Topic: Battery size

[ekim98]

 What would be the smallest cranking amp / physical size battery I could use for a cb750. I'm wanting to relocate the battery and save any weight I can.

[TwoTired]

The electric starter draws 120-150 Amps while engaged.  How long do you plan to hold down the stater button?

[ekim98]

 Well of course that would depend on how good the bike is running/tuned. The less time trying to start the better. I'm not trying to sound smart, and I don't know what a stock Honda battery has for cranking power nor what kind of reserve would be sufficent. Also how much weight you actually could save by going to a smaller battery. If the weight gain was only minimal then, I guess the physical size is more important for my purposes.

[relative q]

I'm curious as to whether or not you can eliminate the battery altogether if you're not using the starter.

[Gordon]

I can't give you specifics, but if you don't mind kick-starting your bike you can probably get away with a much smaller battery than normal.  Many others here have had success with smaller gel or agm batteries that fit under the hump in a solo seat and can still use the electric start.  Do a search and you should be able to find several threads on the subject with more specific information on battery sizes and names.

[Gordon]

I'm curious as to whether or not you can eliminate the battery altogether if you're not using the starter.

No.  Not unless you completely change out the charging system.

[TwoTired]

Here are some very crude and rough calculations.
The stock battery is rated at 14 amp Hours.  1 amp draw for 14 hours, or 14 amps for 1 hour, 28 amps for 30 minutes, 56 amps for 15 minutes, 112 amps for 7 minutes, 224 amps for 3.5 minutes,  whereupon the voltage will drop below 10V.

Cold batteries produce less power, 65% at 32 F compared to 100% at 80F , and cold engines with thick oil draw more current to turn over.


Cheers,

[kghost]

Gawd I love the twotired responses.

[ekim98]

 Thanks for all the answers. That tells me alot. I did find the topic about eliminating the battery and that gave even more info, but I never really wanted to eliminate the electric starter, even though it would save a good amount of weight. And without the electric starter I could get away with a much smaller battery. A lot of info for me to think about. I would like to cut as much weight as possible.

[relative q]

So I was out buying fiberglass for my cafe seat today at Boater's World, and right next door is a place called Batteries Plus.  They make custom batteries and have a shop full of just about any sort of battery you'd ever need.  Awesome.

Anyway, I ended up leaving with a $20 battery that's about 5"x3"x1", rated at 12v, .8Ah.  Since I'm getting rid of my broke-ass starter and only really using the battery to run lights, I figured I'd give it a shot.  By my calculations, it should work.  It's really tiny compared to my stock battery, and will go well with my new electronics package mounted under the seat.  I'll let y'all know how it goes once I get my electronics in the mail and put together.

In any case, see if there's a place like Batteries Plus where you live.  There are at least two in the Pittsburgh area, so your town may have something like that too.  For any Pittsburgh SOHCers, I recommend them highly.  The staff really know what they're talking about and are really helpful.  It's also really nice to be able to see the selection of batteries right in front of you, rather than buying them off the internet where all you have to go on is a crappy picture.

Attached is a crappy picture of my new battery.

[TwoTired]

So, I get a PM saying my back-of-the-envelope battery capacity calculator was wrong.  Which I actually knew, but, was trying to keep the explanation simple.

The batteries chemical conversion to electrical energy has a speed limitation, in that if you take out too much too fast, less is available. Soooo....

"The 14 amp.hr capacity of a battery is quoted on what is known as the '10 hour rate'  ie The battery will deliver 1.4 amps for 10 hours.  However if you increase the current draw the effective capacity goes down so that if you draw 14 amps the battery will be flat long before the theoretical 1 hour.
If the starter draws 100+ amps you will flatten the battery long before 7 mins. "

It's true.  And, there are other variables in the calculation as well.  Temperature, manufacturing tolerances, plate technology, electrolyte composition ...  it's a long list.

I believe I once calculated that a healthy stock battery on the 750 will crank the motor for 90 seconds in adverse conditions.    This, of course, you should not do, as the starter motor is only rated for 30 sec continuous duty.  Then, breaks in the current draw, will allow a bit of chemical recovery and increase the draw duration.  So, the real absolute battery capacity under starter motor cranking is kind of a moving target.

Your crankiness may vary. 

Cheers,

[tsflstb]

I've been wondering about these smaller batteries.  What about the charging system?  They're still designed to juice up a 14ah battery so there's going to be some excess right?  Does the extra energy go to the regulator?

[relative q]

I've been wondering about these smaller batteries.  What about the charging system?  They're still designed to juice up a 14ah battery so there's going to be some excess right?  Does the extra energy go to the regulator?

As I understand it, yes, but I'm going to run this past my roommate soon.  He's and EE and builds battery chargers.  If this isn't the case, I'm sure he could draw me up a quick and easy charging circuit.

[relative q]

After doing some extra research, I've answered the question:  Once the battery reaches the proper voltage, the Zener diode in the regulator circuit dumps off the extra voltage as heat.

[TwoTired]

The regulator does not absorb extra energy.  It does not current limit the alternator output stream directly.  It will tell the alternator not to make so much energy based on the voltage level of the battery.
 This means the battery technology you chose should have the same cell voltage characteristics as the lead acid type found when in stock form.

For example, the Vreg knows the battery is full when six cells in series reach a voltage of 14.5 V which is just over 2.4 V per cell.  This keeps the battery from damaging overcharge.  If you select a battery technology that has, say, 2 Volts as it's peak charge voltage, then the bike's regulator will let the alternator overcharge it routinely and shorten it's life.  The battery may behave nicely early in life, but will wear out faster than it could have.
Conversely, If the battery technology you choose requires a peak charge level of, say, 2.6V per cell, then the battery will never get fully charged.  This also can decrease it's service life as part of the chemicals inside don't get fully involved in the conversion process.

Besides peak voltage, the charge rate is also of concern.  Ramming a lot of power into the battery quickly may also damage batteries that can't convert the chemicals inside fast enough.  These develop a surface charge voltage high enough to make the charger back off before all the chemicals inside can become fully converted.  In essence these batteries overheat during the fast charge, making the chemicals overactive in attacking the anode and cathode material, but because of physical distances from the energy entry point, the battery never achieve a full and thoroughly saturated charge.

There are a lot of different battery technologies out there, and they all have their idiosyncrasies.  There are a lot of different voltage regulator designs, too.  This post uses the stock SOHC4 regulator as a reference.  It has no zener diode. And unless a regulator manufacturer publishes his internal design, or you take it apart, you can't guarantee there is a zener diode in any specific example.  A Zener diode regulator is only one method of voltage regulation.

cheers,