Understanding battery power vs economy??

[richdeloup]

I'm trying to wrap my head around how battery economy equates to lower volts/amps.

I'm putting together a new 72v 10AH pack which I want to run at around 50amps

I understand that it is best to rate all packs in watt hours i.e
36 volts X 20 AH = 720 WH's is the same as
72 volts X 10 AH = 720 WH's

What i'm struggling with is, if i do a 5 mile run at 72v/50amps, would It use twice the WH's as if I did the same 5 mile run at 36v/50amps?

Presuming it does, then how would it work if when running the 72v/50amps, I halved the throttle to go the same speed as the 36v50amp set up? Would I then roughly use the same WH's on both set ups?

Just trying to work out if in the real world, a bike can still be economical whilst still having lots of power available if needed?

 

[Russell]

You only use what you need. Just because you have a 50A controller doesn't mean the motor draws 50A, it'll draw what it needs to meet the load. So say you ride a steady 20 mph you may consume 400W, ride 25 mph and it's more like 750W, at 30 mph about 1200W, at 35 about 1900W and so on.

What you need to do is decide how fast you want to go and look at how many hills you have to climb to determine what kind of power will be required. Then select the combination of motor/controller and battery that will achieve your goals. The best place to play around with different combinations is here;

http://www.ebikes.ca/tools/simulator.html

-R

 

[richdeloup]

Thanks Russell, great that's what I thought.
I've just had a few people telling me not to run more power, as I wont get far but I've always thought that this must depend upon how much throttle I use.
I'm just trying to equate it to driving a car, i.e driving a Ferrari at 20mph would be extremely uneconomical compared to driving a small city car at 20mph? Obviously weight causes some of this but also the engine size and capabilities.
Would having a motor wound for torque rather than speed help with economy at lower speeds? As they are meant to run more efficiently at lower speeds, im guessing that this equates to more economy as well?

 

[Russell]

Power use generally doesn't depend on how the motor is wound as long as you are able to keep the motor running in an efficient band. For example I can ride with a fast wound motor at 36V and achieve the same economy as a slow wound motor at 48V. If I use a fast wound motor at 48V it can however be more difficult to ride at slower speeds as the motor wants to go faster with just a bit of throttle movement (a 3-speed switch on the controller can help) . If you go to the extreme and run a real fast wound motor at a high voltage in a big wheel you'll never go fast enough to be in the motor's efficient operating range and efficiency will suffer. And unless you're climbing Pike's Peak you probably don't want to use a slow wound motor in a 20" wheel, you'd need too much voltage to reach decent speeds, plus you'd probably wheelie the bike from a stop. Personally I find a slower wound motor run at a higher voltage/lower current to be more to my liking, but again it depends on your needs and your specific combination and the terrain you ride. You do have to pin down the parameters of your build first; desired speed, power, bike wheel size, etc. Again the simulator at ebikes.ca can be a big help to sort things out.

-R

 

[danielrlee]

(a 3-speed switch on the controller can help)

+1

For me, this is key to keeping a high speed ebike both usable and economical in most usage scenarios.

 

[melodious]

Understand power addiction too. Took me over a year to lay off 1200+ watts to the wheel. It was such an E-grin to go so fast at first, I think because it was "the new experience" bug.

A wise setup (if your system allows it) is to have multiple power setups. The first being a zero to low power. Why? Reminds you that you are in accordance to the law. Also, if you are approached by authorities in a high power mode, it's a simple on-off-on system reset back to law abiding power. I can let any person ride my bike, hell, it's only 300 watts max. But the wise rider knows how to switch my CAv3's modes and tap into 2000 watts. :wink:

 

[dogman dan]

A three speed switch is a good thing, or a DP CA that you can set to less amps, or feather the throttle on the starts if nothing else. If really wanting to save, pedal the first few feet from a stop.

It's the first 20 feet off the line that you waste your power, if your eventual cruise speed is identical. At cruise, nearly all types of motors will run at nearly identical efficiency. So 20 mph is 400w more or less, no matter what your voltage or amps.

The slow rpm winding motors are definitely more efficient leaving the stop sign, in that first 20 feet. If you ride both, you will see it on the watt meter at start. Amps will drop at a slower speed, while the fast rpm motor will keep pulling hard to a faster rpm. Makes sense, it's more efficient rpm is a higher rpm than the slow motor. By 20 mph both are efficient, but the slow motor gets to it's more efficient rpm at a slower speed. So it stops grunting sooner. I feel they accelerate from a stop much smoother too, if you hear your motor grunting, it's definitely in the inefficient rpm under the load at that moment.

But once you go to 72v, just try to keep speed the same. good luck with that. :wink: Above 30 mph, you blow your wh like mad.

 

[richdeloup]

Thanks guys (where's the ladies on here?) that answers it. A three speed switch and try to keep it around 30mph, above that maybe pedal some to get to 40. I wondered what those things my feet were resting on were called

 

[dogman dan]

You can't pedal to get to 40 for several reasons. You might be able to pedal from 30 to 32.

If you have some very tall gears, as a pro race sprinter does, then you can still pedal at faster speeds. But you won't be Cavendish, nor be able to keep it up at that level for long.

Pedaling really increases range when you can contribute 25-75% of the power needed with your 100-150w of effort. But since 30 mph takes 1000w or so, at that speed its more like 10%.

30 mph is not particularly efficient, needing around 1000w for an upright rider. But 20 mph can be 400w. Each mph more is a huge jump in wattage.

All this is very clear once you start to ride with a watt meter display.

 

[richdeloup]

Arrh okay, thanks Dogman. I presumed that because i can pedal a normal bike at 10mph then I could just add this onto my top speed but obviously not. I am fitting some very tall gearing and have a C/A V3 just come through, time to put it all together then.

 

[arkmundi]

Wind & rolling resistance is not linear, so those higher speeds need more power.

 

[Russell]

Arrh okay, thanks Dogman. I presumed that because i can pedal a normal bike at 10mph then I could just add this onto my top speed but obviously not. I am fitting some very tall gearing and have a C/A V3 just come through, time to put it all together then.

What's interesting is how little extra speed can be gained by pedaling. Here are two plots from the simulator at ebikes.ca using the 72V and 50A as you are proposing. The first is with no pedaling at 29.9 mph, the second is with the same throttle setting and contributing about 200W pedaling effort (more than I can sustain these days ). The difference; 0.8mph and a 12% drop in battery power consumption.




-R

 

[richdeloup]

That is pretty interesting. Here i am putting loads of thought into gearing in order to pedal right up to max speed and it seems not to matter much.

Humm, how about a mini pedal generator to charge as you go. Im sure this isn't feasible as someone would have done it.

On a completely different subject and pardon my novice questions. Mosfets, what is the advantage of having a big bulky 24 fet controller over a small say 6fet which has been modded and reprogrammed for the same volts and amps? Been searching all over but often can't find the answers to the basics! Trying to build a fairly lightweight build and a huge controller always stumps me as where to put it.

 

[Drunkskunk]

A generator takes more power to turn than it produces, so if you could produce 100 watts worth of power with your legs, you might be able to put back 60 to 75 watts into the battery to then go out into the motor, with more efficacy loss, and end up with only 50 watts of your leg power reaching the wheel. Or have skipped the generator and had all your leg power reach the wheel.

Above 30mph, that 100 watts won't mean much. And having your legs flail around in circles to pedal is less aerodynamic than keeping your legs still, so by the time you reach 40mph, you likely will be using more battery power just to push your bike through the air and turn a generator than you would if you just skipped it and let the battery handle it.

As for FET count, it's all about how much power they can handle. Think of a FET like a valve through which the power flows. It's only so big, and can only handle so much power. A modified 6 FET controller should handle 40 watts just fine. But much more than that can cause it to overheat. A 24 FET controller can let 4 times more power through so it will handle 4 times more power.

 

[rscamp]

Yes, the whole idea of the human generating energy that can be used later is a major victim of inefficiencies. To be optimisitic, the generation and consumption conversions might each be 80% efficient. The two conversions used together are then 64% efficient. Pretty dismal!

Here's something that might help with understanding...

Handy terms and formulae for E-Power
Current is measured in Amperes (Amps or A or milliamps or mA)
Output and charging “C” ratings = maximum output and input currents for the battery
Nominal voltage = average discharge voltage (Vnom)
Capacity = Current × Time (Ah)
Power = Current × Voltage (Watts or W)
Energy = Power × Time or Capacity × Vnom (Watt-hours or Wh)
Fuel economy = Energy ÷ Distance (Wh/km or Wh/mi)
Range = Energy available ÷ Fuel economy (km or mi)

Examples:
A 5C discharge battery rated at 10Ah capacity can deliver 50A of current
A battery that can deliver 5A of current for 2 hours has 10Ah capacity
A motor that is consuming 5A of current from a 40V nominal battery is consuming 200W of electrical power
A 10Ah capacity battery with 40V nominal (average) voltage can store 400Wh of energy.
A 400Wh battery used in a vehicle that consumes 10Wh/km has a range of 40km.

If you know your way around these terms, you also really appreciate a device like the Cycle Analyst!

When discussing or describing batteries here and elsewhere, the unit "amps" is often incorrectly used to describe Capacity which is measured in Ampere-hours. The former is a measure of instantaneous electrical current flow and has no meaning relative to the battery 'size' or ability of a battery to store energy. But often someone will say they have a "15 amp" battery or similar to indicate how "big" it is (or how much capacity it has). Literally, this statement means the battery can perhaps deliver 15A of current. What they really mean is the battery has 15Ah of Capacity (the characteristic in bold above).

Even more meaningful with regard to defining the size of an electric "fuel tank" is the Energy storage of a battery. This can be pretty accurately calculated from the product of Capacity and nominal battery pack voltage (Vnom). Capacity alone doesn't give the energy or 'fuel tank' size because voltage is not considered. If two batteries have the same Capacity but one has twice the nominal voltage, one battery actually has twice as much 'fuel in the tank'.

The gas/petrol "fuel economy" analogue for electric power is electrical energy consumed per unit distance. Knowing this and the energy that can be stored in the battery, the range of the electric vehicle can be calculated quite easily and accurately as shown above.

 

[richdeloup]

Thanks for clearing this up for me.
Fet count makes sense and is kind of what i thought. Ive just read about some people using top high quality fets with a low count and getting pretty high currents running through them.
Agreed that a good understanding of battery economy etc is really useful to know and will enable me to stop scrolling through every C/A setting that i don't understand!

The human generator was sort of a joke as ive just come back from a festival where someone was pedaling like mad in order to keep a light bulb going. I was hopeful that generators may have moved on in technology, with some magic internal boost but obviously not
The self charging cell is a very old idea, wonder if it will ever happen?

 

[dogman dan]

Once you start thinking in watts it becomes quite clear. On a long ride, such as more than one hour, most people can pedal up about 100w. 400w might be possible in a sprint, but not for a long time.

So if 30 mph takes 1000w, your 100w pedaling is 10%. Pretty close to that simulator number of 12%.

But drop your speed to 20 mph, and you only need 400w. Suddenly your 100w is 25%. 25% more range IS a big deal. Definitely worth pedaling from about 0-25 mph. At 25mph, its about 16% of the usual total watts.

If you are young and strong, then a 200w effort is possible, and the % gets better. But few of us are really that fit. Before I got sick, I could tap out 150w, but only for about 30 min. I was 53 then.

However, even gearing to pedal 30 mph does something, 10% more range can be that last mile to home. If you can gear 48-11 or 52-11, then you can help the bike reach 30 mph, and help your range. Above 30mph, I find that pedaling just starts high speed wobbles and is not much worth it.

On the other hand, any pedaling with the motor off, or motor running at 100w or less is going to affect range hugely. Being able to pedal at 30 mph on mild downgrade will help range immensely. That is when gearing to pedal 30 mph can really add to your range.

 

[richdeloup]

Thanks dogman, that's a great help