HI Ian
If they are 9.6v motors then I would expect they generate quite a lot of sparking on the brushes at 11.1volts. Over time this will if not properly suppressed damage the ESC's. I suggest you fit or check carefully any suppression capacitors, they should be ceramic disk type and can become detached if the motor gets too hot.

I had similar problems with Graupner 600 motors, very powerful and current hungry and prone to overheating. Dave Milbourme suggested I switch to 700's but they were too heavy so I changed to brushless. Used in my Trent lifeboat and I wanted a nice big speaker to use with the Action sound unit so had to really.

I'm sure your persistence will pay off. 😀

Cheers
Dave

HI Dave,
Thanks for the long and comprehensive post and I understand some of what you say! I must say I have had pervious failures with Mtroniks esc's-as you say, they are very compact. I do understand about the low internal resistance of lipos and this may be the problem on initial startup but I don't think I'd want to be faffing around with bulbs! I have used an Electronize 30 amp unit in my Wavemaster and that has been trouble free- so far but baulked a bit at the cost of two. To be honest, I can't remember which species of 700 are in the boat- I think they were rated at 9.6v but read advice that they could be pushed a bit- from non other than Dave Milbourne of Action (as was) and I respect his knowledge.
I seem to have had a lot of difficulty with this boat which makes me even more determined to persist until it's reliable and effective-whatever the cost!!

All the best...Ian

HI Ian
Just read your post.
There are several variant's of the 700 motor and I wonder which one's you have in your model?
The 16.5 amp current draw does seem to be above the 12.5amp quoted as max efficiency in the 700bb 12v specs. (7.2 - 19.2 volt). However this is still within the range of the motor and ESC.
If you have 700 race motors the spec is for 7.2 - 9.6 volt and max efficiency is at 17.5 amps.

The Mtonic ESC's are useful but are known to fail and I have experience of several that have failed at my model club site. The design is very compact and in every case the unit had shorted at the output wires exit point.
Failure often occurs at switch on and this could point to a sudden current burst causing some local heating resulting in a short.
3 Lipo's have a working voltage of 11.1volts but initially there will be a higher open circuit voltage which could exceed the 12v spec of the Mtronics. As Gregg says their new Eco range are designed for Lipo use but are still only rated at 12v and suitable for 3 cell Lipos. I note they are now using some fast skotty diodes which may have been added to address this problem. Your monitors on your existing ESC's are quite practical and will protect your Lipo's.

I suspect that the power available from the Lipo's is the real problem. As you may know Their internal resistance is incredibly low and as a result almost the entire amperage is available Immediately. SLA's and NiMh have much higher internal resistance and therefore don't deliver the same amperage Immediately. I'm talking in electronic terms regarding time where milliseconds are Important.
Perhaps a 12v car bulb across the battery input would drop the voltage whist you switched on, the bulb could then be disconnected, can't say I've tried it but it may help,

As a final aside there is also the possibility of back emf causing problems. Voltages generated can easily exceed the max woking voltages of the internal MOSFETs which usually fail in the dead short condition. Used to be a problem with some 5 pole motors with many windings so possibly not with high power motors. However if you shut one prop down at speed and the other is still powered there could be back emf generated that could cause cumulative damage that eventually causes failure. Try spinning a motor shaft in a slow battery drill and measure the voltage across the motor contacts to see the effect.
Good luck
Dave

Have used nimh and 12v gel ok, took nimh upto 16.8 two packs ok

Oh dear! Got 'em now so I will use them 'til they blow up and then think again. I may take up crochet!!!

I should watch out with using lipos with the sea sprint esc as had two blow up and hobbystores had to replace them. they were being sold cheap as printed material said that you could use lipos but the power does them in after a little while. regards steve

Thanks both for the replies.

I too thought it unlikely that the voltage was too much. I fact, Mtroniks have suggested to me on a previous occasion that they are good to about 13v.

Gregg, as for cooling, the motors were run for 10 seconds at most, just to check every thing was working before I went out. I have done this many times before when setting up and checking the boat.

I'm not quite sure what you mean when you say the standard controllers are not rated for Lipo's. I'm aware that they don't have a low voltage cutoff but I use low voltage alarms, one on each battery through balance lead extensions, which are visible in the windows.

I thought a volt was a volt and an amp was an amp wherever it comes from- perhaps you could Illuminate- I'm always prepared to learn.

I was in receipt of two Ripmax Sea Sprint escs which are rated to 19.2v this morning. A friend of mine uses these with success on a 700 with 3s lipos, so I'll give 'em a go- they are relatively cheap.

In the interests of increasing the body of knowledge I will post my experiences with them in the fullness of time!

All the best...Ian

Try an Action electrronics dual motor controller, they are rated up to 24v !

But there is other reason why simply running these motors up on the bench [dry]. Do the motors have internal cooling fans? if not, how were you cooling them, whilst running for this test, plus how long did you run the motors for?

Incidentally, mtroniks "standard" controllers are not rated for lipo's, they have only recently introduced a special controller for these.

I doubt if the burn out was due to excessive voltage. A fully charged 3s LiPo will be 12.6V which is less than a fully charged 12V gel cell which can be up to 14V.