Help me make a A123 battery pack...
 

I need a simple and small A123 battery to replace my current lead acid car battery. This is because I need it to be able to drain a lot more than this lead acid since I will be going alternatorless in a bit and dont want to get a huge and heavy optima.

I have a few questions;
-How many A123 would I need to have THE SAME amount of storage as my current battery?

-How many A123 would I need to have 50% MORE storage than my current battery?

Well, first off we need to know what your current battery is. :)

I've been alternatorless in the Paseo for a couple years now. I have a group 24 (~70Ah) lead acid deep cycle battery and it easily lasts 50 miles in summer driving (even with headlights on and wipers going). It cost me about $70.

If you go with lithium, CALB or thundersky batteries are much cheaper than A123. Quality is also lower, but IMO this won't be a problem in this relatively simplistic system you're looking to create (it isn't for most EV owners using these cells). Even still, a 60Ah 12V battery from CALB will cost you around $330 plus shipping. Now, this battery will probably last much longer than the $70 lead acid battery. It could possibly last the life of the car, or longer. Now, you also have to get a lithium battery charger. Some are as cheap as lead acid chargers, so its kind of a wash there. The weight you save will be in the area of 20-30 lbs. So, there are plusses and minuses.

Lets say I want a 60 AH capacity, to keep going for a while. I can get these for $2/battery;
10 x GTL 2000mAh CR123A 123A rechargeable battery Blue | eBay

They are 2000MAH (so 2AH?) at 3.6V. So I put them in series in 4 (14.4volts) to run the car (thats what my car uses running with the alternator). So each series of 4 only has 2AH? So for a total of 60 AH I need 20 series = 80 batteries?? That sounds like a lot of batteries... my math has to be wrong lol

Nope, your math is right. Those cells are only 2Ah.

Also, those are lithium polymer batteries, they're a bit more volitile than LiFePo4 (lithium iron) batteries and are known to overheat and cause fires (laptops) much more easily if they're not strictly maintained. I don't think it would be a problem for this application as you aren't stressing them much, but its something to keep in mind.

Another thing to keep in mind is you do not want to cycle your lithium batteries past 80% depth of discharge, 70% is preferable as a shallower cycles increase the amout of cycles you get out of the battery. With lead, this number is only 50%. So, my 70Ah lead acid battery really only has a usable capacity of 35Ah where as your 60Ah battery has a usable capacity of 42Ah.

check on customfighters.com

There's aDIY there on A123 Packs, including links and where to buy quality parts.

Ahhh that makes sense.

I've been looking at a123 reviews and tests and they say you can discharge them 100% and retain full power... I've read this a bunch of times online now, is it wrong and I can really only do 70%-ish.

Also, can I just connect the pack straight to my alternator to have it replace my current battery? (simply replace the + and - in my current battery and put them on the + and - of the a123's?)

I would like to clarify something before you really put 20 of these in series and blow everithing electronic up in your car : )

Staying with your battery example:

You would either need to put 4 n series (14.4V 2Ah) and then 30 of these series packs in PARALLEL (2Ah X 30 = 60 Ah )

OR
30 cells in parallel (60 Ah at 3.7V) and 4 of these packs in SERIES (60Ah at 14.4V)

So 120 cells either way.
I would do the first version.

Not being picky just making sure you got the theory down regardless of your battery choice.

The nominal voltage of the type of battery that A123 produces are LiFePO4 batteries. The nominal voltage is 3.2 volts, not 3.7(lithium polymer and lithium cobalt have the higher 3.7 and 3.6v nominal, which is a bit higher than what we can use for automotive retrofit). They are fully charged when you bring them to 3.6 volts. They seem to tolerate anything up to about 3.85v but there is no reason to bring them that high. When charged most LiFePO4 settles to 3.4v or a little lower so you will be at about 13.6 when parked. Draw some load for awhile and they are basically around 3.2v for almost the entire discharge, lower is discharging at a heavy amperage. After they are below 3v without much of a load their voltage drops off a cliff quickly. Just for the basic understanding of these cells.

A123 may be a decent option but IMHO, expensive and hard to get.
A better bet would be to get Headway cells, cheaper and 4 of them in series will start pretty much any car or motorcycle. 10Ah each, just parallel them to get the capacity you need. The last I checked I think you can get them for about $18 each so each paralleled 12.8v 10Ah set would be $72 plus shipping. Most of the people selling headway cells have little plastic frames you can buy to hold the cells together along with busbars that can be bought to connect them in the series/parallel configuration you need. Your Focus probably has a 40Ah or 50Ah battery in it.

The tough part about this is to figure out the capacity you need. Do you ever drive across the country or even the state? If so, do you have a plan for that?

^Ok ill look into those cells in a sec.

I do drive pretty far sometimes (18 hours max). Although to complement this alternator-less thing I have; 35w headlights and LED everything (taillights, interior lights, etc). Also, I plan on fitting a solar powered roof in about a month or so (ill make another thread for that). What capacity do you think I can get away with? Do I even need it that high?

The only way to answer that is to measure the actual amp load with all yer accessories on. ;)