36v to 48v - Is it worth it?

[sportcoupe]

Seems some of us suffer from CRS Syndrome. Myself included.

[yurtle]

Mine's so bad I was about to Google CRS, before I figured it out, even though I knew what the context was, and have seen "CRS" before.

[JohnnieB]

Originally Posted by mhynd

So I called and talked with a guy at D&D about a high torque motor for my cart build. Told him I was going to 48v and he asked why. Told me that I would not see any real gain as far as torque but maybe a little more run time. Is this true? I keep reading about everyone saying make the jump to 48v.

Unless the motor is not receiving all the amps it can draw for the RPM it is spinning at due to ampacity of the controller, high current cables, solenoid contacts and F/R contacts (in a series drive system) or has excessive resistance in the high current connections, upping the pack voltage won't up the maximum low end torque significantly.

The often mentioned analogy that Amps = Torque is basically true, but more correctly Watts = Torque. Watts being the actual unit of power and is calculated by multiplying Amps time Volts. Therefore, if the maximum power a motor can draw at a given RPM is 1kW (1,000 Watts) you can get it by sending 100A through the motor at 10V or by sending 10A through the motor at 100V. The torque generated will be roughly the same, but slightly higher at the higher voltage due to less heat loss in the windings. (Heat is generated exponentially based on current flow - W = I^2 X R)

It is true that a 400A controller effectively becomes a 533A controller when you up the pack voltage from 36V to 48V, but that only increases torque if the motor is amp starved.

On the other hand, upping the pack voltage will increase the top speed of any cart, series or sepex. How much it increases depends on many factors, but it will increase.

This is going to sound contradictory, but it really isn't. With a higher voltage battery pack, you do have more high end torque, so the cart won't slow as much climbing hills at speed.

The torque curve for a motor has two main data points. Stall Torque (Zero RPM - Max Torque) and No Load Speed (Max RPM - Zero Torque)

Upping the pack voltage will not change the Stall Torque point unless the Amp deliver system has a bottleneck in it, however it will up the No Load Speed, so the area under the curve changes and the net result is higher torque at higher RPM.

Granted, the low end torque will also increase, but the increase will be slight because the angle doesn't change much with just a 33.3% increase in voltage.

[yurtle]

Wouldn't it be true that eight 6 volt batteries "may" provide more torque - both high and low end - than six 8 volt batteries, assuming both are standard amp-hour batteries - 225 for 6 volt and 170 for 8 volt? As you stated, it depends on if you are amp starved. I'd think you'd see less voltage drop for a given load.

[yurtle]

For those who struggle with electrical terminology, I think a crude analogy is total pounds of lead. It's really total available energy stored, but I think I've heard JohnnyB state six 225 amp-hour 6 volt batteries is roughly equivalent to six 170 amp-hour 8 volt batteries, as for as energy stored or run time.

Am I off here? Coffee hasn't kicked in yet.

[JohnnieB]

Originally Posted by yurtle

Wouldn't it be true that eight 6 volt batteries "may" provide more torque - both high and low end - than six 8 volt batteries, assuming both are standard amp-hour batteries - 225 for 6 volt and 170 for 8 volt? As you stated, it depends on if you are amp starved. I'd think you'd see less voltage drop for a given load.

Originally Posted by yurtle

For those who struggle with electrical terminology, I think a crude analogy is total pounds of lead. It's really total available energy stored, but I think I've heard JohnnyB state six 225 amp-hour 6 volt batteries is roughly equivalent to six 170 amp-hour 8 volt batteries, as for as energy stored or run time.

Am I off here? Coffee hasn't kicked in yet.

A 36V battery pack made up of 6 x 6V-225AH stores about 8.6kWh of energy while a 48V battery pack made up of 6 x 8V-170AH batteries stores about 8.7kWH of energy, so run-time is roughly the same.

If the motor was drawing 1000 Watts motor from the 36V-225AH pack, there would be a current flow of 27.7A, which is roughly a C/8 discharge. If the motor was drawing 1000 Watts motor from the 48V-170AH pack, there would be a current flow of 20.8A, which is also roughly a C/8 discharge, so the voltage drop under the same load would be about the same for both the 36V-225AH and the 48V-170AH battery packs.

However, a 48V battery pack made up of 8 X 6V-225AH batteries stores about 11.5kWh of energy, so a 1,000W load would only be a C/10.8 discharge and the voltage wouldn't drop as far as it would on battery pack with 8.6 or 8.7 kWh of storage capacity.

In general terms, you will get better performance from a battery pack that has a higher storage capacity, but increased storage capacity means more weight, which in turn reduces performance.

It does get complicated, doesn't it?

[kgsc]

Boy I am sure glad we bought 190Ah 8 volts

[yurtle]

Certainly a LOT easier than trying to squeeze two extra batteries under the seat. If you can live without the bagwell, that's a FAR easier way. You can easily add one battery under the seat and either go 42 volt or put one under the seat and one in the bagwell. That would leave a little storage in the bagwell.

[JohnnieB]

Originally Posted by kgsc

Boy I am sure glad we bought 190Ah 8 volts

That gives you about 9.7 kWh of storage.
The only thing that I know of that will give you more storage in a 6 X 8V configuration is the Trojan Ranger 160. (8V-204AH)

[JohnnieB]

Originally Posted by yurtle

Certainly a LOT easier than trying to squeeze two extra batteries under the seat. If you can live without the bagwell, that's a FAR easier way. You can easily add one battery under the seat and either go 42 volt or put one under the seat and one in the bagwell. That would leave a little storage in the bagwell.

I'm lazy, so I went with 42V (7 x 6V).

However, I did go with 245AH batteries, so I have about 10.9 kWh of storage.

Ironically, 42V is the max voltage I can run with my tire height and gear ratio and stay under the Max recommended RPM (6,500) for my motor when WOT on level ground. But I must confess, I didn't plan it that way, it just happened to work out that way.