48v 11ah@20mph vs 36v 15ah@20mph

[Blueshift]

I plan on using a cycle analyst to cap my speed at 20mph. Which setup will give better range?. I was thinking about going 48v because wind and hills are causing me to slow down too much. What would you pick if you capped your speed to 20 mph?

 

[ulrichw]

Others are sure to give you better answers, but just wanted to point out that the 36V 15 Ah battery has 540 Watt-hours to the 48V 11 Ah battery's 528. So assuming the controller/motor are well-matched to the 36V battery, that's the one that should give you the longer range.

I think you're missing a lot of the information needed to answer your question (e.g., what motor/controller are you talking about?) - but someone with more experience should prompt you on those.

 

[nutnspecial]

Hmmm I don't know either . . .

I'd have thought it's the one that runs closer to 90% max rpm (without going over) @ 20mph. I don't see how higher 'gearing' (more volts) can help a whole lot @ 20mph in place of some more amps for hills and wind. Long run is the motor is eating more watts running at more inef ficient speeds, not to mention starts/stops.

 

[Sunder]

I plan on using a cycle analyst to cap my speed at 20mph. Which setup will give better range?. I was thinking about going 48v because wind and hills are causing me to slow down too much. What would you pick if you capped your speed to 20 mph?

Not a simple question, but the simplest answer is 36v @ 15a

Why I say this is that motors are most efficient near their unloaded speed. So if @ 36v, your motor goes 30km/h unloaded, then at 48v, the unload speed is 40km/h. If you govern both to 20km/h, then the 36v motor is operating at 66% speed, and the 48v motor is operating at 50% speed.

Of course you need to consider loss through the copper, as well as through the various motor parts (windings, stator, etc), but as a broad rule, I'd say go the lower voltage to get the governed speed closer to its no-load speed.

 

[Rassy]

I would always opt for the 48V system, unless you are trying for a minimal battery pack. You never have to use all the speed/acceleration you have available, but if you get the 36V system you may end up with less speed/acceleration then you desire under some conditions. As far as range, you will use the same Watt Hours on either system if you ride at the same speeds.

I just read Sunder's post after typing the above, and I agree, except I always think in terms of a mid drive now, such as a BBS02, where you can select the gearing that lets your motor run near its sweet spot at any speed.

 

[BikeFanatic]

48 volts because 36 volts just isnt enough most of the time, especially with voltage sag.

 

[Sunder]

48 volts because 36 volts just isnt enough most of the time, especially with voltage sag.

Eh? To cruise at 20mph, you only need 200-250w depending on profile and tyres. That's only 7a, or 1/2C on his 15Ah suggested battery size

Even going up stupidly steep 10% grades without slowing, you need about 1000w, or about 2C. Even LiFePo4 and cheap 18650s can survive that for a short time (and no hill lasts forever)

If you're getting voltage sag at those levels, all I can say is "AA pencil batteries weren't designed for eBikes".

Seriously though, people seem to forget that the relationship between speed and power is exponential. Your comments would be very valid at 30mph+, but 20mph is a very easy target to hit with an electric bike.

 

[neptronix]

36v x 15ah = 540 watt hours.
48v x 11ah = 528 watt hours.

There ya go.. your dilemma is solved using calc.exe..

 

[neptronix]

If your bike is getting bogged down, but can normally maintain 20mph.. you can increase the amps to increase the watts, rather than buying a new battery to increase the volts to increase the watts.

 

[d8veh]

The voltage you choose depends on the maximum speed of the motor. If you have a 36v motor that can spin up to at least 23 mph and you want to cap it at 20 mph, 48v would be a bad choice. On the other hand, if it maxes out at 19 mph at 36v, then 48v would be better.

A bike with a 48v battery and 15 amp controller and 201 rpm motor will give the same speed and power as a 260 rpm 36v one at 20A.

There's three important characteristics that you need to know to make your choice:

Motor maximum RPM
Controller current (can be adjusted up to a point, say +/- 30%)
Battery voltage