downsides to a higher voltage

[gothicsurf]

I would like to know if there are any downsides to having a higher voltage battery pack for run a relatively small motor (Bafang MG310).

Would the higher voltage and thereby lower current help maintain cooler temperatures considering a CA would maintain the same wattage irregardless of voltage?

Embarking upon my first ebike build and things seem pretty straight-forward except for the battery.

A simple pedelec for minor assist while hauling a trailer+dog -- figure upon a rear, more discrete motor, easily locked through the rim. A downtube battery either mounted or carried in a bag bungeed to the rack. It's just the voltage of the battery I am stuck .. would there be any advantages to running 36v vs 52v?

In the case of possibly two motors down the road, would a larger motor be limited by 36v?

 

[Voltron]

As a very general thing, the 52v will make less waste heat. I didn't look up your motor, but assuming it's brushless, with brushed you get a lot more sparking and heat at the brushes, so that could change the equation. I'm running what was originally a 36v on 72v (20s) fyi.

Going up into the hundreds of volts, there start to be some downsides, like dying etc.

 

[Balmorhea]

Any given motor is going to have a specific RPM per volt. So match the motor and your battery voltage to get the speed you want, within the power that your system can provide.

In a well matched system, 50% of your no-load speed is where you get the highest power output, and 80% of the no-load speed is where you get highest efficiency (but at a much lower power output). So that's the range you want to do most of your business in.

 

[Matador]

Also. At equal power (watts), higher voltages mean less current (amps). Thus you can get away with much tinner gauge wires (save on weight!) And minimise voltage drop.
Matador

 

[gothicsurf]

In trying to discern downsides to a higher voltage, what I gather is that a 52v system may be less efficient at let's say a power output attaining 20 mph compared to a 36v system limited to the same power. This being the case, the 36v system may actually attain slightly higher speeds at the same power.

Are there any other reasons not to utilize a higher voltage? Seems the $/Wh remains fairly flat, so the cost wouldn't be affected much if one already has a controller, wiring, etc. capable of the higher voltage.

I guess it doesn't really matter from an efficiency standpoint -- the simulator on ebikes.ca shows the difference of efficiency as negligible when matching power to attain similar speeds. Though it seems the data gathered is gathered for the simulator on an unloaded wheel, given the speeds do not decrease until well below 50% throttle.

Nevertheless voltage seems all about attaining a top speed, while the main downside may be risk of overheating smaller motors? The simulator shows the 52v (vs 36v) overheating the Bafang G311 at anywhere above 30% throttle -- does this make sense, even with less current through the motor at similar power levels?

 

[Balmorhea]

Nevertheless voltage seems all about attaining a top speed, while the main downside may be risk of overheating smaller motors? The simulator shows the 52v (vs 36v) overheating the Bafang G311 at anywhere above 30% throttle -- does this make sense, even with less current through the motor at similar power levels?

It makes sense in that controllers can send more phase amps to the motor than amps they draw from the battery. The higher the voltage from the battery, the higher the current to the motor can be.

Also, if raising voltage means the motor’s attainable speed range falls into a less efficient part of the motor’s efficiency curve, then a larger proportion of battery energy will be converted to heat rather than motive power.

 

[999zip999]

52 volt.

 

[LeftieBiker]

Going up into the hundreds of volts, there start to be some downsides, like dying etc.

The risk of an ebike killing you starts much lower than that - at about 60 volts.

 

[Voltron]

I would say the voltage is more about maximizing the efficiency of the components in the controller. I'm no expert, but have read there's efficiency plateaus with the fets and capacitors, but there's so many considerations, including things like trading off a little efficiency during use to get reduced charging times from higher voltage, or maybe some hyper efficient motor you want to use needs a certain voltage, so you're willing to lose efficiency in the controller to use that voltage, or vice versa. Using more voltage is a great aftermarket way to get more speed out of existing motor, but if you're designing a system from scratch then it's balancing the whole package out to decide a voltage.

 

[Voltron]

Going up into the hundreds of volts, there start to be some downsides, like dying etc.

The risk of an ebike killing you starts much lower than that - at about 60 volts.

I was somewhat kidding with the hundreds number, and if it happened just right, yes, you're correct that 60v could get you, but it's not real likely. Over 80/100ish it's starts being more aggressive about penetrating dry skin, breeching insulation and jumping air gaps.

I'd say the risk of most 60v bikes is more in the crashing than the electrocuting.

 

[gothicsurf]

Also, if raising voltage means the motor’s attainable speed range falls into a less efficient part of the motor’s efficiency curve, then a larger proportion of battery energy will be converted to heat rather than motive power.

Though this scenario involves capping the speed/power to lets say 20 mph. Efficiency difference is probably negligible, but with the simulated temperatures overheating this motor at 52V anywhere above 30% throttle, would I still risk overheating the motor at the capped speed? The 36V simulation shows the motor speed well below overheating at 100% throttle.

So it two systems at different voltages have different temperatures at idential speed/power (and similar phase current) the increased heat would be a result of inefficiency. Therefore a smaller motor seems better off run at a lower voltage, especially if modest assist is more the goal than speed?

 

[Balmorhea]

Something's fishy about your simulator graphs, because the power output isn't the same. When you reach the capped speed, you'll only be getting enough motor power to maintain that speed-- which is the same amount of power regardless of system voltage.

 

[gothicsurf]

Maybe there is someone familiar with this simulator who can help explain why the temperatures are so vastly different at the same speed.

Is it because the data was gathered for the motor at an unloaded state and therefore the rpm that is most efficient. Thereby looking at the values of the curve with less RPM would reflect a change in the control value of the load/grade?

I am really just trying to figure out whether to go with a 36V system or something higher .. but also wish I knew how to interpret this data better.

 

[serious_sam]

Maybe there is someone familiar with this simulator who can help explain why the temperatures are so vastly different at the same speed.

Is it because the data was gathered for the motor at an unloaded state and therefore the rpm that is most efficient. Thereby looking at the values of the curve with less RPM would reflect a change in the control value of the load/grade?

I am really just trying to figure out whether to go with a 36V system or something higher .. but also wish I knew how to interpret this data better.

Because this:

Something's fishy about your simulator graphs,

You haven't got the settings right, or you changed something and didn't update. The load and power output of each setup should all be equal (I.e. Systems in equilibrium at that speed, not accelerating or decelerating).

For all intents and purposes, if you compare both systems at the same speed, it makes no difference if you run 36V vs 52V.
https://www.ebikes.ca/tools/simulat...b=cust_52_0.2_15&hp_b=0&bopen=true&throt_b=21

 

[gothicsurf]

would the higher voltage pack actually provide 50% more range as shown in your simulation?

 

[serious_sam]

would the higher voltage pack actually provide 50% more range as shown in your simulation?

In that comparison, the 52V pack is the same Ah, but higher voltage. So it has more energy, and therefore more range under the same conditions.

why is the torque line mostly flat for the 36V system?

It's not. The flat torque curve is the 52V system, because that's what the controller is putting out at 21% throttle.

It would be disingenuous to compare both of the full curves directly, since the 36V system is shown with 100% throttle, and the 52V system is shown with only 21% throttle (those settings produce the same top speed of ~30.5km/h). If you really want to compare peak performance for the full speed range, then set both systems to 100% throttle.

 

[gothicsurf]

so basically a wash running this motor at modest speeds -- really no difference between a 36V and 52V system running on a pack of identical Wh?

Would a higher voltage be more useful in a dual motor system even if both also modest 250-300w bafang motors?

 

[Voltron]

It probably is close to a wash. Ultra high efficiency is great and all, but most basic controller and motors are pretty efficient to start with these days. It also seems like with how much power is packed into a modern battery, efficiency for range extension isn't as big a factor any more.
52v has become really common now, and for good reasons, just punchy enough, but not too dangerous and hard to keep insulated, so lots of associated gear choices like batteries, chargers, controllers displays etc that weren't around years ago, so less price and availability negatives that might have made one think twice about upping voltage before now.