Battery pack help, way out of my league

[scmountain]

I have bit off way more than I can chew, In my desperate struggle to figure this out, I stumbled by this site, and the knowledge base is amazing, problem is, it is a bit beyond my expertise. So please bare with me...
I am building a 12v system, The plan was to make a pack with A123 26650 M1's. The pack setup would be two, 4 cells in series (13.6v avg) wired in parallel. (4 single cells in series, two of those in parallel)
This pack would then be wired in parallel to a 12v 20ah AGM boat battery. (not sure what kind of controller i need for this if it is at all possible) The boat battery will be fine with the 13.6 volts as it is made to handle alternators cranking beyond 14.2v... I haven't found any info on people doing something like this, which has me scared that there might be a reason for it.
I have the A123's already (dewalt dc9360) and using these LiFePO4 batts would increase Amp hours by 25% for almost nothing in weight...I did save the BSM, but not sure how I could use it to help my pack as im not keeping the original voltage 36v.

and here is the juicy part.
I have a 6watt 12v solar panel hooked up to help reduce power drain. Having this many variables has really made my google quest quite difficult. (trying to up to solar wattage)
Any advice or places to look would be awesome. Sadly as 90% of internet article regarding the use of the M1's is for EV's which takes the voltage way outta my range haha.

the application if you are interested...
A portable cooler stereo! I am on my 3rd version now, trying to do a fine dance between weight of cooler w/o "refreshments" and play time of tunes. this is why the M1s would be so awesome if I could get it to work. I know I need some type of controller to shut the batts off before they deplete themselves beyond repair as the boat batt is deep cycle it can run till the stereo auto shuts off do to low voltage.
version1 of the cooler had 13watt solar and 50AH (optima yellow) that thing could blast any level for days, just weighed a million pounds!

Thanks in advance to any insight I recieve, this is my first foray into the battery world, real packs, not duracell duct tape packs from my youth

 

[dogman dan]

Well, for starters 12v lead and 12v lifepo4 are really pretty different voltages. So depending on what you use to charge, the lead will be overcharged 1v or the lifepo4 undercharged 1v. This makes things a bit awkward, both on the top of the charge and at the bottom, when it will be easy to overdischarge the lifepo4 a bit more than would be good.

Here's how I would try to simplify it. Run the cooler on the lead, and then run the tunes on the lifepo4, using a bms, or at least a low voltage alarm from the RC toy stuff.

I'm a bit confused by your explanation of the A123 pack. But if I get it right, you have 8 cells. Make them into 4 sets of parallel pairs, then do your series connections. Then you have a 2p-4s , 12v pack that charges to 14v. It can be charged as high as 15v to get it fully charged.

Since the A123 pack is the smallest, I'd put the solar panel on the lifepo4.

At some point, possibly not too long into the run time, you will lower the voltage of the lifepo4 pack to very close to the voltage of the lead. At that point, you could go ahead and connect the two batteries in parallel. Still using some kind of monitoring on the lifepo4 voltage of course. At some point at the end, you will need to unplug the lifepo4 because 12v is completely discharged for lifepo4.

 

[scmountain]

Dogman,
Thanks for the reply. Yes you are Correct on my plan for the A123 pack.
I like the idea of the batteries (AGM and LiFePO4 pack) being independent as to protect the LiFePO4s. I will look into a bms that shuts off their relay when nearing depletion.


Is there any special solar controller needed for the LiFePO4s? I have a standard controller, but now reading how these little batts are Divas, albeit high power Divas, I wonder if they need a specific module.
As far as the top end voltage goes, Neither battery is switched on unless the stereo is on, so I was thinking that in theory, the voltage differential would be negated by the draw from the stereo.

Also let me clarify, the cooler is the housing unit, the only electronics are the LEDS, relays, and the stereo.

Thanks
SC

 

[dogman dan]

Ahh, I was thinking it was a portable 12v refrigerator and a stereo you were running. In that case, the only issue would be a surge of current when you connect lifepo4 charged to 14v to lead charged to 13v, or less if it's tired lead.

I'd say go ahead and run them together, with either a bms or a low voltage alarm on the lifepo4. You can also connect a cheap DVM to the lifepo4, so you have a voltage display. When the lifepo4 matches the leads voltage, then connect up.

I wouldn't consider the lifepo4 a diva, your lead gets just as ruined if you overdischarge it. The only real issue is zapping either battery with too much current if you connect them with the voltage too different. Get em within half a volt at least before you hook em together. Then stop discharging at about 12v. or like 11.9v with no load.

Figure out how much sag you get under load, so you know when to stop. Hobby king has little alarms that are adjustable in voltage. Or just keep an eye on it, when you know it's getting close to done.