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[Paul R]

HI...

Has anybody out there built one of these yet, as I may need some help with the setup. Basically its two brushless motors (robbe 30amp each) and two 40amp ESC's drawing off one 9.6v 4700Mah Ni-Mh battery. The problem is this, after about 10mins on the water the battery is too hot to handle, almost at meltdown. If i put on another identical pack the same thing occurs after about 10-15 mins on the water, battery gets too hot. Is there a solution to the problem, or should I just put it on the water for no more than ten mins then bring it off. Would a Li-Po battery of some sort help or not,

Thanks

[MikeA]

2 brushless motors would over work 1 nimg battery you would need either a 2S 50C lipo or 2 nimh batteries 1 for each motor

[Nige52]

That 9.6v NiMah battery is way too puny to handle those motors, I'm surprised it hasn't melted yet.....you need a Lipo or Lipos of at least 25c discharge, better with 30c or 35c. The capacity should be around 3500 each, or if using one battery about 5000mah. It's your choice as to whether you go 2 cell or 3....What do Robbe recommend in the destructions? 3 cell would be manic, especially if you use one per motor....

[Paul R]

Believe it or not, Robbe only recomend one 8.4v Ni-mh battery of 3700mah or better. If i use one 2s li-po of 5000mah 25-30c , how long approx would that give me on the water?

[AT Boatyard]

Hi Paul,Does the Robble sea-jet come as brushless set up or have you just gone down this road.
Batterys getting too hot means big time over load.
Is it jet drive ?
It could be that the kv on the motors are to high.What KV are the motors and are they outrunners or inrunners.
I have done some playing around with brushless so i may be of some help to.
regards
Alan

[MikeA]

hard to say without getting the amp draw rate. if i was to make a guess judging by your original post:
4.7ah  X 60mins = 282 / 15mins run time = 18.8amps     sooo
5 X 60 = 300 / 18.8amps =

16mins!!

2s lipo is a lower voltage than a 9.6 so it might make your boat slower. secondly you say robbe sugest 1 8.4 nimh is that for 1 motor of both?

[Nige52]

Right,
I've just looked at the original Robbe instructions for my Sea Jet, which is only the single brushed motor version. They also recommend an 8.4v NiMah for this too....which also gets warmish and is underpowered in my opinion.

So for your boat, which is the twin brushless motor, this battery or a 9.6v will be woefully inadequate (IMO) I fear Robbe may not have altered their instructions from the earlier boat to your version too much......

I run a 3 cell 11.1v Lipo in mine, I have a selection from 2200 up to 4700, of course this only gives me a longer run time, but the 11.1v gives me full power. I have to watch that I don't run the battery down too far as the esc is a brushed one with no cut-off if the voltage drops too low.

In my opinion, you should be running a Lipo on each of your motors......

[Patrick Henry]

I have a Mk1 Sea Jet which is pretty quick on an 8.4v niMh pack, a bit rapid on a 9.6v pack, and totally manic on a 12v pack. I get around 10-15minutes run time on the 8.4v/9.6v packs which are 2200a/h, and around 15-20minutes on the 12v pack which is 3700a/h capacity.

The smaller battery packs do get warm but nothing drastic, but the 12v pack warms up considerably, and it heats the motor up...so much so that the fail safe on the esc cuts in if I charge about at full power all the time.

I don't know what the motor is, can't see it under the rudder servo mounting plate, but it's not a stock one...



Rich

[Paul R]

Robbe specify only 1 x 8.4v Ni-mh batt for the Evolution, so yes , its beginning to look like a missprint from Robbe's point of view. In responce to an earlier post, the Evo comes as a twin brushless inrunner setup. Have just found specs for the motors in a robbe catalogue, Robbe ,BL inrunner 2845-07, 3500 rpm/v, voltage range 10-15, load current 38amps (5mins @ 20degrees C).  Li-po batts 3-4. so looks like li-po is the way to go, just got to decide how many.

[MikeA]

get a 3s with a high c rate, enough to supply juice for both motors. i think 45C look threw this to find 1 at your budget:
http://www.hobbyking.com/hobbyking/store/uh_listCategoriesAndProducts.asp?idCategory=378&LiPoConfig=3

[Nige52]

As there are 2 brushless motors with 2 ESC's.....I would suggest 2 x 11.1v lipos with a minimum of 30c.
If you use a single battery, you'll either be supplying each motor with 5 1/2 volts or 11.1v for only half of the time.....someone correct me if I'm wrong.....

Get a battery for each motor  

Oh, and you can go to 4 or 5 cell packs with those motors, most of them are rated up to 18 volts anyway.

[Paul R]

OK guy's, you've given me lots of info,

So what I'm going to do is try a 3s li-po 35c rated at 5000mah powering both ESC's and see what happens. Powering each ESC with its own 11.1v li-po sounds good but they have both got to fit into the battery cradle within the hull which was designed for a 7 or 8 cell ni-mh battery, access is poor at best and the batt cradle cannot be altered now that the boat is fully built. Perhaps li-po batts are super slim, are they?

[MikeA]

35c will still overheat, the problem is that you need a  30c for each motor, and you have 2 so therefore you need a combination of 60c to run both. looking threw the net you can run 2 motors off 1 battery, but the run time will be halved that of running one motor of course. 60c 3s lipo:
http://www.overlander.co.uk/sport-2200-2s-7-4v-25c-683.html?___store=default