jonescg
100 MW
Both are important, but if you really had to rank them, I'd say capacity. You can adjust the throttle all you want to change the discharge rate, but you can't do much about your range unless you increase capacity.
which is important? capacity or discharge rate ?
- Thread starter jonescg
- Start date
spinningmagnets
100 TW
A Direct-Drive hub on a hilly commute will have a high amp-draw when starting from a stop. This is why dogman and others recommend at least a 15-Ah pack for a fast-winding 9C, a larger pack makes the amp-draw easier on the pack regardless of the battery-chemistry C-rate, so it will last longer and gives you more of its actual capacity. A flat commute with a geared hub might be fine with a 10-Ah LiFePO4 pack. For LiPo, even the lowest C-rate available is more than enough to not worry about C-rate and pack size.
If you buy high-quality LiFePO4 cells with a higher C-rate (A123, Samsung?), you can get away with using a smaller pack, but...if you can afford it, a larger pack eliminates the problems with a multitude of deficiencies.
If you buy high-quality LiFePO4 cells with a higher C-rate (A123, Samsung?), you can get away with using a smaller pack, but...if you can afford it, a larger pack eliminates the problems with a multitude of deficiencies.
jonescg
100 MW
How to get both?
Spend more money! :lol: That's literally all it is. You can get cells which have the capacity and deliver the amps AND don't weigh a ton, but they cost more.
Spend more money! :lol: That's literally all it is. You can get cells which have the capacity and deliver the amps AND don't weigh a ton, but they cost more.
Ypedal
100 TW
Discharge rate is directly linked to Internal Resistance " Ri "
Depending on your motor/controller and conditions, you need a battery with a high enough discharge rate so that the voltage does not sag too much causing excess heat.
Capacity means having enough to make it to your destination.
Depending on your motor/controller and conditions, you need a battery with a high enough discharge rate so that the voltage does not sag too much causing excess heat.
Capacity means having enough to make it to your destination.
etriker
100 kW
ice_robin said:
Make a battery pack that will fit your needs.
That depends on how far you want the bike to go and how fast you want to go around where you will be driving it.
If you live around a lot of steep hills then you may need a setup that will only go about 10mph steep up hill and some real good brakes for going down hill.
You can make a small test pack out of A123 cells and use a watts up meter to tell you what your power needs are.
Most of us use the power on and off when riding so you really don't need to run the battery full blast.
I use a custom battery pack built for the riding I do to go the places I need to go.
A combo of a 123 cells for high discharge rate mixed with laptop cells for capacity works for me.
Here is a Panasonic answer
http://www.orbtronic.com/batteries-chargers/imr-panasonic-cgr18650ch-10a-18650-rechargeable-battery-cell-high-drain
wesnewell
100 GW
Capacity is only important for distance. Discharge rate is what will determine if you ruin your pack. So imo, discharge rate (C rate) is 1000 times more important. C rate times the ah rating of the battery determines how many amps you can safely draw from the battery. How much you draw is determined by your controllers max amp rating, and not it's wattage rating. Don't think that if you have a 48V 800W controller that it will only draw 800/48=16.7A or you may find yourself with a dead battery quick.
The Mighty Volt
1 MW
Big capacity and big voltage over C rate any day of the week.
20Ah of shitty 2C LiFe gives you an easy 20Amps, with no risk to the battery.
Now 20Amps isn' shit.....but at 72v in a small wheel, it is plenty.
20Ah of shitty 2C LiFe gives you an easy 20Amps, with no risk to the battery.
Now 20Amps isn' shit.....but at 72v in a small wheel, it is plenty.
dogman dan
1 PW
The key thing is to have enough of both.
For example, take a typical ebike kit sold in the USA. They come with a fairly big direct drive motor, and a 20 amp controller.
So for starters, you need a battery that can put out about 30 amps. ( a 20 amp controller will briefly allow 25-30 amps) So knowing that, if your discharge rate is 1c, you need a 30 ah capacity to make 30 amps. If your discharge rate is 2c then you only need half that, 15 ah x2 will do it. Now you can chart the capacity you need to have enough amps.
1c 30 ah
2c 15 ah
3c 10 ah, and so on.
But now you have another question to answer, how far will I get on 10ah? For 36v, if you travel about 22 mph you will need about 1 ah per mile. Some days more, some days less depending on wind.
So if you need to ride 20 miles per charge, you want a 20 ah battery.
Cut short on amps, you kill your battery. Cut short on capacity, you run out before you get there. So you have to have the one that gets you both. My usualy recomendation for a typical kit with a 20 amp battery is a 36v 15 ah, or a 36v 20 ah lifepo4. Usually these are 2c cells, so 15 ah is the smallest recomended size.
Take another example. Here in the USA, we like to build bikes that go really fast. So a 40-60 amp controller is common. Which needs a battery able to put out about 60-80 amps. So now a 20c battery might be considered a bit weak. To hot rod, you want as close to no voltage sag as you can get. So it might be worth the money to buy batteries with a c rate close to the amp rating of the controller. The more you overkill it, the better, especially if in a race. You don't want to carry a 50 ah battery to have enough amps with 2c cells, so you get something where only 10 amp hours will provide 80 amps or more.
For example, take a typical ebike kit sold in the USA. They come with a fairly big direct drive motor, and a 20 amp controller.
So for starters, you need a battery that can put out about 30 amps. ( a 20 amp controller will briefly allow 25-30 amps) So knowing that, if your discharge rate is 1c, you need a 30 ah capacity to make 30 amps. If your discharge rate is 2c then you only need half that, 15 ah x2 will do it. Now you can chart the capacity you need to have enough amps.
1c 30 ah
2c 15 ah
3c 10 ah, and so on.
But now you have another question to answer, how far will I get on 10ah? For 36v, if you travel about 22 mph you will need about 1 ah per mile. Some days more, some days less depending on wind.
So if you need to ride 20 miles per charge, you want a 20 ah battery.
Cut short on amps, you kill your battery. Cut short on capacity, you run out before you get there. So you have to have the one that gets you both. My usualy recomendation for a typical kit with a 20 amp battery is a 36v 15 ah, or a 36v 20 ah lifepo4. Usually these are 2c cells, so 15 ah is the smallest recomended size.
Take another example. Here in the USA, we like to build bikes that go really fast. So a 40-60 amp controller is common. Which needs a battery able to put out about 60-80 amps. So now a 20c battery might be considered a bit weak. To hot rod, you want as close to no voltage sag as you can get. So it might be worth the money to buy batteries with a c rate close to the amp rating of the controller. The more you overkill it, the better, especially if in a race. You don't want to carry a 50 ah battery to have enough amps with 2c cells, so you get something where only 10 amp hours will provide 80 amps or more.
Russell
1 MW
ice_robin said:
Those aren't the questions you should be pondering.
The real questions YOU need to answer are:
1) How far do you want to go?
2) How fast do yo want to go?
3) How many hills do you need to climb?
Once you answer those questions you will have a general idea of the capacity you will need.
Then you need to decide:
4) Is there a particular voltage you want to use?
5) What motor type and winding will you use?
6) What is the maximum current draw?
Once you answer those questions you will have a better handle on the discharge or 'C' rate you will need.
But you still have to decide what characteristics are most important to you in a battery:
1) Safety?
2) Weight?
3) Size?
4) Charging/balancing?
5) Cost?
You see there are a LOT of things to consider when choosing a battery so FIRST you need to lay out the parameters of your build.
The short answer however is; you want enough capacity to go the distance and enough 'C' rate to not overly stress the battery.
BTW, My recent battery purchase was based on the following top priorites:
I wanted a small and lightweight battery.
I wanted around 48V
I didn't want to spend a lot
I wanted a range of at least 30 miles w/moderate+ pedaling on one bike (17-19 mph and 10-12 Wh/mi) and at least 16 miles at around 20 mph on the other with minimal pedaling (up to 20 wh/mi).
My controllers are 20A and 27A peak
...I chose 8Ah of 12S LiPo which weighs 5 pounds and gives me a maximum of 360 Watt-hours of capacity.
-R
wesnewell
100 GW
20ah 48V pack should do fine. A123 is lifepo4, just with higher C rates. The more ah the battery, the less the C rate matters since C rate times AH = amp draw. The motor rating doesn't matter. It's the controller that determines the amp draw. You need to know the max amp rating of the controller to determine the minimum C rate you need. Without that spec, anyone would just be guessing. That said, most 48V 500w kits will come with a 30A or less controller. So as long as you get at least a 15ah pack with a 2C rate, you'll be fine amp wise. You can get 30 miles with 15ah pack if you ride slower and/or pedal assist. But if you plan on going wot most of the way, then get a higher ah pack.ice_robin said:
Chalo
100 TW
The Mighty Volt said:
But a small wheel gives a terrible ride (plus poor traction, high rolling resistance, poor selection of tires, etc.) that you can do nothing to fix. So it's a tradeoff.
I'd accept the tradeoffs of running high amperage for the benefits of large diameter wheels every time.
Chalo
dogman dan
1 PW
Now we are getting somewhere. 30 miles, at the top speed of a 48v direct drive hubmotor.
It will take about 25 ah, but slowing down just a bit to about 25 mph will get you to where you make it on 20 ah of 48v lifepo4. Lower c rate lifepo4 such as typical pouch cell packs that discharge at 2c, will be good enough for your 500w controller.
No particular need for a123's, unless you want to ride really fast and get a stronger controller with more volts. If you speed up much, you won't double your cost, you'll triple it. 30 miles at 40 mph will take a huge battery! So stick to 48v. The faster you go, the faster you suck down that battery.
It will take about 25 ah, but slowing down just a bit to about 25 mph will get you to where you make it on 20 ah of 48v lifepo4. Lower c rate lifepo4 such as typical pouch cell packs that discharge at 2c, will be good enough for your 500w controller.
No particular need for a123's, unless you want to ride really fast and get a stronger controller with more volts. If you speed up much, you won't double your cost, you'll triple it. 30 miles at 40 mph will take a huge battery! So stick to 48v. The faster you go, the faster you suck down that battery.
Chalo said:
Try running a 16" moped / motorcycle tire on a 20" bike wheel. I did it and loved it. Felt like i had a rear suspension. Rolling resistance was only higher because it was super high. Tire selection is good, you just have to look a little harder.
The smaller the wheel, the more power a hubmotor can make. Ask the greats - they'll tell you that it's the only way to fly.
The only negative i would say is that if you run a 20" wheel on a bike that wasn't designed for it, you're going to have geometry and pedal scrape problems. My magic pie race bike was held up due to this. Not fun.
Chalo
100 TW
neptronix said:
Fat tires have excellent qualities and benefits that do partially offset the drawbacks of small wheels. But you don't have to choose anymore. Now that we have 29ers, you can have the benefits of large wheels and fat tires at the same time. There's more on the way, too-- Surly just introduced a bike with 700c wheels and 3" tires, for a category they call "29 plus".
Bicycles aren't made to fly. They are for gliding! More power is not necessarily better; it just results in something other than a bicycle.
But high winding count hub motors are natural companions for large diameter wheels. That's why I chose X5305 for my own 29 inch wheeled hub motor bike, many years ago. At that time, I had to order it from Kenny in China, because everyone in North America only had faster windings.
There are a few 20" wheeled bikes to choose from these days. The Origin 8 Bully is the one I think is the most appropriate for an e-conversion.
To tie this stuff back into the topic at hand, one of the implications of large wheels is low hub motor rpm, which implies low voltage, which implies high current at any given power level. So for a big-wheeled bike, battery discharge rate becomes more important than it is for a small-wheeled bike.
Chalo
The Mighty Volt
1 MW
Put an X5305 in a 20" wheel and with basic 72v settings, you can go anywhere and up pretty much any hill.
Capacity for me is vital because a little stiffness goes a long way. 10Ah of 20c LiPo is always a bit more fun than 5Ah, right?
Capacity gives you margin for error, taxes the cells less, gives you range, gives you stiffness {ha ha}
Of course, there are limits. 20Ah of 1c LiFePo4 can potentially give you 60 Amps....but its a pain in the ass. Its a lot of weight and a lot of size.
What you need there is a balance.
Some good 5C LiFePo4 will save you no end of bother.
Capacity for me is vital because a little stiffness goes a long way. 10Ah of 20c LiPo is always a bit more fun than 5Ah, right?
Capacity gives you margin for error, taxes the cells less, gives you range, gives you stiffness {ha ha}
Of course, there are limits. 20Ah of 1c LiFePo4 can potentially give you 60 Amps....but its a pain in the ass. Its a lot of weight and a lot of size.
What you need there is a balance.
Some good 5C LiFePo4 will save you no end of bother.