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Told battery is too small and will burn out my ESC?

Old 10-22-2015, 11:32 AM
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sfmongor
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Default Told battery is too small and will burn out my ESC?

Hello all,

I have been flying electric planes for a while now but fairly new to RC Cars.

I just got a Traxxas Stampede VXL with Castle Sidewinder 3 esc/ 4pole 3800kv brushless motor and was looking to run it right away, but did not get a new battery yet.

I have 2 lipo's that I use on my brushless plane that are both 1500mah, 3 cell. One is 20c the other is 40c.

When I called castle to ask a question about the ESC setup, he had informed me that if I used the 1500 battery it would burn out the ESC, saying it was too small and needed at least 5000mah? I am confused, I thought MAH was only about the longevity of the run, not about how quickly power was released. The guy had said something about 1.5amps x 20c that I didnt quite understand.

Could someone explain this to me please.

I do have a 5000mah 11.1v 35c battery on the way, but wanted to play with the car this weekend.

Thank you in advance for any/all help,

Gary
Old 10-22-2015, 06:19 PM
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collector1231
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Pretty sure he's wrong. You can play with it, but only with the 40C 1500 MAh, and for maybe 4 minutes before checking voltage and heat.
Old 10-24-2015, 07:34 PM
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Nope Castle is correct, using too small of a battery can damage esc's. The reason is something called ripple voltage. If you look at a datalog of battery voltage you'll see voltage drop when you hit the throttle but that datalog is more of an average even if it is sampling every second. Consider that on a 4 pole motor the esc has to fire 12 times per revolution. So for giggles let's look at 1 revolution per second of motor speed which on a 4 pole motor would have the esc fire 12 times per second creating 12 distinct drops in battery voltage per second. This is obviously faster than the datalogger can record so we just see an average not the extra dips of each motor phase firing which is called ripple voltage. Also depending on the battery and motor speed the battery voltage may not be able to fully recover to resting voltage before the next phase of the motor is fired and voltage decreases again.

Now we know AC current alternates at 60hz in the range of our hearing but the reason it creates sound is because the voltage fluctuation is wide enough to create harmonic frequencies. So we know fluctuating voltage creates harmonic frequencies and a 4 pole brushless system at 1 revolution per second has 12 distinct voltage fluctuations, see where I'm going with this? Lower capacity batteries will have a greater voltage drop each time a motor phase is fired creating a wider voltage fluctuation with a greater chance of creating esc damaging harmonic frequencies. The frequency range of which can get quite high when you do the math of multiplying pole count by rpm. A light bulb at 60hz we hear the filament vibrating. An esc is mosfets and other circuits that don't want to vibrate at any frequency, make the gates in a mosfets vibrate and they're going to let go at some point.

Castle has equipped the datalogger in their 1/5 scale XL (and XL2) esc with ripple voltage monitoring capability because the system is a power hog. It can stress lower end 5000mah lipos or those with lower c ratings creating high ripple voltages. The same is true for smaller systems with lipos that aren't up for the task. I don't think you really need 5000mah lipos for your system (unless they're 20c or lower) but the ripple voltage phenomenon does exist and I don't think you have the amp capacity with 1500mah 40c to prevent it from being an issue. Sure you can power it on, maybe run around a little on flat ground and half throttle or less but it's definitely not ideal and with heavy throttle is potentially damaging for your esc and batteries as they'll likely be over taxed as well especially if your stampede is 4wd.

Last edited by Maj_Overdrive; 10-25-2015 at 10:19 AM.
Old 10-24-2015, 08:09 PM
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The other issue with using lower capacity packs comes into play when braking. When you hit the brakes in a brushless vehicle voltage and current is created by the motor, just like the regenerative braking you hear about with full size electric and hybrid cars. In fact most esc send this power back to the battery as the esc simply cannot absorb the power created and it has to go somewhere. Now if the battery cannot take the current quickly (NiMh or a low capacity/low c rated lipo) the esc will be stuck with absorbing it. If the esc cannot absorb it then obviously there's going to be a problem.
Old 10-25-2015, 10:36 AM
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Originally Posted by sfmongor View Post
The guy had said something about 1.5amps x 20c that I didnt quite understand
Was reading your post again and saw that we missed and didn't answer this part. 1500ma is the same as 1.5 amps hours meaning with a 1.5 amp draw the pack should last for an hour. The C rating is meant to help determine the maximum amp output the pack is capable of without damage. For a 1500mah 20c pack we use 1.5amps multiplied by 20c = 30amps the pack is capable of delivering. For 1500mah 40c the pack should be capable of delivering 60amps.

This is is supposed to be what the pack is capable of delivering, not what it will actually be delivering. Amp draw is determined by the motor, vehicle weight, gearing, surface being run on etc. So for a given combination the motor will try to draw a specific number of amps, the esc will regulate how much voltage and amperage the motor will get while the battery tries to deliver what is requested of it. If the battery can't keep up it will heat up, puff and potentially catch on fire if it goes into thermal runaway which occurs somewhere north of 130f in lipos.

Now in reality C ratings for rc batteries are a joke, every manufacturer inflates them to try and one up the competition. There have been multiple tests by users who have attempted to put loads on lipos equal or less than the c rating and the batteries have failed in spectacular fashion with evidence on YouTube and other sources. You're particular brushless system is going to have amp draws right around 60amps putting your 1500mah 40c battery right at its rated limit. Feel free to test and see how inflated the ratings of your lipo actually are...

Last edited by Maj_Overdrive; 10-25-2015 at 10:41 AM.
Old 10-26-2015, 08:38 AM
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So from one answer the braking sending power back improperly could damage the ECS, but as far as powering the car, a 40c 1500mah battery should be fine to run it.

By the way, I do NOT run my car flat out non stop, its usually some playing around in the yard with a few second breaks between turns and the occasional flip over (stupid stampede) that really cools down the motor and esc till my lazy ***** gets over to the car to pick it up.

Thanks again for the answers, it has relieved me a bit. I did get a 3000mah 30c battery this weekend and my 5000mah battery comes this week anyway. The 3000 battery drove longer than I was willing to play with it, so I think the 5k battery will be way overkill
Old 10-26-2015, 12:11 PM
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Originally Posted by sfmongor View Post
So from one answer the braking sending power back improperly could damage the ECS, but as far as powering the car, a 40c 1500mah battery should be fine to run it.
I know I threw a lot of theory and information out there but I did not say the 1500mah 40c would be fine powering the car. All three of my posts advise against it, this makes four.

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