Advice on 6s2p pack

[ThomDE]

Hi Guys,

New to the forum, I am Thomas form Belgium and I have just started building my own Li-ion packs mainly for use in big RC planes.

I am now planning on building 2 pieces of 6s2p molicel p45 packs. I don’t like soldering on those things so I have bought a sequre sq-sw1 mini spot welder a couple of weeks ago.

So 2x 6s2p will be used in series to power a 6000w brushless engine. Currently I use normal lipo’s on 45v, this gives me an amp draw of 45A in cruise, when performing aerobatics It peaks to 90A.

As I assume just using 0.15 or 0.2 nickel strips will cause to much loss to heat so I am looking for alternatives to spot weld.

• I read about copper sandwich but I am afraid my spotwelder is not strong enough?
• I thought about just spot welding the parallel connections and then solder on 10awg wire on the nickel strips for series connections?
• Maybe I could use Braided wire as a solution for serie connections?

Any advice would be much appreciated!

 

[From-A-To-B]

CONSTRUCTION: .15 x 10mm nickel will be good for ~6amps constant. Very much not up to spec for your needs, even if two strips are used, or even if 4 strips are used.

If you’re laying out the battery 2 cells by 6 cells wide, you have space for about 40mm width of copper or nickel, if spanning the whole pack. Doing the math with Matador’s ampacity chart:

.1mm copper X 40mm wide is good for 60Amps constant (plenty for you.)

.15mm nickel x 40mm is good for 24amps constant. (Not enough.)

If you’re serious about battery building as a hobby, buy a KWeld and work with .1mm copper and .1mm nickel plated steel strips for the sandwich.

Otherwise, maybe you can get away with your idea of soldering 10awg wire or braided wire to serve as your serial connections. Best practice is to do your soldering then spot weld onto the cells, to avoid cooking the cells.

DEMAND ON CELLS: none of the above addresses the fact that asking for 90amps from 2P of P45As is a big ask. Even at 45amps constant draw, you’re just about at peak performance from those cells, IMO. The sag will be real.

Speaking as a former RC’er, FPV’ers who need 60Amps on launch and 10 amps in cruise are the perfect use case for li-ion packs. I encourage you to consider adding more cells in parallel, buying a KWeld or hiring someone to build this pack, or stick with lipos if you need 45-90amps from a small form factor.

My two cents! I hope you succeed in your project and have lots of fun along the way. Ask more questions and/or share where you land with this!

 

[From-A-To-B]

EDIT to the above ^^ I see you plan to use 2 packs of 6S2P to achieve 45A cont. 90A peak.

Sorry for the misread. That changes things a little bit. I still recommend you build with copper, as the math on nickel is still marginal IMO.

But 4P of P45A cells will now happily meet your power needs, where 2P wasn’t gonna cut the cheese.

 

[ThomDE]

Thanks for your reply, it’s indeed 4p, I ordered a 0.1 copper strip (8mm) to test if my spot welder can handle the ‘sandwich method’

I already build and tested a 6s1p (with p45’s), welded together with 2x 0.2 (10mm). it handles very well and has the same flight time as the equivalent lipo. It’ on an edf jet which pulls 85a peak and 40a continues.

Currently I am playing with the idea to use 2x 0.15 (because this is available in square format), but welding in the braided wire between the two nickel strips on the series side.

 

[From-A-To-B]

Wonderful— if it’s working for you, then it’s working! I’m surprised that you’re able to get 85 peak amps from a single p42 cell. File under: learn something new every day.

As long as you aren’t passing the upper temp limits of the cells, it’s all probably fine.

RC’ers generally expect shorter lives from their batteries than the DIY ebike crowd. Most hobbyists replace their lipos every year or two, and 100 cycles would be terrific; whereas ebikers expect three or more years and hope for 500+ cycles.

Not to mention, we’re talking about 25-50 dollars worth of cells in a small jet pack, instead of hundreds sunk into a bike battery.

Cheers! I’m personally curious to hear how this goes for you.

 

[ThomDE]

i keep you posted on the new bigger pack, ordered a kweld yesterday.

just to be clear: it's the P45's they are much better then the p42's