit them all in (depending on the car).
The first big question is: transmission or no transmission?
If you ditch the transmission and differential, you'll lose a lot of weight and complexity. The tricky bit is how integral the rear drivetrain is to the suspension...If it's all isolated, all you need to do is position a Tesla/Leaf motor so the output is even with the wheel hub centers, bolt it up, and modify or purchase axles to fit. Gearing will likely take you over 100mph, depending on the diameter of your tires. I built this little visualizer for that: Gear Wars
If it's not too bad to pull the rear diff and fit a Leaf/Tesla motor that will drive those rear wheels...that's the route I'd go, even if you have to cut up the trunk some. Redesigning suspensions is beyond my abilities.
If you don't ditch the transmission and differential, the Tesla motor becomes a lot more difficult. The Leaf motor unbolts easily from its gearbox, and can be mated to the OEM transmission with an adapter plate and something for the shaft (this is not trivial). The up side here is that, with a transmission in place, it will be fast off the line with even a Leaf motor. Automatic transmissions require some extra hoops to jump through...They're not well suited to EV conversions, but finding an OEM manual transmission for this car will be tricky (if it even mates up to the rear driveshaft without modification.
For 50mi of range, you're looking at maybe 15-20kWh worth of battery. For 100mi, it's just double that. Range is greatly impacted by speed, weight, cold days, and heat/AC. Leaf batteries are about as cheap as they get, and they're not bad at all. Tesla batteries have greater power density and longevity, but cost much more and require liquid cooling.
There are more motors and batteries that are viable, but anything from the last fives years will cost way more than earlier stuff. There still aren't that many factory EVs to pull parts from.
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