LiFePO4 starter battery

[scribble]

Hi All,

I want to make a lifepo4 starter battery so that it can be a flat shape and be inside the passenger cabin.
I already have the cells, 24x A123 26650 type.
I plan to make a 4S6P pack.  Each cell is 2.3AH and can do 60C so thats plenty of current for the starter without stressing them.

I was looking at something like this:
http://www.ebay.com/itm/12V-14-6V-60A-LiFePo4-Battery-BMS-LFP-PCM-SMT-System-4S-eBike-battery-Pack-DIY-/321046691218?pt=LH_DefaultDomain_0&hash=item4abfdfbd92
But I'm not sure on how to wire up the pack so that all the cells would be balanced?

Also, since I haven't found a BMS will handle the starter moter current draw, would it work to run a relay that takes the BMS out of the picture when the key is turned so the load is drawn directly from the battery rather than through the BMS?

Cheers,
Mark

[Zepnat]

Just curious what are you starting? Is it a portable jump pack? I'm thinking aeroplane from the wording of your post dunno why.! 6P is some serious power for Diesel engine?
I wanted to build 4S2P jump pack when going out to dead cars at one time, there was a good deal on a123 cells from dewalt packs but I had no money and by the time I had the money of course they were all sold 

[scribble]

It's for a petrol car but I want plenty of reserve capacity so as to not drain it too low when using power without the engine running.
It's also going to be in the boot so probably around a 3m cable run which will increase the current draw.

You can get a123's from hobbyking although not cheap.

[Legit-Design]

Also, since I haven't found a BMS will handle the starter moter current draw, would it work to run a relay that takes the BMS out of the picture when the key is turned so the load is drawn directly from the battery rather than through the BMS?

You could use a contactor. You can find contactors which should reliably switch 100 odd amps on ebay. But car alternator doesn't supply clean stable power. You can easily have 80 volts on load dump. Usually lead acid battery just eats away all the peaks and supplies the valleys to make it more stable. Try taking car battery out of a running car... kidding don't do it you will probably just damage everything electrical... or not, who knows? I've only heard stories.

[Rick Law]

Starter batteries are "dime a dozen" in the USA.  You can get an 800A cranking power ones in the $100 range, complete with battery jumper clamps, cable, and all the necessary safety setups for something carrying 200Amps and up.

[ConKbot]

IVe put together a 4s 4S A123 26650 (or whatever their chinese made offshoot was? olevin power?)  jumper pack that works great. Ive started a V8 Jeep Grand Cherokee with the main battery removed.  It sounded a touch slower than the normal starter battery, but I did have ~10 feet of 6 or 4 awg wire and jumper cable clamps instead of a proper low resistance connection.

Since its a jumper pack, no BMS in mine at the moment, just use a hobby charger to balance charge them, and a 100A thermal circuit breaker (curve on it says its good for a 4x overload for 30 seconds)  As you mentioned reserve capacity, the battery would only be 13.8 AH, only a "33 minute" reserve capacity. Vs 80-100 or higher for normal FLA starter batteries.

You can get a starter battery with plenty of power,  but to get the energy in the battery for a good reserve capacity is $$$

As far as diesel engines, most definitely not   I tried to jump a diesel skid steer and it just laughed at my 4s4p pack.  Diesels need 100-200A to run the glow plugs/heater grid for 10-15 second, and then need to run a very large starter that has to crank a very high compression engine. I'd expect a jumper pack to need to do ~300A for 30 seconds to even think of starting a small (4 cyl)diesel, and 600+ for a V8.  For a main battery with a good lifetime, 800-1kA + capability, even more for when its cold (diesel pickups come with 2 800+ CCA batteries for a reason)