Ways to protect MMM when used with subpar batts

[Bondonutz]

I Own 10 Zippy/FlightMax packs and ultimately decided to pull the heat shrink off and inspecting all solder joints and wrapping the cells together with nylon packing tape and re-heat shrinking them. This may have not ben nessasry but I surely feel better inspecting them prior to use. It's a little work and a small investment for peice of mind but the end result has ben being able to save nearly a thousand dollars .
My only real complaint is that 3 of my packs (2-4s and 1-5s) and several of my friends whom also have over dozen packs collectively have had issues with the balance plugs being faulty. I bought replacement plugs with wires from MaxCramps and soldered them on and now they work and balance near perfection.
This isn't really a complaint because I enjoy soldering so much so replaceing the wire is not a problem, but why do they use 12G with some of the Flightmax 4s 30C packs I bought and one of my buddys who has 3s Turnigy packs have 8G wire.
WTF is up with that ?

[himalaya]

Originally Posted by Bondonutz

why do they use 12G with some of the Flightmax 4s 30C packs I bought and one of my buddys who has 3s Turnigy packs have 8G wire.
WTF is up with that ?

Wire gauge in LiPo packs usually reflex the current level. for Turnigy, say a 5Ah pack, I found they use 10AWG on 20C but 8AWG on 30C models, 'cause their supposed current handling capability differs, 100A vs 150A.

Back to the topic I agree with BrianG, ADD EXTRA CAPACITORS.

MMM 's power smoothing capacitor totals only 990uF, electrolytic type. Yes they are high quality Rubycon ZLH type for switching power supplies but still electrolytic. In such a 100A+ switching/pulse application, 990uF electrolytic is far from adequate.

According to Rubycon datasheet, the ZLH35V330uF handles ripple current of 1.33A, three on board totals 4A, only. And the reality is ESCs are all switching type, currents are all in pulse form. This may not be a huge problem in RC cars for its not fully loaded continously. A high quality battery with extremely low source resistance will help A LOT smoothing out the ripple current, making the voltage relatively constant.

When subpar packs are used, because they have higher IR, extra capacitors are highly recommended, 4700uF or greater. Make sure they are high quality type designed for switching power supplies. They will iron out most voltage ripples to help the on board caps survive as well as the whole ESC.

This thread has been very informative, and I'm glad it turned into a healthy discussion.

Update - I borrowed a 4s pack to try and found that the car was just too fast for the indoor track with it. So, I picked up a 3s Zippy (25C 5000) for <$50 and it seems to work quite well. I figure the lower voltage will reduce the amp draw, voltage stress, ripple, etc., and be safe to use. (?) I've kept the gearing and ESC settings the same so far, and I get about 25 mins. runtime until 3.5V/cell LVC kicks in. The motor and ESC come off just warm, and the pack a step above stone cold. I am slower on the straight than the 4s guys, but not by that much, and from the looks of it, I should even be able to go up some on the pinion for a bit more speed. Other than that, the tame setup and budget battery seem to be fine together. Do I still need to add more capacitors?

[suicideneil]

I figure the lower voltage will reduce the amp draw, voltage stress, ripple, etc., and be safe to use. (?)

Nope, the opposite is generally true, but only if you use a lower kv motor at the same time; voltage drop and current ripple is purely to do with lipo quality rather than voltage.

Given your setup and the size/type of your track, 3s lipo & 2200kv should be fine- 4s would probably be too fast as you experienced. A more common setup is the 2650kv motors on 3s, many people swear by that setup for alot of tracks.

If the motor, esc and lipo are only coming off warm, then it sounds fine, no real need for extra caps unless you really want to, but it doesnt sound like you need them.

[BrianG]

Originally Posted by elcheapo

I figure the lower voltage will reduce the amp draw, voltage stress, ripple, etc., and be safe to use

Originally Posted by suicideneil

Nope, the opposite is generally true, but only if you use a lower kv motor at the same time; voltage drop and current ripple is purely to do with lipo quality rather than voltage...

Wait a minute here. Maybe I'm reading something wrong, but for a given setup, a lower voltage will result in lower current (and lower speed). And since current draw is lower, ripple current and voltage drop will be less as well.

Originally Posted by BrianG

Wait a minute here. Maybe I'm reading something wrong, but for a given setup, a lower voltage will result in lower current (and lower speed). And since current draw is lower, ripple current and voltage drop will be less as well.

That is exactly my thinking. Now, if I go trying to gear it way up to match 4s speed, or something silly like that, then it would be a different story.
In any case, I found these if I need 'em...

[Bernie Wolfard]

To answer the question earlier in this thread of how to you tell how good a battery is (copied from another post I did):

How do you tell if your battery is a good one and how to do track your batteries ability to discharge as they age? Several Lipo chargers can also measure battery resistance. The ones I know of are several Hyperion and NeuEnergy chargers. I am sure there are more that have this feature, I am just not aware of them. Internal resistance is what controls a C-Rating, the lower the internal resistance the faster the battery can discharge before it overheats. A batteries internal resistance (C-Rating) also determines the amount of ripple current the controller will have to deal with. The lower the batteries internal resistance the better the battery. With one of these chargers you can simply plug the battery in and records its resistance. The lower the number, the better the battery. If you track a batteries internal resistance over time you will see its resistance going up as the battery reaches the end of its lifespan.

With a charger that measures internal resistance it is interesting to plug in different packs of the same size and type from the same battery seller and see how different they are. If you do this you will find that the better battery companies have much less variance in their batteries because they QC the cells they use. You can also go through several cheap batteries and separate the good from the bad. Another interesting thing to track by measuring internal resistance is how many good cycles you can get out of specific packs. Again, you will find that the better batteries (except for bleeding edge supper high discharge cells, which typically don't last very long) will last much longer than cheapos. If you then calculate cost per run you will find the more expensive better packs are actually much less expensive in the long run.

The equipment it takes to measure battery internal resistance makes doing it yourself expensive and impractical, but it can be an easy job for a charger. Look for this feature in your next charger.

[Bernie Wolfard]

El Cheapo,

You are correct. If you lower the system voltage with everything else the same the max RPM of the motor is lower (Kv * voltage). It will spin up less aggressively when pulling the trigger and have an overall lower amp draw meaning less heat, more run time less stress on the whole system etc. You only get into trouble if you try to match the speed of a higher voltage system with a lowering one by lowering the gear ratio.

Bernie

[shaunjohnson]

that i didnt know, C rating is related to internal resistance...i should have known

what is a GOOD low resistance?
i'm gonna test the resistance of my turnigy today

[BrianG]

I would say 0.003 ohms or less per cell. To get overall pack resistance, simply add up all your cell resistances. Again, the lower the better.

Don't forget, the cell resistance from discharging at a low current will probably be higher than discharging at 100A. The resistance curve is not linear. It generally decreases somewhat as current goes up until you hit the dicharge limit of the cell, at which time resistance climbs fast. Also, towards the end of the discharge cycle, resistance starts to climb as well.

[Pdelcast]

^^^^

[Bernie Wolfard]

If we had zero resistance cells their C-Rating would be infinite.

Bernie

[BrianG]

Yeah, wouldn't that be nice! You guys would have a heck of a time trying to make an ESC that would be able to take advantage of that. You can call it the "Mamba Infinity"

[Bernie Wolfard]

Zero resistance in everything else and we would have perpetual motion.

[Pdelcast]

Originally Posted by Bernie Wolfard

Zero resistance in everything else and we would have perpetual motion.

Well, but you couldn't produce any POWER without batteries - -even with zero resistance...