Regen braking: confirmed!

Voltage jumping back up after going from under load to no load?

Battery voltage can jump like crazy going from full load to no load...

[whitrzac]

Originally Posted by Bolt_Crank

Voltage jumping back up after going from under load to no load?

Battery voltage can jump like crazy going from full load to no load...

no... some of the peaks are above the voltage of the pack starting out....

I've seen my truck go from the 12 - 13 volts when sitting without the truck running, to 16 - 17 volts when I turned off my rack-o-lights... Though, that might be a strange backflow due to the relays themselves...

[pinkpanda3310]

Originally Posted by whitrzac

but you can see it a little with the voltage spikes...

my question is, how come it has those voltage spikes with a mech. brake setup and a clutch

Could it be that breifly the motor is still doing high rpm hence the clutch is still engaged

[BrianG]

Originally Posted by Whitrzac

my question is, how come it has those voltage spikes with a mech. brake setup and a clutch

When you go quickly from any throttle to neutral, the magnetic field in the motor will collapse and generate a small voltage. Won't be near the amount that a brake setup has though, unless the motor IS in fact doing a little braking without you realizing it.

Originally Posted by Bolt_Crank

I've seen my truck go from the 12 - 13 volts when sitting without the truck running, to 16 - 17 volts when I turned off my rack-o-lights... Though, that might be a strange backflow due to the relays themselves...

Relays are coils and generate inductive kickback when shut off. Most circuits simply place a reverse-biased diode in parallel with the coil to shunt that spike.

[teknorc]

Originally Posted by whitrzac

my question is, how come it has those voltage spikes with a mech. brake setup and a clutch

Are you sure the brakes on the ESC are completely disabled? No drag brake, brakes at 0%?

[whitrzac]

Originally Posted by teknorc

Are you sure the brakes on the ESC are completely disabled? No drag brake, brakes at 0%?

I highly doubt it....

[maneba]

This scientific article by a MGM Ing. explain everything...
http://mgm-compro.com/pdf/en-motor-braking-050409-g.pdf

[BrianG]

Originally Posted by MGM Article

a) Use batteries that have truly very small inner resistances (as e.g. Kokam 30C with a suitable capacity).
It is therefore not enough that the seller or distributor states „high Cs“ for the battery and that it can supply high
currents!!! An example of such batteries are packs made of A123 cells, which can provide high currents without damaging
themselves, but the inner resistances are high. At the same time, it is necessary to make sure that conductors between the
battery and the controller are as short as possible and use quality connectors, that is no "4 mm golden plated banana plugs" or
"Dean" etc., but at least MP JET 3.5mm or better 5.5 or 6.0mm. Conductors with a cross-section of at least 4mm2 are also
essential (that is 11 gauge in US units) and quality soldering (our concern are not 1:18 models, but the bigger ones, of course).
Nevertheless, even batteries with very small resistance might not be enough – in some combinations with powerful motors and
bigger models, even with a very good battery, voltages higher than the controller can take can be generated.

b) Use controller for higher voltage than it seems to be needed from the point of voltage on the pins of the battery (that is for 6Lipols use controller suited for at least 8 Lipols).

c) Use „controlled shunt load“ for partial braking current stream diversion, additional module for controllers – under development.

d) Brake softly – this however is quite hard to achieve correctly in practice.

I love this. I don't know how many arguments I've seen at that "other place", but the above really drives home what many more technically inclined people have been saying.

It seems to me that point A is really talking about the Zippy vs any-other-battery debate. High C rates mean little. Resistance is what really matters.

Point B makes simple sense. We all know that brake voltages can be substantially higher than the battery, so if someone is using batteries to the max ESC spec, it's only natural things can go wrong during braking. This is exactly why I NEVER run max spec voltages on ANY ESC. And between you and I, think this is why some people have been blowing MMM ESCs. Gearing a relatively heavy vehicle for stupid speeds on 6s, and then braking fast makes me shudder. Just imagine the kinetic enerfy contained in a 10lb+ vehicle traveling at 60mph+ and trying to stop on a dime.

Point C is basically talking about using some kind of TVS device to shunt overvoltages to a safe level. The MMM uses this, but like any electronic device, it does have limits on what it can absorb. I was rather surprised to see such high braking currents though; I expected values around 20A. And since those current spikes are so short and so large, it makes sense that you don't see much runtime increase; the battery simply cannot charge that fast. Maybe someday we'll see battery charge rates in the 30C range, but certainly not today.

Point D. Well, that is just asking too much of users IMO. When I'm running full tilt and then a curb happens to "jump in the way", the last thing on my mind is whether the ESC will handle the voltage/current spike if I brake hard. It's just instinct (for me anyway) to jam on the brake.

[maneba]

I would add an "E Point"

e) use mechanical brake if you can!

[ticklechicken]

Originally Posted by maneba

This scientific article by a MGM Ing. explain everything...
http://mgm-compro.com/pdf/en-motor-braking-050409-g.pdf

I'm a little confused regarding the comparison between the Kokams and the "not very quality" lipo. These are the values during braking:

---battery-------battery mΩ------voltage rise----peak amps
Kokam--------------80---------------12.5-------------140
low quality---------170--------------17.5--------------80

It seems to me that using a ESC that is spec'd to more voltage than your batteries should fix the "voltage rise" problem. The other issue is the high braking currents. The data suggests that a battery with higher internal resistance will produce less amps and be easier on the ESC. This goes against what else I've learned which is that you should always aim for the lowest battery resistance possible. What am I missing? To me, the safest solution seems to be an over-spec's ESC and high resistance batteries.

Voltage is more likely to burn something out than current is...

[BrianG]

Usually, too-high voltage basically "punches" through whatever insulating material there is, which in turn generates high current. At the end of the day, it matters little if the ESC is a pile of ash.

[bcltoys]

So the MMM is for 6cells right that to me means you should use 4cells to be real safe 5cells max and always at least 35c right or wrong. And nothing but 6.5 connectors or larger. And gear for say 45MPH max.

[Unsullied_Spy]

The MMM is perfectly fine on 6s, I've abused mine on 6s and it handled it well. It's the really hard braking that kills them, I have my brakes down at 30% and I try to take it easy on the brakes. I killed my first one with really hard braking on 4s, my 2nd one has been rock solid for ~50 hours of use so far (about 1 hour of use on 6s and there's nothing left of my tires).