[flash]

yup - it's only the aftermarket ones i'm worried about. it's the casing around them and the cooling systems that makes them work. the plastic shelled aftermarket ones don't have either of those.

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[PorscheG96]

The Battery Tender continues to be awesome. Today began with a cold start at 6:30a so about 50F ambient and the starter cranks with as much power as the old lead acid battery. I drove about 5 minutes away to pump a couple gallons of fuel, started the car again then drove 5 more miles to pick up a friend to autocross with me today. Started the car again and drove 45 minutes to the autocross site at Alameda Naval Base where the Myth Busters TV show is hosted. Now begins the good part...



A dozen additional starts at 75 ambient, all with the same power as the old battery. Basically started the engine, warmed it up, did 1st autocross run, let engine idle for a minute while measuring tire temps, then shut the engine off. Repeated this for my 5 autocross runs, turned off the car, then my friend did his 5 autocross runs in the next group, shut engine off and started again in between each lap. After each of my autocross laps I used my IR digital thermometer to take the battery case reading and it was ambient every single time. 72 degrees, 76 degrees, etc etc. My buddy drove back to the paddock, turned the engine off and we packed up for the day, then I started and drove 5 miles for fuel and started one more time for the drive home.



The 968 drives great with 50 less pounds in it and I've very pleased. I'm done driving this car now for several weeks so we'll see if the battery has any problems holding charge for that period of time. I may buy a trickle charger, we'll see.



I'm going to start on a better mount in July so we'll see how that goes but the proto mount worked perfectly today - SO STOKED!!!

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[Cloud9...68]

Cool; that crushes the performance of my 12 lb Deka lead acid battery. Just to confirm, your fans do run after you shut the engine off, right? I know that's a dumb question - there would have to be something wrong with your fan switch if they didn't - just wanted to make absolutely sure, since it's the fans that kill my little battery. Definitely an encouraging result.

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[PorscheG96]

I can't guarantee the fans cycled every time today but sometimes they surely did. It looks like your Deka is only 12 Ah while the Battery Tender is 2-3x that capacity and weighs 1/3 as much. <img src="/forum/images/smilies/968/smile.png" class="smilie" alt="" /> I'm not sure how many amps the starter draws but if your fans draw 12 amps for 1-2 min [let's say 10min * 12 amps for 2 Ah] after each shut down then it seems like you'd have over 80% capacity remaining and I'm surprised that your battery dies so quickly. Do 5-6 starts really use the remaining 80% capacity?

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[Cloud9...68]

Yeah, 5 starts is about all I get out of it on a hot day. Here in Austin in the summer, the fans run a lot longer than 1-2 minutes after shutting the engine down. I'll time it the next time I drive it. Maybe if the starts were spaced out enough to let things cool off, I could get another start or two, but that's about it. Your battery's performance is amazing considering its weight.

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[flash]

lithium batteries have a number of advantages. they also have their limitations. overcharging is the big one. they vent when that happens. that means you need some rather expensive protection circuitry to prevent that.



ups:

they charge faster



they do last longer (as much as 3 times longer)



they do provide more voltage per volume (about double)



they weigh less for the same amount of power (about half)



downs:

they vent upon overcharge earlier than lead-acid - very dangerous



they require expensive circuitry



they require cooling



they are not as effective in cold temperatures



they cost more over the lifetime, though they are closer in warmer areas



there is plenty of information out there about this. i'm sifting through it now. i think this can be made to work, but it's not going to be cheap, and the weight saving won't likely be as much as some might hope, after factoring in the protective circuitry, the casing, and the cooling.



here are some pages that really point out some things:

http://www.altenergy...plications/1884

uploads-downloads-files-li-ion-vs-lead-acid-WEB_1.pdf Size: 34.02 KB  Downloads: 4

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[PorscheG96]

You can't lump all lithium ion batteries into the same category. Lithium Iron Phosphate is so much safer and more stable than Lithium Cobalt Oxide, for example:



"Li-cobalt cannot be charged and discharged at a current higher than its rating. This means that an 18650 cell with 2,400mAh can only be charged and discharged at 2,400mA. Forcing a fast charge or applying a load higher than 2,400mA causes overheating and undue stress. "



http://batteryuniversity.com/learn/artic...ithium_ion



"Li-phosphate offers good electrochemical performance with low resistance. This is made possible with nano-scale phosphate cathode material. The key benefits are enhanced safety, good thermal stability, tolerant to abuse, high current rating and long cycle life."

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[flash]

i'm not. i was just pointing out some of the stuff i'm finding. phosphate cathodes are much safer than others. they still require some safety precautions though.



i'm having conversations right now with tesla. i'll have a lot better information shortly, but so far they are cautioning against use without proper design precautions.



in the meantime, here is a decent page on them:

http://www-scf.usc.edu/~rzhao/LFP_study.pdf

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[Eric_Oz_S2]

If you do a gentle cool down lap (high speed, low load) you should be able to get your temps low enough that the fans don't kick in on shut down. On very hot days, yes, they still do cycle on though.

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[flash]

he autocrosses, not tracks, so there aren't any "cool down laps". lol - heck, isn't the fastest autocross timed lap really more like a cool down lap? couldn't resist



i have no love for the gel type battery i am running now (on my third in 7 years). i would love to change it out to something smaller and lighter, and more importantly, something that lasted longer. i really hope this turns out to be something cool. i'm just very cautious, given my experience with lithium batteries, and what i am finding out there about them. hopefully i can get some definitive scientific information about safety. batteries are not generally something to screw with when it comes to that. learned that one the hard way.



it's looking promising though, and i look forward to seeing what comes up in long term results. i'd love to see yet another thing come up for the members here as a result of a member developing something.

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[ds968]

No idea if there is a connection between battery weight and longevity, or just a coincidence but that 68.8 lbs Mercedes anvil ( aka battery ) lasted a full seven years , whereas I have never gotten much more than three years ..rarely four.. from any other battery I have ever had.

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[flash]

volume equals storage capacity, regardless of what type of battery.



on that note, i am having a hard time finding a lithium battery with anything near the 70ah capacity of the interstate or 55 ah of the optima. the best i have found so far is the braille with 40. i've killed the battery a few times, with the stereo on for just 15-20 minutes. i'm definitely not looking to go with less capacity.

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[PorscheG96]

Of course, if your priority is a nice stereo then this battery setup isn't for you but when your priority is going as fast as possible within the rules that allow lightweight or smaller batteries, then it's just the ticket.



For Andy: I did 10 more engine starts with the Battery Tender while running errands around town - we have 90 F ambient temps today. I timed the fans running for a min after each stop and took my IR temperature gun to the battery case again. Inside the battery tray it measured anywhere from 99-114 degrees and the battery was 99-104. No battery overheating or charge / load problems whatsoever and perfect starts every time. I even re-started the engine a few times in the garage once the fans turned off to see if I could kill it - nope! <img src="/forum/images/smilies/968/smile.png" class="smilie" alt="" />



The blue car is now parked for a few weeks but I'm thinking that I'll remove the battery next weekend to top-up with the battery charger and report back how much capacity was required to fully charge it.

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[flash]

not necessarily. i needed to move weight to the rear, as the rear was too light, so i moved the battery



frankly, if i were to autocross, i would remove the battery altogether, and use a remote to start the car.

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[PorscheG96]

[quote name='flash' timestamp='1404104361' post='159329']

frankly, if i were to autocross, i would remove the battery altogether, and use a remote to start the car.

[/quote]

And you would get protested. Or you would race in modified class and get your ass kicked by formula cars. <img src="/forum/images/smilies/968/ohmy.png" class="smilie" alt="" /> <img src="/forum/images/smilies/968/laugh.png" class="smilie" alt="" /> <img src="/forum/images/smilies/968/rolleyes.gif" class="smilie" alt="" />

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[flash]

lol - actually, i would probably be driving a formula car

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[type968]

So whatever came of this?  Anyone test the battery charging output? Car explode?

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[PorscheG96]

So whatever came of this?  Anyone test the battery charging output? Car explode?

 

I don't drive my 968 very much these days - just 3x since Nov - and while theory is nice and all, I've been very happy with my Battery Tender in real life.  If I don't drive the car for 3-4 weeks then the battery is dead and I have to charge it, so now I just leave the negative terminal disconnected.  When I was driving the car more often mid-late last year then I didn't have to worry about it discharging.  I drove the 968 today with the Battery Tender and autocross it again this Saturday.  Cloud got a Battery Tender as well and loves it last I heard...

 

My '94 968 is still a street car but now I'm building a 968 race car with my '93 coupe and will use the same battery.

 

Also have a VPH-750 in my 2004 GT3 which has fancy shut-off electronics and smart software to prevent high rates of charge/discharge.  I guess that's better than Battery Tender but also costs nearly a grand.  For just $250 the Battery Tender is a good deal and saves 30 lbs.

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[ds968]

What kind of ( $ 250 ) battery tender are we talking about here ? The name brand " battery tender " s are about $ 50 just about everywhere you look. I have two of them hooked up ; one to tne 968 and the other to the MBZ both cars are rarely driven now, but the trickle chargers work like a charm to keep those batteries like new. I must have missed something in an earlier post - is this $ 250 a high end special type of charger with a lot more bells and whistles ( functional features ) than the usual battery tender ?

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[PorscheG96]

Dan, it's a 3.75 lb wonder battery.  http://www.amazon.com/Battery-Tender-BTL...B00F9LPL5E

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