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PostPosted: March 10, 2020, 6:32 pm 
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Joined: October 10, 2010, 10:26 am
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Location: Santa Maria, CA
A little advice guys as I try to size my battery for an unconventional need.. My BEC will have a 2000 Honda VFR800 engine and wiring harness.. It will be wired as though it's a bike, just stretching the turn signals and such to reach the corners of the car.. There will be no 'extra' anything on this car.. no stereo, no alarm, no seat warmers- nothing..

Except..... there will be the rare occasion when I need to use my high-torque Small Block Chevy starter to act as a reverse for the car (using a tiny ring gear and SBC starter at the diff)... this will be a rare occurrence, and will likely only last for about a 3/4 second burst- just long enough to back up the 900lb car (1100lbs with me in it)..

So my challenge is this: To select the smallest, lightest battery that will run the VFR motor AND occasionally power the SBC starter in brief, rare spurts..

My biggest OEM electrical draw is the bike's (car's) headlights, which I can switch over to LED bulbs if it's helpful (they're H4 now).. other than that, I don't think there are any other corners to cut.. I would like to avoid putting a 40lb battery into such a light car, but on the other hand I don't think one of those fancy Lithium batteries is up to this (is it?)... Maybe there's a 'just right' battery for this job that's between 2lbs and 40lbs??

So- as light as possible, and able to do the job... any suggestions?

Thanks-

-ccrunner

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PostPosted: March 10, 2020, 8:18 pm 
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Joined: April 23, 2006, 8:26 pm
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"As light as possible"... okay...
"...The ETX900 is 4.9 pounds. Max charge current rating is 80-amps. Delivers 840 Peak Cranking Amps..."

https://earthxbatteries.com/product-category/race-cars

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PostPosted: March 10, 2020, 8:45 pm 
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Joined: July 17, 2008, 9:11 am
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Location: West Chicago,IL
From what I read, the VFR800 alternator is good for 34 amps. That should be enough to run the engine and all ancilliaries, even with the H4 bulbs. There will still be more left for other accessories. For comparison, my MGA only had a 28 amp generator but ran a heater fan and wipers which you won't have.

The battery is only used for starting and recharging the battery after starting. The alternator should power everything without having the battery to provide any of the operating load. (maybe it would when operating the horn).

In your case, will you be using the reverse motor while the engine is running? Do you have any idea of what the reverse motor will draw? Hint: it will be different from when used as a car. Can you test the current draw in situ while you are in the car and actually running in reverse? Uphill? All of those will affect the current draw. From there it is an easy calculation based on the AH rating of the battery and the time you plan on running in reverse and how quickly you will want to top off the battery after reverse.

One point of reference: I ran one of the Odyssey PC680 gel cell batteries in my Locost. it started the car most of the time, when it was fairly new.

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PostPosted: March 10, 2020, 9:51 pm 
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Joined: October 10, 2010, 10:26 am
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Location: Santa Maria, CA
I reached out to Martin (Mgkluft) who has already successfully used this combo/design to get his SBC starter + tiny ring gear reverse to work... this was awhile ago, and I didn't think to ask about battery size/weight/cca's, but he did mention that when he engaged reverse with the Hayabusa engine idling, the draw created by the reverse would kill the engine (the Busa alternator wouldn't run everything)..

It would be preferable to be able to have the VFR engine idle while reversing, but it's not crucial.. I think I'm after a small, light battery that will move the car for 3 or 4 seconds (VFR engine turned off), then immediately start the 800 cc motor and be on my way..

If the VFR800 alternator will in fact run both the VFR engine AND the SBC starter/reverse at the same time that would be nice, but if not, I'll settle for a battery that can briefly run reverse unaided, then start the Honda motor...

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My '72 Honda N600 build log (bike engine in a microcar)...
http://www.locostusa.com/forums/viewtop ... 36&t=14452

My '63 Volvo 1800 with a turbo inline 4 build log (LNF Ecotec compliments a Svelte Swede)
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

My '59 Berkeley SE492 build log (bike engine in a microcar)... "A Berkeley With Bite!"


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PostPosted: March 11, 2020, 8:50 am 
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Standard lead acid for most applications is usually well under 40 lbs. Don't go by weight listing for marine batts that have the same group size.

The higher the capacity, the heavier they are and the specialty battery salesmen don't usually test their products to the same standard to come up with a CCA rating. It used to be just testing the battery at warmer temps for an unrealistic CCA rating that should not be compared to standard auto batts. Now it is a PCA based on an extremely short load duration.

A standard auto batt ticks all your boxes at well under 40lbs. Take a scale to where ever you go for batts and pull down a 24, 75, and a 78 series to get some real world weights versus CCA.

IMHO, all you need is a 24 with around 500cca and a weight of 15-20 lbs.

Extra cca (and weight) is not wasted money as a battery with excess capacity will still meet the need that much longer since the degradation curve is essentially the same, assuming you use it regularly.

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PostPosted: March 11, 2020, 1:38 pm 
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If I was making the decision I'd try a standard lawn tractor battery. They're small, tough and cheap. They live in a less than ideal environment (only used once per week in summer, sits all winter, crappy little charger on the engine, stored outside in a variety of ambient temperatures) and usually last 3 to 4 years or more. I used one in my Formula Ford in the 1970's and starting fully charged on Friday night it would start and operate the car all week-end...no alternator or generator on the engine. For $40 I'd give one of them a try in your car. Good Luck!

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PostPosted: March 11, 2020, 4:00 pm 
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Location: Kamloops, BC, Canada
I second the lawnmower battery. That's what I'm using, and it was plenty powerful for starting my 3.0 V6, even for multiple weekends of autocross without an alternator. I now have a 30 or 40A alternator from a Geo Metro, and it works great. I think my electric fan probably draws 25 or 30A when it kicks in, plus I usually drive with my headlights on during the day.
Kristian

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PostPosted: March 11, 2020, 4:42 pm 
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Joined: October 10, 2010, 10:26 am
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Thanks for the feedback guys.. I'll go ahead and try the cheap/low risk route of the lawn tractor battery... Seems they average about 20 lbs and ought to be up to the task :cheers:

--ccrunner

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My '72 Honda N600 build log (bike engine in a microcar)...
http://www.locostusa.com/forums/viewtop ... 36&t=14452

My '63 Volvo 1800 with a turbo inline 4 build log (LNF Ecotec compliments a Svelte Swede)
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

My '59 Berkeley SE492 build log (bike engine in a microcar)... "A Berkeley With Bite!"


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PostPosted: March 12, 2020, 4:59 pm 
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Joined: August 11, 2013, 6:03 am
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Location: CNY
I'd reckon one of the cheap "Jump starter" battery packs from Harbor Freight would work.

Harvest the cell, and slip it in place.
They're gel/AGM, reasonably compact, light-ish weight, and as long as you have the receipt you can slap it back in the enclosure and get an exchange if it goes frumpy.


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