Battery gurus Help - reserve capacity question

[JohnnieB]

Originally Posted by BigWes

Bingo Johnnie! Those links provided the info I was looking for. I found the one for the premium, just couldn't find the economy ones I have. Not sure why though.

Anyway, thanks for the help folks.

When searching the 'net, you get better results when you only type with two fingers and one thumb.

[bigstik40]

Interesting that the GC2 designation does not pertain to the A/H rating, but only to the case size!! Can you say "dirty pool" ?? Here's hoping that my Trojan T-105's only have one A/H rating and are not the "economy" version. - RAY

[cgtech]

Trojan gc2 economy version is called T-605, you dont want them...

[JohnnieB]

Originally Posted by bigstik40

Interesting that the GC2 designation does not pertain to the A/H rating, but only to the case size!! Can you say "dirty pool" ?? Here's hoping that my Trojan T-105's only have one A/H rating and are not the "economy" version. - RAY

BCI (Battery Council International) Group Number identifies:
A. Maximum case dimensions (L x W x H)
B. Voltage (GC2 is 6V and GC8 is 8V, but I'm not sure if there is one for 12V Golf Cart batteries)
C.) Post location.
D.) Terminal type(s).

The term is often misused, but I'm not sure that constitutes "Dirty Pool".

[bigstik40]

Johnnieb - Thanks for the information. OK, I see now. My T105's are also group number GC2. - RAY

[jjance]

If you multiply your AH rating by your pack voltage that will give you how many KwH your battery pack has. Then you multiply the Hp of your motor by 740. That will give you how many watts your motor uses.

In my case I have a 3.3hp motor and that works out to be 2500watts. With a 10KwH battery pack that will run the motor for 4 hrs. You don't want to go below 50% so that leaves 2hrs. Also there are other factors how fast,up and down hills stopping and starting how much you weigh. So this 2hrs is is on flat land pedal to the metal. And this is what they advertise 30 to 40 miles/charge.

[JohnnieB]

Originally Posted by jjance

If you multiply your AH rating by your pack voltage that will give you how many KwH your battery pack has. Then you multiply the Hp of your motor by 740. That will give you how many watts your motor uses.

In my case I have a 3.3hp motor and that works out to be 2500watts. With a 10KwH battery pack that will run the motor for 4 hrs. You don't want to go below 50% so that leaves 2hrs. Also there are other factors how fast,up and down hills stopping and starting how much you weigh. So this 2hrs is is on flat land pedal to the metal. And this is what they advertise 30 to 40 miles/charge.

You are on the right track, but it is a bit more complicated than that.

A couple minor corrections:
If you multiple AH by Volts you get Watt-hours. You then divide by 1,000 to get kWh.
Technically, 1 mechanical (or Imperial) Horsepower = 745.699872 Watts. Some engineers truncate and use 745 while others round up and use 745.7 or 746.

Also, due to the Peukert Effect (As discharge rate increases, available storage capacity decreases), determining what the real life storage capacity of your battery pack is challenging.

The yardstick we typically use to compare golf cart batteries is the 20-hour discharge rate. Which would be 11.25A for a 225AH battery, however when we are cruising in our carts at a steady speed on level ground, we are drawing about 5 times that rate, or more.

Some battery manufacturers publish the kWh stored by their specific batteries, but the numbers they publish aren't based on real life golf cart usage. IE: Trojan states their T-105 battery stores 1.50 kWh at a 100-hour discharge rate. I have two issues with that number. First, the 100-hour rate is only a 2.25A discharge rate while we discharge our batteries at 50A to 75A while cruising. Second, they use the battery's nominal voltage, which is only about 50% SoC. A more realistic kWh storage would be AH measured for a 60A discharge times 6.37V.

Using Trojan's published data, a T-105 has 185AH at the 5-hour discharge rate and has a voltage of 6.37 at 100% Soc, so 1.18kWH is about as close to real life as we can get without doing our own discharge testing.

--------
Another thing to take into consideration is cart efficiency. Since no two carts are exactly the same, the same motor in a different cart will use more Watts-hours or fewer Watts-hours to travel the same distance over the same terrain.

------------
When estimating my cart's range, I start at 100% SoC, measure the distance traveled and measure the % SoC at the end of the trip. That gives me average % SoC used per mile, and dividing that number into 50% gives me an estimate of how far my cart will go without dropping below 50% SoC.

Originally Posted by JohnnieB

------------
When estimating my cart's range, I start at 100% SoC, measure the distance traveled and measure the % SoC at the end of the trip. That gives me average % SoC used per mile, and dividing that number into 50% gives me an estimate of how far my cart will go without dropping below 50% SoC.

i think i said that

Originally Posted by crash test dummy

your batteries will run til they are depleted .. you should never run them til 50% or 36.3v
there is so many things to the "run time" .. driver ,wires ,weight ,road conditions,heat /cold ,tires ...ETC
the best test is for you to drive them around and see when they get to 36.3 v

when doing your real time test .. wait a few min after you stop to get an accurate reading

[jjance]

You are on the right track, but it is a bit more complicated than that

.

The average cart owner or the LSV owner would only need a rough estimate of their mileage. This is what the KwH rating of the batteries and the HP of the motor will give you.
There's no way to be able to calculate precisely how far a cart will travel.

[JohnnieB]

Originally Posted by jjance

.

The average cart owner or the LSV owner would only need a rough estimate of their mileage. This is what the KwH rating of the batteries and the HP of the motor will give you.
There's no way to be able to calculate precisely how far a cart will travel.

You are absolutely correct, there is no way to calculate precisely how far a cart will travel. There are simply too many variables and too many unknowns.
You are also correct that the average cart owner only needs (or wants) a rough estimate.

And, using motor HP and Battery kWh does produce a rough estimate, but only if you can find the motor HP at the voltage you are running the motor and the kWh storage of your battery pack.

My motor is rated at 11.8 HP at 48V, so it would be about 10.3 HP powered by my 42V - 245AH battery pack.

10.3 x 42 = 432.6

Exide does not publish kWh their batteries store, but Trojan say their T-125 (240AH) has 1.60 kWh, so my battery would roughly have 245/240 of that amount, which is 1.633 kWh or 11.431 kWh for my seven 6V battery pack

11,431 divided by 432.6 = 26.4 miles.

However, that calculation is based on the assumption my motor is generating 10.3 HP during the entire trip, when in reality it puts out anywhere from 2.3 HP up to 13.1 HP and averages 6.7 HP while Throttle is at 100%. (This is based on the output log of my controller on typical a 13.5 mile trip) Of course, the throttle isn't on the floor all the time either, but when I use the average HP at WOT, I get 281.4 W and 11,431 / 281.4 = 40.6 miles.

Using the battery's AH at the 5 hour rate and volatge at 100% SoC, the kWh stored in a battery pack can be estimated fairly accurately and consistently, but motor using HP in any calculation is a very iffy proposition.