Question for the Battery Gurus

[kobx]

I've been trying to establish a range for my cart, figured installing a digital voltage meter would provide me with real time readings for diagnosis. Unfortunately, its opened up more questions than answers. So, here they are:

1. My pack reads 38.7V after its charged, and sits for over 12hrs. Doesn't this seem a little high? (I'm under the assumption it should be 38.3V, fully charged)

2. After about 6 miles of carting around the neighborhood. Stopping, accelerating, cruising at bursts of full speed, with the whole family, it read 37.3V. This was after I let it sit for about an hour after cruising. Doing the math, this means I get 4.3 miles for every volt. And assuming I run it down to the 50% SOC (36.3V), this means I only have a range of approx 10 miles?? Does that seem right?

For the record, they are Trojan batteries (T-105) , with a J13 code stamped on them (Batteries are over 2yrs old). My cart is lifted, with rear seat, upgraded cables, F/R switch, and 500A controller with the "Raptor" motor. And these readings were taken with about 575lbs of cargo (me, the wife, two kids, and the dog).

[Volt_Ampere]

How accurate is your meter???? It may be reading slightly high. You need an accurate meter to predict SOC from voltage readings. SOC is not linear so you need the chart to convert voltage to SOC. It also changes with temperature. Ten mile range is very possible given your batteries are not new. Especially if it is running a 500A controller with upgraded motor.

[kobx]

The meter is accurate, in that it reads the same as my actual multi-meter that I checked it with.

Thank you for the insight on it not being linear though, I was wondering about that. Am I correct in saying I need to go by SOC percentage, instead of actual volt readings to try and establish a range?

Again, thank you for the input/response.

[Volt_Ampere]

Yes. And you need to temperature compensate your voltage readings. Use the SOC tables provided for your batteries. I would fully charge and let it rest to get a good full charge voltage. Check against the SOC chart. Then drive it down to about 50% based on the voltage. That is your usable range that will give you decent battery life.

[JohnnieB]

Using your numbers converted to SoC, I calculate the range at 11.9 miles and if the battery electrolyte temperature was 60°F, the range corrected to 80°F is 13.7 miles.

My range estimate is longer because I used 100% as the start SoC, the same way I do when the battery pack is coming off float charging.

I have knocked any surface charge off my battery pack be driving about 1/2 mile, waiting 1/2 an hour and using that voltage.

However, the resulting range estimate of both ways is pretty much the same.

For temperature, I use the average electrolyte temperature of 3 or 4 cells in the middle of the pack before and after the run.

I also use a DVM that resolves the voltage to 1/100V.

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The voltage at 100% SoC is a bit ambiguous since it is the average of numerous mature (Broken in) batteries at roughly 80°F. New batteries tend to have a higher voltage, and so do batteries close to the end of their lifespan, especially if the charger used shuts off at a specified voltage rather than using dV/dT technology.

From what I've been able to determine, the Voltage vs SoC below about 90% SoC is fairly consistent whether the batteries are new or old.

Also, the voltage vs SoC below 90% is fairly consistent between battery brands even though the voltage at 100% varies between brands (IE: A 36V pack of Trojan batteries ought to be about 38.2V while a 36V pack of US Battery batteries ought to be about 38.5V)

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The AH mileage of a cart and the fuel mileage of a car depend on many of the same variables. Heavy loads, high speeds, rapid accelerations, hills and stop/go driving all lower the mileage, along with other factors like air pressure in tires and final drive ratio.

[scottyb]

Note that cold weather will reduce the battery activity and capacity adversely effecting your results

[kobx]

You guys rock. Thank you.....