36V to 72V switch (range vs power)

I have seen suggestions about building a switch to go from 36V for those times when distance is a priority and 72V when speed is more of an interest.

Can someone explain how this might work or whether it has already been done?

I'm leaning towards a Clyte 504 with a 72V battery pack (2X36V 12 or 18Ah ) and it seems like an idea worth exploring if it makes sense.

Greg

There's been quite a bit of discussion and at least one thread here dedicated to this topic of a series/parallel switch. I searched but couldn't find it.

In short, switching for example between 72volts 10ah and 36 volts 20 ah won't buy you much if any increased range because either way it's still a 720 watt-hour battery pack and on any given system it takes X watts to travel Y speed. For example, going 20mph with 72volts will take roughly 400 watts, which means 400W/72volts = 5.5 amps. Going 20mph with 36 volts will take 400W/36volts = 11 amps.

There is a small efficiency gain from the controller input voltage more closely matching the controller average output voltage, but in practice the gain is too small to make a parallel/series switch worthwhile for most people.

One good reason to implement such a switch is if you want finer throttle control at low speeds. At 72 volts, for example, half throttle might accelerate you to 20mph, whereas with 36 volts, full throttle will accelerate you to 20mph. Hence, low speed throttle control is better. But this hasn't been an issue for me or most other people with 72V+ systems.
Another reason may be if you're "heavy handed" on the throttle, and want to constrain yourself to a lower speed limit because holding only partial-throttle becomes too tiring, or it's too tempting to flick full-throttle.

I have a 10S15P lithium pack on my back rack, wired in series to a 10S15P lithium pack on my midframe. If I wanted I could quickly disconnect and reconnect the wires between, or add a little relay box, to make the entire pack 10S30P instead of 20S15P. Just not enough reason for me to do so, though.

I think its been done a few times. a simple mechanicel switch would do the trick.

But there isn't much advantage. All running at 36v will do is force you to run at slower, more efficent speeds. But running at those same speeds at 72vwould be essentualy the same. *1* As for extending the range, your range is watt dependant, not amp dependant, and regardless of how you configure the batteries, they will still have the same total watt hour rating.

2, 36v 10ah batteries in paralel makes a 36v 20AH, 720 watt hour battery.

2, 36v 10ah batteries in series makes a 72v 10ah, 720 watt hour battery.


A far less complicated, and potentialy more usefull device would be a limiter on the throttle, to keep you from going to fast when you want the range.






*1* Actualy, there is a small increase in efficancy for most motors to be run at higher voltage

I think it may have been Buzz on ye old V forums, had made a 36/72v relay thing, worked out the kinks too if I recall properly. I'm interested in 36/72v switch for my 409 bike that's coming together, the idea would be to have a 4th brake for descents. Flip it in 36v and gun the throttle... It could work. Maybe 24/72v would work better, though.

I use one on my bike.. Apart from the extra efficiency achieved running slower, also the battery is leaded only half as much, meaning you lose less energy to Mr Peaukert. At 30A in series my pack sags as much as 12v (6v per 36v pack), but in parallel only 2v. 8v@13Ah more is lost in series...a whopping 104WH, or 4.6miles (@ 25wh/m).

Here's a diagram (given me by another user)

I didn't find any significant difference between lowering the supply voltage to the controller vs. using the DrainBrain speed limiter and turning the current limit down. There should be a small gain due to less controller losses though.

Apart from the extra efficiency achieved running slower, also the battery is leaded only half as much, meaning you lose less energy to Mr Peaukert.

Click to expand...

Not true. At the same speed, amp-draw is one-half at double voltage. Hence this method can't fool Mr. Peukert.

Thanks to all.

I guess I misunderstood the application.

Which when combined with my lack of electrical skill added up to a faulty premis.

Thanks for redirecting me.

Greg

F.Y.I , the relay i am using is for switching a dual speed motor, using a single speed controller. not for 36/72v switching. :wink:

[/quote]
Not true. At the same speed, amp-draw is one-half at double voltage. Hence this method can't fool Mr. Peukert.[/quote]
At a steady cruising speed this is true, but to accelerate to that speed you will likely use all available power, drawing full power from the batts whilst the larger part of the power you are going to consume is used (accelerating). The hardest use on the batteries will be use at full amps either way (in my case 30A)...but the parralel pack will fare better.
Having only half the power available at half the voltage enforces slower acceleration of course....this is the main reason why range is gained.
*rambles on..* you get my point

Jozzer said:

*rambles on..* you get my point

Click to expand...

Yes I do. We agree we agree...mostly. There's also the offsetting matter of decreased resistance with increased voltage.

So then, it's settled -- nobody disagrees with Xter?

1) I need not bother with a switch circuit to go from 36v to 72v because I will not gain range? I might as well just have a 72v system. If I go a bit easy on the throttle when accelerating i will have same range as a 36v system -- just confirming.

2) What would be my best controller for a Clyte 5304 at 72v?

3) As long as we're on controllers -- what is the best controller for 36v Heinzman?

4) Can a Heinzman take 72v? If so, what is the best controller for it?

Thanks,
Al

I think Skunk nailed it: Fechter's current limiter to keep you from speeding your range away.

Works for me in the car... I get much better fuel economy by slowing down and using cruise-control on the highway. (politely drafting trucks helps too 8) )