Battery gurus Help - reserve capacity question

[jjance]

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.

I used 740 because I didn't want to look up the exact number again.

[jjance]

It doesn't matter how much of the motor HP you are using at a time. If you are driving 10 miles an hour instead of 20mph you are going to get the same total number of miles only it's going to take you longer to get there.

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

Just what does this mean? What does HP X Volts equal Mathematical Mumbo Jumbo

[JohnnieB]

Originally Posted by jjance

It doesn't matter how much of the motor HP you are using at a time. If you are driving 10 miles an hour instead of 20mph you are going to get the same total number of miles only it's going to take you longer to get there.


Just what does this mean? What does HP X Volts equal Mathematical Mumbo Jumbo

Not quite true. It is akin to an automobile's MPG in many ways, but for different reasons.
All carts will have a "Sweet Spot" speed at which they are most efficient and get the best range at.

----------
In and of itself, HP doesn't define much of anything since it is a calculated number rather than a measured quantity.
True electric motor Horsepower is Torque times RPM divided by 5250.
(5250 is a constant that is derived from the formula that converts circular displacement into linear displacement because motors spin and horses pull in a straight line.)
HP can also be calculated by the Watts used by the motor, but motors are not 100% efficient, so that number will be higher than HP based on measured torque and measured RPM. (The Watts not converted into motion are converted into heat)

In fact, an electric motor's efficiency is the HP output calculated by the Torque and RPM divided HP input calculated by Watts. Depending on how it is wound (High-Speed or High-Torque or someplace between) and the RPM it is spinning at, the efficiency of a cart motor will vary from down around 50% up to about 90% while the cart is being driven.

[yurtle]

5252.

[JohnnieB]

Originally Posted by yurtle

5252.

I've seen both 5252 and 5250 on-line, but it has been so long since I did the math myself that I don't remember which one it is.

5252 will make HP calculations about 0.04% lower, so I'll use it in the future.

[BigWes]

Ok, now that we have all that cleared up, ( yall went way over my head the last few posts) and I've wired up my digital volt meter, it led to another question.

When the cart is moving, the volt reading shows to be lower than when at rest. I guess that is normal from what I've read a few places here. The more I have loaded and the faster I go, the lower the reading. When I stop, it goes back up.

Example, yesterday, I took the cart out and put it to work. Loaded it up with a little over 1100 lbs of people and stuff, and took it back in the woods, up some hills, etc. I started off with the meter reading a full charge of 38.2 volts. Driving maybe 1/2 speed loaded up with all that weight, the volt meter fluctuate in the upper 35 low 36 range and going up hill loaded down it got as low as 34.7. The cart never seemed to struggle, just noticed the volts dropping. I'd go from one place to another do get work done. The full distance traveled was probably about 1.5 - 2 miles. When we finished and loaded the cart up to leave, the volt meter showed 38.0 volts. When I got home, I plugged it in to recharge overnight.

So here are the questions: Is that a SOC reading while under load or is that something else? Should I be concerned about this?

[JohnnieB]

There are three types of voltage readings for batteries. Each has a different purpose.

1. On-Charge: Voltage measured while the battery is being charged. It typically climbs into the 44-46 volt range for a 36V battery pack, but may go up to about 50V with some charger technologies.

2. Under-Load: Voltage measured while the battery is being discharged. The more amps being drawn, the lower the Under-Load voltage. The amount of voltage drop is determined by the battery's storage capacity (AH) and State of Charge (SoC) and well as the number of amps being drawn.

3. At-Rest: Voltage measured after the battery has set for a length of time (rested) without being charger or discharged. At-Rest voltage is the only one that can be used to estimate SoC.

A battery has to rest about 12 hours after it has been charged to get an accurate SoC estimate.
A battery has to rest about an hour after it has been discharged to get an accurate SoC estimate.

------------
The voltages and voltage fluctuations your are seeing are normal. In fact, only dropping down to 34.7V going uphill with the weight you were carrying was pretty good. Of course, you were still pretty close to 100% SoC and it would have dropped more if the battery pack had been discharged more.

You are running a 36V pack, so try to stay above 31.5V Under-Load.
The absolute minimum Under-Load voltage for a 36V pack is 27.0V.
Go below that and lead will start getting sucked off the plates.

-----------
When I'm out and about and want to know what my SoC is, I just wait until the voltage stops climbing so rapidly (5 to 10 minutes) to estimate the SoC. It will be lower that what it is, but that is on the safe side.

[BigWes]

Thanks JohnnieB. You are a wealth of knowledge.