Understanding lipo c burst and 2s/3s

[RickM]

I have done a search for this but to no avail. To start to get to my question here is what I am trying to accomplish. I am trying to find the smallest possible lipo battery to put on the front axle of my MRC. That being said here is what I am trying to determine. A 3s pack has much better wheel speed than a 2s but is that because of the 3s or the C rating on boost output? 2s packs are usually smaller but is there a way to get good wheelspeed out of a high C rated 2s, or is the only real way to get the same wheelspeed to go to a 3s pack? Thanks everyone.

[SMR 510RR]

Ok, there are tons of people on here who know more about batteries than I do but I think I can get you pointed in the right direction.

The "S" number (2S, 3S, etc.) refers to how many cells are in series. This determines the voltage of the pack because no matter the size of the cells they all have the same voltage (4 volts per cell fully charged or 3.7 volts nominal). So a 2S pack is labeled a 7.4V etc.

The C rating you are looking at is the discharge rating of the battery. Mah is the rating used to describe how much energy is stored in the battery. To find one C you basically move 3 decimal places to the left (1C of a 1000mah pack is 1 amp) this is a very important number because most packs require charging at 1C. So lets say that you have that same 1000mah pack that happens to have a 20C discharge rate. That pack could put out 20 amps without harm to the pack. When looking for a lipo look for one with a 20C or higher discharge rate, obviously this depends on what you are running and how many amps your crawler will pull.

What pack you need really depends on what your electronics are like. Are you running stock electronics? If you are running the stock ESC you can not run a 3S pack, also the ESC DOES NOT have a low voltage cut off so you will either need to be very careful not to discharge the battery too far (not advisable) or you will need either a pack with a cut off built in or an external alarm or cut off. We will need to know more about your setup before we can advise you of a good battery to fit your needs.

Dont forget that you will need a lipo specific charger (hopefully a balance charger) to charge a lipo. It takes right around an hour to charge a lipo from empty to full so you may want a couple packs also.

Check out the stickys at the top of this section for more info on lipos and check out the MRC section for more MRC specific tech.

[Terranaut]

Higher wheelspeed comes from higher voltage, not from higher mAH capacity or from a higher C rating. So 3S is better than 2S for wheelspeed.

For any given size battery pack a higher C rating is better than lower, get the best you can afford. Higher C gives more 'punch' with less stress to the battery.

Cheers.

[oldhippie]

The C rating is related to the amperage the battery can discharge safely. The continuous C rating is just that. The maximum safe discharge rate through the discharge cycle. The burst C rating is the maximum safe discharge rate for a very short period. To get the discharge amps you multiply the battery mah and C rate. A 1500 mah battery with a 20c rating can only safely discharge 30 amps over its discharge cycle but may have a 35c burst rating allowing it to safely discharge 52.5 amps for short periods. Anytime you exceed the maximum discharge rates you damage the battery. To confuse matters more there are no standards for C rating and some manufactures don't list burst ratings.

[RickM]

Just for good measure I am running a tekin FXR with a 3s pack now and it is a 400 thunderpower and it works great and gives me good wheelspeed with a holmes motor but. I have it mounted at an angle on the side of my full size servo on a mrc bu I am simply curious if I can get an even small battery and mount it flat without killing my run time. Truth be told I hardly ever burn off a pack as it is even with a 400 mah.

[Terranaut]

Originally Posted by oldhippie

The C rating is related to the amperage the battery can discharge safely. The continuous C rating is just that. The maximum safe discharge rate through the discharge cycle. The burst C rating is the maximum safe discharge rate for a very short period. To get the discharge amps you multiply the battery mah and C rate. A 1500 mah battery with a 20c rating can only safely discharge 30 amps over its discharge cycle but may have a 35c burst rating allowing it to safely discharge 52.5 amps for short periods. Anytime you exceed the maximum discharge rates you damage the battery. To confuse matters more there are no standards for C rating and some manufactures don't list burst ratings.

Not crossing swords here, just something for people to think about. On a popular forum for R/C airplanes a fairly knowlegeable guy did some tests and found that at maximum discharge, as per C ratings, batteries can only sustain that rate for one, maybe two cycles at best. They are so damaged that they are not able to do it repeatably. The burst rate is only achievable if the battery is being discharged well below max C rating before and after the burst, and burst means only a couple seconds.

If you expect your batteries to last you need to stay well below max C rates and try to avoid burst rates altogether. High discharge rates reduce battery life, max discharge rates kill them quickly. This is just a heads up, do your own research.

Cheers.