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Old 05-06-2013, 04:09 PM   #1
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Smaller battery closer to the starter = weight savings?

Hey, I tried doing a search and couldn't find much regarding this subject so please bear with me if this has been answered previously.

After having replaced the battery in my portable jump starter, and later making a motorcycle battery work in my Miata, I began to wonder if anyone has come up with this idea.

So we all know that smaller batteries put out much less cranking amperage and we also know that longer and thinner gauge wiring will decrease cranking amperage. So in that sense, if you were to solder a 2 gauge welder's cable to a smaller motorcycle battery's terminals, you would have the best connection possible. Now if you were to limit each cable to maybe one foot from the battery to the starter, you would have maximum potential cranking amperage (without having the battery bolted on the starter).

Just curious as to whether or not anyone had tried such a mod. I understand that the car would most likely go dead if the radio or lights were left on for a small duration of time but you could always carry a small jump starter in the trunk with you.
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Old 05-06-2013, 04:16 PM   #2
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While it may be possible to do this, I don't think its practical for every day use. Something as simple as sitting in a loading zone with your hazard lights on for 5 mins may prevent you from starting. I also don't think that a motorcycle battery will last long term in a car. It would prematurely wear out.
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Old 05-06-2013, 04:36 PM   #3
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Actually, motorcycle batteries were being used in the Miata by a number of people in the Miata forums. The original battery that came with the car was an AGM (Absorbed Glass Mat) battery. They tend to last longer, remain maintenance free, and absorb violent vibrations much better than the standard wet cell batteries.

The AGM battery replacement in the Miata held about 18.5 amp hours when converted. I had switched mine to 30ah and according to O'reilley's load tester, it puts out about 50% more cranking amperage as expected (400 cranking amps vs 600)

I suppose the downside to AGM batteries would be a slower recharge rate required when reaching full charge and that they are more prone to thermal runaway if overcharged. However, both of these should be accounted for in motorcycle batteries.

As far as the scenario with hazard lights left on is concerned, you could always switch your bulbs to led bulbs so it uses much less electricity. I don't think leaving led hazard lights on for an hour would lead to a dead battery that easily unless you had a 5ah battery installed. I think ideally, a 14ah battery would work fine.
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Old 05-07-2013, 09:55 AM   #4
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Carrying a jump starter would negate the weight savings of going with a smaller battery. Which is the only advantage I see with this.

A smaller battery will eliminate any engine off techniques as an option. I've actually considered putting in a second battery for these.

I agreed with Jay on premature wear. Lead acid battery life is shorten with deep discharging. The smaller battery will be expending a greater percentage of its charge per start.
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Old 05-07-2013, 01:20 PM   #5
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Motorcycle batteries are usually taxed pretty hard anyway, and its common for motorcycle users to use trickle chargers to extend the life of the battery. I know with my motorcycle, if it doesn't start after about 30 seconds of cranking, it needs to be put on the charger. I'd imagine a car would require even more starting power, and my guess is if you can't start within 10-15 secs, forget it.
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Old 06-17-2013, 06:51 AM   #6
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This sounds like what you may need:

Mini BoostPack 12V Capacitor Car Battery Update - YouTube

I've ordered some of these ultracaps to piggyback onto my regular battery just for extra diesel cranking power.
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Old 06-18-2013, 10:39 PM   #7
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I think a 7AH motorcycle battery is just too small to handle cranking a car engine reliably. I P&G my car, and a 40 mile Winter round-trip can drop a 55AH AGM deep cycle battery's voltage down from a healthy 12.6V down to 12.0, and down below 10V when cranking the engine over. Two of those 55AH batteries can propel an electric scooter with a 180 lb. rider 52 miles. Two 7AH batteries should drive that scooter only ~6.5 miles.

I'd rather have the cranking capacity of the bigger battery than the 30 lb. weight savings and the smaller battery.
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Old 06-19-2013, 04:09 AM   #8
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Would that weight saving count anything measurable? A passenger weighing ~165lbs (plus gear) would deteriorate a ~400-lb motorcycle's FE by less than 10 percent. And a passenger (weighing 40 percent of the vehicle) on the back seat changes aero too!

Now what would a 30-lb (1.4 percent) weight saving do to a 2100-lb car? OK, it's definitely not zero, but you will probably never ever see the difference.

The other thing is, I think it wouldn't like FAS glides with starter usage and/or with lights on (mandatory on the highway here). With Teresa I often do ~15% of my commute FASed with a handful of starts and I don't know if I'll ever have a full battery. She's a 650 EFI single with the necessary electronics and one 55W bulb (no switch!), with a 12Ah battery. At winter I rarely FAS because even my heated gloves put enough extra load on the battery to gradually drain it (I think the idle 1500rpm - I glide much - can't charge it anymore).
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Old 06-19-2013, 05:03 AM   #9
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I know guys that have been using lawnmower batteries for years. They are into autoX and for them, weight can be a huge issue. There was a story circulating about a guy that spent an entire day cutting off the excess length from all the bolts he could find on the car. His total weight savings was around a pound and a half. I originally thought it was a joke and laughed... then got funny looks.

I think there is an advantage to losing weight in a car but I think the added security of higher battery capacity is more important.
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Old 06-19-2013, 11:22 AM   #10
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Quote:
Originally Posted by alvaro84 View Post
Would that weight saving count anything measurable? A passenger weighing ~165lbs (plus gear) would deteriorate a ~400-lb motorcycle's FE by less than 10 percent. And a passenger (weighing 40 percent of the vehicle) on the back seat changes aero too!

Now what would a 30-lb (1.4 percent) weight saving do to a 2100-lb car? OK, it's definitely not zero, but you will probably never ever see the difference.
Agreed. While significant weight changes can make a difference, I think people interested in saving fuel devote far too much effort to saving small amounts of weight that will never make any difference. Further, if you ask me, a much better way to save 30 pounds is to put 5 gallons less fuel in your tank, though that makes it difficult to measure fuel economy.

If you could reduce weight by 20% and implement mechanical changes that save fuel by sacrificing acceleration that you otherwise would be unwilling to do, that's when saving weight can really make a difference. Just dumping your spare tire and using a battery that will leave you stranded isn't going to do anything for anyone who drives in a reasonably efficient manner (which includes everyone who is reading this), in almost any car on the road.

I suppose a 2% reduction in weight could make a 0.2% difference in fuel economy for someone who drives like a complete idiot in city traffic all the time, though.
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