Lipo Vs Ni-mh

[Simon170]

Hi guys, I'm new to lipo's and need some help. Heres my situation:

I have just bought an Aquacraft 2030KV motor and 7011 ESC http://www.offshoreelectrics.com/proddetail.php?prod=dh-aqcom-2030
I need to sort out supplying juice, I was going to run 2 Ni-mh 7 cell packs to get 16.8 volts..
But now I'm looking at Lipo's (I'm new to these) but if I only run 2 x 7.2 packs I'll only get 14.4 volts, and if I run 11.1 volts packs I'll have to use a BEC as you have to use one above 20 volts I believe... which I just cant be bothered with as its another battery to sort out.
So heres my choices...

1) 2 x 7 cell Ni-mh packs 16.8 volts

2) 2 x 2 cell Lipos 14.4 volts

3) or maybe... 2 x cell lipos 18.5 volts each, but wired Parellel, not series like the normal ESC setup


any help you can offer would be great, as I'm a little lost... things have changed since I was last into RC, when 2000mah cells were the latest thing lol!

Many thanks,
Simon.

[kookie_guy]

This is a great opportunity to walk away from NIMH. They simply don't have the power output that FE boats require. Lipo is the way to go.

Consider this, the 16.8v from your nimh packs will QUICKLY drop to well below 14.8 (2s lipo is 14.8, not 14.4), because they simply can't supply the current you will need. Your packs will get VERY hot, and likely die pretty quick.

The beauty of lipos is that they can supply the current draw. That means no voltage drop. So you are getting a solid 14.8 volts out of the packs.

So to answer....

1) ditch that option, sell off all your nimh, and get on the lipo train. They are really cheap nowadays.

2) yes. run 2x 2s lipo for 14.8v. That's what the ESC is rated for. A word of advice, even tho it says you need packs that can put out 80A, get the highest C rating packs you can. The way it works is C rating multiplied by the capacity is the discharge rate. So a 20C 5000mah pack will put out 100A continuously. A 40C 5000mah pack will put out 200A. This will give you play room in the future. If you get a bigger ESC, you will already be covered in the battery department. Also remember that just because your packs are rated for 200A, does not mean they will put that out. They will only supply what your ESC pulls. The ESC pull is dictated by the motor, and the amount of load (or prop) you put on it.

3) this will add a significant amount of weight. So not the best option. If you wanna run 5S for 18.5v, get a 2s pack and a 3s pack, and run them in series. You can safely do that with lipos, AS LONG AS they are the same brand, C rating, and capacity. Lots of guys do this.

Sorry about the novel of a reply. But hope that helps.

[Simon170]

Kookie = LEGEND! :)

Thanks man, thats just about the perfect answer. At least I now know to ditch the Ni-mh idea.

I wonder if the extra weight might be good as the boats 36" long and very light, worried that it might be a bit too unstable/lively when that lipo powered brushless motor comes on song... This is my first boat project, so learning as I go really.
Not going racing or anything, just want to get from one end of the lake to the other as quick as possible lol

[kookie_guy]

There are guys on here that will be able to chime in on the setup, but that system may not be big enough for a 36" hull. Now that you have the lipo vs nimh settled, post in the building area, with your hull/ESC/motor specs, and guys will be able to tell you whether or not it's going to be underpowered.

[Simon170]

Oh man, dont tell me that... I thought I might have over done it on the power side of things. DU-OH!

Will put another post up later on today. Thanks again for your help.