Does a crawler need need a high C rating ?

[mprudic0404]

" A battery doesn't supply current, current is product of the motor creating torque."




"Doesn't matter what battery is attached to the motor, it's gonna demand the current it wants to torque it's trying too, even if it causes failure."




i raise you 2.


1)the first point is true for a dirrect hook-up motor, IE: switch on/off.

The battery DOES supply current in our application due to the FET'S switching on/off. Current is Pulled, in essence into each winding, but limited by each gate switching on/off/on... etc, other wise, every throttle input would be 100% full on-state (no gate/fet switching) The motor ITSELF wants every amp&volt it can muster, EVERY TIME and ALL THE TIME.
Even 100% trigger (wide open throttle) is still choked by the Esc, because even the cheapest speed control out, still switches at high frequency, limiting in-rush to each winding. They do this to feather the steped range of current into the motor and give smooth transistion from crawling to WOT. brushless setups CAN go ever further and vary the frequency of each phase to create a smooth curve. (think 3phase- VFD) A high end brushless ESC handles current and voltage waveforms seperatly. ( leading or lagging - similar to generation of grid power )
brushless motors LOVE reactive power, so its a complex gate dance, to switch, and still maintain a smooth feeling.

2) the motor (dc or ac) ALWAYS wants every bit of power it can take, 100% of the time. SPOT ON.
the ESC will not let that happen, because if it did, the gates wouldnt last 10 seconds(FET'S) These are after all, very small little junctions, and the capability of them to handle such high power levels comes from each on/off/on... cycle.
Even in the 80's when RC cars went FULL electronic, the battery still had massive requirments for reactive & inductive load (motor and regen braking). The old days of a wiper type res-type-esc are just a memory.

C ratings were born from the concern of lithium. great battery, still quite dangerous. You could pull any amount of current you want from any battery, but at some point, heat and losses rise too fast, and its a total loss. DC is always 100% maximum power, on or off is how its regulated.(battery will always put out maximum voltage and current=wattage)
ac is great because depending on when you clip the waveform, depends on your current and or voltage power levels including power factor.

in really high voltages, this 'cliping' becomes critical. think the grid. A breaker isnt going to open or close at the peak of the waveform, instead it times out to catch 0-phase -or very close to it-, so it limits the current/voltage interupted.

c ratings keep people safe, because most of the world dosent want to use Ohms law every time they buy a battery.

 

[Browneye]

Check out the iChargers. They can charge 30 amps and up on a single channel "thumbsup"

I find this battery technology amazing.
Charging and discharging sessions are easy, and im just never in a hurry so 1S has been fine so far. Some planning is required if there’s a stack of them to charge.
Run times are always amazing as well. I come from the carbon or nicad era. :mrgreen:

 

[carpetfuzz17]

I find this battery technology amazing.
Charging and discharging sessions are easy, and im just never in a hurry so 1S has been fine so far. Some planning is required if there’s a stack of them to charge.
Run times are always amazing as well. I come from the carbon or nicad era. :mrgreen:

Oh yeah the good old days of discharging your battery on a bunch of bulbs then putting them in a special tray to discharge each cell down to its cutoff point :roll:
And this was all done to gain what I little bit extra run time :lmao: don't miss those days at all

I also like the 1c charge value for my batteries. I also pit with several others that charge at 20 amps and up :shock: but then again I break every other rule when it comes to charging like leaving and not paying attention or charging in the truck and not using a lipo safe bag don't own one and never will.

I have seen some also warming up their lipos before they are charged :shock: but that is what it takes to be fast

 

[Xx Losungen xX]

Current is never supplied, it's ALWAYS demanded, unless our natural understanding of physics is wrong. Voltage is the complete opposite.

You could hook up a AAA battery with paperclips to your rc motor, if that motor is rated to draw 60a at FLA (full load amps) it's going to demand 60a from your AAA battery. Will it? No, the supple will fail long before the load can reach it's FLA.

That is overly simplified of course, not nearly enough emf (volts) to create the magnetic field in an rc motor.


Think of it this way. Fuses and breakers are not designed to protect the load (motors, lights, etc). They are designed to protect the wire. Reason being that the load can, in a short or ither failure draw an unlimited amount of current from ANY source, until something fails. Wire is usually more costly to replace then the load.

A standard residential circuit breaker with a trip current of 15a has a fault current of 10,000 amps, yes 10,000 amps. Why? Because the load can draw what it wants.

There is no magic in rc motors or escs that make them act differently.

 

[Hydrocarbon92]

Realistically a tiny AAA battery doesn't work since it's internal resistance is so high. Before amperage can go very high, voltage drops & the battery burns energy as heat.



The amperage a battery can supply is based largely on internal resistance. Duracell lists their AAA at 114 mohm of impedance (rising over 400 when depleted) compared to 3mohm for a great lipo. A no-name lipo will be over 10mohm.


The real problem people face is 2-fold. First people buy batteries that are likely over-rated. If you provide the minimum C rating, but it's over-rated, then you are really hurting the lipo. Heat is built up internally, usually damaging (out-gassing) the electrolyte. They puff & your cells are junk. The second is how DC motors can draw a LOT more amps than you expect. For a fraction of a second a motor at rest will draw a lot more that it's rating, then settle down near it's rating. At low speed, this happens several thousand times a second!


Here's where the problems overlap. You're pulsing the battery with a high peak but low average amperage. It doens't get hot, but each pulse is more than it can take. This is like death by needles.

if you get a high C rating, your battery will be the last issue to worry about.

Exactly. I'd rather worry about burning out a $10 motor than a $20-50 battery. If you short a motor with a good battery, it melts the windings well before the battery. Also, dying motors rarely breath fire like Lipo's.

A personal experience is my 100A+ boat. With sub-3mohm Gens Ace batteries it will go 50mph+ (45C 5500mah 4S, 12awg wire). With my 2S SMC batteries (50C, 5000mah, 10awg, 4mohm) in series it rarely hits 48mph. My old Turnigy 4S are about 5mohm and it hardly breaks 40mph. Also, the later two hit voltage cut a lot sooner than their capacity would indicate.

 

[Xx Losungen xX]

Yes the AAA comparison was of course over simplified, points still valid. The point is that the load will draw what it wants, regardless of what the battery can handle.

 

[Xx Losungen xX]

Realistically a tiny AAA battery doesn't work since it's internal resistance is so high. Before amperage can go very high, voltage drops & the battery burns energy as heat.



The amperage a battery can supply is based largely on internal resistance. Duracell lists their AAA at 114 mohm of impedance (rising over 400 when depleted) compared to 3mohm for a great lipo. A no-name lipo will be over 10mohm.


The real problem people face is 2-fold. First people buy batteries that are likely over-rated. If you provide the minimum C rating, but it's over-rated, then you are really hurting the lipo. Heat is built up internally, usually damaging (out-gassing) the electrolyte. They puff & your cells are junk. The second is how DC motors can draw a LOT more amps than you expect. For a fraction of a second a motor at rest will draw a lot more that it's rating, then settle down near it's rating. At low speed, this happens several thousand times a second!


Here's where the problems overlap. You're pulsing the battery with a high peak but low average amperage. It doens't get hot, but each pulse is more than it can take. This is like death by needles.








Exactly. I'd rather worry about burning out a $10 motor than a $20-50 battery. If you short a motor with a good battery, it melts the windings well before the battery. Also, dying motors rarely breath fire like Lipo's.

A personal experience is my 100A+ boat. With sub-3mohm Gens Ace batteries it will go 50mph+ (45C 5500mah 4S, 12awg wire). With my 2S SMC batteries (50C, 5000mah, 10awg, 4mohm) in series it rarely hits 48mph. My old Turnigy 4S are about 5mohm and it hardly breaks 40mph. Also, the later two hit voltage cut a lot sooner than their capacity would indicate.

Internal resistance will have an effect on voltage, current goes up, voltage goes down. Higher voltage, lower current, and of course vice versa.

Higher resistance equals more heat, not the other way around. A 200C battery will always handle a load more efficiently than a 1C battery (C rating is mostly about internal resistance), much less heat being wasted inside the battery. Heat is always a by product of resistance, no way around it.

 

[mprudic0404]

excellent fun, cheers!

if one wanted to go a cheap route and get a low C rating battery, then pick up a small ESC (castle micro perhaps) .

this will protect your low C battery, by limiting its flow. big motor, small motor, dosent matter, the micro esc would only allow so much before it reachs its thermal limits and shutdown.


NiCds dont have much love, which makes sense due to the limiting Mah factor, but i miss them for the sheer power. The old bulb discharge trick was used to reach a almost perfect discharged state. people took this too far and would often leave them bulbs connected way too long. A proper nicd discharge is best when done quickly, and then switched to a LED, to prevent polarity reverse. I STILL have nicd's from the 80's and 90's that are near 95% of rated value. crystal growth inside the battery happens if not properly treated, but is easily prevented. Of all the rechargeables ever used for RC, NiCd's are still the only type with almost unlimited lifespan. (google this yourself)
goverment testing of batteries at the time (70-80's) showed that nicds lived well past 5000 cycles with very little losses. They actualy stopped testing nicd's not because of failure or loss, but because nimh and lithiums became popular with much higher storage, and at less cost. (nicd isnt expensive, but its not eco friendly)
To me, its sad that my old dewalt 18v nicd tools are fading, but the batteries are still near perfect condition. There is a artform to maintain NiCd's and they must be either always charging or always discharging (only charging when needed) Even my early 18v packs show 95% of their rating on the CADEX & turbothirty.

DC (dirrect current) is fun, for a good side read, look into DC breakers. They are quite expensive and only used in a few modern applications. AC won, and today we have AC everywhere including our homes, but DC is still more important ever, and fun fact, We still have a few DC TRANSMISSION lines in the usa. (a few other countries also have DC transmission lines as well)



note: the AAA battery ref is neat, has anyone ever plotted that delta of current/voltage over a short time range ( QUARTER, HALF SECOND or 1 SECOND OF TIME ?) i would suspect, that for most of the frames, the current would show a endless rise, until voltage began to swing down, Due to IR / heat build-up.

 

[pyroM!KE]

If your gonna skimp, skimp on tires or cosmetic etc.... Dont skimp on anything electric....

 

[Hydrocarbon92]

pick up a small ESC (castle micro perhaps) .

this will protect your low C battery, by limiting its flow. big motor, small motor, dosent matter, the micro esc would only allow so much before it reachs its thermal limits and shutdown.

Not really. A tiny mosfet can still pass a huge load of current until it thermals, potentially enough to be bad for your battery. It might be less energy, but the voltage drop that happens can cause higher current draw. Voltage drop can be so bad that you hit LVC very early or even the first time you go full throttle.




It boils down to this: skimp on the battery and you'll be buying them much more often; skimp on ESC and you'll be buying more than 1; skimp on motor and you won't stress the batt/esc/wallet as bad. I'll run cheap motors, but I always run a "too big" esc and high-end batts.

 

[Voodoobrew]

The gear reduction in crawlers lowers the amp draw in crawlers for the most part, but when you bind up a tire it will amp draw will spike. Honestly why would you want the battery to supply enough amps to burn up your esc (rhetorical?)? Because a high c rating while not critical, it is a rough indicator of quality (if its not an outright lie).

 

[Teng]

Slow like crawler 40C is more than enough. However I do have Revolectric Lipo packs in 40C and 70C though.