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Old 01-26-2017, 06:56 PM   #261 (permalink)
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I looked what hobbyking has on offer nowadays, and found this:
https://hobbyking.com/en_us/turnigy-...case-pack.html
5Ah, and a 70C (-->350A) 10 second burst rating, and only 32 bucks. For a bigger engine, just use two. I guess I'll have to buy quickly before you guys buy 'em all.

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Old 01-26-2017, 07:11 PM   #262 (permalink)
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I went a different route; 4s4p bank of LiFePo4 cells at 8 AH each.
This is for my daily driver/race car (daily racecar?) and weighs around 12 lbs. I went this route after looking over lead acid/cap combos as well as just straight caps.

If I were constrained for space/weight, I'd probably do a boost converter setup with a single larger iron cell and a charger dedicated to that and a cap bank just big enough to start whatever it was I'm working on; however the size of this setup is rather small at less than 6.5" x 6.5" x 6" high, with posts.

With my recently added requirements of being able to run the water pump and fans with the engine not running, caps no longer made sense.
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Old 01-26-2017, 09:46 PM   #263 (permalink)
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Quote:
Originally Posted by AlexanderB View Post
I looked what hobbyking has on offer nowadays, and found this:
https://hobbyking.com/en_us/turnigy-...case-pack.html
5Ah, and a 70C (-->350A) 10 second burst rating, and only 32 bucks. For a bigger engine, just use two. I guess I'll have to buy quickly before you guys buy 'em all.
Not sure that would be a good fit for 12v vehicle battery replacement. The reason people choose LiFePO4 despite the lower energy density is that a 4s configuration very closely matches the voltage of the lead-acid battery and alternator output of a vehicle. Not only that, but it's more tolerant of the non-managed charge/discharge cycles that the battery would be exposed to.

You would probably need to build a charge circuit to run the battery you listed, and it might be damaged by the low voltage since it wants to be at 15v.
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Old 01-26-2017, 10:12 PM   #264 (permalink)
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Old 01-27-2017, 02:17 AM   #265 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
The reason people choose LiFePO4 despite the lower energy density is that a 4s configuration very closely matches the voltage of the lead-acid battery and alternator output of a vehicle. Not only that, but it's more tolerant of the non-managed charge/discharge cycles that the battery would be exposed to.
This is the other reason I went lithium-iron, and got a 270 amp underdriven alternator internally regulated at 13.8v hot. Ancillary testing by another person with an 18650 iron pack on his car showed that the overcharge current was negligible at the factory alt/ecus 13.8 float voltage, so it all makes sense for me to go that route.
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Old 01-27-2017, 05:02 AM   #266 (permalink)
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The battery, between 17.6V full and 12V empty indeed averages at 14.8V and thats a little high for the average car.
I wouldn't mind modifying my alternator to charge to 16 volts instead of 14, running a slightly higher voltage would also increase the performance of the fuel pump and other components, my only worry would be the lightbulbs, though those are cheap, easy to replace, and I could always hook up a dc-dc converter for stuff that really doesn't like 16V.

3 lihv cells (13v full) is just a little bit too low, unless I modify the alternator to put out less than the full 14+ volt, which would then further reduce the power going to the fuel pump, which in my application would be bad, not better.

LiFePO4 is a good straight swap for a car battery, but they're either pricy or not very good so you need more of them in parallel and then they're still pricy, or you need a cap bank thats pretty pricy too.

Oh, and balancing would either be done by just balance-charging it every now-and-then, or by building a little balancing board to permanently connect.

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