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LiFePO4 batteries ?

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  • #16
    I always cut my batt disconnect while the car is in storage. Even for a night. Do you have a link or more info on the current limiting resistor/ultra capacitor option. I want to make this happen! I'm looking to be spoon fed, haha... Too many projects, not enough time...

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    • #17
      This guy has some info:

      https://www.youtube.com/watch?v=qweEl8Wypzg

      I can say that a 6x 350 F ultracapacitor bank (Maxwell) + a 4.2 Ah LiFePO4 Zippy Flightmax battery with a 0.1 ohm resistor (50W capacity needed) could start an SR20DET with a standalone in all but <32 deg F weather. The higher compression SR20VE would not start at all with that setup, even with the current limiting resistor dropped down to 0.04 ohms or so, which probably doesn't limit charging current much.

      The idea behind the current limiting resistor is to limit charging amps, but it also makes the battery contribute less amperage during starting.

      The 8.4 Ahr battery can take twice the charging current, and if you're not starting things in freezing temps often, you can be more aggressive on charging amps. Lithium batts do not like charging amps when very cold.

      Or you could go with a cheap sealed lead acid batt like 7.2 ahr that weighs a bit more but is dirty cheap and use that plus ultracaps to help take a lot of the high starting amps. The SLA will give you the charge capacity, and you don't have to worry about charging amps really. Although the 4.2 Ahr LiFePO4 batt is pretty cheap ($45ish), so you could always just replace it if it gets toasted with no current limiting resistor.
      '18 Chevrolet Volt - Electric fun hatch for DD duty!


      DefSport Koni Sleeve and Spring Perch Buy!!!
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      • #18
        Awesome! Looks like the 8.4ah zippy is my best bet as I'm about 10.3:1 compression. I'll take the 1.2lb penalty over the 4.2ah for the added reliability. I'll probably put the resistor in, I can always bypass it or take it out if it won't start. I'm in Phoenix so freezing temps aren't really a concern for me. Thanks for the detailed info!

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        • #19
          You can only put in a resistor if you use ultra capacitors to do the heavy lifting. If you put a resistor in with only that battery, you'll never crank the engine and roast the resistor as you put a brief pulse of hundreds of amps through it. P = I^2 * R, so even at 0.05 ohms, you're dissipating 2,000 W for a brief moment with a 200 A current (which is probably about average).


          What engine do you have again?


          The eBay 500 F caps are totally Chinese rebrands of stuff that failed quality control, but they're not horrible if you get lucky enough to get some that aren't crappy. That will start to get you in the range of what you need to help the battery start the car. I'd recommend a 0.02-0.05 Ohm resistor, as I felt like the 0.1 ohm resistor buffered even the 4.2 Ahr battery a bit much (lots of time to charge the cap). Just don't let the battery draw down a lot and the charging amps will be less once the engine starts.
          '18 Chevrolet Volt - Electric fun hatch for DD duty!


          DefSport Koni Sleeve and Spring Perch Buy!!!
          http://www.nissanroadracing.com/showthread.php?t=5902

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          • #20
            I definitely plan to do the caps. I'm running an sr, 10.3:1 compression.

            I hear with these batts it's temps that kill them. Do you guys run them in the engine bay without problems in 120*f ambient temp? Here in Phoenix, low temps aren't really a problem. I'd say 32*f is about the lowest it'd see.

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            • #21
              32 F, low battery, and full charging amps = dead LiFePO4 battery

              Same thing happened to me, it was about 32-34 F when I started the car on the trailer before a track event. It took a try or two given how cold the engine was, and I think I didn't have enough cold start enrichment on the Infinity then. Once I shut it down, I knew the battery was dead. At least a couple dead cells. Towards the end it was like a dead short and wouldn't start even with a jump box connected in parallel.

              I'm in Houston, so it probably doesn't even get as cold as in AZ as often.
              '18 Chevrolet Volt - Electric fun hatch for DD duty!


              DefSport Koni Sleeve and Spring Perch Buy!!!
              http://www.nissanroadracing.com/showthread.php?t=5902

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              • #22
                Hey everyone long time no see! I hope it isn't inappropriate for me to hijack this a little bit. I have been using a Big Crank AGM (probably either the ETX16L) motorcycle battery for over 5 years now and I've been staying half an hours drive away from where my car is. Between some issues I've had with the ignition switch and the age of the battery coupled with the extended intervals between starts (no tender, stored outdoors...) I've been thinking a new battery (and ignition switch) are in order. I had originally planned on upgrading to a Shorai LFX18A1-BS12 but based on what I'm reading here it shouldn't be a "drop in swap" or might not last the "two to four times" longer than the existing battery that it's advertised as, especially not without significant investment in additional components like a BSM or current limiting resistors and ultra caps.

                So I guess it's better that I stick with what works rather than hope for a lithium battery to last me 10-15 years?

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