Battery Connector Question

[Fuzzy Whumpkin]

Hey all, I found these LiPos on AliExpress pretty cheap, but i'm not totally sure about them... mainly what the connectors are coming off the battery. here are some pics:




it's a 2200 mAh 15c 3-cell. I'm pretty sure about the red being a JST plug and the white being the balance plug, but what's the extra black one?

Thanks!!

 

[Craftydan]

Red = JST

Black = Futaba -- standard servo connection, but check the voltage before you plug that badboy into an RX. It's not uncommon to see that on older transmitter battery ports and old-school gear where the servos and RX ran directly off of an unregulated battery. Most were 2S lipo or neighboring sized NiMH, so 3S Lipo seems a bit high for this, except as a TX pack. If they tapped off as a 2S it might be fine, but full 3S is bad mojo for most radio gear these days.

White = JST-XT Balance plug for 3 cells.

Long story short, the black wire can be replaced by a standard plug if you like -- check the voltage -- but keep in mind 15C isn't much current . . . and might be optimistically rated at that.

 

[Montiey]

So It's a battery that can power your RX if you aren't equipped with a BEC? Yes, check the voltage before you plug it into your RX.

 

[Craftydan]

I'd lean more towards it being a transmitter pack at 12.4v than a receiver pack, but essentially, yeah.

A common strategy with very large airframes is to have multiple RX's simultaneously bound, with each connected to a subset of servos isolated out on the wings or tail, each subset having it's own receiver battery -- keeps your wire runs to a minimum. Those servos tend to be a bit more power hungry than the typical and can tolerate higher voltages . . . maybe not 12.4v, but that's not impossible. As usual, the weak link is probably what the RX can tolerate.

You can also find large fire-breathing helis (you know, the ones with smoke coming out of them) sporting an RX battery pack like this. All the servos run unregulated -- No ESC, then no built-in UBEC for power. So long as the servo has a high enough voltage, their precision shouldn't change (commands are time-based) and the response time and torque should level out around it's max. At that point, voltage regulation is overkill.

 

[Fuzzy Whumpkin]

so i could replace the JST with another connector (XT-60) and just let the black connector hang out on my upcoming FT Flyer build, right?

And the 15c rating - just to be sure i understand that right - that means that for this 2200mah battery, it would be right around 30 amp output, right? i'm planning on running the following motor/esc combo:


seems to me like these should all work together...

 

[Craftydan]

Effectively, yes . . .

. . . and they will "work", but . . .

A 3S 2200mAh pack is very heavy for a plane the size of an FT flyer. The 30A ESC is a bit oversized as well -- weight being the concern, not capacity. you likely will not have the needed power to remain airborne under that much weight.


Keep in mind the FT Flyer is spec'ed for anything between a 2S 500 and a 3S 1000, which is an enormous span -- the largest battery is 3 times bigger than the smallest. Despite that large span, anything in between will work. You've chosen a battery that's more than double the size of the largest battery, which flies on the heavy side as it is. I'd highly recommend getting a much smaller battery. The added weight of the larger ESC might be ignored, but that's a big battery for a small plane.


BTW, the ESC is not sized by the maximum possible current the battery can source, but the maximum current the motor can safely (or is expected to) draw at WOT (Wide Open Throttle). The battery's C rating is also speced for this as well. So how much can that motor safely draw? 10A continuous, but 12A burst. Picking a 12-15A ESC would be more inline with what that motor needs -- larger will work but it will be added weight and cost.

So, the specs tell us the maximum safe draw, but . . . how much do you *expect* the motor to draw? That will depend on the prop. The recommended prop for a motor at a given voltage should put you near those maximums. Put a shorter or shallower prop on or drop the voltage (3S->2S), and the current needed to sling that prop at WOT will decrease, as will your thrust output. There's a handful of very non-linear relationships here, which is why many people will recommend using a power meter to verify the current draw, but some go by "if the motor isn't burning Hot, it's fine" method.