battery booster
 

would something like this work as a battery booster?

DC 12V 9800mAh Super Rechargeable Lithium-ion Battery - eBay (item 220740015259 end time Mar-17-11 02:30:22 PDT)

plug into the cigarette lighter to help keep the alternator off for a while,
the only thing i can think off that it wouldn't work is the output voltage,12.6-10.8, but one can always add a dc to dc converter to get higher voltage

it provides about 125 watts, that would take off 250 watts from the alternator, thats a third of a horsepower.not too bad
and it weighs only one pound

what do you guys think?

You would have an easier time finding a dc-dc converter that steps down from a higher voltage, so get two batteries and run more then twice as long because you will also be putting a lighter load on each battery.

i wouldn't recommend using a Lithium battery for that, they require the proper kind of charger and can be damaged if drained too low.

the listing says it has overcharge/discharge protection

i might as well get a controller then,like this

Amazon.com: HQRP 20A Solar Panel Battery Charge Controller / Regulator 12V / 24V 20 Amp 300W with PWM Type of Charging plus HQRP Coaster: Patio, Lawn & Garden

the two batteries is series to the controler to the car battery

i can mount a switch form the batteries to the car ,that stops the imput when driving down a hill or breaking, that way the alternator can come on

I think there's merit to the idea.

Ideally you'd want to support the system voltage at alternator levels (~13.4-14v) while running without the alternator. But of course that complicates things and raises the cost.

If saving money is your goal (vs. tinkering for fun & outright efficiency) you'd have to do the math on this. I'd be surprised if the cost of the battery (consumable too) will offset the savings in fuel.

"Ideally you'd want to support the system voltage at alternator levels (~13.4-14v) while running without the alternator. But of course that complicates things and raises the cost."

i'm planing to leave the alternator on all the time,

i'm hoping that the extra pack will "full" the alternator that there's enough power not to come on
the switch will be for the battery pack to turn it off and then the alternator will come on seeing that there's less power
Now of course the alternator might come on at any time, i'm just not sure how this will play out.

the battery pack can provide 250 watts , that should be plenty to full the alternator and keep it off for a while

Aha.

Well, it won't work that way. The alternator will simply try to raise the voltage of the added capacity to whatever its setpoint is ~13.5-14v.

You'll just be carrying dead weight.

If you want to leave the alternator connected and add a "boost" pack, it has to provide more voltage than the alternator puts out for it to automatically reduce its output.

also keep in mind that what ever you use will have to be able to handle the current dra you are looking for. 250 watts at 12 volts is almost 21 Amps, i don't know many small battery packs that can handle that kind of current draw for extended periods

A single pack of the size that he's looking at will not be able to handle 21 amps, nor will the wiring to the lighter plug (fused at 7.5 amps on most cars) but it might be enough for day time driving and the alternator is still going to be hooked up so when the batteries die or an over current protection kicks in the driver will most likely never notice a change.