Does acceleration decrease with charge ?

[Cd]

I know - stupid question right ?
Of course acceleration decreases with battery charge

......or does it ?

I think back to my days as a kid with an R/C car. As the batteries weakened, the car would slow to a crawl.

However, I notice that the electric forklifts that I drive at work seem to go strong, right to the last charge bar on the meter.
( I notice that I can lift a full load even right before the machine finally goes to red. I assume that this requires quite a bit of electricity, but I realize that it is mainly using the hydraulics. I'm confused about how much power the batteries use in a situation like that. )

So I'm puzzled : With a full size electric car, do your 0-60 times drastically increase as the charge is depleated , or are electric cars built in such a way that the power stays pretty level as a safety feature.

I would hate to think that a 12 second 0-60 MPH electric car would become a dangerous 30 + second 0-60 MPH car once the batteries are near dead.


I'd really love to find out more about electrics, and this was one question that I was wanting to ask.


Thanks !

[MetroMPG]

I can't speak for other chemistries, but in the flooded lead-acid ForkenSwift, it gets slower as charge is depleted.

To be more specific, the battery voltage sags more under load when the charge is low, and if you push the car hard and pull the batteries under 5.25v (per batt) under load, you can damage them.

When you say the forklift works well until the last bar, that tells me you're probably seeing an engineered display which is meant to protect the batteries (ie. the last charge bar goes out well before you're actually in danger of damaging the pack). That would explain why you don't see a massive performance drop at that last bar.

Modern, production EV's will likely have electronic protection that prevents you from draining the pack so far as to seriously hurt acceleration and/or damage the pack. (They'll also come with something other than lead acid chemistry, which may have better performance characteristics as the charge is depleted.)

[Ryland]

yes, as charge drops top speed and acceleration drops as well, come to a stop and let the battery rest for a minute and it will have regathered it's self a little and will give you a nice little boost.

[bennelson]

Also, keep in mind that, more or less, Volts = Speed and Amps = Power.

But, because voltage sags as you pull lots of amps, this can lead to a strange situation in which if you are going up a hill, pushing the pedal further down increases amperage out of the batteries, which drops your volts, which lowers your speed!

You could actually go faster by letting UP on the gas pedal a bit.

Since volts x amps = work, and volts drop off as your battery drains, you pull more amps to do the same work on the lower end of the battery pack. Now that you are pulling more amps, you drain the batteries that much faster. I think that's a big part of the reason why batteries seem to drop off quickly at the end of the pack - it's sort of a vicious cycle.

The thing is to have a large enough pack, and DON'T BE RUNNING IT ALL THE WAY DOWN ALL THE TIME! They don't like that.

[aerohead]

Alan Cocconi's AC Propulsion',EV- conversion CRX used inverted 3-phase AC current in his car,from 28, 12-volt spiral-wound batteries.The inverter was 98% efficient.His CRX maintained full"performance" right up to battery depletion threshold.On-board "look-up" real time diagnostics allowed state-of-charge and remaining range display capability.-------------At$ 70,000(US) for the converted CRX,it was pretty serious electronics.He's the guy that designed the initial controller for the GM Impact.

[Cd]

Bummer . That really puts a damper on my enthusiasm.

So if I get on the highway and drive flat out ( 65 MPH ) till I'm at half charge, my batteries will be so depleated that merging back onto the highway could be dangerous ?

[bennelson]

No thats not quite right.

A fully charged lead-acid battery will have something like 12.8V (divide all these numbers in half if you are using 6V batts!)

As you use up your charge, the voltage will drop, but half a charge is NOT going to be 6 volts, it's going to be just over 12.


This image if from THE 12V SIDE OF LIFE, which explains how power systems on RVs work.

So, with half a pack of power, your car may not be able to have the same top speed, but it's not going to be something weird like only HALF your top speed.

For example, when I converted my motorcycle from 36 to 48 volts, the new top speed was 45 mph. I think I could get it to 47 mph on the flat with a fresh charge. I ran the cycle hard for 20 miles - that's basically the whole pack. Then I could only get to maybe 35 mph on the flat. It really felt like the bike was tired and trying too hard.

Now 10 miles in to the ride (half the pack) I could still go 44 mph no problem.

If your EV is going really slow compared to normal, you either have pushed the batteries too far and need to charge right away, or you have some other problem.

PS: Keep in mind it's dangerous to get on the freeway just before running out of gas too!

[almightybmw]

Anyone know if current production EV's have voltage regulators to keep a stable/constant voltage to the motor? I also wonder if such a circuit could be built that when climbing a hill with say 50-80% charge, and you want to increase your speed up it (the term accelerate bothers me since you're already accelerating :P) could the circuit be built that senses the increased demand for amperage and determine what voltage/amperage combination would produce the desired outcome?

Something using an accelerometer, V/A meters, TPS, VSS, motor RPM, possibly altimeter. That's all the variables and related parts I can think of one might use, but not need. Not sure if an accelerometer can determine altitude. A rediculously smart one might.

[The Atomic Ass]

Originally Posted by Cd

However, I notice that the electric forklifts that I drive at work seem to go strong, right to the last charge bar on the meter.
( I notice that I can lift a full load even right before the machine finally goes to red. I assume that this requires quite a bit of electricity, but I realize that it is mainly using the hydraulics. I'm confused about how much power the batteries use in a situation like that. )

I've driven those types of forklifts before, and I have to say I noticed a performance dip if a battery was down to 2 green bars. (2 orange below green).

It may still lift a load, but compare it side-by-side with a fully charged machine. It's going to take longer to get that load up in the air, longer to get moving with that load, and so forth. Hydraulics just make the truck cheaper to build/more compact, they don't reduce the energy needed to lift a given load in a given time frame.

Conversely, I was the only one in my warehouse to discover that our lifts had regenerative braking. The trucks we used were very stable, (fixed mast), and starting a lift while decelerating from top speed forks trailing yielded an obvious increase in the hydraulic motor whine that would taper off when the truck came to a stop.

Man I drove crazy on them things.

[MetroMPG]

Originally Posted by aerohead

Alan Cocconi's AC Propulsion',EV- conversion CRX used inverted 3-phase AC current in his car... He's the guy that designed the initial controller for the GM Impact.

Here's another link to Cocconi that addresses this thread's subject:

I happened across the web page of the owner of an OEM GM S10e, which uses essentially the same drive train (including front wheel drive) that the production EV1 used.

Regarding acceleration performance, GM quoted it at partial charge to give drivers an indication of real world performance (rather than best case fresh-off-the-charger acceleration):

GM estimated 0-50 mph times of 13.5 seconds at 50% charge; "even less" when
the truck had a full charge.

Source: 1998_s10e_