lifepo4 c rating

E

ejonesss

10 kW
Joined
Aug 31, 2008
Messages
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i have been seeing the c rating talked about too and i was wondering if that is the continuous rating then.

let me see if i got this right

lets say you have a single 3.3 volt 1 amp hour cell .

normally if you draw 1 amp continuous for 1 hour the battery is dead.

a lifepo4 cell claims a c of 30 for example.

does that mean i can draw 30 amps for 1 hour and it is dead?

or i can draw 1 amp for 30 hours and it is dead?

because i am seeing some ebike users using tool and rc (radio control) batteries mainly the new lifepo4 like used by dewalt.

surely a tool battery cell cant put out more than 3 amp hours of power?
 
Engineers have given batteries a standardized C rating to describe discharge characteristics and safe operating ranges. This rating compares the amount of current drawn from a pack to the overall pack Amp-hour (Ah) specification. To figure the rate, multiply the C rating by total pack Amp-hours. This product determines the maximum current that can be drawn from each cell without fear of cell damage or explosion.

LiFeBATT cells have a theoretical peak 20C rating and, within a 10Ah pack, LiFeBATT cells can discharge impulses of 200 Amps for 10 seconds. LiFeBATT cells also have a 12C continuous rating, which translates to 120 Amps discharged continuously from a 10Ah pack. At 5C and less, LiFeBATT cells have such a solid voltage that for as long as the pack is in shallow discharge cycle (70% of total capacity or less) the battery pack voltage will remain unaffected by the current draw. Basically, you can draw 50A continuously for every 10Ah in the pack and the pack voltage will be sustained until current capacity is 70% depleted.

The more constant internal chemistry and voltage characteristics of LiFePO4 battery packs provide a more constant supply of total power to a system, as a function of the balance between voltage and current where Power = Voltage * Current. Since the voltage does not drop out as quickly in LiFePO4 chemistries as it does in PbA chemistries, the current does not need to increase proportionally in order to maintain power. This means the power delivered to the motor will be very consistent until the very end, and less power will be lost when accelerating and cruising in an electric motor vehicle.

Hope this helps.
Don Harmon
 
will that mean that on a phoenix brute a lifepo will give double the hill power than lead acid?

because for now i have been seeing on ebay there are lifepo 48 20 batteries on buy it now from china for 585.00 by a seller called free_shopping88 i am thinking of getting and according to another post it looks ligit.

seeing it is 1/4 of the cost of the Eonyx battery i am considering getting it.

have you heard of any bad things about that dealer?


Don Harmon said:
Engineers have given batteries a standardized C rating to describe discharge characteristics and safe operating ranges. This rating compares the amount of current drawn from a pack to the overall pack Amp-hour (Ah) specification. To figure the rate, multiply the C rating by total pack Amp-hours. This product determines the maximum current that can be drawn from each cell without fear of cell damage or explosion.

LiFeBATT cells have a theoretical peak 20C rating and, within a 10Ah pack, LiFeBATT cells can discharge impulses of 200 Amps for 10 seconds. LiFeBATT cells also have a 12C continuous rating, which translates to 120 Amps discharged continuously from a 10Ah pack. At 5C and less, LiFeBATT cells have such a solid voltage that for as long as the pack is in shallow discharge cycle (70% of total capacity or less) the battery pack voltage will remain unaffected by the current draw. Basically, you can draw 50A continuously for every 10Ah in the pack and the pack voltage will be sustained until current capacity is 70% depleted.

The more constant internal chemistry and voltage characteristics of LiFePO4 battery packs provide a more constant supply of total power to a system, as a function of the balance between voltage and current where Power = Voltage * Current. Since the voltage does not drop out as quickly in LiFePO4 chemistries as it does in PbA chemistries, the current does not need to increase proportionally in order to maintain power. This means the power delivered to the motor will be very consistent until the very end, and less power will be lost when accelerating and cruising in an electric motor vehicle.

Hope this helps.
Don Harmon
Click to expand...
 
The "current capacity/juice-capacity" is the AH rating. The amount of current you're allowed to draw can be calculated from the battery by the "c-rating" times "AH rating/juice-capacity". ("allowed current" = "C rating" * "AH rating")

So a 10 AH battery with a c rating of 2 can draw 20 amps(without over-stressing the battery, tripping to over-current protector, whatever.). The same 10AH battery with a c rating of 10 could draw 100 amps(10*10=100). A 20AH battery with a c-rating of 2 can draw 40 amps(20*2). That same 20AH with a c-rating of 10 can draw 200 amps(20*10).

A battery can continuously discharge at its c-rating by 1/C hours. So if you're continuously discharging at 2C, the battery will discharge for roughly 1/2 hour before cutting out. If you're discharging continuously at 1C, the battery will discharge for roughly 1 hour(1/1). The larger capacity you have for a given motor-wheel/controller, the lower the C-rating you'll be running at so you'll last longer.

Did that help?
 
ejonesss said:
will that mean that on a phoenix brute a lifepo will give double the hill power than lead acid?
Click to expand...

Running at the same nominal voltage, and assuming an appropriate amount of capacity for each(And, dang, that'd mean that lead acid would be heaaaavvvvvyyyyy.), the lifepo4 will not have double the hill power of lead-acid. What limits hill-power is the current rating of the controller and, if not that, the motor itself. But, the LiFePO4 will usually be more "powerful" since it usually has less voltage sag at higher currents and the voltage doesn't drop off as much as the lead through-out the ride. And it'll last longer, assuming the same nominal AH, due to lead's dreaded puekert effect(and to make this effect minimal would require far too much lead for the current a phoenix motor requires).
 
Comparing Amp-hour ratings between PbA and LiFePO4 is tough. Peukert’s exponent explains the capacity expected from a PbA cell at given discharge currents. The equation is

I^x * t = Ah

I = current discharged

x = exponent

t = time

Ah = total Ah's

Most PbA cells have a Peukert’s exponent rating between 1.3 and 1.6. Good PbA cells have an exponent the gets closer to 1.15. We can’t assign this exponent to LiFePO4 because of chemistry differences, but an approximation of the Peukert’s exponent for comparison would be 1.05 for LiFePO4. As an example, a 20Ah LiFePO4 cell will produce approximately as much or more power as a 50Ah PbA cell at the same rated voltage.

Don Harmon :mrgreen:
 
A battery discharged at 1C will be dead in one hour, 2c in 30 minutes, 10C will be dead in 6 minutes. A battery discharged at 20C will be dead in 3 minutes. 30C, 2 minutes, etc.
 
Don Harmon said:
As an example, a 20Ah LiFePO4 cell will produce approximately as much or more power as a 50Ah PbA cell at the same rated voltage.

Don Harmon :mrgreen:
Click to expand...
so an Eonyx 4820 will be the equiv in power to 50 ah of lead acid?
 
is there a formula for calculating what ah of a lifepo4 is needed to give the same power as lead acid?

because right now i am using 15 amp hour lead acid deka agm extreme power sports batteries and it may be possible that a tool battery like a dewalt 36v batteries and get the same run time as the 15ah lead acids do.
 
There is a basic formula of 1/2 PBA Amp Hours to 1 Amp Hour for LiFePO4, but is is not completely accurate. You still have to factor into everything the type of motor and drive you are using plus the desired top speed & range you are looking to acheive as well as the weight of the vehicle and the rider/passenger load. The 1/2 formula therefore is only theoretical and should be tempered with a total analysis of your particular system requirements. This can be done with a cool little tool we have developed to work out a LiFePO4 vehicle spec. If you would like a copy to play with you may write me off list and I will send it to you by email. It is Excel based, so you need to have Excel and be familiar with using the program.

Don Harmon
don@lifebatt.com :mrgreen:
 
you mean if i have a 1 amp hour lead acid it will be 2 amp hour with lifepo4? a 1 to 2 ratio?
 
Yes, a 12v 2AH Lead battery will give as much power as a 12v 1AH LiFePo4. In fact, i've found real life range to be about 60% higher using the same AH/voltage of LiFepo4 instead of PBA. A good example is a customer who came for a recharge at my workshop recently. On brand new 50AH PBA at 96v his vehicles power meter showed that he had only drawn 3000wh instead of the 4800wh he was hoping for (50x96=4800). My 72v 40AH LifeBatt pack returned 2795wh out of the calculated 2880wh on its last full discharge. My meter read that I had used 38.8AH, 2795/38.8=72. THis shows that the average voltage throughout the whole journey was 72v (not bad considering that the cells had seen peaks of 10C discharge rates and continuous of 4-5C)
 
thanks it looks like all i need is 2 dewalt 36 volt batteries for now to get me by until i can get up the money to get a better lifepo4 battery.
 
Though some of the newer lifepo4 cells in the economy bracket, such as Ping, Cammy, Freeshopping etc, may claim a discharge rate of 2 or 3 c, you see test results conducted at 1 c. As far as I am concerned these are 1 c batteries till you show me how long they last at 2 or 3 c. So buy one big enough to give you the amps you need at 1 c. Most bikes need the 20 ah sise in my opinion, to minimize the chances of early battery death. Besides what is not to love about a long range? A big battery is the cheapest one, over the long haul.
 
That is why this is a particularly relevant post - a good one IMO for people to read, because ALL LiFePO4 Cells are NOT created equal! The "C" Rate is one of the key points that must be understood and validated. A few other comparison values that folks often overlook are:

Maximum Discharge Current (Continuous)
Initial Internal Impedance
Gravimetric (Watts / Kg)
Warranty

These are topics for more posts! :mrgreen:

Don Harmon
 
Fit 48V Motor Wattage: 500 Watt to 1200 Watt, 800 Watt suggested
Voltage: 48 Volts
Capacity: 20 Amp Hours
Dimension: 260×140×140mm （If need other sizes, possible to make order, please provide the size）
Weight:10.8 kg
Charging Voltage: 61 Volts
Charging Current: <5 Amps
Rated Discharging Amperage: 20 Amps
Max Continuous Discharging Amperage: 40 Amps
Maximum Discharging Current: 60 Amps
Discharging Cut-off Protection: 40 Amps
Lifecycle of the whole pack: >85% capacity after 1000 cycles.
Lifecycle of single cell: >85% capacity after 1500 cycles, >70% capacity after 3000 cycles. (<1C discharge rate and <1C charge rate)

a confusing thing why is the cut off 40 amps when the maximum 60 amps?

this is from ebay item 220282686154
 
A typical curious spec. from an Ebay Vendor. I can't judge from what they quote here a good comparison. Other than their Max. Contiuous Discharge is very low and their results are based on a "1C" rating. You might want to ask the seller what the Wh / Kg spec. is on his cells ?

Don Harmon
:mrgreen:
 
i did find the power to weight ratio you may have heard of a seller dy-power with the brand hi-power

High theoretical capacity of 170mah/g and high practical capacity as high as 165mah/g. from item 170263033161

the full specs are

Battery Voltage: 48 Volts
Battery Capacity: 20AH (Amp Hours)
Charging Voltage: < 58.4 V
Charging Current: < 6.66 Amps
Normal Working Discharging Current : < 20 A
Max Continuous Discharging Amp. : 40A
Peak Discharging Current. :60A
Battery Pack Life cycle :>85% capacity after 1000 cycles
Battery Weight: About 12.36kg
Battery Dimension:266*202*165mm ( small size)
The Battery Pack Using Suggest: 800 -- 1000 Wattage

the first line is from the bit of text above the specs i dont know if that will help .


Don Harmon said:
A typical curious spec. from an Ebay Vendor. I can't judge from what they quote here a good comparison. Other than their Max. Contiuous Discharge is very low and their results are based on a "1C" rating. You might want to ask the seller what the Wh / Kg spec. is on his cells ?

Don Harmon
:mrgreen:
Click to expand...
 
What a vendor claims on an Ebay auction and what independent testing shows can be misleading. Independent tests not being available - the next best thing is actual user reports.

Don
 
dogman said:
Though some of the newer lifepo4 cells in the economy bracket, such as Ping, Cammy, Freeshopping etc, may claim a discharge rate of 2 or 3 c, you see test results conducted at 1 c. As far as I am concerned these are 1 c batteries till you show me how long they last at 2 or 3 c. So buy one big enough to give you the amps you need at 1 c. Most bikes need the 20 ah sise in my opinion, to minimize the chances of early battery death. Besides what is not to love about a long range? A big battery is the cheapest one, over the long haul.
Click to expand...

True, except for the batteries that aren't purchased for the "long haul" and one whose range is usually less than half the "large size" provides(I.e., even if the battery cycles are cut by 2/3 for the small battery compared to the larger battery, if don't use that extra half, you're essentially paying more per usable cycle for the larger battery. This becomes untrue (assuming the large size is double the small size) once the cycle-life is less than 1/2 the larger battery's, which I doubt it's going to be.). I'm giving up my current lifepo4 within 2-3 years, and I have a feeling it'll easily see 700 cycles at its usual 1.6 C.
 
Sounds ok, as long as you are willing to pay a bit more per cycle. Here's the rub though, if you are hammering a pack, and one little pouch dies, you lose one third of the capacity on a 12 ah pack. Your bms will cut out as if the whole pack is only 8 ah. So I don't want to risk killing a cell to save a couple hundred bucks. With the duct tape prismatic cells, conservatisim will pay in the end I think. Maybe you kill a cell in 100 cycles.

If you are doing 1.5 c, I would consider that pretty safe. I think I discharge about that when I ride up the hill. and about 1 c when going down. I'm cautioning about above 2 c discharge rates. 2-3 c is what I mean when I say hammering a pack. If I get 700 cycles that will be about 2 years. I do hope for a lot more than that, but I expect I will also be running something better on my commuter. I can take the old ping and beat it on an off road bike.
 

LiFeBATT cells have such a solid voltage that for as long as the pack is in shallow discharge cycle (70% of total capacity or less) the battery pack voltage will remain unaffected by the current draw.
Basically, you can draw 50A continuously for every 10Ah in the pack and the pack voltage will be sustained until current capacity is 70% depleted "

right before I start riding on my HPS 36V10Ah LifeBatt pack voltage on WattsUp reads tipically 40.1V , after even feather touching throttle /Iam still not on the bike/ voltage drops to 39.8-9V. Right at the beginning of my trip to work there is slight hill . ON that hill voltage drops to 38V under less than 20A for NO more than 10 second, it varies but for sure between 10 and 20A.

If I undrestand literally
"voltage will be sustained" - does it means voltage will not drop when you draw 50A from my pack?
No way voltage will simply stay put when you draw 50A from my pack, on any 10Ah pack, any chemistry.
WattUp I checked against Fluke multi is very accurate or my run of 2times 90cm of AWG wire would cause such a drop under 20A - very unlikily.
Am I missing something??
I think on any 10Ah battery draw of 50A would for sure cause voltage of that battery to drop.
everybody has to agree.
What I agree is that voltage do come back on my HPS pack to beggining of trip level IF you give enough time after I draw 48A FOR view second. And it is very nice future, how wonderfully voltage recovers, it is amazing how voltage would recover /after example 8km/ to almost exactely pre-trip values WHEN you give it like one hour to rest.
MC
 
Maybe they really mean, voltage won't progressively drop as you ride. Surely all chemistries drop some voltage once you start to use them. But the lead, just keeps dropping and dropping. The lifepo4 seems to stay the same for the whole ride, till just before the end.
 
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