Electric car conversion: Project ForkenSwift

[MetroMPG]

Someone recently asked me:

What kind of current draw and voltage do you tend to be at around 30mph or so? I ask because I'm wondering if I can trust the output of this calc for low speeds.

I can't say for sure, because I spend very little time actually cruising at any constant speed. I'm usually either accelerating (slowly) or coasting to a turn/stop.

On one trip with longer stretches I tried to limit battery amps to roughly 80-100 and seemed to top out around 45 km/h (28 mph) on the flat bits.

[roflwaffle]

Did (could) you see any voltage sag during the 80-100A portion?

[MetroMPG]

Sadly, I don't run a voltmeter - just the LED battery pack monitor. So I can tell you that the voltage does sag, but I can't say how much (except that it's well above 5.25v per batt). Sorry.

[roflwaffle]

That's right, you best be apologizing!

But naw, that's plenty of info to go off of. If only I won the lottery and could buy a larger motor, Paul's controller kit, and some LFP cells to dump into the ForkenSwift. I'm really interested in how the Wh/km or mile figure would improve.

[MetroMPG]

If you won the lottery, I'd gladly accept all the aforementioned items to upgrade the car. Just so you know in advance.

Seems like LRR tires would be my best first step to improving the car's energy use: Test: comparing Blackfly vs. ForkenSwift rolling resistance (Goodyear Invicta GL)

[bennelson]

My car, the Electro-Metro, is pretty similar to the Forkenswift, except that it is currently running at 72V.

It tends to draw around 100 amps while crusing at about 35mph.

Voltage always drops a bit while the motor is under load. The main thing is that voltage drops VERY little when the batteries are fresh off a charge, and A LOT when the batteries are getting low.

When the battery voltage sags a LOT on acceleration, it's time to get to an electric outlet.

[roflwaffle]

If you two are at ~200Wh/mile at a steady ~30-35mph, not even counting the losses due to Peukert's law, then controller/motor efficiency must be pretty bad. Someplace around 40-50% or so. Even then, going w/ a new 9" motor (Assuming it's low current/~2000+rpm efficiency is pretty good at ~85% or so), and the ReVolt would take a couple hundred thousand miles to pay itself off so there isn't much justification for it given a low speed EV. Something highway capable at ~20k miles/year would certainly justify the greater cost though.

[vpoppv]

I apologize if you have already covered this, but how did you deal with the brakes on Forkenswift? I didn't see a vaccuum pump listed, so I assume you went with "puncture the diaphragm". I know that Dr. Tillman used this method and seems to work fine, but I am having trouble figuring out exactly how to do it: where do I drill the hole? How far? Are the brakes fine after this, or is further modification required? Thanks in advance....

[MetroMPG]

vpoppv: sorry for the delayed response - I didn't see your question (was away sailing when you posted it and it got lost in the volume of messages).

The car has "unassisted" brakes like Dr Larry's car, but we didn't puncture the booster diaphragm. Instead we upgraded the smaller rotors & calipers from the 3-cylinder car to the larger caliper & rotors from the 4-cylinder car. The brakes are still firm, and a fair amount of muscle is required to stop quickly. (How hard? I wouldn't feel comfortable letting my mom drive it.) But it's noticeably improved over the "unassisted" 3-cylinder setup.

I'm reluctant to puncture the diaphragm because once I do, if I decide later to add a vacuum pump to reduce braking effort, it can't be undone.

I know Dr Larry swears by his punctured diaphragm method, but I'd like to hear a 2nd or 3rd opinion of someone else who's done it before I'd take that step.

[MetroMPG]

In response to Ben Nelson's "2 bikes on my electro-Metro photo"...



I'm upping the ante with "two colour-coordinated kayaks" on the roof of the ForkenSwift:



Took my nephew kayaking yesterday around some of the park islands on the St Lawrence river just west of Brockville. Round trip by EV was about 10 km. (No comments about the destroyed aero efficiency - and yes, it was noticeable!)