AMPS: 12,24,36volts = most efficient

[Bill Snow]

My last post was a listed as "watts" and I learned a lot from this forum. I am writing this to check on my thinking from some outside information I got, before I really get into this project.The information applies to trolling motors but think it is useful knowledge that can be applied to bikes or trikes as in my case, so, here goes.

12V = 1.1 to 1.2 amps per pounds of thrust.

24v = .85 to .95 " " " " "

36V = .5 to .55 " " " " "

Would this not mean that each amp would have more "punch" the higher the voltage. My understanding is voltage = speed, but would not this also mean that at a low fixed speed a 36v system would have more range per amp than a 24v system ? BUT, using a higher wattage motor at the same fixed speed and voltage, I have been told will absorb more power. Will that cancell out the gain of higher voltage and more efficient use per amp ?

Should the above be correct, then would not a higher voltage system ( 36v, 48v, 72v ) with a lower wattage motor ( 5-600w ) have a much greater range per amp ? Would the range increase be so minor that the additional gain would not be realized do to a much increased weight if SLA batteries are used ? This is probably some pretty basic stuff, but my little mind is still working on the basic, basics.

Batteries: I have been sticking With the SLA battery idea, partly do to cost and problems with the high tech batteries. Those who have a much better understanding of the electrical world than I do are probably more qualified to properly use and care for these batteries than I am.

Tennessee Bass Guides say they put 1000's of hours on electric drive trolling motors each year and can not afford failing batteries. The owner of this business has been doing this for around 40 years, is saying the Interstate deepcycle btteries are the most reliable. That would be good for me, they can be purchased all over the place, at lease in California, Here in Fresno, their is a big distributor with all kinds of shapes and sizes.

Well, there you have it, now I am ready for your thoughts and corrections.

 

[dogman dan]

It seems like you may be blending amps and amp hours together in your mind. Or at least it reads a bit that way.

Say you have a controller and throttle setting that puts out 12 amps.

A 48v battery is run for 1 hour. So if the 48v pack takes the motor 25 mph, you'd go 25 miles.

A 36v battery is run for 1 hour, but 12 amps of 36v is slower, at 20 mph. You'd go 20 miles.

The 36v battery is more efficeint because its slower. Wind resistance makes the 48v setup take more power to go faster using the same amp draw.

For efficiency at the same speed, then things get more complicated and harder to pin down. You have to measure watthours with a good meter to even get started.

But in general, slow is more efficient simply because of wind resistance. Also in general, a motor is more efficient spinning fast compared to very slow and under heavy load. So a 36v system run closer to full thottle may be more efficient at running a certain speed, say 15 mph, than a 48v system running closer to half throttle. But all this depends on the load on the motor. Even the 48v setup at half throttle may be pretty efficeint if the rider pedals enough to reduce the load on the motor. Or it may be inefficient if the rider is poking along towing a huge load on a trailer.

So in a way, the answer to the most efficient voltage depends on how the bike is to be used. The most efficeint voltage for cruising on flat ground may not be the most efficient voltage for climbing a hill with a heavy load. And vice versa. But in general, 36 or 48 volts is the butter zone. A compromise between the extremes of 24v and 72v.

As for the trolling motor example, a heavy trolling battery is no problem in something that floats. The rolling resistance of tires when weight is added punishes rubber wheeled vehicles a lot more. A relatively low power tug can handle huge loads in a barge. Why sla's work so good for the trolling guy is also related to the ease of pushing the weight across water. His motors may be relatively low hp compared to the size of the battery. A bike, needing the battery to be smaller, has a bigger motor relative to the battery. This leads to high discharge rates that are hard on small sla's.

Lastly I don't think there is a "better punch per amp" at any voltage. Really its all watts, or watthours. Each watt , at any voltage is just one watt, and can do just one watts work. Efficiency differences arise when things happen that cause the watt to be turned into motoion, or into heat. When you make heat, you aren't getting motion. An example is climbing a big hill. When the motor is loaded to the max like that, climbing the hill, it makes more heat for sure. On flat ground, the motor can cruise, drawing less watts per hour, and makes less heat.

 

[spinningmagnets]

Higher volts are more efficient (better range per a given battery size)

Its true that Watts is Volts X Amps, so...480W is 24V X 20A, or, 48V X 10A

But in the real world, lots of amps turn your controller and motor into an "air heater". If you have ever been drilling into some wood with a cordless drill, you may have noticed that when the bit starts to bog down, the drill begins drawing lots of amps to keep the RPM's up, and gets very hot (will also deplete battery very fast).

Many years of electrical engineering has determined that around 3,000-RPM's is near the sweet spot concerning efficiency with electric motors. A motor can be designed to get its "best" efficiency at a lower RPM, but it will still be less efficient than a higher RPM motor. The benefit in that situation may be the elimination of the cost and complexity of a gear-down stage or two (bicycle wheels turn at ~100-300 RPMs).

A small direct-drive hub motor has poor efficiency (Clyte 408?), but it will remain the simplest and least expensive E-bike option. A 5:1 geared hub (such as a BMC, Bafang, eZee) provides 1,000 motor RPM's per 200 wheel-RPMs, so...better efficiency.

Matts RC dual-stage drive is about 10:1, so it provides 2,000 motor RPMs per 200 wheel-RPMs. I would use that if I was in a range-per-battery competition. Even with the added friction of a dual-stage, the increased efficiency would be a net positive (and also at higher volts, lower amps).

 

[Bill Snow]

Yes, I think you are right about blending amps and amp hours. 3000 rpm motor speed, higher voltage and of course, wind resistance. Thank you for pointing this out, It will help me out on the system I settle on.

I can see the type of riding and the area ( hills or flatland ) all comes into the picture.

 

[dogman dan]

Still don't understand 90% of it myself. But the amps vs amp hours gets confusing at first. I like to compare it to a bucket with a hole in the bottom. Amps is the rate the water flows out the hole. Amp hours is the size of the bucket. C rate is a limit on the size the hole can be, or how fast the water could be allowed to flow out the hole. A 10 ah battery would have a 1c limit of 10 amps, a 2 c limit of 20 amps. A 20 ah battery would be 1c-20 amps, 2c-40 amps.

 

[davespicer]

I like to compare it to a bucket with a hole in the bottom...

If I could try to extend the comparison a little - imagine trying to take a shower under the bucket, and look at voltage as being how high the water is in the bucket. So a big stream of water from a shallow bucket would pour out but not very forcefully. That'd be a lot of amps at low voltage. On the other hand, a small stream from a very tall bucket would be a stinging jet that didn't cover much area. That'd be low amps at high voltage.

Now, how quick a shower... um, I mean, how quick an ebike trip... would you like to take? And how dirty are you (how steep are the hills?) The best combination of water pressure and flow - of volts and amps - depends on how fast and how thorough a shower you need. And how big the bucket (battery pack) is.

There we go, something for us visual thinkers. Or maybe just something to make matters more confusing, which would mean that I'm, um... all wet

 

[TylerDurden]

Actually, as voltage increases, more 'punch' per amp is not a total misunderstanding... punch=watts. Watts are power, so an amp of higher voltage has more power than an amp of lower voltage. (amps x volts = watts)

Power delivered over time... that's energy, usually measured in watt-hours (Whr).

Since each battery has a known voltage, its capacity (stored energy) is usually expressed in amp-hours (Ahr).

Ahr x Volts = Whrs.


So... Let's take a bite at a time:

My understanding is voltage = speed, but would not this also mean that at a low fixed speed a 36v system would have more range per amp than a 24v system ?

All other things being equal, yes... a 10Ah36V battery has more energy than a 10Ah24V battery. Use the controller to limit speed to get more range.

BUT, using a higher wattage motor at the same fixed speed and voltage, I have been told will absorb more power. Will that cancell out the gain of higher voltage and more efficient use per amp ?

Not really. A higher rated motor can handle more watts, but you don't have to use that ability. A controller allows you to determine how few or how many watts the motor will get.

Should the above be correct, then would not a higher voltage system ( 36v, 48v, 72v ) with a lower wattage motor ( 5-600w ) have a much greater range per amp?

A motor's rating is typically its max load continuous, not usually the peak power draw. If you limit the power draw (watts) by lower throttle settings (or by current limiting), a higher voltage battery will run longer than a lower voltage battery of the same capacity (Ahr).

Tennessee Bass Guides say they put 1000's of hours on electric drive trolling motors each year and can not afford failing batteries. The owner of this business has been doing this for around 40 years, is saying the Interstate deepcycle btteries are the most reliable.

Deep cycle are preferable, if using PbA (lead-acid) batteries; they have thicker plates that give them longer life. Flooded cells are more common in golfcarts and large EVs, where rollover potential is less likely. For small EVs, scooters, bikes, etc., Sealed Lead Acid (SLA) batteries that have gelled electrolyte are recommended (spilled acid is nasty). SLAs are also called 'unspillable' batteries; some SLAs are AGM (Absorbed Glass Mat). Lead batteries are heavy, have comparatively short lifespans and exhibit major voltage sag under load (Peukert effect)... newer chemistries are about as easy to use and have fewer drawbacks for most applications.


FWIW,

3,000rpm is common for some electric motor applications; but a motor can be built to operate at rated load at just about any rpm. Different application types use different motor strategies where certain rpms are typical.

The bucket analogy could be further extended...
re: exceeding a battery's C-rating:
A situation where the motor draws more current (amps) than the battery capacity can safely supply might be more like a transfer-pump drawing fluid from a vented tank: if the pump draw exceeds the ability for the tank to supply it, the tank walls can collapse or crack and be permanently damaged. When batteries are discharged with amps beyond their c-rating, they usually get hot... and if they don't burn up, they will later have reduced capacity and a greater risk of overdischarge in the future. The same can happen when batteries are discharged beyond a percentage of their total Ahr capacity. Most batteries will not tolerate 100% depth-of-discharge (DoD). The C-rating and optimal DoD are determined by battery chemistry and construction.

 

[Bill Snow]

It is swell the way you are explaining the use of volts, amps, amp hours, watts and watt hours in a way for me to understand. I think you know I appreciate it.

So, on a bicycle or in my case a Terra recumbent trike, higher voltage ( 36 or 48v) with a 1000-1500w motor is not a bad thing at a 15- 20 mph speed and the motor is turning slowly ( maybe 1000 rpm, I am thinking of the ideal 3000 rpm ) but want the more power and speed there if needed. Range at these speeds would much greater with a 48v 12ah, 1000w system than a 24v 12ah, 450w system. It also seems that motor design and controller ability with good quality is important to tolerate this kind of useage.

I understand why the SLA batteries over marine deep cell and the reason for the more advanced batteries. Weight, 48v SLA battery can be pretty heavy. As pointed out, "this is not a fishing boat", and weight means a lot.

Clearly explaining the above gives me some pretty good basics to work from.

Best Regards

Bill Snow