The Math...

Volts times Amps equals Watts

Amp: the practical meter-kilogram-second unit of electric current that is equivalent to a flow of one coulomb per second or to the steady current produced by one volt applied across a resistance of one ohm

Watt: the absolute meter-kilogram-second unit of power equal to the work done at the rate of one joule per second or to the power produced by a current of one ampere across a potential difference of one volt :  horsepower

Watt/hours are usually just called Watts in layman conversations

BATTERY MATH:

Let’s say that you make a 36 volt  battery consisting of thirty  1.2 volt 7 amp “F” cell nicads.

Wired in series,  30 x 1.2 volts = 36 volts

. 

36 volts multiplied by 7 amps equals 252 watts  ( We’ll ease the math and call it 250 watts).

The value of watts occurs over the time period of one hour.

So, your battery has enough energy to illuminate a 250 watt light bulb for an hour,

or illuminate a 25 watt bulb for 10 hours,
 
or illuminate a 500 watt bulb for a 1/2 hour.

Your battery can also run a 500 watt motor for a 1/2 hour.

If,  you design your bike to go 20 Miles Per one Hour, then your battery will make it go for 10 miles.

Only Ten miles???  Yup!!

Then why do so many bikes claim 25 mph for 40 miles??

It’s a “crock”

Quite simply, it’s false advertising, unless the caveat “with some assist” is included.   You know, the same way my Chevy Suburban gets 80 mpg coasting down Pikes Peak, (assisted by gravity).  Or if I run out of gas and someone gets behind and pushes me (that’s even better mileage).

THE SALES PITCH:

The following importer’s testimonial was copied from the website that sells these miracle bikes for $799 plus shipping. 

 ...........Are you going to believe this???

“When I turned the throttle, immediately I could feel the substance and power of the 48v battery powering the efficient brushless hub motor. I took off and immediately began looking for the nearest hill.
I drove around town till I found the nearest hill, and with no pedalling I cycled right up the incline, not just once, but several times! After being satisfied with the hill performance, I took off down the street and back to the shop. I was looking to really challenge this heavy duty bike. My friend was anxious to hop on so she sat on the rear seat and we took off! We rode about 4 miles to our next destination. I dropped her off (she had some work to do), and I took off looking for more hills and just having a ball with the new pollution free, quiet running bike. I guess this was about 3 miles. No noisy gas engine. No expensive gas, just 5 cents to charge the battery!
After touring for about an hour, I picked up my friend and we drove back home about 4 miles and had to go inside Later we took another ride... about 7 miles, that's right, with BOTH of us on the bike. Me at 180 pounds and her at about 110 pounds, and all 290 pounds travelled to the party. We took a walk and did another 7 miles back. When we rode back it was dark! No problem this bike has a built-in headlight that is more than enough. Later my friend took the bike back to work, another 4 miles.
The next day, another friend who weighs about 200 pound, decided to take this powerhouse for a ride. He said that he rode another 15 miles. So I stopped to count this up: 4 + 3 +4 + 7 + 7 + 15 = 40 miles. My friend saw that his battery was running low, but now he was 15 miles from home! Think 55 miles! He got home, but had to do some pedaling. All in all, this bike went for many miles -- get this -- on LESS than a full charge with minimal pedalling!”

     Wow!! I wonder how far it will go on a “Full Charge”??

OK, the truth?? I did the math on this bike, and it’d be pure luck  if went 15 miles on it’s 48 volts of “substance and power”.   

“Some assist” (or as he puts It; “had to do some pedaling”) accounts for 40 miles of the 55 mile journey, which ain’t minimal for me... 

BTW, there’s a video on his website of him riding the bike, and it rattles like a shopping cart on a gravel road.   The build quality is about that of a ride-around Barbie toy electric jeep. 
 

The "Q" factor:
http://en.wikipedia.org/wiki/Q_Factor_(Bicycles)

Another thing about many of these poorly engineered bikes, is, that the bike frame is so wide, that you can fit a milk crate between your pedals.  So imagine having to spread your legs that far apart and comfortably pedal.  Unless you're an orangutan, you won't be pedaling very far.  Another consequence of such a wide "Q" factor is that you can't lean the bike into the corners and pedal, because the pedals hit the ground.  These poorly designed bikes compensate for this by having ineffectively short crank arms, not much longer than a fishing reel crank.   Without enough leverage, the crank arms are very tiring. 

Crank arm length is so critical to efficient cycling, that it is measured in 2.5mm increments from about 145mm to 185mm.  A single manufacturer might offer fifteen different arm lengths.  Understand how this affects "with some assist".

These kinds of bikes harm the progress of electric bike transportation.  People that sell this junk are opportunists that (perhaps unwittingly) take advantage of the wishful and poorly informed. 

MAKE YOUR OWN BATTERY:

A good place to find cells to build your battery is The Nicad Lady.  Sanyo cells are pretty good, That's what Heinzmann uses in their Nicad Packs.  
http://www.nicdladyonline.com/



Search around Sanyo Nicad and you'll find a great diagram of cell sizes available.  It comes down to how much do you want your battery to weigh??  How do you want to fit it to the frame??  How often will you want to recharge??   http://www.batterystore.com/


I have purchased cells from these guys, and they work fine so far...  I really haven't had a problem with the cheap China cells. 
http://www.all-battery.com/index.asp?PageAction=VIEWCATS&Category=166



If you can solder, you can configure your battery any way you like.  I use flexible silver coated 3/8" wide braided copper straps to connect the cells tip to toe.  They look like miniature car battery ground straps.

There's not much mystery to soldering.  Pictured below are Sanyo "F" cells that have been inserted into paper tubes and shrink-wrapped.  Cut some inch and a half silvered straps, scuff the ends of the cells with a file or emery, "tin" all the ends of the cells and straps with solder, then solder together plus to minus in whatever shape that you want your battery. 

The duracell is shown so you can see the size difference between "D" and "F".
You can have a place like "Batteries Plus" build a battery for you.  They will spot-weld your cells together.  I prefer doing it myself the old fashioned way of soldering because I feel it's a more durable bond.  I have never had a failed connection.  I prefer the flexible straps to the rigid "Deans" style. 
You'll hear a lot of caution about soldering cells together 'cause they might blow up in your face.  I've never had a problem.  Wear a face shield just in case...  Pay attention and work fast and neat. 

Pictured above is inside the batterypak that I built for the snow bike shown on the home page.  You can see the soldered flexible cable connection between the "D" cells.  The black covering is "ice guard" butyl roofing material.  The diamond plate is aluminum sheet metal. The controller is Asian, rated at 40 Amps. There's a toggle for on-off, and a common charge port.  You can build this for less than $200US. It is 24 volts, 5 Amp Nicads.  The actual "battery" weighs 7 pounds.  The battery case doubles as a luggage rack, houses the controller, and incorporates LED tail lights.  The whole thing weighs under 10 pounds and can be transfered to another bike in a few minutes.  This battery powers a 400 watt Heinzmann motor.

ANOTHER 24 VOLT 5 AMP NICAD BATTERY WITH LED TAIL LIGHTS. 

Please understand the distinction between the words...CELL and BATTERY

The Battery is made from individial cells

You'll get better service from electronic people if you understand the nomenclature.

Your typical flashlight can need a new battery (if it has more than one cell), but it doesn't need new batteries

You need thirty individual cells of 1.2 Volts to make a thirtysix volt battery

 One cell is one cell, not a battery.
 
You need two or more cells to make a battery

The 12V battery in your car is made from six 2v lead cells
(actually it's closer to 2.2V each times six = 13.2V)





The following is copied from a nicad site.
There's a lot of detail.

The Care and Feeding of NiCd Batteries

 The care and feeding of NiCd batteries, as NASA Learned from 30 years in space satellite operations.

Storage of NiCd Batteries

Guideline No. 2 Flight batteries should be maintained in a discharged and shorted condition and stored at cold temperatures when not required for "critical" spacecraft testing. Optimal temperature is around 0 degrees C. NASA does it this way:

1.        Discharge at C/2 constant current rate to first cell at 1.0 Volts

2.        Drain each cell with a 1 ohm resistor to less than 0.03V

3.        Short each cell with a bar

4.        Place batteries in a sealed bag with dessicant (stops condensation)

5.        Store in cold temperature (about 0 deg C)

To re-charge such a stored battery Guideline No. 7 A battery stored discharged and shorted for a period greater than 14 days should be activated with a "conditioning cycle" prior to placing it in use. The conditioning cycle (20 deg C) is defined as follows:

1.        Remove from cold and allow to come to room temperature

2.        Charge at C/20 for 40 hours +/- 4 hours (Deliberate over-charge)

3.        C/2 discharge until first cell reaches 1.0V

4.        Drain each cell with a 1 ohm resistor to less than 0.03V

5.        Short each cell with a bar for 4 hours

6.        C/10 charge for 16 hours +/- 1 hour

7.        Do steps 3 and 4 again

8.        C/10 charge for 16 hours again ***

*** Steps 4,5,7 and 8 are not needed if the batteries haven't been stored for an extended period of time. See note in Guideline 9 below.

What does this mean? Well it means that to store our packs we should use the lightbulb style (or similar) method to discharge our cells down to .9V per cell. Then we should use a discharge tray to bring them down to 0V per cell and then short them and store them in the freezer in a baggie with a dessicant. Then bring 'em up before we race with 'em if we want our packs to be in peak condition. Quite a process huh?

 As for memory in ni-cads, it was described to me as follows re-charging a cell that is only partially discharged say 50% often enough will cause the battery to only accept a 50% charge. If you then discharge more than 50% say down to 25% the best a re-charge would get you would be 75% (25 + 50).

Unfortunately this conclusion is not entirely true. The real reason for his NiCd early death was probably over-charging. Most chargers that come with equipment that use NiCd batteries aren't very sophisticated and over-charging is very easy to do. This is why the manuals tell you to fully discharge your NiCds packs before charging, their chargers aren't peak detecting and the only way to "know" that a pack is properly charged is to drain it first. The reason that our NiCds lose their capacity are manifold and complex but usually come down to voltage depression and crystal formation on the electrodes. The former is caused by age and over-charging and the latter usually by low discharge rates.

The most common problem is the formation of large crystals on the electrodes, this shows up to us as a "soft" pack, that gets better each time you run it on race day. According to one paper, this is caused by trickle charging, according to this NASA publication it is caused by low level discharging (leaving the battery sit open circuit or under light load). High discharge rates tend to bust up large crystals though, so that is a good thing for us.

To avoid this low level discharge we should reduce the charge of our batteries as much as is feasible before storage. The procedure above details this process in far more detail than any of us would probably care to take. But it does give us pointers to way to keep our cells in good condition within the level we each choose to afford.

Voltage depression usually occurs because of overcharging. Fortunately, this condition can usually be overcome by a few discharge/recharge cycles. I got this tidbit of information from an article the Navy published on reconditioning its massive supplies of NiCd packs and the money they saved by doing it. Unfortunately, I can't find that article any more so I can't quote much from it.

Other Useful NASA Guidelines for NiCds

Here are some useful bits of info in the form of other guidelines from NASA:

Guideline No. 3

The use of flight batteries after an open circuit stand of 4 hours or longer should be initiated with a short (3 to 5 minutes) discharge prior to initiating battery charge. Using a normal load.

Guideline No. 4

During short periods when the battery is not needed to support spacecraft integration and test, it should be maintained on a low rate trickle charge. Low rate is defined as C/60 to C/100.

Guideline No. 8

Batteries should not stand on open circuit for more than 7 days without being charged. Charging should be initiated only after implementing Guideline No. 3.

And finally, this is a useful guideline for us racers:

Guideline No. 9

A battery should be "reconditioned" if it has been on open circuit, subjected to intermittent use, i.e, open circuit, trickle charge, occasional discharge, etc., for a period of 30 days. Reconditioning is effected by performing the following sequence at 20 deg C:

1.        discharge at C/2 constant current rate to first cell at 1.0V

2.        Drain each cell with 1 ohm resistor to less than 0.03V

3.        Short each cell for a minimum of 4 hours

4.        Recharge battery at C/20 constant current rate for 40 hours +/- 4 hours (see NOTE below)

NOTE: The re-charge method following step 2 is not critical if the cells have not been discharged and shorted for extended periods. After a few hours (4-8) at the C/20 rate, charging at high rates is acceptable. If the battery is integrated into the spacecraft, final charging can be accomplished with the spacecraft battery charger.

Some Comments

Some things come up again and again, you may have noticed them. The first is heat is the enemy, do everything you can to keep your packs cool. The second is Overcharging is bad, the use of a good peak charger is probably the single best investment you can make to protect your batteries. The final one is our most common fault, don't let your packs just lie around. When not in use they should be either stored shorted, or, if you are about to race with one, on a C/60 or C/100 trickle charge in preparation for use. When I say shorted I mean that each cell is shorted, not the whole pack. You can't short your whole pack at once without risking damage to it! Also note the careful process used to short cells.

If you are interested in more details on the care and feeding of NiCds, you can get this publication through an interlibrary loan or through NASA, its called:

NASA Reference Publication 1326, February 1994 Handbook for Handling and Storage of Nickel-Cadmium Batteries: Lessons Learned by Floyd E. Ford Swales & Associates, Beltsville, Maryland) and Gopalakrishna M. Rao, Thomas Y. Yi (Goddard Space Flight Center, Greenbelt, Maryland) Published by NASA Scientific and Technical Information Branch

OK, So what's it all mean??
These NASA techies have their own scientific methods, but I truly believe that the guys that race radio control cars know tons more than NASA when it comes to getting power out of a battery pak.
These guys have closely guarded secrets, and it might be unethical for me to reveal what I've been told...but I'll give you a clue to start your research...

Matched and Zapped 

I get a lot of argument about which cell chemistry is best.
YES, "someday" you'll plug your motor directly into a carbon fiber lithium poylmer bicycle frame.  In the meantime...

NICADS RULE !!!
CONSIDERING...  WEIGHT, COST, LONGEVITY, ABUSABILITY, STORAGE, RECHARGE TIME,
HIGH AMP DISCHARGE RATE...

This chart is for "weight weinies"
I've used the 4/3D aka "F" cell, and it's great.  7amps is plenty of power.
Interesting that some of the smaller cells have best power to weight.
I went to work and back (3miles) on a pocketful of  3.3A SUB C NIMHs

Don't get too concerned about the weight of your battery!!
 
With full suspension and electric assist, you have a much different dynamic than pedaling a rigid frame bike.  Your suspended electric bike will have much less "unsprung weight", and you must factor in your higher and more constant power to weight ratio...
The motor doesn't really care if you're carrying an extra five pounds in your battery pak or your ass.

FORUM