LiFeP04 24V 100ah Cells

[ronald44181000]

I'm looking at assembling battery pack for my latest E-Bike project and I need to know he value of using 100ah cells over 20ah cells. The reasoning for a 24V system is that is what is legally allowed where I live. Is there any advantage and if so what are they for using 100ah cells in a parrallel fashion.? You'll need to understand that for myself when the design of anything electrical comes into play. I lose all sense of reality. In short I'm scared to death of the subject, though I will be having someone else do the actual wiring.

 

[Link]

For one thing, 24V and 100Ah will be a bigger pack than anyone here is running on their bikes. Even DocBass's is only 2.1kWh.

Why 100Ah cells, anyway? 30-40Ah would be much more reasonable for a 24V system...

 

[dogman dan]

For weight and cost, 100 ah is a lot of battery for a bike to carry. Just how far do you need to go? My bike, at least the way I ride it is just about as inefficient as they get, and I get 20 miles out of a 20 ah 36v, so you should get at least 12 miles out of a 24v 20 ah, and most likely more than that since you will be going slower than I do.

Perhaps you are confusing amps with amp hours? The controller can put a lot of amps out at 24v if it is designed to, but for lifepo4, duct tape type batteries, the rule of thumb is one amp hour of battery sise for each amp the controller puts out on average. For example, my 35 amp controller can put out more than 35 for a short burst, but on average, it uses about 20 amps. If you have a 200-300 watt motor, it is likely that you would average around 15 amps or maybe less.

Basicly all you would get out of 100 ah cells is really long range, but much slower carrying all that weight. Like Link says, I can't imagine a need to go bigger than 30 or 40 ah. My butt would get pretty sore after 3-4 hours on the bike.

 

[Sacman]

I'm looking at assembling battery pack for my latest E-Bike project and I need to know he value of using 100ah cells over 20ah cells. The reasoning for a 24V system is that is what is legally allowed where I live. Is there any advantage and if so what are they for using 100ah cells in a parrallel fashion.? You'll need to understand that for myself when the design of anything electrical comes into play. I lose all sense of reality. In short I'm scared to death of the subject, though I will be having someone else do the actual wiring.

First of all, I'm just curious where you live that they restrict operating voltage on an ebike to only 24V? ... And why? I've never heard of that before. And I agree with Link and Dogman that running around with a battery pack 5 times the size of what most people run is just too heavy and inefficient. You really don't need that size of a battery unless you plan to be riding continuously for at least 4 hours.

 

[ronald44181000]

The reason that I'm looking at using a 24V 100ah setup is not because I'm limited to using 24v. The only limit that Ontario Canada has is that the motors wattage can't exceed 500 watts. I can even go higher as far as the amperage is concerned. I'm trying to get the most " bang for the buck".

 

[dogman dan]

Time for the newbie post. Amp hours is like the capacity of a bucket, while Amps is like the ammount of water that can flow out of a hole in the bucket. The sise of the lifepo4 pack can be important, since too many amps flowing out of a small battery can damage it, so a lot of us are using 20 ah packs with the cheapie lifepo4 cells so we don't damage the cells. Other than that limitation, there is only a benefit in increased range, but carrying the extra weight will slow you down. The extra amp hours will not make you any faster. More volts will.

Will anybody actually be likely to check your bike? if not, just run what you want, like 36v and stick a 24v sticker on the side of the battery pack. Pretty much what we are all doing in america where we are supposed to be under 20 mph and most of us can hit 30 mph. Just slow down if you see the heat. Other places have to get a licence plate and do have to toe the line, at least untill the get the plate anyway .

If you do want to keep it legal, get a 24v 20 ah, unless you want to ride over 20 miles between charges.

 

[JinbaIttai]

The advantage is greatly improved travel distance, with a big weight, handling, and wallet penalty.

With 24V, 100aH of LiFePo4, you're looking at a 55ish pounds of batteries, and expect to pay around $2000 for the batteries alone, probably more. If you go SLA, well I think that might exceed 150 pounds.

I'm no expert on these things, but from what I've read around here, I gather:
For your battery and motor combination to squeeze the maximum number of charges & discharges you can out of the batteries life, choose one where its 1C rating coincides with however many amps your motor will draw continuously at full throttle and at cruise speed.

36V 20aH batteries paired with hub motors sold in the 500W flavor is popular because this combination approximates that.

The inexpensive 20aH LiFePo4 duct tape batteries allegedly have been tested to last for 1000+ charges, but that testing was at a steady current draw of 20 amps, or the 1C rating. A typical hub motor at 36V should put your discharge rate pretty close to 20amps. A battery with much less capacity, say 10aH, or much more capacity, like in your 100aH example, takes you out of the best-bang-for-buck zone you could get by matching the 1C rating to the continuous discharge rate.

 

[dogman dan]

Good explanation. The only reason to pack more than 1c of amp hours is to go farther without a recharge.

 

[smithinparis]

Regarding the laws in Ontario, you are right, the maximum power is 500 W ... but this isn't the critical law. What's important is the maximum speed of the bike, and in Ontario it's 32 km/h.
I too live in Ontario, and I'm running a 36 volt system. What will regulate the power are current limiting components within the controller. Assuming a 36 volt system, to stay within the 500W limit, the current needs to be limited to just under 14 A (500/36). What needs to be pointed out is that you can still exceed 500 W on a 24 V system - assuming you are going up a really steep hill, and your controller doesn't limit your current, if the load allows you to draw 21 A (21 A * 24 V = 504 W) then you are exceeding the limit.
So all that rambling to state that, the intent of that law (as far as I can tell) is to simply make sure we aren't building monster bikes. The practical part of the law is to keep the speed under 32 km/h.
A typical hub motor will run (just a ballpark rule of thumb) just under 1 km/h per volt. So for example a 24 volt system will run just under 24 km/h, and a 36 V system will run at just under 36 km/h (which is a pretty reasonable approximation of the 32 km/h limit).
Given that the power limit of the system is internal to the bike (and completely invisible to the outside world), it's not a practical way to measure the bikes compliance to the law. However the police can easily set up a radar trap (or more practically other cyclists and pedestrians can complain about an ebikes excessive speed).
Don't fret about the power of the bike, and focus more on the speed. Or better yet, get a 36 volt system and don't worry about anyone splitting hairs over the difference between 32 km/h and 34 km/h.
Hope that helps!