Electronic questions....

[10kman]

Just about to get my brushless MGT conversion wrapped up, and realized that I had a few questions.

When using a Quark 125 controller, the manual says that if I use more than 10 cells, to disable the BEC and use an external one, or a hump pack. I have a BEC, and I have it wired up to be used, but last night when I was programming/testing lightly, I had a 9.6v (8-cell) pack hooked up to the truck. Is using the BEC with "only" 8 cells going to harm anything? Can I use the BEC no matter what (as long as I am putting in more than 6v I guess)?

Second question - when I was using the 9.6v pack for testing, I was wondering if having such a low cell count was driving up my amp draw. I have a 1515/y motor with the Quark 125 mentioned above. There was no heat buildup, the ESC was maybe a smidge warm after the test session, but it was fine. The 1515/y says in the manual that it is a 2700W motor, but I'm assuming that it isn't actually at that amount unless the voltage is maxed out.

Thanks a lot, pics will be coming soon after I get it prettied up a bit.....

10k

[jollyjumper]

well, the motor will have 2700w if the batt's can deliver it.
or more then that.
if not it will have the power that your battery can pump out.

[10kman]

That's where I was getting dizzy trying to calculate this stuff. The W = V * A equation is nice and peachy, but the only thing I really know for sure is that my 8-cell pack is 9.6v. It is also 3000 mah if that helps with anything.

Can I see my wattage output from that pack knowing those 2 things?

I'm still wondering about my BEC situation as well but I'd really like to grasp this wattage calculations. Online references just keep throwing that same equation around but I need a more "RC Specific" explanation....

10k

[jollyjumper]

the bec is ok to use anytime.
about the power, one of the other guys will have to chime in.

[BrianG]

As you may know, using a switching type BEC is much more efficient than the one built into the ESC since those are linear types. Not sure if you wanted to figure out main amperage draw from just the BEC, but basically it's:

((servo_amps * bec_volts) / 0.85) / v_battery

figure 3A as worst case for servo_amps, bec_volts is 5v or 6v depending on the BEC, 0.85 is the average efficiency of a switching BEC but it depends on load and BEC quality, and v_battery is the voltage of your battery pack. And yes, the current from your battery will actually be less than the servo_amps.

A motors wattage figure is typically the amount of power the physical wires are capable of handling. However, as jollyjumper said, it depends on the applied voltage. You won't generate 2700w with 9.6v for two reasons:
1) power is the product of voltage and current and to get 2700w at 9.6v, you'd need over 281A. Most batteries cannot do even near this amount for very long at all.
2) the motor has a certain impedance of the coils, but this varies with load and motor speed. At stall, the impedance is just the DC resistance of the motor coils, which is VERY low. At higher rpms, back EMF will increase the impedance. Mush of the current draw will be as you are accelerating because the motor is trying to change speed. The quicker it tries to change speed under load, the more current will be required.

Because there are so many factors in current draw making it almost impossible to calculate, most simply select a high-enough voltage to spin the motor at ~35,000 rpm. Then, load the motor (through proper gearing) so the motor, ESC, and batteries don't get too hot. If you really want to know the current/power, it would be best to get an Eagletree device, which plugs in series with your battery to measure voltage and current (and calculates the power) at the battery. You can then upload the device data to a PC and see the voltage and current levels over time.

[10kman]

Thanks guys. Assuming 35k rpm for that particular motor, 4s will be close, or 14 cells. 5s will probably be my max, even though the ESC can take up to 6. No point in frying the motor.

Honestly, the 9.6v test pack was impressive, I got the front wheels off of the ground a couple of times in the street. I was using a 16t pinion, and the stock MGT 52t spur.

I ran my MGT with the 46T or 48T, can't remember which, spur with a 16T pinion and 8XL on 4S and love it.

[10kman]

About your MGT -

You replied to my first post a little ways back, and you posted pics of your truck. I just re-looked at them to see how you had things placed on the chassis, and I am going the same way pretty much.

I noticed that you don't have the stock roll bar on there. Have you had any issues with running like that, especially if you have had a crash?

The truck on 9.6v is fun in my little test area, I can't wait to figure out LiPo's and get a 4s pack. I have a MRC 989 coming, but am confused as heck about the balancing taps and such, so I'm still holding off on buying batteries just yet.

10k

Someone gave me the MGT as sort of a thank you for converting his Losi 8ight. When I got it, it didn't have a roll bar on it and I didn't realize that it was supposed to. I haven't had any issues with it at all.

You already have the motor and esc covered and since you went with the 1Y, you're pretty much stuck (if you want to call it that) with 4S. Get yourself a Hyperion LBA10 balancer and order a couple of 4S2P 8Ah packs with the hyperion taps on them from max amps. This will get you running and keep you running.

[BrianG]

10kman; if you want to get a visual explanation about how lipos are wired, check out this page. It might help...