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Old 02-09-2012, 11:18 PM   #1 (permalink)
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LiFePO4 setup for gasoline car?

I'm considering a LiFePO4 to power everything but the starter motor. The starter lead to oem battery would be hooked up normally but the 2nd smaller battery cable that goes to the fuse box would instead be hooked up to the Li battery pack's + terminal.

That cable (and its current) are small enough that I could put a switch in the circuit to power the car off the oem battery if I do run down the Li pack.

Both batteries would be grounded to chassis. IMHO they would be isolated from each other. Correct??

Oem battery would be still connected to the alt via the starter/alt cables and so would be charged for service. I can cut the alt circuit so I won't need to apply ~14V to the oem battery constantly, only for a short while after starting the car, and every now and then to keep the battery topped off.

BATTERY PACK with PCM (Protection Circuit Module):
LiFePO4 Prismatic Battery: 12.8V 40Ah (512Wh, 100A) with Balancing PCM (48.0)
Battery has a 15.2V peak; I think that would be the open circuit voltage and what it provides to the clock & computer etc. at low current draw?? Seems a bit high to me, for use in a car.

Charger, 10A
Smart Charger (10 A) for 12.8V LiFePO4 Battery Pack, Worldwide use, CE listed

Does this combination make sense? Cost would be well over the cost of a "regular" battery but similar to a ~65AH AGM deep cycle Odyssey battery + charger. The disadvantage of the AGM is that to protect battery life you really only want to discharge about 25%, down to approx 75% charge level, on a regular basis. Thus I'm considering a LiFePO4 setup instead.

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Old 02-10-2012, 02:07 AM   #2 (permalink)
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I guess I’m not sure what you want accomplish with this. I’m guessing you are trying to take load off of your alternator and apply that to your lithium battery.
First off, your lithium battery will degrade faster the deeper you draw them. With only a 20 percent draw these batteries last A LONG TIME! Unfortunately nobody wants to pay for a pack 5 times their current draw.
Second, a LiFeP04 is about the most exotic expansive battery out there. This is great for large electric motors as they have very low internal impedance (high maximum amperage). This is not something you would need for accessories.
I would just by an inexpensive RV battery for this. In fact, I would get two and a contactor. Then I could start a small car engine, run all my stuff, and then charge up at night. Finally if you get low on the road and need a charge, flip a switch and start charging. All for less then $500.
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Old 02-10-2012, 06:42 AM   #3 (permalink)
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Yes the idea is to de-load the alternator. I've already worked out the details for switching the alt off electrically by interrupting the field wire (Civics have an unusual 5-wire circuit,and none of the 5 is a ground). My commute is a bit over an hour each way so I need some significant capacity to cover that.

LeFePO4 really only likes a 20% discharge?? That's my concern with the AGM. Wouldn't be much different if I used a FLA deep cycle, I don't think. For any lead-acid, I think I could discharge it down to ~20% but just not very often. LiFe could discharge that far all the time, I thought?
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Old 02-10-2012, 08:54 AM   #4 (permalink)
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For lead most EV users like to use a 50% dod to maintain decent cycle life (800 cycles but thats usually optimistic). LiFePo4 are rated at usually 2-3000 cycles at 80% dod and 3-4000 at 70% (probably an optimistic number as well), but there isn't too much info out there on actual long term use yet.

The resting voltage of a single lithium cell is ~3.35V. That gives you a starting voltage of 13.4V vs 12.7 with lead. Its not huge, but it will keep your headlights and other things a bit closer to the target voltage, although there will still be voltage sag (more or less depending on temperature and load, but still probably less than lead acid). You will not be anywhere near 15V though. The max charge voltage is 3.65V/cell for most lifepo4 cells (if you want to maintain good battery life), but once they're off the charger they settle around 3.35V.

My question to you is why even keep the lead acid starter battery? The lithium batteries can easily start your car. If you need to go on a long trip there is no reason why the alternator can't be switched on to maintain them. For the rest of your driving you can just run without the alternator and charge them when needed.

I also wouldn't buy those cells. I'd buy some individual thundersky or calb cells. Those don't even have a name brand and the TS and CALB cells are rated for more cycles and higher C rates. I have cells very similar to those in my Prius PHEV kit (40ah mottcell cells), so I'm quite familiar with using them.
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Old 02-10-2012, 12:46 PM   #5 (permalink)
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Quote:
Originally Posted by Daox View Post
My question to you is why even keep the lead acid starter battery? The lithium batteries can easily start your car. If you need to go on a long trip there is no reason why the alternator can't be switched on to maintain them. For the rest of your driving you can just run without the alternator and charge them when needed.
This is correct, it could start your car. I don't think a car alternator would do for charging Lithium batteries. They require special charge conditions, and I think a higher charging voltage (not sure off the top of my head, I do not the charging cycle is not linear).

The lithium batteries CAN be discharged all the way if you like, it just makes them wear out faster. Its something you would have to weight out, size and cost of pack vs. expected life time.

I don't know your tech abilities, but building a pack can be troubles. Lithium batteries need to be protected during charging, discharge, and from short circuits. If you do not get these things right, you will ruin your cells.
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Old 02-10-2012, 11:02 PM   #6 (permalink)
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Thank you to Electictracer and Daox.

My electrical skill is um, medium? I have more knowledge than an average person but absolutely don't feel up to designing and building a BMS. I've seen a couple writeups of what went into a BMS and I got lost almost immediately. Unfortunately I'm not ready to acquire half an Elec Engineering degree to get through the project.

I can solder and read simple circuit diagrams. I can calculate Ohms Law if I use a calculator (haha). I know what ohms, volts, amps and amp hours are which is very helpful on a project like this one.

If you can suggest a LiFePO4 pack of ~40-60AH, I'd be interested to see it. I do think the BatterySpace 40 AH pack is a pretty decent unit, more notes are below..

The BatterySpace 40 AH pack in post #1 is based on this pack, $245, sold by electricmotorsport.com. Anyway, BatterySpace also sells the $245 version which is without the PCM - protection control module. Electricmotorsport also sells some Thundersky stuff but not built up as a size pack I would need.

BatterySpace PCM: Protection Circuit Module (PCM) with Equilibrium Function for 4 cells (12.8V) LiFePO4 Battery Pack at 100A limited

Most importantly, the PCM does these jobs:
  • Keep 12.8V LiFePO4 Battery pack from overcharge ( 3.90 V/cell )
  • Balance each cell at Max. 3.60V/cell +/-0.03V ( required voltage tolerance within 0.2V for each cell)
  • Keep 12.8V LiFePO4 Battery pack from over-discharge (2.00V/cell)
  • Limit 12.8V LiFePO4 Battery pack's discharging current below 100 A.
Please see the link for all that it does.

Does it seem more sensible?

I feel I need an assembled pack with some kind of BMS, and of course a charger. The 40 AH pack would certainly start a car but the PCM limits current to 100A. Most days that would start it but on a real cold day with thick oil etc. etc. and everything working against you, the PCM limit of 100A might stop the show. BUT if it keeps the pack healthy without me needing an engineering degree, it might be a good way to go.

As noted by someone else in this thread, LiFePO4 should tolerate 80% DOD (20% remaining) and give ~2K cycles. Lead acid would only give a few hundred cycles at that rate. For this project, that's the main benefit of LiFePO4. With lead acid, I'd need maybe 150AH of battery to get a usable 30AH of charge, and ~200AH if I want >30AH. The weight and cost of that would be insane. The 40AH Li pack could give me 32AH without strain. And if I'm fortunate enough to be able to buy a second one a year from now, the pair would give a usable 60-64 AH, enough for many hour's drive.
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Last edited by brucepick; 02-11-2012 at 12:07 AM..
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Old 02-11-2012, 08:03 AM   #7 (permalink)
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The Lithium pack is also light enough to carry for those times you park your car without access to an outlet.
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Old 02-11-2012, 08:48 AM   #8 (permalink)
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Quote:
Originally Posted by jakobnev View Post
The Lithium pack is also light enough to carry for those times you park your car without access to an outlet.
Yes. And that's another reason to keep a battery in the oem underhood location. To keep the computer and MPGuino powered while the Li pack is out at my office desk being charged. Normally hopefully, I can just charge it at home each evening but the flexibility to charge outside the car is nice.

It's true that a much smaller 12V battery would keep the electronics going, but a standard one will support the entire car if needed in a pinch.
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Old 02-12-2012, 09:21 PM   #9 (permalink)
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You might be able wire in the starter in before the BMS so it doesn't pull the current through it.
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Old 02-12-2012, 11:07 PM   #10 (permalink)
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Quote:
Originally Posted by Daox View Post
You might be able wire in the starter in before the BMS so it doesn't pull the current through it.
True, if I hack into the BMS package. I think the battery pack with BMS might be a sealed unit, from viewing the photo and reading the description. Link to pack is in first post in this thread.


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