Acceptable voltage drop T-105's

[Dixie Chopper]

Since I had the blown fuse over the weekend I've been worrying myself to death (probably for nothing) about the way the motor sounds now and what kind of voltage drop I'm getting. The performance of the cart seems normal. Batteries are brand new AUG 2017 T-105's. I just wanted to get confirmation from the experts about how much my pack is dropping. Fully charged pack after resting for 12 hours shows 38.89 so that looks great but on a hard take-off on flat ground it drops to 30.7 for a split second then jumps back up to around 34.5 and stays there basically until top speed is reached (24 MPH). Are these numbers normal for my setup? Readings taken with a Fluke 115 DVM. I didn't have time to do individual checks on the batts today.

Mods are 23" tires, 6" lift, XCT400 PDS. Thanks

[Freckles]

Was wondering the same thing. I have a dcs with alltrax 500 and mine drops to 31 for just a split second then comes up to 34ish.

[rib33024]

Under-Load voltage. Battery voltage decreases while amps are being drawn and the more amps drawn, the greater the decrease.

When doing non-destructive testing, battery manufacturers typically use 1.75VPC (Volts per Cell) as the cutoff voltage, so that is a fairly safe goal to stay above while your cart is under heavy load. That works out to 31.5V for a 36V pack.

If the batteries are loaded down to 1.5VPC or below, irreversible plate damage can occur. That is 27V for a 36V pack.

The amount the battery's voltage drops per amps of current drawn varies with the AH capacity of the battery and SoC. Older batteries have a lower AH capacity due to the natural aging process and the SoC is lower on the return leg of the trip, so keep an eye on the meter while climbing hills on the way back to the house.

Also, Batteries have less capacity (AH) when the weather is cold. The loose about 10% of their AH capacity for every 15°F below 80°F.





http://www.buggiesgonewild.com/elect...tml#post984660

[JohnnieB]

In and of themselves, the batteries in a 36V pack will drop 1.0V/100A of current draw from them. The peak amp throughput of a XCT4800 is 460A, so the batteries alone will drop 4.6V.

An XCT controller drops about 0.18V /100A. (0.828V at 460A)
15' of 4Ga cable (copper conductor alone) drops 0.373V/100A (1.716V at 460A)

That is 7.144V at 460A in the intrinsic voltage drops of a nearly perfect amp delivery system and virtually all of our amp delivery systems are far less than perfect due the the ring terminal connections on each end of each high current cable and the metal to metal interfaces throughout the high current circuit.

Of course, the above is theoretical since you motor probably never draws 460A, or if it does, it doesn't do it for very long.

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Your Fluke 115 has a Min/Max function, so measuring the minimum an individual battery drops to, is fairly easy.

Try to avoid going below 1.75VPC (Volts per Cell), which is 5.25V for a 3-cell (6V) battery and never go below 1.5VPC (4.5V for 3 cells). Irreversible damage starts occurring at 1.5VPC. Those numbers are for individual batteries only and cannot be multiplied by six for the entire battery pack due to the resistance if the cables and connections connecting the batteries in series.

However, the added voltage drop of the interconnecting cables makes the error on the safe side for the batteries.

[Dixie Chopper]

Johnnieb - so I gather that individual readings will provide better data than reading the pack voltage. I’ll try to get that done tomorrow. The drop to 30.5 only happens from a dead stop so I can try not to do that.

Also, I upgraded to 2Ga cables on the batteries but not the controller and motor wires. I wonder if that would help any?

[JohnnieB]

There are two main reasons for tracking the voltage drop under load.
1. Battery health. (Monitoring individual battery voltages)
2. Cart performance. (Monitoring pack voltage)


Battery health:
In their non-destructive testing battery manufacturers only take their products down to 1.75VPC, so drawing more amps than what is required to lower our cart batteries to 1.75VPC should be avoided to get the batteries to last for more years before needing to be replaced.

If enough amps are drawn to drop the voltage to 1.5VPC or lower, active material is sucked off the plates and the battery lifespan is greatly shortened if immediate death does not occur.

Lower storage capacity batteries (AH) drop more Volts per Amp than higher AH batteries and a partially charged battery drops more Volts per Amp than a fully charged battery.

If your cart routinely drops below 1.75VPC you should buy higher AH batteries the next time the battery pack is replaced, or upgrading to a higher voltage battery pack. (Higher voltage takes fewer amps to do the same work.)

If your cart approaches 1.5VPC in adverse conditions (mud, sand, hills etc.), tire type and height need to be addressed as well as motor type along with battery AH and pack voltage.



Cart performance:
In addition to the voltage drops of the individual batteries, there is the voltage drops caused by the resistance of the of the high current cables, connections, solenoid contacts (and F/R switch in series drives) and controller that are located between the batteries and the motor. The voltage drops across these resistances is caused by some of the amps passing through those resistances are converted to heat and amps converted to heat in the amp delivery system are amps that do not reach the motor where they would have been converted to torque and motor heat. (Cart motors have a peak efficiency of 85% depending on RPM) The fewer amps lost to heat in the amp delivery system, the better the cart's performance.


The high current cables form a series circuit, so if the cables between the battery pack and controller or between the controller and motor as thinner gauge than the interconnecting cables in the battery pack, the cart's performance is being limited by the thinner cables to controller and motor.

In a PDS cart all ten high current cable ought to be same gauge. (The original cables to the Field windings are okay, if they are in good condition. They only carry up to about 30A, so 10Ga is sufficient.)


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Analyzing the data logs from an XCT can give you a pretty good I idea of how efficient you amp deliver system is up to the controller and there are only the two high current cables connected to the motor not included in the mix.

I haven't checked recently, but in 2015 my battery pack (3 years old at the time) was dropping 2.9V/100A at the controller.

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The number of amps a DC traction motor can draw is dictated by the RPM the armature is spinning at. The max amp draw is at 0RPM and the minimum amp draw is at the max RPM the armature can reach with the mechanical load being imposed upon it at the voltage being applied to it.

Worst case scenario is trying to move an immovable object. Second worst is a cart with tall, low pressure tires, starting from a standstill on a steep incline.

[Dixie Chopper]

Johhnieb. I cannot thank you enough for taking time out of your day to explain things in terms that a fella like me can easily understand!

I will soon be upgrading to 42V which should lower the amp draw on the pack if I’m understanding you correctly.

For right now I’ll try to take it easy on the cart ;)

[JohnnieB]

Your pack is dropping to 30.7V momentarily during a hard start on level ground. That appears to work out to about 1.71VPC, but half or more of that voltage drop is due to the resistance in the amp delivery system, so the actual voltage drop at the individual battery level most likely isn't anywhere near the danger zone, however I'd still check the voltage drop on the individual batteries just to be sure.

Adjust the XCT for a milder take off and you won't have to worry about it.

As for the amp reduction going to 42V. Let's say the cart draws 400A @ 38.2V (Nominal voltage for a fully charged 36V pack - Trojan batteries) for a few feet when accelerating from a standstill. That is 15,280W of energy being used. It only takes 342.6A of current at 44.6V (Nominal 42V pack) to yield 15,280W, so there is a 14.4% reduction in amp flow from the battery pack. However upping to a 42V pack increases torque by 16.7%, which in turn further reduces the amps needed to do the same work, so the percent the amp flow from the batteries is actually reduced will be a bit more than 14.4%.

You will love your cart at 42V.

[Dixie Chopper]

I can’t wait to up the voltage to 42!

I looked on the alltrax website to get the software to run diagnostics. Apparently they only support windows and I only have MacBooks? Or am I missing something?

[JohnnieB]

Originally Posted by Dixie Chopper

I can’t wait to up the voltage to 42!

I looked on the alltrax website to get the software to run diagnostics. Apparently they only support windows and I only have MacBooks? Or am I missing something?

I think it is Windows only, but you can e-mail them at: helpdesk@alltraxinc.com