700w at 24v, Pros and Cons ?!?

[Azzi]

Looking thru Ebay for a cheap controller and I came across this Guy/Item.

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=180363768666

700w is a nice amount of power, but its unusual (for me) to see one at only 24v. I would have expected it to be atleast 36v and perhaps even 48v.

Now, the obvious thing that jumps to mind is that you can get a 24v Battery pack for less money, take up less space and a substancial weight loss than running a 36/48v power setup.
So to me , it looks like a bargain with no drawbacks ........


......... So what am I missing? If it looks to good to be true, it normally is.

Why go for a higher supply voltage if the Hub power rating is the same ?!?

Please Discuss, I'm seriously tempted to press the Buy It Now button .

 

[vanilla ice]

I'm guessing its a 24v 28a controller, that adds up to 700. But if thats the case its input power.. not what you get at the wheel. For example my "450" currie controller is about that.. 24v x 28a. The motor has some say over how many watts get drawn also, but I'm not real clear on that. And certain motors work out better one way or the other with regard to voltage/gearing..

Anyway theres been tons of discussion on this if you search. IIRC there were even some interesting comparison graphs using simulators- 48v@15a vs. 24v@30a for example. IMO for a given wattage its better to go with higher voltage and less amps.

 

[Azzi]

I had a wee search before i posted, but this site has so much information and unfortunatly a lot of clever guys.
I find my brain just turning off trying to understand half of what is being discussed. :?

I'll have another Dig about and see if i can find the topic.

Thanks

 

[vanilla ice]

I'm an ICE guy so I tend to think of it like this- Amps is cubic inches, voltage is rpm. For a car with a tractor-like 2500cc block that can't rev past 2000 rpm chances are your accel isn't going to be that great. Same for a 20,000 rpm 250cc car. But when you have both matched up pretty good, say a car with an F1'er spec 2.5 that goes 20k rpm, now you can really move.. Ok, not a great example, but should help the ICE guys a bit.

 

[deardancer3]

Somethings to ponder considering your post:

Watts is amps times volts, so the lower the volts, the higher the amps. high watts at low volts means high amps.

High amperage wires and connectors are more problematic (especially in high vibration products) than lower amperage circuits.

Good HIgher amperage batteries are harder to find and are more expsenive; sometimes you have to parallel two packs and have diode isolation and charging issues.

48 volts is much nicer for 700 watts. 15 amp wires and connectors, 15 amp rated batteries, just no big deal, common easy to find almost anywhere, quality of construction not as critical. wires are smaller, cost less and are easier to route and hide. 30amp batteries, connectors, wires not so easy and cheap.

two identical ebike trips will take x watt hours no matter the voltage. so if you are at lower volts, you just need more amp hours; watt hours about the same, so we measure trip energy used in watt hours.


At peak load- The controller FET's only needing to handle 20 amps (at 48v) is easier on the controller than 40 amps (at 24v) thru the FET's.


and the real kicker, the higher the voltage, usually the higher the motor speed and your top speed.

hope this helps

d

 

[tostino]

So 24v and 48v in theory could be the same efficiency and go the same speed, and produce the same power. However, since heat is a product of amperage, without bigger wires you will lose more energy to heat in the wires on 24v 40a than 48v 20a. Same goes for motor construction, it is almost always better to have higher volts lower amps because it produces less heat.

 

[dogman dan]

Sometimes what really matters is the watts the thing puts out in cruise mode, usually half what the calculated "potential " total is.

In practical terms, using a 35 amp brushed controller on a WE hub, 24v = 15 mph 36v= 24 mph 48v= 27 mph

Harder to quantify is the torque, but the 24v could barely start itself, while the 48v felt like why pedal at all? 36v is the best compromise, still close to street legal in 20 mph jurisdictions without speed limiters but still pretty usefull on hills. 48v is much nicer, the motors feel better, smoother running, etc, if cops are ignoring you where you live.

Definitely a 48v battery supplying a 700 watt draw is under less strain and will last longer a 24v, unless the 24v pack is twice the ah of the 48v.

That is, if the packs have the same number and type of cells and are the same size physically. The 48v pack would be half the ah of the 24v but the strain on each cell would be the same. The same ah size 48v and 24v packs would have less cells in the 24v pack.

 

[rkosiorek]

another consideration is that the higher the AMPs the higher the resistive losses. for example. just for instance you are using a 4ft. loop of 14awg wire to connect the battery to the controller. this wire will have a resistance of about 0.02 ohms. we will only consider the 700W as power drawn from the battery. at 48V that works out to 700W / 48V = 14.6A. with that current th voltage drop will be 0.3V. at 24V the same 700W will draw 29.2A and drop 0.6V in the wire.

it doesn't sound like much does it? but only loosing 0.3V out of 48 means that 99.4% of the power is still being delivered to the controller/motor. for the 24V system a loss of 0.6V means that only 97.5% of the power is delivered. that is an increased loss of 1.9%. and we are only looking at one wire. to equalize the losses the 24V system needs much heavier (more expensive) wire.

you also need much heavier batteries. finding batteries you can drain at a 14 or 15A rate is simpler than ones that can be drained at 30A.

the other trade off is that lower voltages are inherently safer. you need volts to overcome resistance. it is a lot easier to deliver a fatal shock at 100V than it is at 24V. from personal experience 24V gves a nice sharp tingle. 48V will cause muscles to jerk. 72V will cause a VIOLENT muscle contraction and some burns. i havent tried 100V yet but i don't want to.

as with everything it is a trade off.

rick

 

[liveforphysics]

As far as battery cost is concerned, you need to look at it in watt/hrs. You would need a pair of 24v 10Ah packs to get the energy of a single 48v 10Ah pack.

The higher voltage system becomes more efficient in every part of the system, controller heating, wire losses, voltage drop at the battery, heating in the motor, limited top speed etc.

As far as a real world perspective goes, Dogman put it pretty well.

 

[morph999]

24v is weak. I run my bike at 60v and it's still kind of weak.

 

[OneWayTraffic]

24v is weak. I run my bike at 60v and it's still kind of weak.

I like this.

Define 'weak.'

Weak compared to the motorbike I used to ride?

Or weak compared to the 3-speed non motorized push bike I ride now?

 

[vanilla ice]

physics brought up the most valid point for the OP.. Seems like OP wants low voltage to save bat weight. Like live4phys says, it doesn't work that way. You will need the same amount of battery for half voltage 2x amps, just wired differently.

Higher voltage is also nice for the gearing advantages, just like a high rpm ICE motor is.. torque is nice and all, but power=accel

 

[paultrafalgar]

...
As far as a real world perspective goes, Dogman put it pretty well.

He's right!

 

[64ragtop]

Someone noted that higher watts at lower volts calls for higher current. This translates into needing more AMP-HOURS of battery capacity for the same range, thus more battery WEIGHT. I'd go for higher voltage and keep my fingers out of it! But then I'm the coward that used to work on 5 to eight THOUSAND volt power supplies. The motto in the trade was: There are OLD transmitter guys and there are BOLD transmitter guys - - but there are NO OLD BOLD transmitter guys

P.S. there were darn few transmitter gals back then - but I'm dating myself

ATB

BC

 

[davespicer]

P.S. there were darn few transmitter gals back then - but I'm dating myself

This may be a little OT but reminds me of what documentation and manufacturing errors can do:

In high school (class of '65, speaking of dating oneself...) a buddy of mine and I each had our amateur radio "tickets" (licenses). He chose a 60-watt Knight-Kit from Allied Radio for a transmitter and assembled it, but it wouldn't work properly. Turned out the directions in the manual for tuning it were for one type of meter (plate current) while the actual circuit had an RF power output meter. So tuning for a "dip" on the meter at one step in the process was exactly the wrong thing to do... think the output tube actually glowed red. Allied Radio had stuck a little slip of paper, easy to overlook, into the documentation which corrected the instructions.

Later on (late '70s) I bought a "Sears' Best" set of jumper cables. They looked really good - nice thick stranded wire. But boy did the sparks fly when I tried using them! Turned out that the clamps on one end were reversed - the red was put on the negative cable and the black on the positive. The folks at Sears were amazed. (That was the only experience I've had with spot-welding BTW... )

I'm reminded of things like this when we ebikers run into wire color mismatches, etc. in kits...

 

[jag]

Please Discuss, I'm seriously tempted to press the Buy It Now button .

More important than 24 or 36V is perhaps how good this kit and vendor is. Numerous kits have been reviewed in the various threads here. Going with one of the kits known to work well and having good after purchase support might be the best plan. (b.t.w. while the purchase price looks attractive, the freight charge from hk is rather steep.