Yea i took a look at a thread over on the lexus IS forums where someone tried using one as his DD:

http://www.clublexus.com/forums/is-f...d-battery.html

Looks like he had some good success except for the occasional mess up of leaving lights on and such. Looks like he is using different packs than you are.

I am floating more towards the Deka battery just because I have seen some better reviews compared to the PC680.

 

Interesting.. Wonder if he was using the defective A123 cells? I use the heavy PC680 for DD now and the LiFePO4 for track days. The 'ol 38lb Interstate LA battery is what I use come winter.

 

lol. "heavy" pc680

 

My Voltphreaks VPR40S LiFePO4 battery weighs 4lbs less than the PC680 but has 3x the capacity. It does retail for 15x more though. I had very bad service life and capacity issues with both Braille and Odyssey batteries. I'll never go back to them again.

 

honestly that is exactly what I have been hearing. All the searching I have done on various sites shows that the Odyssey lightweights dont average more than a year or two under normal use. I would love to make a "replica" of the VP LiFePO4 battery you have using individual cells like Spin did...but as we all know, by the time you get the science down with overcharge protection and cell balancing you almost spend just as much as your battery costed lol (assuming you are looking for a battery capable of DD)

 

I have been running an Odessey 925 in the trunk...so far they seem to last about 3-4 years bwfore they crap out. Cant justify the price yet for 15lbs when I need to loose twice that

 

I only ever got a year of service ever and even then it would never make it through a summer event without needing a jumper box due to the fans running hard after shutdown etc. i can leave it all winter uncharged now because my 9# battery has the capacity of a 45# lead/acid battery. I can't justify the price of the entire car any more than hookers & blow ....

 

The rotary's heat surely makes for faster battery failure. Could there be a worse place for a battery box than directly in the radiator wash and a few inches from the block? Sounds like trunk mounting doubles the 1-2yrs a PC680 underhood life at the cost of the extra battery cable weight.

LiFePO4's are similar, too much heat is bad.

 

i never really understood the benefits of the washer relocation; makes far more sense if you plan on getting a light battery to retain use of the oem battery box considering it has a somewhat effective cooling design. Although that wouldn't do you too much good when sitting between track runs in some sweltering engine bay heat and no air-movement lol.

And I never really saw a big deal of battery cable weight. I mean taking a one line 2-4awg cable to the trunk would weigh no more than an extra pound or two. Best of both worlds would be a relocation to under the passenger seat .... unfortunately SCCA doesn't allow that for obvious reasons.

 

I am watching this thread and very tempted to try and build my own battery.
I would be using mine as a daily driver, you consistently talk about it as a track day only application, would it also work on a daily driver.
My understanding is that batteries work better when they are used nearly every day, making this ideal for daily driver. I do have a battery charger/started that I can carry for daily use in case of emergencies then obviously remove for Autocross/track.

 

That is exactly what I am looking to do. I think the issue is going to be how well the alternator can keep the battery topped off when using it daily. I would suspect that you would need at least 12 cells to have enough reserve and charge capacity.

 

'If' you dd and so charge daily, there's no more need for anything more than capacity to easily start the cold car a few times wo/excessive voltage drop. That's the same as for track use. Both DD and racing is 'doing something' and (re)charging the battery regularly. A small 'electrical buffer' is sufficient. The alternator is fine, why wouldn't it do a good job?

A different situation.... sit in traffic with air conditioning on blasting the megawatt amp for hours a week during stop&go commutes. Then you need more capacity. You need low voltage drop, high amperage supply.

 

Sorry, haven't read this whole thread, but has anyone tried adding a supercapacitor to stabilize supply?

Here is a paper on the topic.
http://www.ipcbee.com/vol8/25-S065.pdf

 

The only reason I would say having more cells is "better" is only on the terms of convenience for a DD, you know like a set-it-and-forget it type thing (which of course is something that isn't always the case when modifying a car)

One thing that would be interesting, if you haven't already, would be to connect the LiFePO4 battery on your car and let it sit for a week. It would be interesting to see how much the parasitic losses are on the RX8 and how much they affect the Lithium chemistry.

 

Interesting.....I have tried using 12V BatCaps supercapacitors

http://www.batcap.net/Products/BatCa...0/Default.aspx

and I've had them starting and running the car w/400A for upto 8 seconds, otherwise a 5AH battery essentially. Something caused one of the cells to start leaking, thinking maybe the heat, so removed it. The 400 model I have - inside is 6 light plastic cells wired in series, each a supercap. As I remember it started the car a bit slowly, but ran ok.

At ~5lBs, a LiFePO4 battery would have more capacity. The combo of both as the paper describes might be interesting if it did anything for performance and economy.

I also had tried a 30F Ultracap to buffer the PC680, but didn't find any noticeable improvement.

 

Has nothing to do with LiFePO4 chemistry, but I just ordered a Deka ETX18L to see how that fairs compared to you guys' experiences with the PC680

 

@Spin9k you have inspired me and as my wife's car battery is currently dying I an going to copy your design to make myself a battery and give her mine.
Here is my shopping list
4 x H-38120HP LifePO4 Cells
3 x 2 Hole Bus bars
4 x 2 hole spacers
I will buy the power cables locally
Will I also need the 12V 6A LifeP04 Charger? Would a normal car charger suffice? If I do need the charger it is something I would just connect to the car every couple of weeks as part of maintenance?

Again I use my car as my daily driver and for Autocross. Looking forward to any feedback you might have to offer.

 

I've revised my own setup from that in this thread, but considering you're making the battery as described.... As to charging ...yes..and no. I now use an intelligent LiFePO4 charger (Chargery 410B plus I fabed some battery charge clamp leads and use 2 x the 6" 5-wire connectors... so I can can connect/disconnect the sensor wires easily). While charging from another 12V battery, the Chargery monitors cells individually with the sensor wires, then charges or discharges each cell to maintain equal cell voltage. This balance charge needs to be done before first use and then occassionaly to make sure the cells stay balanced (don't get unbalanced too much).

During normal in-car use, the alternator charges the battery as it would a Lead Acid which is sufficient, if not ideal for a (unknown) period of time. I wouldn't hook a L-A charger to the LiFePO4 battery alone, the charging algorithms are different.

Only other thing I learned is to use 2 AWG vs. 4 AWG wire to minimize voltage drop if you're going any distance. It does make a difference, easp. starting from cold. Hot start is much easier on the battery. Personally I use a PC680 for DD, I'm afraid I'll accidently leave something going and drain the LiFePO4 battery too easily, after all its only 8AH capacity. If you don't want to swap and are careful, it could work for DD I imagine.

 

What about adding a kill switch into the line. Obviously there a pros and cons to this:

Pros:- virtually no-one will be able to steal your car, the battery should last much longer

Cons:- Other then adding weight and another point of failure, you can not use remote central locking, the alarm wont work, the clock will never be right again and I am not sure if the ecu will like to have no power most of the time.

 

You're right, a kill switch would work, adding only the extra wire, weight, and POF. One positive, it would reset the ECU's cruise fuel trim setting every disconnect

 

Well I have ordered the PC680 for now but, I think I will try out the LiFePO4 anyway. I could probably sell a few at the track if it works well enough and then it wont cost anything.

 

http://www.nytimes.com/2012/10/17/bu...ptcy.html?_r=0

In regards to A123 batteries.

 

^ I never liked the A123 form factor, quality problems, counterfiting, sourcing difficulties, and expense. Headway's are better IMO.

 

Their form factor may have sucked but the formulation they developed has changed the lithium landscape significantly. I did a bunch of early proof of concept work with them.

Sad sad day.... and now the politicians are going get involved and turn it into something else entirely.

 

Agreed, one search of A123 on ebay brings up a TON of fakes