Cant figure out the C thing

[zen3]

Just not getting the C thing...Like in 20C, 10C, and so on....I Really need to know because Im in the market for a couple of 11.1 2100 lipo, and 7.4 800 lipo. I need this in regular turms....Its not that im dumb, Its that I can intubate or crack open a chest when it comes to a medical patient, but I cant grasp this....Z

[mjws]

C is the amount of current necessary to deplete a cell in 1 hour.

Since we often measure cells in milliamp hours. Divide by the mah by 1000, and you get C.

e.g. 2100mah cell. C = 2.1 amps. 20C is just a multiple of C and typically used to describe the amount of continuous current the cells can sustain... so for the same 2100mah cells 20C = 20 * 2.1A = 42A.

For 800mah cells. C= 0.8A, Thus a 15C cell would be capable of 15*.8A = 12A

Good luck,

Mike

[Pinecone]

C = Capacity

1C Discharge rate is 1 x capacity for safe discharge. 2100 pack 1C = 1 x 2100 = 2100 mA = 2.1 amps.

10C = 10 x capacity = 10 x 2100 = 21 amps.

etc.

[mjws]

C is NOT capacity. That screws up units and is not accurate. The capacity of a cell is measured in current/unit time. C is a rate of current (amps) not mah or ah.

Mike

[blueviewlaguna]

Amps a battery can put out for a full hour = the mah rating (I.E. 5000 mah) divided by 1000, in this case 5 amps for an hour for a 5000 mah battery. The only requirement from the battery for a full hour of output is 1C

If you want 10 amps (2C), you get half the time (30 minutes now)

If you want to (fully for this example) discharge the pack in 6 minutes (T-rex 600) its 1/10 of an hour, so you get 10 times the current for 1/10 the time (50 amps for 6 minutes) this is 10C

Formula - multiply the current by the same amount you divide the (1 hour) time. 20C = 20 times the current for 1/20 of an hour, etc. 20C discharge will drain the pack in 3 minutes (1/20 of an hour), giving 100 amps in the process (20 times the 1C rating which is the mah/1000).

[MarkD]

To put it simply

Multiply the C value by the packs amp capacity and that is the safe max consistent amps you can draw out of it

You charge at 1C

[mjws]

A pretty good primer on this is here: Point 4. Describes 'C'
http://www.rcgroups.com/forums/showthread.php?t=209187

In practice we just divide the capacity (in mah) by 1000 and multiply by the rating. Good news is most decent cell makers are spelling out max current and charge current explicitly on their packs.

Mike

[Pinecone]

C is NOT capacity. That screws up units and is not accurate. The capacity of a cell is measured in current/unit time. C is a rate of current (amps) not mah or ah.

Mike

Works the same.

But to make you happy, C is capacity-time. So the units work out to amps.

Happy now?

[mjws]

I'm always happy, man. No offense intended.

Mike

[Pinecone]

None taken. Just harrassing you. :)

[kamtsa]

The physical units of C are 1/hour. The max discharge rating of a battery (given in C units) can be used to compute its max sustained discharge current.

For example, a 2100mah battery with max discharge rate of 10C can be discharged at most at this sustained current:

Imax = 2.1AH * 10C = 21A (notice how the physical units match)

The discharge rating can also be used to compute the shortest discharge time of a battery. For example, the shortest time to fully discharge a fully charged battery that has the sustained max discharge rating of 10C is:

Tmin = 1/10C = 0.1H = 6 minutes (again, note how the physical units match).

Notes:
1 The above examples are for max discharge rates. The same applies also to max charge rates (typically 1C for LIPO).

2. Make sure not to discharge more than 80% of your LIPO capacity

3. I am not a battery expert. Just using high school level physics to make sense of the physical units.

Kam