Battery question

[jrod703]

I am currently upgrading and tweaking an old RC boat I have had for about 17 years now. Its a Graupner Hydrospeed Cobra. I started with a graupner speed 500 race motor and upgraded it to a 600. I upgraded the stock 42mm prop with 1.4 pitch to a 42 mm with 1.6 pitch and then upgraded the battery packs from 7.2v 6 cells to 9.6v 8 cells. I also implemented a water pickup and coil for liquid cooling to beat the heat. I run one battery pack at a time and the speed is quite good, but i still want more.

the speed controller in the boat is an old Cherry Electronics microswitch which has connectors for two battery packs. the way i believe it was supposed to work was with two 4.8v packs in series. When the boat is at half throttle only one pack would be tapped into, but at full throttle it would open up on both packs. Reverse would only draw from one pack. As i have never been able to find these packs I have never run it in that configuration. So i have only been using one pack, making half throttle on the controller equal to full power. If i push the control stick to far, the power cuts out as there is no second pack to draw from.

I would like to run more voltage to the engine so as to obtain more speed. I am pretty much out of room inside the boat for engine upgrades, so a bigger can is out of the question. i fiddled around with hooking up four AA 2100 mah NiMh rechargeable batteries in a holder i bought from radio shack. I hooked that to one of the battery connectors on the speed control and my 9.6v pack to the other. this resulted in a little puff of smoke and a melted battery holder. Hind sight is 20/20 and i can now see the error of my thinking.

I have pondered hooking up two 9.6v packs, but i can only imagine that 19.2 volts would be far too much for the motor (best spec i can find for the speed 600 7.2v motor says max 12 volts). the speed control indicates it can handle 10.1 amps and 125 volts, so i am not so concerned about it. it might be old, but it works fabulously.

the question (as i am sure you have all been waiting for) is: Does anyone know of any kind of battery isolation circuit or diode that i could put between the speed control and my 4 AA battery pack that would prevent the other pack from overloading this smaller "homemade" pack? If such a thing exists, i could then run the small pack for slow forward and revers, and both packs for high-speed forward. I have thought about taking apart my 9.6v packs and making my own 12 cell pack, but i'd rather not go that route yet, especially if a simpler solution exists.

Thanks!

ps - if anyone can think of other ways to increase speed please do not hesitate.

 

[jrod703]

I decided that the cost of 600 motor would be worth seeing the boat run on 19.2 volts so i hooked up both 8 cell 9.6v packs. it was awesome!! steering is quite touchy at high speeds, but the worst that happened was a couple of spin-outs. i also found out the hard way that i had not put any threadlock on a couple of screws and almost got stuck in the middle of the pond!!

two runs in so far, neither of them quite a full run. the first fell short becuase i think that the packs were not fully charges, the second because one of the screws in the drive train came loose and the shaft was spinning almost free without turning the prop much at all - barely limped back to shore.

run three today!

does anyone know how long i can expect my graupner speed 600 to last running it like this? it has been holding up quite well thus far, and the liquid cooling keeps it from over-heating...

now i need a way to figure out how fast it really is going.

 

[Old Sloppy]

does anyone know how long i can expect my graupner speed 600 to last running it like this? it has been holding up quite well thus far, and the liquid cooling keeps it from over-heating...
now i need a way to figure out how fast it really is going.

I have been told 140 degrees is the absolute limit for a motor.

Any higher will damage it internaly.

Remember heat is the enemy.

I can't tell you how long it will last.

I can tell you to put a GPS in it to check the speed.

Harry S.

 

[jayt]

does anyone know how long i can expect my graupner speed 600 to last running it like this? it has been holding up quite well thus far, and the liquid cooling keeps it from over-heating...

That many cells on a small format motor will cause damage, period. I have no idea how long it will last - no one I know has tried something so....extreme. You may think that water cooling helps, but the hottest part of the motor - the armature - is not cooled at all. About all you are doing is keeping the magnets a bit cooler so they may last longer. Even with a minimum load on the motor, the bearings/bushings/commutator/brushes will not last long at the excessive rpm that voltage provides.

But like you said the motor is cheap, so if you don't mind replacing it periodically I suppose you can keep over-volting it. But in the long run you'd be better off getting a real race boat setup and enjoying better performance without the touchy handling.

.

 

[jrod703]

Thanks for the replies thus far, I appreciate any information or opinions anyone has.

One thing I forgot to mention that is probably a little bit pertinent: The boat is not direct drive. I have a 2.5:1 reduction gear system in place. This is probably why I have not blown up the 600 motor as of yet. I have half a dozen successful runs under my belt at this power level and must say that I am impressed with how well everything is holding together.

I raced my buddy who has a CEN racing AquaJet 775 and I was able to keep up easily and even pass him, much to his dismay

So far the engine has not gotten significantly hot, at least so far as i can tell. I ordered an infrared temperature sensor that should tell me more precisely how hot I am getting it.

THE PROBLEM (there's always at least one!): I am overheating my battery packs big time! so far i have melted the shrink wrap and adhesive backed velcro off of both 9.6v packs. They will still discharge and recharge, but this cannot be healthy for them. I have had some friends suggest liquid colling them as well, but that seems a bit ridiculous and really is impossible due to space limitations inside the hull.

Is it possible my microswitch speed control is allowing the packs to discharge into each other? Forgive my electrical ignorance, but I figured if this thing was desinged to run two packs, then it should have sort of preventative measure for this scenario. Did I assume incorrectly?

Maybe it isn't the controller at all, perhaps I am just pulling too much power from the packs too quickly or for too long. I get at least 10 minutes of run time, some at full power and some at half...

Any thoughts or suggestion welcomed as always! I'll also try to see if i can get a picture of the speed control attached to this post at some point, maybe that will help.

 

[jayt]

Your packs are overheating because you are pulling too many amps too fast. Reduce the prop size and/or limit your run time to 3 minutes. Exactly what cells are you running?

The fact that the speed control switches between series and parallel (IIRC) can't help but contribute heat. Your comment "I figured if this thing was desinged to run two packs..." forgets that battery technology 17 years ago was stone-age compared to what we have today - we have literally 3 to 4 times more power today from the same number of cells. And you are running way more cells that the speed control was designed for...why are you surprised you're having trouble?

While you are not discharging into both packs at the same time, packs which are not balanced in capacity and/or state of charge when run in parallel can damage one pack. If you want to run two packs of different capacities....don't.

The gearbox is allowing the motor to live a bit longer than normal, but the comm will still wear rapidly and warp out of round. However, if low cost is your goal then that isn't so bad. B)

.

 

[jrod703]

jayt - haha, yeah that is kind of what i figured. the packs i am running are matched powerizer 2200mah 9.6v ni-mh 8 cell packs. they would appear to be more like AA size than sub-c. i was originally only running one at a time, but got a little adventurous.

i'm curious if there is any way to regulate or "cap" the max amount of amps that can be pulled from them when running? probably nothing too cheap or easy to implement, but i'll throw the question out there anyway.

now that the packs have "shed" their shrink-wrap, i am probably going to dis-assemble them and test the individual cells. i can then make a larger single pack and put myself in a more stable position. if that works maybe i'll even invest in an ESC so i can run at varying speeds rather than all or nothing.

as long as the thing doesn't catch fire and sink i'll be alright. three 9.6v packs and the 600 motor combined were less than $50 and i have certainly gotten that much entertainment out of it the past couple of weeks. worst case scenario i just put my 500 race engine back in and go back to the 7.2v setup - all were set aside as a fallback in case of an experiment gone awry.