LiFePo4 C-rating confusion: 1C or 20C?

Murfix

100 W
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Apr 24, 2013
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153
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Belgium
Hello,

I've been searching for appropriate LiFePo4 cells for an E-skateboard battery pack. However I read a lot about those cells having a C-rating of 1. On the other side, I've found cells that claim 20C on the internet.

Here comes the nooby question: which is it? I'm looking into 26650 cells mainly.

Examples of cells claiming a high C-rating:
http://www.abfbattery.com/_c_nEcqWK5pE2eTOAAAUre6QFXDp8tyvdTofUVa22JA-kiLOClQnQoAIK8V7hgTTtStFHUFod5F_GRxAvLzvZsoI3uOOdxZa8a2.jpg
http://www.batteryspace.com/A123-System-Nanophosphate-LiFePO4-26650-Rechargeable-Cell-3.2V-2500-mAh.aspx

All help/info is appreciated. Thanks!
 
Schlafmutze said:
first one is very unlikely 20c

second one (a123) is.
Thanks for the quick reply.

Could you clarify why I've been reading claims of only 1C? My best guess would be because of a different cell type? For example 18650?
 
I'm definitely interested to learn more about this as well. From what I've noticed where it lacks is the actual BMS boards if you plan on adding one to eventually add in a charging port? They are somewhat limited by continuous amps and get much bigger for underneath our eboards. I haven't been able to read yet on the BMS's on this side of the forum however but seems like they're might be a fix for it.

Those 26650's look nice though. 26mm tall 1" and 2.6" long. It would be a huge pack however. That's the main benefit of why I've stuck to lipos and haven't made the switch.

8S - 4x4 and you'll still only have 2500mah if ran in series. Doesn't seem worth it... Hrmm..

Interested to see what other's think..
 
Well the 26650 cells are actually 26mm wide (a bit over an inch) and 65mm long (a bit over 2.5 inches.)
otherDoc
 
18650 and 26650 relate the cell size, 18mm dia x 65mm long or 26mm dia x 65mm long.

LiFePO4 relates the chemistry. Different manufacturers make different cells using the same chemistry. Other features and processes combine with the chemistry to determine the cell rating. This is true for cells of the same chemistry from the same manufacturer. You must examine the specification for the particular part number of the cell. Don't assume anything. Get it in writing :wink:
 
Murfix said:
I've been searching for appropriate LiFePo4 cells for an E-skateboard battery pack. However I read a lot about those cells having a C-rating of 1. On the other side, I've found cells that claim 20C on the internet.

Here comes the nooby question: which is it? I'm looking into 26650 cells mainly....
Not a nooby question at all, since even the best of us here in ESland need help understanding cell dynamics. The 26650 format is a great format for all es'ers. I recently built an A123 26650 36V 12S8P pack (96 cells). Let's take one of those cells, which has a nominal 3.3V 2.5ah capacity. That's 2.5 amps for an hour at 1C. From full to empty, which we don't want to do, but this illustrative.

But my 500 watt motor draws 20 amps continuously and sometimes on full throttle from start its going to spike up to 30 amps. That means I really need 12 times that 2.5 amps, or 12C. The cells are rated for 30C, so I'm well within the operational margins for the cells. Think of the C-rate as a cell robustness factor and must be thought of in the context of your application of that power. A higher C-rate also means you'll likely be paying more for them. When buying cells on the open market, the adage is to know what you're getting - look at the data sheet. If your supplier won't give you a data sheet, then don't buy from that supplier. What's important are: voltage, amp-hour capacity (sometimes provided as watt-hours), the C-rate, and operational temperature range. Take the whole package into account. So, C-rate is one of the tested & stated characteristics that should appear on a data sheet. Different brands & models of cells can be compared that way.

Also be aware that C-rate is bidirectional - one rate for charge and one rate for discharge, and they are often different, but related. So it figures into consideration of your charger selection as well. If I have a 2.5 amp-hour cell rated for 1C on charge, then better not apply more than 2.5 amps charging it. All chargers apply both volts and amps. Mine, for instance is a 36 volt, 5 amp charger, so quite safe for my pack. Best! :mrgreen:
 
My rule of thumb for years has been to cut claimed c rates in half. Not that the cell won't do the claimed rate, it's just that the cell will perform much nicer, (less voltage sag under load) and have a much better chance of lasting at half the claimed rate. So try to design to avoid continuous discharge at the max rate.

Two notable exceptions. Real non counterfeit A123 cells may not need their c rate adjusted near as much. But there are scads of counterfeit A123 out there.

And Turnigy or Zippy brand RC packs should have their c rate cut by 3/4 rather than half.

Again to clarify, it's not that a cell cannot do it's claimed c rate, it's just that it will not like it so much. Then you get hot cells, that result in a more unbalanced pack after the discharge. If your pack gets more than just a bit warm, you are hammering it too hard for maximum lifespan.

Re the lifepo4, there is lots of 2 and 3 c rated lifepo4 out there, and it will last OK, if you try to limit continuous discharge to about 1.5c. Pingbattery cells are one example of this grade of lifepo4.

For example, take a 36v 20 ah lifepo4 with 1c rate. If you have a 20 ah battery, you could get away with a 20 amps controller on a bicycle. At cruise speed of about 20 mph, you will only draw about 12 amps continuous. Just above .5c but close enough to work fine. But with a 30 amps controller, you would have the possibility of amps spikes up to 35 amps, enough to harm the battery with non continuous discharge well above 1c. Even though it would still draw 12 amps at cruise, the spikes when you start up, or climb a steep hill might damage the cells.
 
dogman said:
Again to clarify, it's not that a cell cannot do it's claimed c rate, it's just that it will not like it so much. Then you get hot cells, that result in a more unbalanced pack after the discharge. If your pack gets more than just a bit warm, you are hammering it too hard for maximum lifespan.
Let's underscore that particular point. There is nothing that will shorten the lifespan of a cell than internal heating from over-amping it. And that probably happens way more often than anybody suspects. What happens is what is called plating. The metallic ions (there are a variety of chemistries) began to plate out at the cathode. That significantly increases internal resistance (IR). At some point the cell dies. So, know the operational characteristics of the cell you're buying, apply a margin of safety, and stay within range. Do that and happy cells, happy battery pack, happy you!
 
Thanks for the explanations. It seems there is a lot more variety in lifepo cells than in lipo cells.

Could you recommend a BMS that can handle 50A and peaks up to 100A (for use with 26650's, 40V) ? I'm in Europe if that helps.

I've been reading about the characteristics and found tons of charts for the cell types. How can I source the cells I want? My only option atm is alibaba, but I don't really trust the suppliers...

Thanks!
 
Murfix said:
Thanks for the explanations. It seems there is a lot more variety in lifepo cells than in lipo cells.

Could you recommend a BMS that can handle 50A and peaks up to 100A (for use with 26650's, 40V) ? I'm in Europe if that helps.

why did you decide to restrict voltage so low? then demand huge currents at low voltage. why not use a high enuff voltage you can run with lower currents to make it easier on the components?

i recommend the bestechpower D131 as the best deal for the money. buy two if you order from henry so you have a spare for when you break the first one while learning. go to 24S too!
 
Someone was testing cells a while back, and he did a controlled series of drains on a particular cell that claimed a 10C capability. It performed well for E-bike standards, but...at a 5C rate he logged a significant increase with its internal heat. Nothing horrible, but he didn't even test it at 6C because previous tests after the first measurable bump in heat...they ALWAYS performed much worse at every higher drain level. He called that brand/chemistry/model a 4C cell.

I know offhand that doctorBass and Chris (icecube57?) have both done a lot of cell testing. There are several others, but i don't recall their names...

If you need a high-current battery that will stay cool, em3ev.com is what I compare all other options against. http://em3ev.com/store/index.php?route=product/category&path=35
 
Murfix said:
Could you recommend a BMS that can handle 50A and peaks up to 100A (for use with 26650's, 40V) ? I'm in Europe if that helps.
So, again with the A123 26650 cells - they are nominally 2.5ah, 3.3V. You need a 12S?P for that to work. The question mark is to figure a parallel design for both range AND to deliver the necessary amps when needed. Let's stay on the safe side and use a C-rate of 20. So, minimum a 2P design - 2.5 * 2 * 20 = 100 amps. That's 5 amp-hours, not really much, so go with how ever many you need, taking into account you'll not come close to draining them - I'll not go below 80% discharge.

I recommend designing a battery pack taking into account the continuous draw of your motor and setting it down to more like 1 to 2 C. So I have a 500 watt motor that is about 20 amps continuous. At 2C, that's 20 amps / (2.5 amps * 2C) = 4P, so a 12S4P design works nicely. But I wanted more range, so made it a 12S8P, which is 96 26650 cells.

I don't use a BMS and still recommend that pack builders at least consider that option rather than willy-nilly taking the advise of the pro-BMS crowd. In many instance, if you want your pack to stay balanced at all times (another assumption), you'll need a BMS. But for the A123 26650 cells, staying safely above a 80% discharge, I don't believe it necessary or worth the fuss. They are very robust and quite agile self-balancing. In part its become the high-C chemistry lends itself to just that.
 
Thanks :)

I think my best suitable pack would be 12s2p. It can deliver the amps I need and can still be small and light enough to be carried underneath an eboard.
I will definitely be needing a BMS though. I don't want to hook 'em up on the balance charger every time I've been riding. I just want to plug the power chord and be done with it.

I will definitely check out the A123 cells. To fit underneath and e-board, the BMS can't exceed some specific dimensions. I am considering building my own to solve that problem...
... Time to read about building/buying BMS's!
 
Murfix said:
I think my best suitable pack would be 12s2p. It can deliver the amps I need and can still be small and light enough to be carried underneath an eboard.
The decision is what is your range need? Decide that first - think ahead.
I will definitely be needing a BMS though. I don't want to hook 'em up on the balance charger every time I've been riding. I just want to plug the power chord and be done with it.
That's all I do is ride, home, plugin & charge. I have yet to balance charge any of my packs. Nonetheless, I have a Thunder 1220 should I need to. My path is the human-BMS, active monitoring and proactive intervention only when necessary.
I will definitely check out the A123 cells. To fit underneath and e-board, the BMS can't exceed some specific dimensions. I am considering building my own to solve that problem...
... Time to read about building/buying BMS's!
Research & pick your cell first before finalizing a design. There are other good cells out there, from Samsung, Panasonic and a bunch of Chinese LiFePO4 startups, which should not be discounted. Yea, everyone is faced with the choice to BMS or not-to-BMS and make their own decision. Good luck with that!
 
I would buy the cells spot weld in parelle and then you could solder them in series with the tabs on the cells from the vendor. I wouldn't solder or heat the cells with a solder iron without pre tabs. Use a big solder iron 60-80 watts or a spot welder. As they don't like heat. Also like vendor with this gasket on the pos. side.HY5LP[}J3C`}BTVG0B@]@NM OSN.jpg
Osn had these cells in the past.32113..jpg
 
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