36v vs 28v: Same Capacity?

[PeteCress]

Here's a clip from http://toolhacker.com/2010/01/bosch-dewalt-makita-milwaukee-ryobi-ridgid-hilti-battery-specs/

DeWalt

Line: 36V (Nirvana)
Model: DC9360
Chemistry: LiFePO4, nanophosphate
Voltage: 36
Max cell discharge rate: 30C
Capacity: ~2.3 Ah 10S

Line: 28V (Nirvana)
Model: DC9280
Chemistry: LiFePO4, nanophosphate
Voltage: 28
Max cell discharge rate: 30C
Capacity: ~2.3 Ah 8S

What's got my little non-engineering mind going is the apparently-identical storage capacities and maximum delivery rates of the two batteries: both about 2.3 ampere hours and 30 * 2.3 = 69 amps.

Yet the 28v contains fewer cells... Seems logical from a voltage standpoint: stack more, get more volts, stack fewer get less volts... but it seems like the 28v pack would have to contain bigger cells to get the same amount of stored energy into fewer cells.

Am I making sense so far?

If so, is it fair to say that somehow the voltage determines how fast a motor can spin the wheel, but not the amount of energy that can be applied to the road?

If that's true, would it be correct to extrapolate that if, for instance, somebody didn't care to go more than 10 mph under battery power the lower-voltage battery could deliver just as much energy to the wheel as the higher-voltage battery?

Where I'm going with this is that the 28v batteries cost less than half of what the 36v batteries cost - and if they can deliver the same performance when climbing a hill at less than 10 mph and the user doesn't plan on using them on the flats, maybe they're the more rational choice economically.

??

 

[pwbset]

Watt/hrs...

10 cells x 3.65v x 2.3ah = ~84wh
8 cells x 3.65 x 2.3ah = ~67wh

36v will be faster and last longer, but the 28v are still a better price per watt/hr I think.. if you can still get the 28v for like $45 new and shipped = $0.67/wh

Unless I'm not understanding the question, which is entirely possible as I'm not all that bright.

EDIT: Oh.. and the 36v will provide more overall watts before the BMS trips than the 28v will... so, no, the 28v will not deliver as much hill climbing power, but you could series 2 together for 56v etc. etc.

 

[Mark_A_W]

Umm..is this a joke?

The question should be 48v vs 72v: Same Capacity?



Anything less that 48v is not worth the bother, get a roadbike, seriously, 36v is too slow, I started there (now on 60v).


But the answer is: More volts will push more current through. You will go faster, climb harder. It's just better.

2 x 28v in series = 56v would be good.

 

[dogman dan]

This I why I get hassled for talking in terms of amp hours, but for the newbies and folks with less electronics background it's easier to understand. But when you compare a 36v with a 48v then watthours is the way to compare em.

The lower voltage battery will perform the same as the higher voltage battery if the throttle is set to provide the same watts to the motor. So if you ride half throttle at 36v and are going 10 mph up the hill, then the 28v battery would use the same wattage to also go 10 mph up the hill using more throttle.

But in the real world, the 28v battery would not go 10 mph up a hill with your motor, but rather it would go ughhhh and stop before the top.

There are of course, 24v motors that can climb hills. My 24v heinzmann could climb quite steep hills with the advantage of internal gears.

 

[pwbset]

This I why I get hassled for talking in terms of amp hours, but for the newbies and folks with less electronics background it's easier to understand. But when you compare a 36v with a 48v then watthours is the way to compare em.

You know that's interesting. I can't remember when I stopped thinking in terms of anything but "watts", but I think when you've run your ebike on everything from 18v NiCad to 87v LiMn and everything from 1a through 100+a it all boils down to just watts. Plus, for me, it was always easier with concepts I could understand like, "okay I need to light 22 100watt lightbulbs to climb the mountain at 20mph, but I only need to light like 4 bulbs to cruise into town and back fast-like on the flats". :lol:

 

[dogman dan]

My problem is a stubborn resistance to buying a CA. So I don't have a good way to measure watts used. I did spend $25 to measure watts into the charger when I want to. Using that, I can compare the efficiency of a motor or tell if a battery has lost capacity.

Newbies of course don't have a ca, and always ask what ah size to get for a certain motor or ride. Anyway, the question is always how many ah to buy, not how many watthours. But to answer a question like this one, watthours is the way to understand it for sure.

Mabye the battery sellers should stop selling batteries by the ah and start selling them by the watthour.

 

[PeteCress]

.... Anyway, the question is always how many ah to buy, not how many watthours. But to answer a question like this one, watthours is the way to understand it for sure.

Maybye the battery sellers should stop selling batteries by the ah and start selling them by the watthour.

Can somebody explain what an amp hour really is.

Understood it's one amp over a period of one hour.... and an amp is a rate of electrical flow. But what is the practical implication?

I'm pretty sure that watts indicate a rate of work - like horsepower - and watt hours indicate how much work was done over a period of time.

I'm also pretty sure that watts = amps * voltage.... but voltage varies during the use of the battery.

If that's true, what's left for amp hours in terms of tangible characteristics?

 

[TylerDurden]

Amps are current, Ah is how much current for an hour.

Since a battery is a specific voltage, an Ah is the indication of that battery's capacity: it can deliver x amps for an hour.

As you correctly indicate, watt-hours are energy, i.e. how much power (watts) over time.

more here: http://batteryuniversity.com/partone-16a.htm

 

[Ypedal]

Watts are a measure of power

Watt / Hour is a measure of power over time.

So a 2.3 amp hour battery can put out 2.3 amps for 1 hour

or it can put out 4.6 amps for 30 minutes ( 2C )

or 6.9 amps for 20 minutes ( 3C )

Amp hours only apply to batteries.
...

Now when refering to a charger, you don't use amp/hour " ah " as a term.. only it's rated power in amps or watts. ie: a 2 amp charger will provide 2 amps at whatever voltage and yes, V x A = W

When using a chain and gear type setup, lower voltage means less rpm and in turn makes gearing easier.. with direct drive hub motors ( 9C, crystalyte, GM, etc ) voltage is directly related to rpm so more voltage = higher theoretical speeds.

BUT... the speed controller also plays a factor, just like the charger, it is rated in amps or watts. and this determines how much power it can feed the motor.

So a 20 amp controller at 36v = 720w peak

That same controller run at 24v = 480w peak

If your hill is not steep enough to force the controller to limit the amps and keep withing it's set limit, it won't matter what voltage you run, watts are watts.. 200 lbs dude on level ground no pedaling needs about 150 to 300 w at 20 mph ..

confused yet ?

 

[PeteCress]

...
BUT... the speed controller also plays a factor, just like the charger, it is rated in amps or watts. and this determines how much power it can feed the motor.

So a 20 amp controller at 36v = 720w peak

That same controller run at 24v = 480w peak

If your hill is not steep enough to force the controller to limit the amps and keep withing it's set limit, it won't matter what voltage you run, watts are watts.. 200 lbs dude on level ground no pedaling needs about 150 to 300 w at 20 mph ..

confused yet ?

Thanks for humoring me.

Those last few sentences are exactly what I was fishing for.

I still don't have a clear feeling for what ah means to me on the discharge side, but it seems like the number is irrelevant as a measure of how much energy I used during a ride.

215# dude, doesn't give a damn about assist on level ground, able to negotiate the steepest hills drawing 400w or less on his 9C... sounds to me like the cheaper 24v batteries are still contenders.

What I need to do now is

• Quantify the energy capacity (watt hours) diff between a 24 and 36v battery.
• Start noting the CA's Watt Hours number post-ride.

 

[Drunkskunk]

Can somebody explain what an amp hour really is.

Understood it's one amp over a period of one hour.... and an amp is a rate of electrical flow. But what is the practical implication?

I'm pretty sure that watts indicate a rate of work - like horsepower - and watt hours indicate how much work was done over a period of time.

I'm also pretty sure that watts = amps * voltage.... but voltage varies during the use of the battery.

If that's true, what's left for amp hours in terms of tangible characteristics?

Its true. you have a firm grasp on the technicle side. Understanding it is sometimes easier with Analogies.

Spray Whipcream.

think of the Amp hours as the cream in the can. the voltage as the pressure to spray it out, and the wattage as the amount of fun you can have spraying it all over your girlfriend.

If you don't have enough voltage/Pressure. it won't spray very hard. if you don't have enough amps/cream. it won't spray for long. if either is lacking, you'll have less fun spraying your girlfriend.

Amp hours are a measure of quantity. Its expressed as a rate of flow over time, But realy its a quantity. 1 Amp hour is a fixed number of electrons. ((6.25 x 1018 ) X 3600) to be exact. its a damn big number. it has weight. 1 amp hour weighs roughly 0.000,001 grams.

 

[PeteCress]

What I need to do now is

• Quantify the energy capacity (watt hours) diff between a 24 and 36v battery.
• Start noting the CA's Watt Hours number post-ride.

Just got back from a 3-hour ride.

Seems to me like 40 watt-hours is plenty to get me out of the valley and up to my house without resorting to mouth breathing.

Call it 36 and I'm back to the 1 amp hour that I've seen over an over again (at roughly 36v).

Also, checking my watt hours vs ah on the CA during the ride, I got right around 36 watt hours for each ah used... which seems to agree with the math as long as the voltage doesn't sag.

DrunkSkunk's explaination notwithstanding, I'm still grasping for the distinction between watt hours and amp hours.

Everybody seems to treat AH as "gas in the tank": you know how much gas you started with, you note the ah used so far, and then you compute how much gas you have left.

But it seems to me like the amount of "gas in the tank" would be more correctly expressed in watt hours. The AH works bc the voltage remains relatively constant over the discharge curve, but it's technically not the real deal as far as energy available/expended goes.

Seems to me like watt hours is an absolute measure of how much work is done and that the 28v battery has roughly 2.3 * 28 = 64 watt hours in it and the 36v battery has 2.3 * 36 = 83 watt hours - assuming a constant voltage across the discharge curve for each battery.

In dollars per gallon, assuming certain prices for the two batteries:

• For the 28 , that would be $45/64 = 70 cents per watt hour.
• For the 36, that would be $105/83 = $1.25 per watt hour
• For the 36 to compete with the 28 based purely on capacity, it would have tb purchased at .7 * 83 = $58.

The only penalty for using the cheaper battery is that the rider's highest possible speed will be less, but if they never want to go over a certain speed, it's a wash: both batteries will do the job with no difference detectable by the user - except that if they are using the same number of batteries in parallel the 28v array will, of course, run dry sooner than the 36v array will.

OTOH, in two scenarios:

• A single 36v battery at $105
• Two 28v batteries at $45 each = $90

The $90/28v array will outlast the $105/36v single battery.

Am I making any sense at all?

 

[dogman dan]

Yeah the cheaper battery is cheaper, except your bike is designed for 36v( actually about 44v hot off the charger). So you might need a 24v controller to have the thing even work at all on 28v. If the lvc is 30v for instance, it won't even turn on with 28v.

When I was running my aotema brushed motors, they had a controller designed for anything between 24v and 48v. It was kinda cool since I could run the thing on just about anything. With 4 sla's I sometimes ran 24v for economy, but could switch it to 48v for a hill. The low lvc also allowed you to easily kill your sla's by overdischarging them.

When I started with a brushless aotema, I found it would also run at 24v, but I had to pedal up to at least 5 mph before I used the throttle, and absolutely could not climb even the easiest hill. I suspect your motor will, if it runs at all, have equally poor perfromance at 24v with the stock controller. This is why most motors for 24v are gearmotors, so they don't bog down on a hill with such low voltage. If you get a 24v controller, then look for one with more amps, so you will be able to climb a hill.

 

[TylerDurden]

Whr is an expression of energy: whether consumed or stored or required. Energy is typically expressed as Power x Time.

Ah is literally a division of Whr... it is Whr/V. That is expressing Current x Time.

You can think of battery Ah as 'Capacity-at-X-voltage'.

If you compare batteries of different voltage, use Whr to determine energy stored. (as you have done)

If you compare batteries of the same voltage, you can simply compare Ahr for Capacity relative to each other.

Since most systems will specify a particular voltage battery pack, Ah to describe the size of the pack is the simplest way.

 

[PeteCress]

Yeah the cheaper battery is cheaper, except

Last question, I think:

From context I get the idea that the 28v pack uses exactly the same cells as the 36v pack ("M1" ?)... just 8 of them instead of 10.

If that little logical leap is correct it seems like the 28v packs are a much better source for somebody who wants to build their own battery:

$45 / 8 = $5.63 per cell

$110/10 = $11.00 per cell

Or are the cells somehow different?

For awhile, I was all spun up about the Chevy Volt's impending arrival - figured it's battery pack would be veritable gold mine of a123 cells... until I looked at some diagrams and realized the physical implementation was totally different.

 

[dogman dan]

That much price difference make me leery of thinking they are the same cell. They might be the same size cell, but one may be a good quality cell, and the other a cheaper kind with lower c rate. Often battery pack sellers are pretty cadgy about letting you know extactly what's in the package. They seem to really like not telling you the c rate.

 

[PeteCress]

That much price difference make me leery of thinking they are the same cell.

That was my own reaction, so I pulled the trigger couple days ago on a pair of DC9360's.

 

[webfootguy]

That much price difference make me leery of thinking they are the same cell.

That was my own reaction, so I pulled the trigger couple days ago on a pair of DC9360's.

The Dewalt 28v uses *exactly* the same cells as the Dewalt 36v pack. There are just 8 of them instead of 10. If you are going to use the packs intact, then the voltage match with your controller and cost are the overriding concerns.

 

[PeteCress]

The Dewalt 28v uses *exactly* the same cells as the Dewalt 36v pack. There are just 8 of them instead of 10.

But how can they claim 2.3ah capacity for each pack? Either I've got the claim wrong or I do not understand AH properly.

On the 36v packs I just received, they print "79wh" - which I take tb "Capacity=79 watt hours".

If that's correct, then I would expect the 28v packs to have a lower wh figure - confirming my misunderstanding of AH.

But still, on a cost-per-m1-cell basis, if they really do have the same cells, the 28v packs are clearly cheaper.

 

[evblazer]

The Dewalt 28v uses *exactly* the same cells as the Dewalt 36v pack. There are just 8 of them instead of 10.

But how can they claim 2.3ah capacity for each pack? Either I've got the claim wrong or I do not understand AH properly.

On the 36v packs I just received, they print "79wh" - which I take tb "Capacity=79 watt hours".

If that's correct, then I would expect the 28v packs to have a lower wh figure - confirming my misunderstanding of AH.

But still, on a cost-per-m1-cell basis, if they really do have the same cells, the 28v packs are clearly cheaper.

If the capacity is 79 watt hours on the 10 cell pack than it is 63.2 on the 8 cell pack.
Volts * AH = WH
So given a different voltage (8 cells vrs 10) you'll have a different watt hours for the same amp hours.

From a prior post
"10 cells x 3.65v x 2.3ah = ~84wh
8 cells x 3.65 x 2.3ah = ~67wh"

Personally I think someone in marketing set the prices or the beancounters etc.. They aren't selling them at a set materials + labor + Profit % (cost +?) They are selling them for what the market will bear. Therfor the 36 volt battery pack is the high end model and they charge alot more for it. They aren't thinking of us ebike tinkerers who are using them for other purposes or even taking them apart when they set their tool pack prices.
If these were two purpose built ebike packs then I would more question the pricing differences for such a small difference in cell counts and wonder if it were the quality of the cell.

 

[pwbset]

Personally I think someone in marketing set the prices or the beancounters etc..

Totally. It's all marketing indeed. The best way to know is to look at the DC output of the charger to tell the "hot" voltage. Like the Ingersoll Rand IQv 19.2v LiMn is actually 29.4v hot off the charger, but their NiCad says 19.2v so the public is used to it. A Milwaukee 28v is also 29+ topped off. Etc. An 8 cell 28v DeWalt with likely charge to 29.2v or thereabouts. Keep in mind these are all lower once you bleed off the surface charge... suspect a 28V DeWalt would "settle" around ~26v or so.

 

[PeteCress]

Personally I think someone in marketing set the prices or the beancounters etc.. They aren't selling them at a set materials + labor + Profit % (cost +?) They are selling them for what the market will bear.

It's been a looooong time since Marketing 101, but my recollection is "Never price from the inside out."

i.e. Don't price on time/materials. Instead do what you say they are doing: find out what people will pay and price accordingly.