@mtfunder this contradicts your statement, unless you only use batteries that have <1 mOhm (TOTAL note that the figures above are PER CELL) of internal resistance.
TIL that you can measure battery internal resistance conveniently while charging: What's The Deal With LiPo Battery Internal Resistance? - YouTube
Seems like a great way to check health and performance of lipos.
@xfacta @mtfunder i hope you guys havent misled too many people with your (for some reason, strong?) opinions about connector resistance.
Well I don’t want to make too much of a mountain out of the connector issue (I was just trying to validate that it’s a point worth considering). But in practice they do tend to be much greater than the data sheet specifications. When they are new, they will be around there, but over time they do oxidize and wear out and get much worse. I have seen many very expensive drone failures due to high resistance connectors unsoldering themselves at currents they should be able to handle (not on drones I’ve built fwiw).
Several of the other potential issues have already been brought up and dismissed by you, and I am not going to waste more of my time trying to explain the more nuanced issues, realistically a drone built so fast and loose will probably fail first for some other reason.
I’ve been professionally developing large drone power systems for about 3 years now and I’ve done quite a bit of my own testing and observed many failures to come to my point of view.
hmm well all i see from you and @xfacta are claims that stem fundamentally from connector resistance. when the facts contradict your claims you seem personally offended. ill be sure to replace the connectors when they look nasty and im sure ill be fine 
also i will monitor connector resistance and battery internal resistance. all that professional experience and you dont have any quantitative anecdotes to back up your claims?
I’m really not trying to have a go at you and I’m not personally offended. I just offered my opinion.
It seems to be you who are offended when someone doesnt immediately agree with whatever you are proposing, then you start calling people insane and offering nothing but mocking.
Have you ever disassembled lipo battery packs and tried to swap out cells? You’ll notice that the cell pockets are really seriously joined to the main positive and negative wires, so that the operation is almost impossible. A battery pack with the correct number of cells is quite a different proposition to batteries joined in series by (longer) wires and more connectors.
If you dont have a large expensive copter, or doing commercial work, any battery arrangement you feel comfortable with would be perfectly fine to try out, and we’ve all been there.
i didnt call you insane. i said the proposition that 14s would offer nothing over 12s due to connector resistance is insane (which is still my opinion). and i never called you guys inexperienced or amatuers or anything like that, but you guys did. i dont really care, but i just thought you guys got offended because you resorted to personal insults.
I offered evidence to back up my claims. i compared internal resistance figures to connector / wire resistance figures (with help from Dave). but its as if i said nothing. at least @mtfunder offered another explanation (oxidized contacts). but aside from that, i dont think what you guys are saying is sensible.
I must say I respect that you looked up the facts and figures.
well at the end of the day im sure we ALL learned something. 
After reading through the thread it looks like 12S is off the table - which is a shame because it is probably the best route.
7S batteries are not that common and come with increased cost - but you seem to be on a budget.
I personally wouldn’t combine two 6S and one 2S battery in series to achieve a 14S pack - but no one is going to stop you from doing that. Best to just test the makeshift pack in a safe area before throwing it on the bird.
thanks for your input. I would like to go 12s, but I have not found a motor-prop combo with better performance, all things considered. I definitely will test 14s on a test stand of course, as I need to verify motor-prop performance against manufacturer specs anyways. I do have interesting options with FoxTech batteries; they make 14s packs.
No one has told me the “why” behind the claim that packs in series are not feasible. I will either find out exactly why they are not feasible, and report here, or find that they are feasible (and probably also report here).
managed to find a 12S arrangement (thanks to new motor/prop offerings), and put 6s in series. tests on my thrust stand matched that data provided by the manufacturer (actually exceeded because density at my altitude is higher than that of manufacturer tests). no problem with series connector.
HOWEVER. I did notice in all the series connectors I bought online that the soldering seemed subpar. you have to break open a sacrificial connector to see it; otherwise its hard to tell (maybe ohm measurements will tell you). so i ended up having to solder myself. i did not test to see if the subpar soldering actually affected performance, but i did not care to use it anyways. also soldering myself i was able to better control the battery wire length.
note: manufacturer use dc wall power supply (e.g. not batteries), so this adds to the evidence that series connectors done right are perfectly fine.