general info battery pack question...

You are right about first two claims (well, sort of - you really want to have some reserve , battery peak Amp rating wise , because putting maximum allowed stress on cells too often , shortens their life).
Amount of Amps (energy) in your (future) cells is constant (by its rating) and it doesn't have almost any relation with C rating. It is just the way it's released. Think of it as a nozzle (C rating) on garden hose , attached to a water tank (battery cell) - when you put it on the hose you get more pressure on the flow (more water power) but you get more pressure on the hose itself and if it's too much, hose/tank will burst or nozzle will come off and the amount of water is the same . Now , if you put bigger nozzle(2C) , you'll have less pressure on the hose/tank (batteries) , but you'll be quicker out of water (energy). If you want to put some fire out (god forbid), meaning you want to ride up hill , you just want as much water as you can possibly have, therefore bigger nozzle is better. Tap on water tank is your throttle.
C rating is about internal resistance of cell. More resistance - less work (more heat-wasted energy) and if you push it , cell will short out (blow up , crystallize or change polarity). You had an experience with your AA NiMH pack - it would spin up the wheel on it's own , but as soon as you sit on the bike it would do nothing (accept heating the cells - which you didn't check). Bigger C rating - the better and it'll affect range only by keeping cells in good condition , so you can use maximum energy from those cells (other inefficiencies are another matter).
I know that this sounds stupid , but I'm having trouble expressing myself properly in English (in short period of time).

Also I have heard the definition of burst (impulse) C is no more than one 5 second burst every 30 seconds. Not much of a hill but maybe enough to get across an intersection. For a hill you need to rely on regular power. I'm pretty sure the Prius NiMH are rated at 10C constant, so it's possible to get NiMH that are powerful enough. There probably is an online market for Prius cells out of wrecks, etc if anyone is interested in researching that - jd

Anyway I think for length of time that the battery will put out the required amps, it might be better to go with the lower c and just add to the pack till it works.

Click to expand...

Deacon , how you came up with that conclusion? I mean , you will do as you like (naturally) , but I'm interested to learn how (or from which facts - since you're running the tests) you imagined that lower C is better?
As I've explained (somewhat clumsy) bigger C is better (you can check out endless sphere for confirmation). Adding cells in parallel is more expensive than buying cells with higher C rating. JDC is right about short bursts , but time (said 5 sec) is depending on particular cell structure (and chemistry) - it can be up to 30 sec or more .
Once again(maybe this is more understandable): if you have leaking boat , in the middle of the lake , which would you prefer to take the water out - drinking glass or a bucket (representing C rating)?
It is very hard to put a lots of maths and physics (and chemistry , for that matter) into something that is "every day" looking and understandable and comparable (although I'm not a certified expert for that stuff - I've just paid attention in school and college and done plenty of reading).

Aaahhh , now I get your point of view. You , basically , try to preserve energy in order to extend your range. Am I right?
First let us remove fire from discussion (please , don't use words fire and house in the same sentence any more - it gave me chills). Second, bigger water tank isn't an issue here (you'll get as much AH as you find necessary for your trips - we are talking about availability of Amps ). Third, energy(in batteries) is preserved in various ways : efficiency of the system , reduced weight , regenerative braking , coasting as much as possible, pedal assist...
Let's change the analogy. If you have truck with 3.0 l engine (I don't know how much is that in cubic inches) and you want to preserve gas (energy) you can put carburetor from 2.0 l engine on it , right? It will get much better mileage , but slower acceleration (not enough power , engine over heating and lots of problems with it) and if you put some load on the bed it can even stall on some hill. Now , your solution is to put carburetor valves from 2.2 l engine on the carburetor, than from 2.5 l , etc...until you can make it over that hill with the load on the back and if those valves are smaller then it used to be, you'll be happy and if not you'll put old ones (3.0 l) back in place. That is analogy for C rating. Bigger tank on the vehicle (meaning , you can go further , not over that hill) is AH. Now , if you keep old valves (3.0 l ones) , you can still preserve petrol by driving slower , coast on downhills , not using AC (making it more efficient)...But you can make it over that hill (with a load on the back) any time , without trouble for your engine.
What I'm saying is - don't mix C ratings (power availability) with AH (power storage). If the system is underpowered (insufficient C rating) , it WILL kill the batteries prematurely and their demise will pull out more money from your pocket. We are talking theoretically , since you never measured your power consumption (it's based on assumption) , and you don't know how much is your system efficient (not very much , sure) and you try to conclude what happens with your batteries by measuring Volts and feel and eye ball.
That being said , you know that motor uses less amps with higher speeds and you drive pretty slowly. So your trouble isn't with capacity (AH) - it is in other things. With 15 AH batteries(any chemistry) you can go at least 15 mile ,with hills (and that is a fact - people do it almost every day and I know few of them) without pedaling. If you "adjust" your system (meaning more speed on the motor, with better efficiency , geared low for more torque and low driving speed and you can always limit Amp draw of the motor) assuming that everything is correctly wired(no loose ends) you can make it easily. But , even with that kind of setup , batteries must be able to deliver enough power for peak demands (which is unknown), otherwise , you'll be experiencing the same things that you did in past few weeks. I'm not saying that you should buy batts with 30 C (which people do use on e bikes - Lithium ones) just few times more than 1 C.
This concludes my posting about this subject and further experimenting with widely known issues (C rating related) is solely up to you (and your wallet). However , I would love to see you do it your way (relatively cheap and without BMS and proper chargers) and move on to other (bike related) projects and I will try to replicate your success (which would make me wrong, but who cares, if that means that I can ride for less money).
Please , don't get me wrong , I'm not criticizing, just expressing my opinion.

NiMH Packs and Cells
NiMH - AERO high performance, long lasting charge discharge (nimh ni-mh) nickel metal hydride rechargeable batteries of AAA, AA, A, SC, C, D battery sizes for power sources. We wholesale rechargeable batteries similar to panasonic sanyo saft gp nicke
Charging NiMH Cells
Speciality cells > ESP NiMH cells > Buy a cheap C NiMH cell 4000mAh from MDS Battery (see the discharge rate)
I've put this , just because it is in better English than mine. Check it when you find the time.
And I fully understand what are you doing , maybe I'm just too inpatient (knowing what I know).

I think ditto to most of what zabac70 said.

Great links zabac70, thanks. From the first link NiMH Packs and Cells

"The disadvantage of NiMH cells is that they usually have a much higher internal impedance. That means that if you try to draw a lot of current from NiMH cells, they will drop excessively in voltage which can cause poor performance or the device they are powering to shut down."

This seems to explain exactly the performance problem Deacon is experiencing with the NiMH battery packs he is building.

Have you read it Deacon? What do you think?