Range calculation

[Helirich]

I would like to know what my range is. I know that lots of things come in to play here, but I would still like to get a general idea. Of course I could drive it till it quits and then tow it home, LOL. I'm hoping I can do it a little more scientifically.

So if I charge it up till its reading 50.9 volts and then I drive 2 miles. Now the volt meter reads 50.6 volts. Does that mean if I drive 4 more miles, it will be at 50.0 volts? If so, I could just figure miles till I get down to 50% soc.

If this plan won't work, is there a better way.

[cgtech]

What are you currently using for SOC estimates? You could drive it around until the pack voltage is at 60% SOC while at rest (give a few minutes for voltages to stabilize), then calculate double that mileage as your "max useful range", because the most you want to discharge to is 20% SOC without doing destructive damage to the weakest battery in the set. (even a really, really good set has a "weakest battery"). Everyone says "never discharge below 50% for max battery life", while i agree, there are situations where more may be needed. Just try to do "as best as you can" to minimize deep discharges, but it still happens. When you get below 20%, performance will suffer anyway. If you are creeping back in the driveway at 5mph, you are ensuring a short life for the batteries.

[Volt_Ampere]

You need to use the SOC charts. Voltage is not linear with SOC. Charge it and let it set for a few hours and read the voltage. Get the SOC from the chart. It should be near 100% if your batteries are good and your charger is good. Now drive it at least 5 miles or so and read the voltage - look up the SOC. Now you can estimate your range under THOSE conditions. Remember - to prolong the life of your batteries you don't want to drive it to much below 50% SOC.

[JohnnieB]

I use SoC to estimate max range and I let the battery pack rest for at least half an hour before taking the "After" voltage for the end SoC.

Based on the numbers you've provided, your cart uses about 3.5% of the battery pack's SoC per mile, so if use use 50% Soc as the max discharge, you range estimate is 14.3 miles. (If you went all the way down to 20% SoC, the range would be 22.9 miles.)

Those are some mighty low numbers for a modified PDS cart, so I suspect you may not have let the battery pack recover fully at the end on the run, so you got an artificially low SoC.

Attached is a study I did on battery voltage recovery over time.
The batteries were 3.5 year old T-105's that would only take a 85% charge when I got the cart 18 months earlier, but would charge to 100% after I played with them some.

The "0" time is actually the few seconds after stopping the cart that it took me to lift the seat and attache a handheld voltmeter that would resolve to 0.01V. (Dash meter only resolved to 0.1V and I was looking for highest resolution I had available for the task.)

As you see, there is a 13.7% difference in SoC between the time the cart stopped and a one hour rest, which is huge when estimating range.

The cart was a stock Plain-Jane PDS except to 2Ga cables and 17" tire height. Running in Mild Hill Mode, so there was some energy returned to the battery pack during regen braking. The cart was driven 10.2 miles (GPS) over a mixture of fields, gravel roads, paved roads and city streets with very little level ground and some inclines >15°. Basically, the test route was typical of the moderately long trips I take with my cart.

(FWIW: Max recommended incline for an EZGO cart is 14°)

Using 50% SoC as the max discharge, I would have gotten an estimated range of 16.1 miles if I used the voltage immediately after I stopped the cart and 28.3 miles if I used the voltage after an hour's rest.

Measure the distance traveled with a GPS and wait at least 30 minutes after the cart stops to get the voltage and you'll probably see something closer to 20 miles, maybe over 20.

The longer the trip, the more accurate the estimate.

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added

Also, temperature effects range. The battery's AH are fewer below 80°F and greater above 80°F, so you cart will go further during the summer months.

[Helirich]

I understand about low soc and battery life. I didn't know at what point the cart quits. Worse case senerio. Now that I think about it, I believe my controller can be set to a particular voltage. (I'll have to check)

I was mainly wondering if the soc goes down 1 volt in ten miles, will it go down another volt in ten more. It appears you guys are saying it will. I wasn't sure if the second volt would only get six miles.

Has anyone done testing to deturmine what the best speed for mpg is? (Should that be mpv? Lol) I sappose there would be some wind resistance at twenty plus mph. But between five and fifteen mph would there be any benifit to a particular speed?

[Volt_Ampere]

NO it won't! It is not Linear. You need to look up the SOC on the charts! It is hard to estimate what speed is more efficient. Slower is normally better since drag losses are lower. The motor may be slightly more efficient at higher speeds due to the way the controller regulates the motor voltage.

[JohnnieB]

For Trojan batteries, the voltage drop per 1% SoC varies from 0.0019V to 0.0026V, so as VA stated, it is not linear.
Of course, it depends on how accurate you want your range estimate to be.

Using Trojan Battery data, if you went 10 miles and the pack voltage dropped 1.0V, the SoC would be 79% or 2.1% per mile. The "MPG" for the next 10 miles would probably be very close to 2.1% per mile also, so the voltage at the end of it ought to be about 48.85V, which is slightly more than a 1.0V drop. In other words, the distance traveled for the second 1.0V voltage drop would be slightly less than 10 miles (9.9 miles??).

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As for best cruising speed for max range.

Series wound motors tend to be most efficient towards the upper end of their RPM spectrum while SepEx motors tend to be most efficient toward the middle of their RPM spectrum, but the efficiency of a SepEx motor is also dependent on the field mapping.

I don't have a speed/torque curve for your motor, but based on a similar one (high-torque/mid-speed) my guesstimate is that you'd get the best mileage in the 14 to 15 MPH range (about 2500 to 2600 RPM).

[Helirich]

A couple of things I might mention. I typed my second post without seeing JohnnieB's post. He types faster than me. LoL. Also, all the numbers I have given so far are "made up". I haven't done any testing yet.

My plan is to start with the battery at less then 100 % soc.

1. Check voltage with volt meter. (Not the dash one)

2. Drive cart about four miles at about 15 mph.

3. Measure the distance with my car odometer.

4. After a bit, check voltage again.

5. Use some math to get my 50 % range.

I'm thinking that if I start with less than 100%, I will get some extra miles per volt when fully charged and less miles when lower then my test soc. That way it should average out pretty close. Ideally, I should start with about 80%.

Sound good?

[JohnnieB]

What SoC you start at doesn't really matter much, just as long as you've got enough juice to get back home.

Your cart will average very close to the same Wh/mi if driven in the same manner over the same terrain if the battery pack is 100% or 80% or 70% or 60% or 50% SoC. The difference, if any, might be a small fraction of a mile at most.

Here is the spreadsheet I use. (Both Quattro Pro and Excel formats are in the zipped file)

Just enter the data in the yellow highlighted cells and it does the math.

[Helirich]

I did the test. A couple issues came up.

First, my handheld multi-meter only goes to tenths of volt. Funny, the meter I got from Scotty reads exactly 3 tenths of a volt higher. I have a second meter from Scotty. I guess I need to check it. Of course, I don't know which is right. My multi is not a high doller unit, but not junk either.

The second thing, I didn't realize my GPS had a odometer. The only problem is it only shows miles. (Not tenths)

The third thing, I just realized my Volkswagen's odometer doesn't work. The car is only forty two years old. I wonder if it's under warentee.

I checked the run with my other car and it was 3.8 miles, but the GPS said 4.

In any case, it would appear I get about a mile per tenth of a volt. That works out to about twenty four miles from 100 to 50 % soc.

I did this test at 15 mph. (As best I could) Next test will be at full throttle. How much do you figure that will change.