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[flack]

Afraid I am a bit of a novice when it comes to modern electrics in radio control so please excuse my ignorance. I am in the process of building a Metcalf Waveney class Lifeboat which will is fitted with two Turnigy D3542/5 1250kv motors with two hobbyking 50 amp esc's connected to an Action Electronics P40D mixer. I am not a fan of lipos and I don't think lead acid will give me the power required fast enough so have opted for NiMH, probably around 4600 to 5000mAh. Is there any kind of advantage of using 12volt over 9.6 volt?

Cheers Shaun

[Stavros]

on 9.6 v your motors will be spinning at 12000 rpm on 12 v it will be 15000 rpm an increase of 3000 rpm thats what I would say it the biggest difference.......in other words it will be quicker on 12v than 9.6
 
 
Dave

[tica]

"I am not a fan of lipos" Why not, they have come a long way since the first fire crackers. The last 2 years I have only burnt out 2 packs and that were my own fault.

With the BL setup you are using you will get approx. 15K rpm with a 12V set-up and that's pushing the physics, if you use a 7.4V Lipo you will get approx 9250 rpm which is close to the max for a submerged prop. With a NiMh 7.2V you will have approx 9K rpm.

So if you will stick with NiMh the max would be a 8.2V 7Cell = 10250 rpm.

Please note that I have calculated the rpm at the nominal voltage and not the max for a fully charged pack.

BR
Carsten

[flack]

Thanks to both Dave and Carsten.

Carsten,

You hit the nail on the head, as a retired firefighter I have seen too many nasty fires involving electrical equipment and "human error" and so work on the principle of not tempting fate.

Trying to understand the info about NiMH where do the figures for rpm come from with regard to the voltage for the various types of battery and does "So if you will stick with NiMh the max would be a 8.2V 7Cell = 10250 rpm." mean that's the maximum voltage NiMH pack I can use.

Cheers Shaun

[tica]

Hi Shaun

When I state Burned out it means that the packs stoped working, No fire. I simply overloaded them with a very high A/h load so the became bloated 

RPM calculation: The data on your BL motor states 1250KV = 1250 rpm/volt unloaded.

For the fire hazard we so called "normal" persons uses a Lipo safe-bag when charging  LiPo's and do NOT leave them unattended unless they are left in the middle of the green.

BR
Carsten

[john44]

Hi. Shaun, if your esc,s are like my 90amp versions they won,t run on 7.2v they stop and go to the
Set up mode, they run on 8.4v and above very well. Lipo,s are lots safer and cheaper now,
But do need treating with more care than the Nimh batteries.


John

[malcolmfrary]

The real question is whether the batteries chosen will stand the load imposed by the motors for long enough.  With LiPos you can calculate from the specification what you should be able to get from any particular pack because you get to see a "C" rating.  You don't usually get this with NiMH, what you do get is the chance to select more closely what voltage you want by using the right number of cells to get the performance you want from the motor.  At 3.7 volts per cell, LiPos give quite big steps, NiMH are 1.2 volts so give a smaller step per cell added.
If the batteries can't deliver the current demanded, the voltage will drop in use and might cause problems with the ESC, usually the thing will basically switch off, invariably when the boat is out of reach.

[flack]

Thanks for all the knowledgeable replies, it certainly has me thinking about whether to give the liPo route a try and ballast the boat another way.

Malcolm, what is a "C" rating please.

Where does Li-Fe / LiFePO4 battery technology fit into the discussion?

Cheers Shaun

[tica]

Hi Shaun

Regarding the C rating: You will typical see the marking like 2S 1500mA 25-35C, decode this means:

2S = 2 cells 2X3,7V => 7,4V, 1500Ma = 1500mA  25 = Max continuous load 35 = Excessive load for typical 10seconds. C = capacity of the pack.

So the above example gives the following information:

7,4C, 1,5A, max load 37,5A and 52,5A/10 Sec

Li-Fe and LiFePO4 is new battery technology that is slowly mowing into business. They do fit the transmitter voltage better than LiPo's and they tend not to be as fragile as Lipo's.

Regards
Carsten

[flack]

Thanks for this information Carsten,

Still a bit nervous about the LiPO but willing to give it a try and have ordered one to test.

Cheers Shaun

[Fastfaz]

 Hi Shaun,
      If you send me a PM with your Email address I will send you some photos of the Perkasa I had which was powered by a 12 volt NIMH (I don't seem to be able to post photos on this site being a PC Dinosaur) and it planed and looked right on the water. I borrowed a Lipo and to be honest there wasn't a great deal of difference apart from the battery weight, like yourself I am wary of the fire risks involved. Hope this helps.
      Regards,
           Peter.