Dual Battery Speed Controllers

[Mountain]

In my Super Clod I have an Evx powering the standard Clod motors (which I thought were 550). The problem I have is that I am causing one of the two batteries always end up overheating and losing a cell. This is getting quite expensive. I am trying to find the root cause of the problem. The batteries are the same brand and rating.

Is it possible that the motors are not up to the increased voltage and are drawing too much current on the batteries. The cell is always the 3rd cell in the pack (any physical reason for that). I have tried 8.4V batteries and this causes the 4th cell to go.

The EVX is a reconditioned one because the original I had new failed after 3 runs. Could it be that the original fault with the reconditioned one was not fixed correctly. Its probably out of its 30 day warranty period anyway so it would cost me at least 25 to get fixed and probably 50. Should I send it back and argue the case for another one in case it is at fault.

The other thing I could try would be swap the motors to the Titan ones from the Emaxx. Does anybody know if these fit the standard clod as they are a bit longer than the norm. Also what RPM are they and how would they compare with the speed that I am currently getting. (its insanely quick but I have independant steering so its still controllable [}] )

Has anybody got any other experience of batteries overheating etc with this or different configurations. Is it worth upgrading to the MTroniks Super E Truck?

Any advice would be greatful.

[miramar]

If you're running the batteries in series and the original motors in parallel, your clod must surely be quick (as you indicate), but the current drawn will be quite high and especially under high load conditions, it would not be unnormal for wiring and heat shrinking to fry. The motors coming with the Clod are not designed for 14.4V (or 16.8V!) and as the current will roughly double when voltage is doubled, you will end up with roughly 4 times the wattage when compared to running the motors in parallel with just one 7.2V battery (or several in parallel). This puts an excessive load on both batteries, wiring and motors, and probably the ESC too.

It still puzzles me that one particular cell is always the one to take the punishment (first), but connecting either both the batteries and the motors in parallel or both batteries and motors in series should calm things down quite a lot.

[Mountain]

Thanks for the reply,

I think I will end up swapping the motors for the Titan from the Emaxx which are designed for 14.4V (and 16.8 at a pinch).

I don't believe you can run the EVX with anything less than 14.4V (btw the ESC is warm but not hot like other speed controllers that I have seen/used) so the only option would be to put the existing motors in series. The only question then is how I should wire it. The EVX has 4 motor power cables which I guess I could wire together and then onto the motors in a series arrangement, the only thing it is it will be back to regular clod speed which after seeing it going fast will probably be a disappointment.

Anybody know of a good source of the Titan's or what there RPMs are like compared to regular 550 motors.

[Backlash]

If you are going to run 2 x Titans, then it should simply be a matter connecting them up to the ESC as normal, that is, 1 motor off each output of the ESC. The only thing you will have to do is reverse the polarity of either the front of rear motor, so both the front and rear wheels drive the same way.

As for connecting the 2 standard motors in series, its a simple matter of running a wire from the ESC to the + of one motor, then another wire from the - (negative) of that same motor, to the negative of the 2nd motor. Then run another wire from the ESC to the + of the 2nd motor. The reason for doing the wiring this way is so one motor will rotate in reverse. Then simply plug these 2 wires into 1 output of the ESC. Give it a test run, If the car goes in reverse, simply swap the wires over on the ESC.

[miramar]

quote:

Originally posted by Mountain

I don't believe you can run the EVX with anything less than 14.4V

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I didn't either, but I just stumbled upon this:

http://www.rccrawler.com/evx.htm

Apparently the EVX works OK with 7.2V too. However, the tip to use the connector from an old battery to short one of the ESC's connectors is not what I would recommend. An old battery will probably have an old worn connector too, with increased resistance (= voltage drop = less wattage and less RPM) and will possibly also be a root cause for radiogear glitching. A good connector is just as much needed for shorting as it is for the battery itself. Also, the jumper cable on the "shorting" connector should be kept as short as possible.

[Mountain]

Thanks for that it may stop me chewing thru batteries until I can get some Titan's at a sensible price. Anybody know what the length of the Titan can is (will it fit in a standard Clod?)

[miramar]

quote:

Originally posted by Mountain

Thanks for that it may stop me chewing thru batteries until I can get some Titan's at a sensible price. Anybody know what the length of the Titan can is (will it fit in a standard Clod?)

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Unlike most other RC-car motor, the Titan motors are designed to be run with 14.4V, so if you modify the EVX to run 7.2V, the Titan motors won't be suitable.

Mountain, it's not an argument against anything you've written, but it's generally important for the topic this thread is about, so as a sidenote it should be added that, expect that a higher voltage will mean a lower current at a given wattage, which again means lower percentage voltage drops, a high voltage motor is not superior to a lower voltage motor. Accordingly, the typical hype that "a motor designed for a higher voltage is better" and similar, is rather senseless. It's the compability of all components in the circuit (motor(s), battery/ies, ESC and wiring/connectors) that really counts.

[soulglow]

One cell pushing off is almost always a sign of extreme current draw...whichever cell has the highest internal resistance will get hottest first, at which point its IR increases, so it sinks more current, gets even hotter, etc etc spiraling off into oblivion. I'll be thinking about why it's always the middle cell...I have some ideas and my curiosity is "all kinds of" piqued

[miramar]

quote:

Originally posted by soulglow

I'll be thinking about why it's always the middle cell...

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My colleagues and myself experienced this phenomenon for a series of batteries at the time RC and batteries were my daily business (daytime working with batteries, moonlighting RC). The only thing we could think about was that the middle cell (both in stick and in saddle packs) has a larger contact area with other cells than any other cell in a the battery has (as long as we're talking 6 cells as 3x2 saddles or 2x3 sticks). The larger contact area would mean that there is less air around the middle cells and thus a smaller area from which to dissipate heat into the air. As heat is dissipated between the cells as well and the steel casing of the cells is a better heat dissipator than air (although the air would be cooler than the neighbour cell(s)), the above argument seems a little vague, but it's the only possible reason we could think about during a long series of experiencing the problem.

[Mountain]

Sorry when I meant the middle cell it was if you physically count 3 in from from one end then it would be the 3rd cell from positive connector (or 4th in case of 8.4V pack). Since the 8.4V one is at the end it would probably get the most air circulation and so should be coolest. The plot thickens ...