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### Author Topic: I need help with units, Battery Rate of Discharge "C".  (Read 13008 times)

#### CliffordK

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##### I need help with units, Battery Rate of Discharge "C".
« on: 26/09/2011 06:08:40 »
When I've been looking at batteries, especially those made in China, invariably I see a rate of Discharge listed with Units "C".

I've seen it a few times.  Here is a listing for LiFePO4 batteries on E-Bay.

300 Ah battery cells
Operation voltage of cell is: 2,8 V - 4,0 V
Max charge current: <= 3 C
Max discharge current (constant): <= 3 C
Max discharge current (impulse): <= 20 C
Standard charge/discharge current (for long life time): <= 0,5 C

Cycle life (estimated):
3000 charging cycles at 80% Depth Of Discharge
5000 charging cycles at 70% Depth Of Discharge

The units I would expect would be Amps.  Or, perhaps watts, but that depends on the array config whereas Amps are easier to deal with parallel/serial array configurations.

This one lists the discharge current as CA for a higher voltage, lower AH battery (several smaller cells in series)

I was thinking it might be an abbreviation for hundred amps, but the official metric abbreviation for 100 is hecto-

This one (180 Ah) lists it as:
Recommend Charging-Discharging Current (0.3C,A)      54

Which doesn't seem to make any sense either.

If we take the last example, 54A discharge rate at 180Ah, gives a 100% discharge in 3-4 hours, which would be a little slow, but almost a livable rate.

If this is the conversion factor (0.3C/Amp), dividing the rate for the first battery would give about 10 Amps, which would still give one about 30 hours for 100% depth of discharge....  which again is far too slow for many applications.

If C stood for 100 amps, then for the first battery above, it would be about 3*100Amps/C, and one would get about a 100% depth of discharge in about an hour which wouldn't be bad.

So, WHAT IS "C"???  (no, I'm not planning on accelerating my car to lightspeed).

#### Geezer

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #1 on: 26/09/2011 07:12:26 »
I believe C is just the capacity in ampere-hours. In the first example they suggest charging it at 150 amps (which should take about two hours if it's fully discharged.)

#### MikeS

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #2 on: 26/09/2011 08:23:15 »
When I've been looking at batteries, especially those made in China, invariably I see a rate of Discharge listed with Units "C".

I've seen it a few times.  Here is a listing for LiFePO4 batteries on E-Bay.

300 Ah battery cells
Operation voltage of cell is: 2,8 V - 4,0 V
Max charge current: <= 3 C
Max discharge current (constant): <= 3 C
Max discharge current (impulse): <= 20 C
Standard charge/discharge current (for long life time): <= 0,5 C

Cycle life (estimated):
3000 charging cycles at 80% Depth Of Discharge
5000 charging cycles at 70% Depth Of Discharge

The units I would expect would be Amps.  Or, perhaps watts, but that depends on the array config whereas Amps are easier to deal with parallel/serial array configurations.

This one lists the discharge current as CA for a higher voltage, lower AH battery (several smaller cells in series)

I was thinking it might be an abbreviation for hundred amps, but the official metric abbreviation for 100 is hecto-

This one (180 Ah) lists it as:
Recommend Charging-Discharging Current (0.3C,A)      54

Which doesn't seem to make any sense either.

If we take the last example, 54A discharge rate at 180Ah, gives a 100% discharge in 3-4 hours, which would be a little slow, but almost a livable rate.

If this is the conversion factor (0.3C/Amp), dividing the rate for the first battery would give about 10 Amps, which would still give one about 30 hours for 100% depth of discharge....  which again is far too slow for many applications.

If C stood for 100 amps, then for the first battery above, it would be about 3*100Amps/C, and one would get about a 100% depth of discharge in about an hour which wouldn't be bad.

So, WHAT IS "C"???  (no, I'm not planning on accelerating my car to lightspeed).

invariably I see a rate of Discharge listed with Units "C".  C ie current in amperes.
This is peak current available (short term) but not recommended.
If we take the last example, 54A discharge rate at 180Ah, gives a 100% discharge in 3-4 hours, which would be a little slow, but almost a livable rate.  Yes, 3.3 hours.
If this is the conversion factor (0.3C/Amp), dividing the rate for the first battery would give about 10 Amps, which would still give one about 30 hours for 100% depth of discharge....  which again is far too slow for many applications.  300Ah @ 3A = 100 hours continuous but surge of 20A allowable.
So, WHAT IS "C?  C is current in Amperes

I think you are looking at the wrong kind of batteries for what you want.  You need deep discharge batteries as used in submarines (bit big), electric vehicles etc. boats, mobile homes, caravans etc.
« Last Edit: 26/09/2011 08:32:28 by MikeS »

#### CliffordK

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #3 on: 26/09/2011 08:26:22 »
I believe C is just the capacity in ampere-hours. In the first example they suggest charging it at 150 amps (which should take about two hours if it's fully discharged.)
Ahhh,
That makes a little bit of sense.
Except that current is always measured in Amps, not Amp Hours.
And Capacity is measured in Amp Hours.

But, it would mean that one could use similar specifications for multiple batteries, and it would make it easier to compare different batteries.

#### MikeS

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #4 on: 26/09/2011 08:47:35 »
I believe C is just the capacity in ampere-hours. In the first example they suggest charging it at 150 amps (which should take about two hours if it's fully discharged.)

If you charge it at that rate it will probably blow up!
The recommended charge current is 3A maximum, that's a charge time of 100hours.  300Ah/3=100

I believe with most if not all rechargeable batteries, the slower the charge and discharge cycles the longer the battery will last (the more charge/discharge cycles it will take).  Most lead acid cells are not designed for deep discharge.  Those that are have a different construction to deal with the extra heat and stress that is involved.

#### Geezer

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #5 on: 26/09/2011 09:01:38 »
I believe C is just the capacity in ampere-hours. In the first example they suggest charging it at 150 amps (which should take about two hours if it's fully discharged.)
Ahhh,
That makes a little bit of sense.
Except that current is always measured in Amps, not Amp Hours.
And Capacity is measured in Amp Hours.

But, it would mean that one could use similar specifications for multiple batteries, and it would make it easier to compare different batteries.

Capacity, C, is a "normalized" measurement that applies to any battery. It gives you a pretty good idea of the the energy that can be produced by a battery compared with any other battery. However, different batteries have very different charge and discharge rates, and if you charge them or discharge them quickly, a lot of the energy can be dissipated inside the battery itself (in the internal resistance).

So, a 100 AH battery won't necessarily deliver 100 amps for one hour, although, depending on the type of battery, it might well deliver 10 amps for 10 hours.

It gets more complicated when you try to determine the energy in terms of power because the instantaneous power is the product of voltage and current, but the voltage isn't necessarily all that constant, which is why the manufacturers prefer to work in amps

#### Geezer

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #6 on: 26/09/2011 09:09:08 »
I believe C is just the capacity in ampere-hours. In the first example they suggest charging it at 150 amps (which should take about two hours if it's fully discharged.)

If you charge it at that rate it will probably blow up!
The recommended charge current is 3A maximum, that's a charge time of 100hours.  300Ah/3=100

No, it isn't. A 300 AH battery that took 4 days to charge would be about as much use as a chocolate teapot.

#### CliffordK

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #7 on: 26/09/2011 09:24:29 »
I think you are looking at the wrong kind of batteries for what you want.  You need deep discharge batteries as used in submarines (bit big), electric vehicles etc.
Obviously everything is application dependent.

Most of the submarine batteries are Lead Acid, I think.  Weight has to be in balance, but perhaps isn't as critical with water displacement as other applications.  Forklift Batteries are also lead acid, and very heavy.

I put in a bid on a used Gizmo Car.  I should find out if I won it tomorrow.

Anyway, I think the submarine battery might squish the car.

I have some UPS batteries that weigh about 300 lbs for a 4.2 V battery.  Again they are a little heavy.  My Ford Ranger definitely became a low-rider carrying home 6 of them in the back.

The Lithium batteries are supposed to be less sensitive to deep discharge cycles than lead acid batteries, as well as being much lighter.  So, theoretically they would be superior to lead batteries for automotive applications, and I could potentially increase my range and speed.  But, of course, they are very expensive.

I believe the Gizmo is supposed to have qty: 8 - 6V, 225 AH batteries for a total of about 48V, and about 500 lbs.  Enough to give it around a 40 mile range.

It would take about 16 of the Winston Lithium LiFePO4 batteries above with about a 50% increase in range over the Lead Acid batteries, at a cost of more than I'm hoping to pay for the entire car including current lead acid battery set.

For the SE180AHA Lithium-Ion Batteries on Alibaba above, it would take about 32 batteries at a weight of 100 lbs Less than the equivalent Lead Acid battery pack, and likely double the range.

Hmmm...  Looking back at that Alibaba page
It lists:
Nominal Capacity: 180 Ah.
Maximum recommended Discharge Current: 54A (Amps) (0.3C).  Multiplying 0.3x180 = 54.

So, I think Geezer is right that "C" stands for Capacity, or Capacity/h.

The Tesla uses Lithium Batteries, and can suck down a LOT of power.  I believe with a good system, it can recharge half its battery capacity in about an hour.  I believe that it also uses active cooling with its battery pack for both charging and discharging.

The Trojan Batteries for the Gizmo:
8 batts x 6v x 225 Ah, and one gets about 10 Kwh for about 500 lbs.

The Tesla uses a pile of little battery cells, but a total of about 53 Kwh (5x the capacity of the Gizmo), and 992 lbs (less than 2x the weight).
« Last Edit: 26/09/2011 09:42:27 by CliffordK »

#### CliffordK

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #8 on: 26/09/2011 09:30:28 »
No, it isn't. A 300 AH battery that took 4 days to charge would be about as much use as a chocolate teapot.

Oh, I saw that...  maybe on a BBC special about Rocket Engines.  They made a chocolate Ice Cream Teapot.  Fed propane into a burner.  Used the expansion of the gas to keep the chocolate Ice Cream cold, and were able to boil water in it

#### MikeS

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #9 on: 26/09/2011 09:34:27 »
I believe C is just the capacity in ampere-hours. In the first example they suggest charging it at 150 amps (which should take about two hours if it's fully discharged.)

If you charge it at that rate it will probably blow up!
The recommended charge current is 3A maximum, that's a charge time of 100hours.  300Ah/3=100

No, it isn't. A 300 AH battery that took 4 days to charge would be about as much use as a chocolate teapot.

300 Ah battery cells
Operation voltage of cell is: 2,8 V - 4,0 V
Max charge current: <= 3 C
Max discharge current (constant): <= 3 C
Max discharge current (impulse): <= 20 C
Standard charge/discharge current (for long life time): <= 0,5 C

Cycle life (estimated):
3000 charging cycles at 80% Depth Of Discharge
5000 charging cycles at 70% Depth Of Discharge

Yes it is.  If you want the longest life from this battery then 300Ah/0.5A = 600hours charge/discharge.
This battery is not designed for a high charge discharge rate.  From its slow charge/discharge rate one could probably deduce it is meant to be used as a float battery.  It's just there as a backup in case of mains failure.  You might think its about as much use as a chocolate teapot but it's a very long lasting chocolate teapot

#### Geezer

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #10 on: 26/09/2011 09:54:06 »
I believe C is just the capacity in ampere-hours. In the first example they suggest charging it at 150 amps (which should take about two hours if it's fully discharged.)

If you charge it at that rate it will probably blow up!
The recommended charge current is 3A maximum, that's a charge time of 100hours.  300Ah/3=100

No, it isn't. A 300 AH battery that took 4 days to charge would be about as much use as a chocolate teapot.

300 Ah battery cells
Operation voltage of cell is: 2,8 V - 4,0 V
Max charge current: <= 3 C
Max discharge current (constant): <= 3 C
Max discharge current (impulse): <= 20 C
Standard charge/discharge current (for long life time): <= 0,5 C

Cycle life (estimated):
3000 charging cycles at 80% Depth Of Discharge
5000 charging cycles at 70% Depth Of Discharge

Yes it is.  If you want the longest life from this battery then 300Ah/0.5A = 600hours charge/discharge.
This battery is not designed for a high charge discharge rate.  From its slow charge/discharge rate one could probably deduce it is meant to be used as a float battery.  It's just there as a backup in case of mains failure.  You might think its about as much use as a chocolate teapot but it's a very long lasting chocolate teapot

Mike, this type of battery is intended for rather rapid charge/discharge rates. A 600 hour cycle time would make it practically useless. Have you ever heard of a battery with a 600 hour cycle time?

"C" is capacity in ampere-hours. It does not directly relate to charge/discharge rates. That's why those parameters are quoted in terms of C.

#### MikeS

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #11 on: 26/09/2011 10:06:19 »

Hmmm...  Looking back at that Alibaba page
It lists:
Nominal Capacity: 180 Ah.
Maximum recommended Discharge Current: 54A (Amps) (0.3C).  Multiplying 0.3x180 = 54.

So, I think Geezer is right that "C" stands for Capacity, or Capacity/h.

Not so.
Going back to the original example if C stands for charge/discharge capacity and it is 0.3 then it follows that
from the above figures
"Max discharge current (constant): <= 3 C
Max discharge current (impulse): <= 20 C
Standard charge/discharge current (for long life time): <= 0,5 C"

Max. discharge current is 300Ah x 0.3 = 100A
Max discharge current (impulse) is 300Ah x 20 = 6000A
Standard charge/discharge current (for long life time) is 300Ah x 0.5A = 150A

The standard charge/discharge current is higher than the max. discharge current.  See the problem?
« Last Edit: 26/09/2011 10:17:37 by MikeS »

#### CliffordK

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #12 on: 26/09/2011 10:08:49 »
A 600 hour cycle time would make it practically useless. Have you ever heard of a battery with a 600 hour cycle time?

"C" is capacity in ampere-hours. It does not directly relate to charge/discharge rates. That's why those parameters are quoted in terms of C.
Cell Phones are now up to 200+ hours standby time (less if you are talking on the things).
But, anything above a few hours for recharging is a pain.

I'd like a laptop computer battery that would last 600 hrs without having to lug around a 500 lb brick.  Can't one dream?  Even so, anything over 8 hrs recharge time would be unacceptable.

Anyway,
I'll go with Geezer on this one.  Thanks for the help.

#### MikeS

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #13 on: 26/09/2011 10:11:40 »
I believe C is just the capacity in ampere-hours. In the first example they suggest charging it at 150 amps (which should take about two hours if it's fully discharged.)

If you charge it at that rate it will probably blow up!
The recommended charge current is 3A maximum, that's a charge time of 100hours.  300Ah/3=100

No, it isn't. A 300 AH battery that took 4 days to charge would be about as much use as a chocolate teapot.

300 Ah battery cells
Operation voltage of cell is: 2,8 V - 4,0 V
Max charge current: <= 3 C
Max discharge current (constant): <= 3 C
Max discharge current (impulse): <= 20 C
Standard charge/discharge current (for long life time): <= 0,5 C

Cycle life (estimated):
3000 charging cycles at 80% Depth Of Discharge
5000 charging cycles at 70% Depth Of Discharge

Yes it is.  If you want the longest life from this battery then 300Ah/0.5A = 600hours charge/discharge.
This battery is not designed for a high charge discharge rate.  From its slow charge/discharge rate one could probably deduce it is meant to be used as a float battery.  It's just there as a backup in case of mains failure.  You might think its about as much use as a chocolate teapot but it's a very long lasting chocolate teapot

Mike, this type of battery is intended for rather rapid charge/discharge rates. A 600 hour cycle time would make it practically useless. Have you ever heard of a battery with a 600 hour cycle time?

"C" is capacity in ampere-hours. It does not directly relate to charge/discharge rates. That's why those parameters are quoted in terms of C.

Quoting what the battery was for in the first place would have helped.

#### Geezer

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #14 on: 26/09/2011 10:16:43 »

Hmmm...  Looking back at that Alibaba page
It lists:
Nominal Capacity: 180 Ah.
Maximum recommended Discharge Current: 54A (Amps) (0.3C).  Multiplying 0.3x180 = 54.

So, I think Geezer is right that "C" stands for Capacity, or Capacity/h.

Not so.
Going back to the original example if C stands for charge/discharge capacity and it is 0.3 then it follows that
from the above figures
"Max discharge current (constant): <= 3 C
Max discharge current (impulse): <= 20 C
Standard charge/discharge current (for long life time): <= 0,5 C"

Max. discharge current is 300Ah x 0.3 = 100A
Max discharge current (impulse) is 300Ah x 20 = 6000A
Standard charge/discharge current (for long life time) is 300Ah x 0.5A = 150A

The standard charge/discharge current is higher than the max. discharge current.  See the problem?

Sure do.

The max. discharge rate is 900A, not 100A.

#### CliffordK

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #15 on: 26/09/2011 10:25:38 »

Hmmm...  Looking back at that Alibaba page
It lists:
Nominal Capacity: 180 Ah.
Maximum recommended Discharge Current: 54A (Amps) (0.3C).  Multiplying 0.3x180 = 54.

So, I think Geezer is right that "C" stands for Capacity, or Capacity/h.

Not so.
Going back to the original example if C stands for charge/discharge capacity and it is 0.3 then it follows that
from the above figures
"Max discharge current (constant): <= 3 C
Max discharge current (impulse): <= 20 C
Standard charge/discharge current (for long life time): <= 0,5 C"

Max. discharge current is 300Ah x 0.3 = 100A
Max discharge current (impulse) is 300Ah x 20 = 6000A
Standard charge/discharge current (for long life time) is 300Ah x 0.5A = 150A

The standard charge/discharge current is higher than the max. discharge current.  See the problem?

You've jumped onto 2 different batteries.
The Winston Battery on E-Bay:
Max discharge current (constant): <= 3 C
Max discharge current (impulse): <= 20 C
Standard charge/discharge current (for long life time): <= 0,5 C

Substituting in 300AH (and dropping the H), one gets:
Max discharge current (constant): <= 3*300 = 900 Amps
Max discharge current (impulse): <= 20*300 = 6000 Amps (it doesn't list a duration, but the Alibaba battery listed a 10 second impulse, such as car starting).
Standard charge/discharge current (for long life time): <= 0,5 * 300 = 150 Amps.

Nothing inconsistent there.

The  Alibaba battery was 180 AH.
Recommend Charging-Discharging Current (0.3C,A) 54 Amps
Maximum short-tine Discharging Current (period≤10s,A) 1000 Amps

So, it was a little more than half the capacity of the Winston Battery, but less impressive specs.

If I get an EV, I should get a better idea of the actual amp usage, both short term (acceleration) and long term (normal driving), but ether battery is likely close, especially if one incorporates an active cooling system.

The biggest problem is getting an acceptable price.  Tesla lists their battery pack as being worth \$36,000.  While I don't intend to spend that much, it would not be hard to spend \$10K on a lithium battery pack.  And, abuse it too much and it becomes worthless.

#### MikeS

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #16 on: 26/09/2011 10:41:03 »
Geezer is correct and I was wrong.  What threw me was not knowing what sort of battery it was or its intended use and C being used for capacity and not current.  Also 0.3C being quoted in the first post.

#### MikeS

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #17 on: 26/09/2011 10:57:02 »
From the first example
"300 Ah battery cells
Operation voltage of cell is: 2,8 V - 4,0 V
Max charge current: <= 3 C
Max discharge current (constant): <= 3 C
Max discharge current (impulse): <= 20 C
Standard charge/discharge current (for long life time): <= 0,5 C"

300Ah x 3C = 900A charge in 1/3 hour.  That's one impressive battery, no wonder the price.

#### CliffordK

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #18 on: 26/09/2011 12:10:26 »
300Ah x 3C = 900A charge in 1/3 hour.  That's one impressive battery, no wonder the price.
The price is the Lithium Batteries (low weight and high power density).

A typical car alternator can push out 300A x 12V (3600W).

In this case, that is 900A x 4V, or 3600W

At 240V, it would take about 15A.  Yes, that does sound pretty impressive, especially considering that you would undoubtedly have several of these batteries in the system.  Likely to achieve such a high rate, one would need to be concerned with heat and cooling.  I believe most lithium type batteries include a controller as part of the battery as well as monitoring the internal battery pack temperatures.

New dedicated car chargers run at more than 50A, 240V, or even higher.

Most (new) house Mains are 200A, at least in the Northwest, but I'm not sure they're designed to run at 100%.

#### techmind

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #19 on: 07/10/2011 20:53:09 »
I believe C is just the capacity in ampere-hours...

Indeed. From this it follows that the "1C" charge or discharge current is that which will charge (or discharge) the battery in one hour.

The "2C" rate is the current that will charge (or discharge) the battery in half an hour.

"C/4" is a 4-hour charge/discharge etc.

Owing to battery inefficiency, you usually have to put in 110%-130% of what you get out (so in the detail you have to be a bit careful to check what C means precisely).
Probably "C" is the rated capacity, so pertains strictly to the discharge current/time. The charge time at "1C" will be a little over 1hour.

Of course, depending on the battery technology and construction, it's not physically possible to charge quicker than a certain amount without something going bang and/or catching fire. The terminology also sort-of assumes a battery technology which you can charge at roughly constant-current.

Older batteries (NiCad) can be charged at "C/10" (capacity in Ah divided by 10 hours) in 13-15 hours (typ.) and won't come to much harm if left charging at that rate indefinitely.
WIth careful electronic control to avoid overcharging, NiCd/NiMH can be charged at perhaps C/2. Special NiMH may go to C, with care.
These can typically be discharged at up to the "C" (1-hour) rate without damage.

Lithium Ion / lithium polymer you typically charge at constant current for about 1 hour and put in about 80% of full charge in that time, then "top off" the remaining 20% in the next hour or so. Again typically these shouldn't be discharged faster than about the 1-hour "C" rate.

Some special LiFePO batteries can be charged/discharged in 10C - 6 minutes! Good for power tools where you need a really high power, but only for a short time. However their capacity is somewhat lower for the same size/weight as regular Li-Ion/Li-polymer.
« Last Edit: 07/10/2011 20:55:25 by techmind »

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##### I need help with units, Battery Rate of Discharge "C".
« Reply #19 on: 07/10/2011 20:53:09 »