vstrok

iTrader: (31)

Can someone please help me to understand...

If a LiPo and NiMh were both equally run as hard and both had the same mah capacity....why would the LiPo have a longer run time?

Jason Pelletier

iTrader: (33)

Cause it's better. I think it's due to the discharge rate.


anyways lipo's fade less and last longer.

They are really that much better in almost every way !

SpraydbySprague

iTrader: (64)

LiPo cells have a much higher sustained voltage across their discharge curve compared to NiMh cells. The LiPo cells are considerably lighter than a NiMh cell for any given capacity. This higher voltage propels the motor easier, and the reduced weight makes it easier for the motor to propel the vehicle. These are two factors I believe why a LiPo will have a longer run time vs a NiMh with the same capacity.

NJA82

iTrader: (33)

Lipo batteries have an extremely low internal resistance and a strong burst and sustained discharge capacity ( in the higher C rated cells ) this puts a lot less strain on the cells under load allowing them to sustain the discharge current easier and give longr run times for the same given capacity.

JeromeK99

iTrader: (3)

I have done extensive testing on NiMh and LiPos..

The best 4500 NiMh is as good as any standard 3200 LiPo in runtime and power. The power curve is really the same. Now if you have been buying mediocre or budget NiMh, you will see a BIG difference in performance.

Biggest difference is in the weight.. the LiPo is half the weight of the NiMh. The next most signifigant thing is that there is very little maintenance for the LiPo. If you buy a decent brand like Orion/Kokam.. you only need to balance it every once-in-awhile. NiMh take constant maintenance in discharging, equalizing.. etc... The price differential is big though... LiPos cost at least double if not triple that of a NiMh.

Usually I would get a good year of weekend racing/practice out of a good set of NiMh batteries.. usually a set for me is 3. Then they would retire to the pile o'batteries.. not strong enough for a good stock race anymore. On the other hand, this last year I used a single LiPo for each of my cars/classes I have raced. My Orion 3200 for stock buggy. One pack for practive + 2 quals and the main. It is still as strong as it ever was! So.. going on year 2.. So far its has been worth it for the $$ and more than paid for itself in ease of use! I run it.. then put it right back on the charger! Its done in 30min between the heats!

Jerome

vstrok

iTrader: (31)

Also, what does the "C" mean? Usually it is followed by a 20 or 24. Is this important?

burnineyes

iTrader: (17)

C is the capacity of the battery. So 20C is 20 times the capacity of the pack. This is the discharge that the battery is designed to handle. You have to be careful though as some companies listing of their "C" rating is the maximum the battery can handle for short bursts, where better brands list it as the constant ability. A good brand name Lipo- say a 4000mah 20C constant, is good for 80 amp draw continuously.
Another reason they run longer than a comparible Nimh, whch was listed in a previous post is internal resistance. Since it is so much lower, less energy is wasted to heat. Resistance creates heat. This alone gives huge runtime improvements. Add in everything else mentioned above and you have a much better battery.

SpraydbySprague

iTrader: (64)

The "C" means capacity the number following it is a multiplier of the "C". This represents the maximum sustained current a LiPo pack can handle. For example:
a 5000 Mah 20c pack can handle 5a(5000MaH)X20C=100 amp maximum continuous load.

TRF415boy

You're nearly there.

The internal resistance means that under load there is much less losses in heat thus allowing more of the stored energy to be used to propel the car/plane/boat.

As for the C rating, yes it is relative to the sustainable discharge rate, but it's a little more complicated than that. To be more precise it's the max discharge current under which the discharge capacity loss is less than a certain amount. And THAT is what makes the difference between the manufacturers, as not all have the same standard with regards to the capacity loss.

scrawl

iTrader: (8)

The interesting question is how much belief should be put in a pack's C rating.

Are all battery manufacturers testing using a standardised method? Is a 40C brand A identical to a 40C brand B?

*edit* Just realised TRF415 pretty much already raised this

Duster_360

iTrader: (21)

No, unfortunately there are no testing standards for establishing C rating. You will find that some lipos are over rated and some (prob fewer) are under rated.

Anyone who has run NiMh knows how hot they get during running. That heat comes from the pacs internal resistance and is wasted energy. The IR of typical lipo is much smaller than NiMh, anyone using a good charger that provides IR value can see this. I track it thru time as the pacs gets older and you can see NiMh increase and lipo stays pretty much the same or shows a small increase.