[brian_]

Based on my understanding there is too much sag because of the Lead Acid batteries and is not providing enough power for the motor.

How much sag are you seeing now? That is, what is the voltage from the batteries under load?

The manufacturer of this battery has said not to discharge for than 1.2C. Are they being overly cautious with this number? I thought Lithium ions would allow for a higher discharge rate.

What is the discharge current now?

As major suggested, the small expenditure for instruments to understand what is going on now is the logical step before a much bigger expenditure for batteries that might not fix the problem.

 

[brian_]

Controller:
Golden Motor has at least four controllers for 48 V systems, with continuous current ratings as low as 80 amps. I assume it is one of the suitable models, with 10 kW continuous output rating.
The current (100 amps) is way under the rating of these controllers (200 amps) so the controller's capability shouldn't be the problem... but is there a current limit setting which is being hit? 100 amps is not enough (would be 4.8 kW of motor output at perfect efficiency), so something is limiting the current.

On the other hand, it is probably not the controller or its settings, because the motor characteristics probably explain the problem...

Motor: model - HPM-10KW, SKU - HPM10KW48V-AIR

The product description in the Golden Motor web site is almost entirely unhelpful. They give a range of speeds, but that is in the context of a description which lists operating voltages up to 120 volts. Although the title says "10 kW", the description says 8 kW to 20 kW for that range of 48 V to 120 V. Just from that description, it's entirely possible that the motor can't run effectively at 4000 rpm with only 48 volts, and can't produce even 8 kW with 45 volts input to the controller. On the other hand, they show "8KW-20KW" for all voltage versions

Via a link in another forum, I found performance data in the Golden Motor website for this motor:
data table: https://www.goldenmotor.com/eCar/HPM10KW%20(48V)%20Data.pdf
graphed: https://www.goldenmotor.com/eCar/HPM10KW%20(48V)%20Curve.pdf
... and for the higher voltage versions, including the 72 volt...
data table: https://www.goldenmotor.com/eCar/HPM10KW%20(72V)%20Data.pdf
graphed: https://www.goldenmotor.com/eCar/HPM10KW%20(72V)%20Curve.pdf

All the labels are in Chinese, and this is one of those idiotic data sets that shows only the upper part of the speed range, but I think I see the problem. At 48 volts this motor can spin almost 4,000 rpm, but cannot produce useful torque up there. Get down to about 3700 rpm and it is only using 100 amps, because that is all 48 volts can push through it at that speed; the corresponding torque and power are 11 Nm and 4.3 kW. Power and torque drop off rapidly above that speed and the efficiency falls apart, so when you accelerate it will be like hitting a wall when the motor speed gets that high. I copied the data into a spreadsheet so I could graph it in a more useful way, and attached some graphs.

From that top end of the useful power range down to some point (off the data chart) it is limited by power (go slower and get more torque, etc), and peak power is available at only one speed. It's not in the 48 volt data, but the data for the 72 volt motor shows the expected constant-torque range below the peak power speed (which is 4700 rpm for the 72 volt, but 3300 rpm or something less for the 48 volt). So you can gear to put something like 3500 rpm at the highest road speed desired, but when you go slower you hit the next problem - 200 amps is not enough to get maximum torque and power at lower speeds. I copied the 72 volt data into a spreadsheet, too, and attached some graphs showing the flat torque region at lower speeds; the 48 volt motor should be similar (but at a lower speed).

With a 48 volt nominal system voltage, battery voltage dropping to 45-46 volts doesn't seem like it could be the problem. Even if that is significant sag from the fully charged resting voltage, it's not much less than the full nominal voltage and should be able to drive a 48 volt system as designed. If I have understood this situation correctly, that means that a different battery won't help. Higher current capacity won't be used if the voltage isn't there to drive it.

Getting enough power for the desired top speed means gearing very precisely, since too tall will not provide enough torque and too short will mean the power drops off too soon.

 

[brian_]

5) I have tried gear ratios of 1:10 and 1:5. 1:10 gave us the speed we were expecting (~around 40 km/hr). 1:5 should have theoretically given us around 90 km/hr but we barely went over 50 km/hr using 200 Ah. The moment it reaches 50+ km/hr the speed barely increases, and if it does, it does so at a very slow pace which isn't acceptable.

Running a 600mm diameter wheel (roughly 23 inches). Motor operates at around 4000 rpm. According to the manufacturer of the motor it should be able to get to higher speeds, hence my concern regarding the lead acid causing issues.

A 600 mm outside diameter tire has a circumference of 1.9 metres. This tire would then turn about 350 rpm at 40 km/h, 440 rpm at 50 km/h, and 800 rpm at 90 km/h.

With 10:1 reduction gearing, the motor would have been turning about 3500 rpm at 40 km/h. The motor was running too fast for peak power at this speed, but would deliver more torque to the wheel at low speed than it would with less gear reduction.

With 5:1 reduction gearing:

• the motor would have been turning about 4000 rpm at 90 km/h... but it couldn't get anywhere near that because it can't produce any torque (and thus no power) at that motor speed;
• the motor would have been turning about 2200 rpm at 50 km/h... probably too low for ideal power, so you might hope that it would climb in speed and climb in power to match, but it might have just not had quite enough torque to the wheel to overcome drag. It would have 59% more torque at this speed than at 3500 rpm, but only half the multiplication of torque by the gearing.

You can pick different gear ratios, see where that puts you versus speed on the power curve, and maybe find an optimal compromise. I suspect that it's not going to be 90 km/h: for example, if 90 km/h is 3300 rpm, the gearing would be about 4:1, and we know that would not produce enough power at 50 km/h to keep accelerating.

 

[brian_]

Also investing in a single speed transmission instead of the direct drive I had earlier to provide me the torque I require.

You were using different drive ratios already; I assumed that meant that you have a chain or toothed belt drive, and had changed sprockets. Since you can get lots of reduction (10:1) with the current setup, and can continue to change sprockets to get different ratios, I'm not sure what benefit would come from adding a separate single-speed transmission.

 

[brian_]

I have visited EVdrives.com and selected an entirely new set of motor and motor controller to work at 48 V.

Motor: http://www.evdrives.com/product_p/mot-me1602.htm
...
The motor is rated at 6.4 KW @ 48V. In your estimation is this motor any good? The motor typically hovers around the 3500 mark unloaded. Would you say ME motors have better performance compared to Golden Motors?

Although the higher-voltage versions of the current Golden motor would be more capable, I don't think the issue is that the Golden motor is unsuitable for 48 volts. In the motor specs there are different resistance and inductance values for different voltages, indicating that the motor is wound differently depending on the voltage for which it is intended (higher voltage motors have more turns of wire).

It is good to see some detailed data for the Motoenergy motors. Unfortunately, it is presented the same way as the graphs for the HPM: it doesn't directly show how output power varies with speed. I think this is legitimately a result of the mediocre testing method used for cheap motors, but it also conveniently hides the problem that output power drops dramatically at higher speeds.

The EV Drives product page for the ME1602 explains that it is the same motor internally as the ME1004, so it has the same performance. That means that the ME1004 performance data applies to the ME1602, but they don't publish a table of actual data values for either of them so it is not convenient to plot that data in a more useful way. I read approximate values from the graph, put them in a spreadsheet, and produced a couple of graphs that I find more useful (at the bottom of this post).

The basic problem is still visible even in the Motenergy chart: the ME1004/ME1602 produces its maximum torque at somewhere between 3300 and 3400 rpm, but torque drops off rapidly with speed to the point that it can't do much more than just spin freely at 3700 rpm (or even less).

The peak power for the HPM 10 kW 48 V was 29.9 Nm (21.3 lb-ft) of torque at 3304 rpm, for 10.0 kW
The peak power for the ME1004/ME1602 appears to be 152 lb-in (12.7 lb-ft) of torque at about 3350 rpm, for 6 kW.
Both should have roughly constant torque at that peak level at lower speeds; both drop off rapidly at higher speeds and are useless by less than 4000 rpm.

It seems that the motor is simply not good enough to provide the necessary power.

A motor which provides the same peak power but over a wider speed range would help, but that means a good permanent magnet synchronous AC motor like the full size production EVs use - but much smaller - and I have yet to see anyone offering those. The "brushless DC" or "BLDC" motors such as the Golden HPM are similar in design to the AC PM motors, but don't have the same speed range of performance. The ME1004/1602 is a permanent magnet brushed DC motor, which is a type commonly used for accessory motors (such as fans) in vehicles, but is not used in large motors, because it has the cost of permanent magnets but still has brushes and a commutator to wear out.

It has occurred to me that the ME1602 might have looked attractive because of the torque numbers up to 152, but those are pound-inches, not pound-feet.

I think that Duncan and kennybobby have homed in on the issue: although ideal gearing could make the existing motor or a similar one provide somewhat better performance, and a better speed range of power would make the vehicle accelerate better and be less sensitive to gearing, you likely simply need more a more powerful motor. Changing to the Motoenergy motor is going the wrong way - it's designed for the same 48 volts, and it is lower-powered than the Golden.