New warning insert in MMM box: "DO THE MATH!"

[JThiessen]

Originally Posted by TexasSP

Your right, the Boeing 747 tires cost as much as a corolla.

More, much more....

[hemiblas]

Theres still too many problems with the way manufactures rate lipos. 3200mah 15C batteries should be good for 48 amps continuous. To me, That means you should be able to pull 48 amps continuously during the run, sometimes more, sometimes less but an average of 48 amps. Thats a lot of amps and I can guarantee that most runs wont even come close. Even though I agree that they wont work, the math says it does and I think its the lipo manufactures are the ones at fault for improperly rating their lipos. I have learned , not thru math but trial and error, to always buy more battery than I need, but I still dont understand why we have to. It wasnt until I bought a wattmeter that I finally understood that 15C really means 5C to most manufactures.

[SpEEdyBL]

It's not current that's the problem but the voltage spikes that kill the esc. Even though a 48 amp battery can supply the needed current, it's voltage spikes will be a lot more violent, compared to those from an 80 amp rated battery. C rating mostly decided on being able to hold a voltage, which makes higher amp capable packs better. You'd be amazed at how high voltage spikes can get during braking, easily twice the pack nominal voltage with bad packs. Since the the MMM and MM both have 30 volt fets this was not an issue with the MM as 3s was the rated limit.

[hemiblas]

Ya thats a great point. So really then we should be telling everyone to buy a battery that has a continuous rating of what we expect the surges to be. There is also a huge range on C rating. My 11.1V, 10C 5000mah truerc battery can hold 40-50A with good voltage with bursts of over 120A according to their website. My ebay 11.1V, 10C 5000mah battery can hold 10A at around 9.6V and weighs about half as much. I dont use either of these on my MMM though, but I bet the truerc battery would work fine.

I dont know if this matters, but my velineon ESC weeds out the bad batteries by the use of LVC. Its set very sensetive from the factory and on cheap batteries I get approx 10 seconds runtime from a full charge on a 10C cheap 5000mah battery before it shuts down. When the ESC pulls too much current and the voltage drops it shuts down. Shouldnt the same thing be happening on the MMM? If not maybe setting this feature from the factory so its more sensitive might help to prevent failures due to bad batteries?

[BrianG]

Originally Posted by SpEEdyBL

...You'd be amazed at how high voltage spikes can get during braking, easily twice the pack nominal voltage with bad packs. Since the the MMM and MM both have 30 volt fets this was not an issue with the MM as 3s was the rated limit.

There is a TVS on the MMM which limits these spikes to something around 26-28v.

Originally Posted by hemiblas

...When the ESC pulls too much current and the voltage drops it shuts down. Shouldnt the same thing be happening on the MMM? If not maybe setting this feature from the factory so its more sensitive might help to prevent failures due to bad batteries?

This is a double-edged sword. If you make the LVC too sensitive, it false trips all the time. If you make the LVC less sensitive to spikes, battery or ESC damage could occur. So, the LVC probably looks at the voltage over a set time and if it falls consistently below a certain value, it trips.

Originally Posted by BrianG

I still have my trusty old HP48gx, with a whopping 512k RAM card. lol.

LOL, I still have my HP38G calc. I recently took a chem class after 10 years of graduating college (now applying to med school). It was great, when I took a test, the calc checkers had no idea to look for stored notes in my HP. They were like whatever and gave it back to me. That calc had a great solver function. Sorry OT

[BL_RV0]

Originally Posted by BrianG

On the next version of your ESCs, incorporate a mechanical fist that comes out and punches the user if they try to use such packs.

Naaahhh. Just make a version with HUGE caps so that you get a giant spark...

Originally Posted by BrianG

On the next version of your ESCs, incorporate a mechanical fist that comes out and punches the user if they try to use such packs.

while maintaining the current dimensions

[fastbaja5b]

Originally Posted by SpEEdyBL

It's not current that's the problem but the voltage spikes that kill the esc. Even though a 48 amp battery can supply the needed current, it's voltage spikes will be a lot more violent, compared to those from an 80 amp rated battery. C rating mostly decided on being able to hold a voltage, which makes higher amp capable packs better. You'd be amazed at how high voltage spikes can get during braking, easily twice the pack nominal voltage with bad packs. Since the the MMM and MM both have 30 volt fets this was not an issue with the MM as 3s was the rated limit.

And this is why on 4s the failure rate for the MMM is far lower than on 6s, greater difference between the pack voltage and fet rating... right?

[SpEEdyBL]

Exactly. I think there was also some flaw in the early software that prevented the MMM esc from being able to detect those spikes properly, which lead to the V1 and V2 failures. But of course even with the TVS working properly, the more the esc has to depend on it, the more likely damage will occur (I can only guess).

[pasan]

Originally Posted by SpEEdyBL

It's not current that's the problem but the voltage spikes that kill the esc. Even though a 48 amp battery can supply the needed current, it's voltage spikes will be a lot more violent, compared to those from an 80 amp rated battery. C rating mostly decided on being able to hold a voltage, which makes higher amp capable packs better. You'd be amazed at how high voltage spikes can get during braking, easily twice the pack nominal voltage with bad packs. Since the the MMM and MM both have 30 volt fets this was not an issue with the MM as 3s was the rated limit.

A little confused... I understand most of the intricacies behind brushless technology, but why would bad packs cause higher voltage spikes?

[Pdelcast]

Originally Posted by pasan

A little confused... I understand most of the intricacies behind brushless technology, but why would bad packs cause higher voltage spikes?

Because during braking the ESC has to dump power back into the battery. Packs with poor C ratings have high internal resistance, so to get the power back into the battery the voltage has to rise. The lower the internal resistance of the battery (higher C rating) the lower the voltage drop during accelleration, and the lower the voltage rise during braking.

[jpoprock]

I think my Hyperion EOS0610 II can measure the resistance of a pack, but maybe that's not the right sort of resistance reading? So, can the way you have your ESC and radio set up for braking help minimize this, or does the ESC respond to any braking the same way, regardless of how hard or light it's set to?

My Flightpower Packs are rated at 25C, and they are 3s 5000mah packs wired in series. Since they are higher quality batteries, I'm assuming that 25C is good. But if they were a cheaper made Lipo, 25C for the cheaper pack won't be as consistent? I guess that's the rub.... 25C isn't always 25C, etc. Right?

[pasan]

Thanks for the clarification Patrick.

[suicideneil]

Originally Posted by jpoprock

I think my Hyperion EOS0610 II can measure the resistance of a pack, but maybe that's not the right sort of resistance reading? So, can the way you have your ESC and radio set up for braking help minimize this, or does the ESC respond to any braking the same way, regardless of how hard or light it's set to?

My Flightpower Packs are rated at 25C, and they are 3s 5000mah packs wired in series. Since they are higher quality batteries, I'm assuming that 25C is good. But if they were a cheaper made Lipo, 25C for the cheaper pack won't be as consistent? I guess that's the rub.... 25C isn't always 25C, etc. Right?

The charger can measure IR (Internal Resistance), so thats the figure you want- the lower the better.

Changing the brake strength wont really help much- even when you brake gently there will still be the regenerative braking effect sending current to the lipos, so decent lipos are needed regardless. If that doesnt get ya, the ripple current will...

Your Flightpowers will be fine, but you are kinda right about the cheaper lipos; not so much about the consitancy (10c lipos can be consistant, you just wouldnt use them for a demanding application), more to do with inaccurate/ or exaggerated ratings. A low C lipo cant deliver much current when you start to draw beyond about 15C, the voltage drops like a brick and the temps climb- hence why higher C rates are much more desireable from decent brands.