battery cables

[wingnut2000]

I converted my RXV from 4 12 volt batteries to 6 8 volt batteries earlier this year. I have mixed matched battery cables. how bad will this effect performance?

I want to get all matching cable sizes. what would be a good size to put on this set up?

[sunking]

Originally Posted by wingnut2000

I converted my RXV from 4 12 volt batteries to 6 8 volt batteries earlier this year. I have mixed matched battery cables. how bad will this effect performance?

First question and most important are there any safety issues? If any cable is less than 6 AWG, then you have a safety issue.

A mix of 6 AWG to say 2 AWG if you had instrumentation to measure acceleration forces, then say switch to all 2 AWG you could see a slight performance increase in acceleration, but not speed. Probable not enough to notice and could only measure with an accelerometer.

Originally Posted by wingnut2000

I want to get all matching cable sizes. what would be a good size to put on this set up?

As large as you can afford without going over the top. Factory wiring is 6 AWG and the smallest size you can safely use with a factory controller. So at a minimum I would recommend 4 AWG and that can be used with up to a 400 Amp Controller. Over 400 amps you would be required to use 2 AWG minimum. So the answer is 6, 4, or 2 AWG, as large as you can afford and meet minimum safety requirements.

[JohnnieB]

The batteries are connected in series to the controller, so the cart's performance will be limited to the amp passing abilities of worst cable in the series chain and the RXV parts manual lists the as 6Ga, which at best, is marginal for the application.

I'm not into RXV carts, but as I understand it, the controller's output is three phase AC and the controller's amp rating is for each of the three phases individually and the three phase AC motor can be drawing up to the controller's max rating with each set of windings, but the amp draw for each of the three phases is 120° apart, so the max amp draw from the battery pack is greater than 1 times the amp rating of the controller, but less than 3 times.

I'm to lazy to look up the formulas and to the math, but just looking at a 3 phase waveshape, when one phase is at its peak, the other two are about half way between the peak and crossing the zero line, so I'd guess the max battery amp draw will be about twice the amp rating of the AC controller is.

With that in mind, I'd use a minimum of 4Ga high current cables, and use 2Ga if the cart was going to carry heavier loads or used on hills, mud or sand.

[JohnnieB]

sunking,

Looks like you replied while I was researching.

I'm most definitely not up to speed on 3-phase AC drives, but I seem to remember a post by BobBoyce where he said the amp rating on the 3-phase controllers was for each phase, so total battery amps will be the instantaneous sum of what is being drawn by the motor for each phase.

If that's the case, no wonder a 4x12V battery pack doesn't fair too well in an RXV.

[sunking]

Originally Posted by JohnnieB

I'm not into RXV carts, but as I understand it, the controller's output is three phase AC and the controller's amp rating is for each of the three phases individually and the three phase AC motor can be drawing up to the controller's max rating with each set of windings, but the amp draw for each of the three phases is 120° apart, so the max amp draw from the battery pack is greater than 1 times the amp rating of the controller, but less than 3 times.

You are in the ball park. To make Apple to Apple Comparison AC to DC we have to use the only common denominator of POWER aka WATTS.

Think of a 3-phase AC motor as 3 DC motors. Makes no difference AC or DC 3000 watts is 3 thousand watts, and Power Always Adds in DC and AC circuits. Voltage and Current in AC add trigonometry, not straight addition.

In a 3-phase circuit I1 = I2 = I3. So lets say where it is 1 amp. 1 amp + 1 amp + 1 amp does not equal 3, it equals the square root of 3 or 1.732 amps. The magic number is the Square Root of 3 = 1.732

So we have two 48 volt 7500 watt Peak motors (roughly 7 to 9 hp peak), one DC and one 3-phase motor. They both draw 7500 watts at 48 volts from the battery or 7500 / 48 = 156 amps . In the DC motor we would see 156 amps flowing in the windings. However in the AC motor we only see 156 amps / 1.732 = 90 amps in each of the 3 windings. So we add 90 + 90 + 90 = 156 amps. 156 amps / 1.732. Si if using a DC motor we are looking for at least a 150 amp controller, and only 100 amps required for the DC motor. Anything larger is just overkill and expense. Don't waste your money on 400 amp controller as it would not buy you anything. Only the retailer got a gain at your expense.

I wished I had the full specs to the RVX motor so I can see what the motor is actually capable of. I got rid of of my RVX motor and controller and replaced it with an HPEV 15 motor running 96 volts. My little cart eats RXV for a snack and leaves it hungry for some real competition like a Corvette.

[sunking]

Originally Posted by JohnnieB

I'm most definitely not up to speed on 3-phase AC drives, but I seem to remember a post by BobBoyce where he said the amp rating on the 3-phase controllers was for each phase, so total battery amps will be the instantaneous sum of what is being drawn by the motor for each phase.

If that's the case, no wonder a 4x12V battery pack doesn't fair too well in an RXV.

That has to do with capacity and the battery internal resistance as they are both directly related. A 12 volt GC battery has roughly 150 AH vs say 190 AH of a Trojan 8-Volt Ranger. The total internal resistance difference between the two is the 8-volt Ranger has roughly 21% lower resistance, so it can deliver more current. No real difference in speed, but significant in acceleration because motor current = Torque, and Voltage is speed. Both wil go 25 mph, but the 8 volt batteries get you to 25 mph in less time because there is more force pulling you (torque). Will not touch on the range difference as common sense would tell you higher capacity = longer ranges.

[sunking]

A little math fun for you Johnny. How big of a difference is there between 2, 4, and 6 AWG. Is it worth the expense or not. From a speed point of view. not enough difference to justify the expense. That is because speed is a function of voltage, and that does not change enough to make any real difference.

To prove this requires some math fun. I will use some generic industry numbers for golf carts. A 48 volt system uses 50 amps continuous, and 75 amps for 36 volts. Power wise they are both roughly equal of 2.7 Kw and 2.4 Kw respectively. I am just going to use 48 volts and 50 amps. To keep apples to apples I assume 10 feet of wire used of 2. 4 and 6 AWG A 10-foot section has the Resistance of:

2 AWG = .001563 Ohms
4 AWG = .002485 Ohms
6 AWG = .003951 Ohms.

Now if you are a liberal democrat and voted for Biliary you would scream bloody murder #2 AWG is 252% superior to 6 AWG and there is no contest. Conservative would laugh at Biliary and say you are a fool spinning numbers pissing away money because 2 AWG cost 220 % more than 6 AWG. 6 AWG cost 57-cents per foot, and 2 AWG cost $1.40 per foot.

Who is right? I will leave that up to you. Simple Ohms Law applies. Voltage = Current x Resistance. So cruising along at 50 amps on 6 AWG you loose 50 amps x .003951 = .2 volts loss, and with 4 AWG you loose 50 amps x .001563 = .08 volts a difference of .12 volts. You gave up 0. 25% speed or 1/4 of 1 percent. You dropped your speed from 20 mph to 19.95 mph. Good luck measuring and justifying that Biliary.

Is that the end of the story, no sir. I have not talked about acceleration or safety yet. For that we need to use a real motor. Say a Red Hawk Admiral Motor Mot-B-2 a 10 HP peak, 4 hp continuous. We look at the specs and see the winding resistance and note Rs = 0.22 Ohms. Peak motor current occurs at 0 RPM with full battery voltage applied. Current = Voltage / Resistance = 48 volts / .22 Ohms = 218 amps. So we need a 220 amp controller. Who bought the 400 amp controller for that motor? It will work, but you will never use it. Anyway we use 220 amps and now we can see differences .

With 2 AWG voltage loss at 0 RPM stall current is 220 amps x .001563 = .34 volts, and with 6 AWG 220 amps x .003951 = .86 volts. So Biliary voters what this means is 2 AWG would give you a little more acceleration from 0 mph up to maybe 3 to 5 mph. After that no noticeable difference you could measure or feel. Only things that feel worse or better is your wallet (worse) and the guy selling you something (better for Biliary selling you something) you cannot use.

So it really comes down to safety of what is safe or not. To determine that we need to find the motor maximum continuous current. Once that number goes above 60 amps, you need to make the move from 6 AWG up to 4 AWG, 2 AWG just cost more and buys you nothing.

So back to the Red Hawk Admiral Motor Mot-B-2 Continuous Current = 88 amps @ 4 hp consuming 4220 watts. 4 AWG is your choice for upgraded motors. I am sorry if you bought a 400 amp controller for that motor, all you need is 220 amps to push the motor to the limits. Me I would use a 200 amp controller and make the motor last longer.

Sorry guys I know some of you do not want to hear that, but math is the Truth and it can hurt sometimes.

[JohnnieB]

Granted Math is truth, but numbers can be tortured into saying most anything. The common name for number torturing is statistics.

The data logs I have saved for my motor indicate that the armature can and does draw 395.2A when is is spinning at 1507RPM at WOT with 33.9V applied. That figures out to an armature impedance of 0.082Ω.

Of course, the data recorded from an XCT48400-PDS controller may be open to doubts, but dynamometer testing by the manufacturer (D&D) indicates the armature draw is at 1914RPM is 444.8A with 40.2V applied, which figures out to 0.090Ω for armature impedance. (Higher impedance due to greater BEMF at higher RPM)

The reason I went with a 400A XCT (460 Peak) controller is because the 500A XCT for a PDS drive had not been released for sale at the time and my data logs from the DCX500 I was running showed my motor never drew more than 460A accelerating from a standstill on a 30° incline.

[sunking]

Johnny you are not comparing apples to apples. I am talking about one motor, and you are talking about another different motor. Besides you are talking AC Impedance, not DC resistance.

I don't care what your specs work out to, nor do I doubt them. Fact is any , and I mean any DC motor draws the highest current at 0 RPM locked rotor. To determine the current is simple Ohm's Law. All you need to know is the motors total resistance of of the winding. So if you connected the battery directly to the motor with a Locked or Stalled Rotor, the maximum current that motor will ever draw is Amps = Voltage / Resistance. That is not spinning numbers or statistics, just plain elementary DC 101 Electric and Physics of Ohm's Law. Man made Laws can be broken at will, you cannot break the Law of Physics.

So lets spin this. If you have a 48 volt motor, in order to pull 400 amps is Ohm's Law again. Resistance = Voltage / Current. So to draw 400 amps would require the winding DC Resistance to be 48 volts / 400 amps = .12 Ohms all day long. Interestingly enough Power = Volts x Amps, well 48 volts x 400 amps = 19,200 watts. That would be the Peak power the motor can withstand for a few seconds before burning out he windings. FWIW 19,200 watts depending on efficiency which for DC motors is 70 to 80% making 19200 watts a 18 to 20 peak horse power. Please show me a Series DC 48 volt golf cart motor that produces 18 to 20 Hp peak. That same motor continuous hp rating would be roughly 5 to 6 hp and that works out to 100 to 110 amps at 4800 watts to 6000 watts input.

You are right Back EMF (Eb) changes things and protects the motor. At ZERO RPM Eb there is 0 volts Eb. As RPM's increase, so does Eb and Eb lowers the voltage which is Negative and subtraction. At some point in a Series DC Motor, usually around 5000 RPM's Eb = Battery voltage, and at that point Current = 0 Amps and no power is being consumed.

Have you ever witnessed this? Most of us have and may not know what causes it. You are going down a steep hill with pedal to the metal. You pick up speed, than all the sudden you notice a braking action despite having pedal to the medal. What is happening is your is one of two things. 1. Motor RPM has gone up to the extent Eb is greater than Battery Voltage. Your motor is not consuming power, it is generating power and charging your battery. That is REGEN Braking. The motor by design is trying to save your and the motors life trying to prevent over speed causing the motor to fly apart and/or you loosing control.. The other possibility if not over-speed, is your controller is lowering voltage below Eb so you have REGEN Braking. I am not a golf cart guy, I am an electrical engineer, but every cart I have tested will do this. Get your cart up to speed, and turn off the key switch. Brace yourself because you will go into Maximum REGEN Braking. Why someone might ask? Because that is how DC motors work. When you turn off the key, you remove power from the motor and it now becomes a generator via Eb and charging your battery.

[wingnut2000]

I am more confused now than I ever was. lol