LiFePO4 C ratings and "Quality"

[powermed]

Can someone either refute or confirm my research? I haven't seen anything to suggest there's a definitive relationship between C ratings and the quality of a battery. For my application (an electric boat) I'm needing few cells with a high AH capacity and [relatively] low C ratings. The cells I'm looking at are 200 AH / cell, with a discharge rate over 200 AH (see below). I'll be charging each of the 8 cells individually at over 60 amps). I never draw more than 35 - 50 AH, so that discharge limit will never get challenged.

What I've read is that while cells like A123's have very high C ratings, it's due to technologies applied to the cathode. The cells I'm looking at are large format LiFePO4 cells with very good test / life cycle data (same as the best I've seen - 1000's of cycles, drop to 80%) with low impedance and low C ratings. I've never seen any data on shelf life - so unless someone has it - I'm guessing such talk is just a bunch of guys sitting around talking philosophy.

In sum, I can't see any justification for spending 2 or 3 x the price to obtain higher C ratings.

z.



3.2V/200AH

Can Material
Plastic Can

Typical Capacity
200Ah

Minimal Capacity
199.5Ah

Nominal Voltage
3.2V

Max. Charge Voltage
3.65V

Discharge Cut-off Voltage
2.0V

Max. Charge Current
1 C(5)A

Max. Discharge Current
2 C(5)A

Battery Size
82*182*278mm

Weight (Approx.)
≤6.40Kg

Impedance（Max, at 1000Hz.）
≤1mΩ (charged status)

 

[safe]

The cells I'm looking at are 200 AH / cell, with a discharge rate over 200 AH... I never draw more than 35 - 50 AH, so that discharge limit will never get challenged.

From everything I've learned in the whole time I've been here and all the practical on the bike experience I'd have to say you are in good shape. By running such a low "C" rate relative to your battery you could practically even use SLA's. You should have no problems with 1C battery chemistries.

Calender life is a complete unknown on LiFePO4. I think that they probably will last a little longer than Li ion (5 years max) but talk of more than 10 years makes me suspicious. Don't forget that all the Lithium chemistries need excellent BMS's if you want them to last. Unlike the Nickel chemistries that are pretty much idiot proof you have to monitor your LiFePO4 for low voltage and be careful not to overcharge.

In a boating application you normally need some ballast anyway. If you could replace the ballast with batteries you might even think about using SLA's because they are very cheap. Just a thought.... is this some kind of racing boat?

 

[powermed]

Thanks for the reply. Actually these are replacements for the SLA / AGM's I've used for the last 5 years. Shaving about 250 pounds off the boat (http://www.PowerMed.com/electric) for some early pics. LV battery cut off not a problem, the [series] battery output goes to an array of voltage converters which takes 25.6v to 48. The converters have an activation voltage that's perfect (17v).

At 17v, the converters "shut off" (17v/8 cells) = a cut off of 2.125 volts per cell. In practice, the batteries give me over a 30 mile range - I'm not likely to test those limits.

 

[powermed]

Ballast....yeah, I have that worked out - it is absolutely critical with an electric boat design that's for sure.

 

[GGoodrum]

That's a very cool looking boat you have there, Bill. I've been looking to do something to put on a 10' inflatable, to be used on Mission Bay, in San Diego, but most of the electric motor setups I.ve seen seemed a bit under-powered, more for trolling. I didn't thing to try using two of them.

Which motors are you using? How much voltage are they normally setup to use, 24V, or 48V?

BTW, those cells will be fine for your application.

-- Gary

 

[powermed]

Hi Gary, thanks for the vote of confidence. The motors are stock Minnkota 101 Riptides running at 48v (heads/controllers removed). Another critical modification was using the stainless steel props from their e-drive series. I did a number of tests in the "pre" - prototype phase and determined (and documented) running a single motor at 48v increased efficiency over 36v, running two improved it again, and using the stainless props bumped it even more. I had put together a great graph mapping amps, speed and running times, and published it on one of these forums 4 or 5 years ago, but I can't seem to find it. The basic picture was each modification moved the "sweet spot" to the right as it were - more speed / range at a given amp drain. The best spot ended up being about 4 mph and about a 15 amp draw with this design (I could hit over 6 mph but it would take 4 - 5 times the amperage). I'm looking forward to shaving 200 pounds of battery weight and getting 3X the range.

The Carolina skiff is a very good eboat hull - as standard boat hulls go (no rounded transom which would be fantastic). I'm considering simply embedding the LiFePO4's in floor of the hull (8" foam / fiberglass).

 

[GGoodrum]

Hi Bill --

I FINALLY found this thread again. I got sidetracked for a few days and then lost this comepletely.

Anyway, I'm very interested in doing something similar, but I have a ton of questions. First, and foremost is what are you using for controllers? You mentioned that you "removed the heads and controllers...", but exactly does that mean?

My second question is why you used the voltage upverters? Why not just run four SLAs in series? What do you plan to do with the LiFePO4 cells? How many will you run in series?

How do you steer?

Looking forward to hearing more about your setup.

-- Gary

 

[johnrobholmes]

The quality and C rating of a cell are not tied together, as the C rating is defined by the balance of anode, cathode, and electrolyte. More electrolyte = more capacity and less discharge rate.

Those cells look fine for your application, I say get em and run it hard. Running at 1/4 of the rated discharge will make them last nice and long.

 

[GGoodrum]

The quality and C rating of a cell are not tied together, as the C rating is defined by the balance of anode, cathode, and electrolyte. More electrolyte = more capacity and less discharge rate.

Those cells look fine for your application, I say get em and run it hard. Running at 1/4 of the rated discharge will make them last nice and long.

I totally disagree with this statement. The true "C" rating has everything to do with the quality of a cell. The C rating defines the discharge rate, not the capacity, which is what I think you may be refering to here.

-- Gary

 

[Link]

It seems that very high C ratings are a given for high-quality cells. It's probably because everything that would equal a long-lasting, efficient cell (good materials, high cathode/anode surface area, etc.) also happens to equal the ability to put out a lot of power.

I've never seen any data on shelf life - so unless someone has it - I'm guessing such talk is just a bunch of guys sitting around talking philosophy.

I'm not sure "guys" (plural) is the right word, here...

 

[johnrobholmes]

I totally disagree with this statement. The true "C" rating has everything to do with the quality of a cell. The C rating defines the discharge rate, not the capacity, which is what I think you may be refering to here.

-- Gary

Capacity and discharge rate are tied together in a fixed cell size, if you want more capacity you must give up some discharge rate. You can keep the same quality and have a cell made as high capacity or high discharge. Discharge rate has nothing to do with quality, it is manufacturing process that determines quality. Quality can determine discharge rate, as only high quality cells can be high discharge and reliable. It doesn't work in reverse however. I see your point, but I was not saying that low quality cells can have high discharge rates. I was saying that C rating is not necessarily indicative of quality.

As an example I can get high quality low discharge Life made in the form factor of an a123, but with 3200 mah. The C rating (discharge) is lower, but when used properly the cycle life and voltage under load is the same. Thus, low discharge rate cells can still have high quality.

 

[GGoodrum]

Capacity and discharge rate are tied together in a fixed cell size, if you want more capacity you must give up some discharge rate. You can keep the same quality and have a cell made as high capacity or high discharge. Discharge rate has nothing to do with quality, it is manufacturing process that determines quality. Quality can determine discharge rate, as only high quality cells can be high discharge and reliable. It doesn't work in reverse however.

Wrong again. They are absolutely not tied together at all. As an example, look at two extremes, a123s, which are rated at 30C continuous/52C burst, and Ping's so-called "duct tape" cells, which are at best rated at 1C continuous and 2C for short bursts. What that means is that a single a123 2.3Ah cell can put out 70A continuous, and 120A for 10-15 seconds, while Ping's 5Ah cell can only put out 5A continuously, and 10A for short bursts. Even putting four of these in parallel, in a 20Ah configuration, means that the limits are still only 20A and 40A, respectively.

I would agree that for Bill's application, those 1C/200Ah cells are going to be fine, because he doesn't need to draw 200A. His situation is somewhat unique, though, in the context of using LiFePO4 cells with ebikes, where you can't carry around so much "extra" weight. Many ebike setups routinely pull 40-50A, so the C rating is much more important, when selecting cells/packs. Using 1C cells really means you need a pack with at least a 40Ah capacity. That's a lot weight on a bike, even with using LiFePO4. Most here can easily get by on 15-20Ah of capacity in terms of range, so for 40A loads, a 20Ah pack with a 2C rating is the best compromise. With higher quality, higher C-rated cells like Headway (5C), LiFeBatt (10C) and a123 (30C), you don't have to worry about the load, and you can simply pick the capacity that meets your range requirements.

-- Gary

 

[powermed]

Hi Gary,
"what are you using for controllers? "
- I use a very nice Zahn servo amplifer I picked it up on ebay for a song a few years ago. Ridiculous capacity, something like 5000 watts. I've run it close to that limit in the ocean and it never even got warm (I was seeing 100 amps on my meter fighting waves and swells). On the lakes I almost never go over 25 amps for any length of time.

"You mentioned that you "removed the heads and controllers...", but exactly does that mean?"
- the motors I use are twin Minnkota 101 Riptides. I removed the tops with the electronics to set up steering cables and one controller.

"My second question is why you used the voltage upverters? Why not just run four SLAs in series? What do you plan to do with the LiFePO4 cells? How many will you run in series?"
- I have used 4 SLA's in series for years - all 300 pounds of them at 105 AH. Design parameter for this version of the boat called for lithium - the 8 200 AH cells in this config are about 100 pounds and offer far greater range. Although it may seem contrarian, my version of the KISS principle was to use as few cells as possible, monitor / charge each one individually, and to not use a BMS at all - fewer cells and shallower discharge minimize chances of a bad cell or balancing problems.

Unless you've played a bit with DC converters some of this may not make much sense...

"Upverters" would be fine and I've got some, but I'm going in a slightly different direction to get the same result - I'm merely taking the series output of 8 large LiFe04 cells (25.6v) and putting it to an array of 8 dc converters (24v nominal to 28v output) that are setup in 2 parallel groups of 4 converters for current sharing (1600 watts total) and the two groups are connected in series to acheive 56v.

The converters outputs are isolated, so all converters in the array are powered from a common bus. The low input range of each converter is 17v - so when the series voltage of the LiFeP04 cells falls below that, the converters won't power up. 17v becomes an effective LV cutoff of 2.125v / cell, so don't need a BMS for that.

Charging is fast and furious - combo of an AC timer, metering each cell and eternal vigilance for about an hour and a half - but keep in mind this is a weekly thing - not to hard for me to do.

"How do you steer?"
I rigged a dual stick helm, one in front and one in back (captains chair can vary depending on the load in the boat). Basically used dual standard throttle cables from a motor boat. I think the pictures on the web are from before I put in the forward helm.

 

[johnrobholmes]

`Wrong again.

Please show me how to fit 10 pounds of shit into a 5 pound bag. I stated fixed cell size. The a123 2.3ah and ping 10ah batteries you cite are not the same size, nor even the same construction. With current technology you can only fit so much power into a cell. New technology can pack more into the bag (make it denser) in both capacity and lower resistance. Comparing two different cells sizes is not useful, unless you will account for volume and density too. With a with a fixed cell size (and chemistry) you trade off capacity for discharge rate- this holds for a crappy cell as much as a nice cell.


I agree on discharge rates for bicycles. I wouldn't want to use anything less than a 15C pack really. A 48v 20ah pack at 15C would be about perfect IMO to balance size, weight, capacity, and very long service life. Even at 100 amps it is only seeing 5C, not enough to even warm it up. I use 9.2ah on my current ride, a123 cells. I hit 80 amps up steep hills because it is only a 6s pack, but the cells still don't even get warm and haven't gotten out of balance after 10 full cycles (Still no balancing, but watching).

 

[GGoodrum]

Bill --

Interesting approach, re: the isolated DC-DC convertors. You are basically doing what a 16-cell, 100Ah pack might provide, but just with fewer, larger cells.

One thing I would still recommend, howver is doing some sort of individual cell low voltage protection. With LiFePO4 cells, it is pretty easy to have one cell die, and still have the total pack voltage above the pack-level cutoff. If one cell got down to 1.5V, for instance, which is right at the point it will become irreversibly damaged, the others would only have to be at 2.64V, or higher, to still be above the 17V cutoff.

Do yu have any more boat pics? I still don't get the steeing at all. I don't see a wheel, so is there some other mechanism. I get that there is a steering cable, or cables, but what to they go to?

Are these motors brushed DC, or are they brushless? I'm guesing basic brushed DC, since you are able to run them off of a single "controller". Are these normally run at 36V? I'm guessing yes, if you haven't had any problem running at 48V. Have you had to do any maintenance to the motors themselves?

What do you use for a charger?

What do you use for a throttle?

Thanks -- Gary

 

[GGoodrum]

`Wrong again.

Please show me how to fit 10 pounds of shit into a 5 pound bag. I stated fixed cell size. The a123 2.3ah and ping 10ah batteries you cite are not the same size, nor even the same construction. With current technology you can only fit so much power into a cell. New technology can pack more into the bag (make it denser) in both capacity and lower resistance. Comparing two different cells sizes is not useful, unless you will account for volume and density too. With a with a fixed cell size (and chemistry) you trade off capacity for discharge rate- this holds for a crappy cell as much as a nice cell.


I agree on discharge rates for bicycles. I wouldn't want to use anything less than a 15C pack really. A 48v 20ah pack at 15C would be about perfect IMO to balance size, weight, capacity, and very long service life. Even at 100 amps it is only seeing 5C, not enough to even warm it up. I use 9.2ah on my current ride, a123 cells. I hit 80 amps up steep hills because it is only a 6s pack, but the cells still don't even get warm and haven't gotten out of balance after 10 full cycles (Still no balancing, but watching).

I guess I just don't get what you are trying to say. If you compare your 4p/9.2Ah a123 setup to a 10Ah ping pack, the differences are even more pronounced (a123 4p = 276A continuous and 480A burst vs. 10A and 20A for the10Ah ping pack...). If, you mean similar can size, compare the Headway 26650, which is about the same capacity (2.2Ah vs. 2.3Ah...) and is the same size as a single a123 cell. the Headway uses the same chemistry, but is really at best about 5C continuous and 8-10C burst. That's 70A vs 11A. I guess one way of looking at it is that it is cramming 30C into a 5C can.

 

[powermed]

Hi Gary, you're right about the possibility of singular cell weakness hiding amongst the cells. I've a single voltmeter and the cells are all on tap via a small 10 position DP rotary switch. That plus the larger bats keeps me safe on my usual jaunts since I know my territory and range very well. This setup is pretty much a 2 day boating trip for me with an extra day in reserve, the bats are unlikely to ever see a deep discharge.

Steering is old fashioned "stick" steering - no wheel or tiller. In the pics it's the handle to the right of my seat. The throttle is a gorgeous piece of stainless steel - some kind of very heavy duty potentiometer with a center detent and handle - it looks like it belongs on a very nice yacht. Ebay find a few years back.

On the forward (second) helm (not in the picture) I used a vintage old gas engine marine helm, but replaced the handles with a 2 foot rod of stainless. Forward moves the the left, back to the right. You can still find stick steering around, I think it's called ez stick or something. I put a 5k pot into the vintage helm with a bit of drilling and epoxy. Updated pics sometime down the road.

 

[GGoodrum]

Sounds like you've got a pretty good handle on monitoring the cells, plus individually charging them helps keep them close in balance. What are you using to charge these?

I'd still love to see more pics of the details of your setup. What I'm going to do is put two of these on an inflatable dingy, from my friend's boat. I don't need anywhere near as much range, as it will just be for tooling around the marina, and around Mission Bay, so I will probably end up using something like a 16s4p a123 setup, to start. That way I can use the same packs that I use on the folding bikes. We're planning a trip to Catalina Island, which is about 25 miles off the coast here, later this summer. The small town there, Avalon, basically has no cars. Everybody gets around on golf carts and bicycles. Our plan is to use my folding bikes to get around town and the island, and then use the same packs in the dinghy.

ANyway, do you happen to know if the motors are the brushed DC variety? I need to research controller options. If there are DC types, I guess I could use something like an Alltrax controller, which are fairly easy to get. I know you are using a big servo amp, right? Is it just one unit, driving both motors?

I've got feelers out to try and find a couple of used Riptides. New, these suckers are about a grand each, at West Marine, so I'm hoping I can find used ones in good condition.

 

[johnrobholmes]

I was making a statement showing the relationship between discharge rate and capacity. Surely the headway cells are of lower quality (and energy density) than a123, and the price should reflect this. If headway were to use the same manufacturing process they could get a higher discharge rate cell at a sacrifice to runtime (same cell size, etc). If a123 wanted to do the same they could as well. K2 has two versions of the "a123" sized cell, one high capacity and one high discharge. I'm not denying the differences in quality between brands at all, just stating the relationship between a fixed size cell and the capacity vs. discharge rate.

 

[Pete]

If you are going to compare , you need to define 'Quality' in terms of.

C rating (Charging and discharging)
Ah capacity
Energy Density
# of discharge cycles (Proven, not claimed)
Internal resistance
Ambient temperature Vs electrical characteristics
etc.

I don't have an answer that gives a numerical 'quality' index figure. My gut feeling is that a particular chemistry can only yield X amount of energy under ideal conditions. Manufacturers tailor the mechanical construction to meet design goals. Headway is a perfect example of comparing apples with apples. I know they have two identical sized cells with different characteristics. I don't know the precise details but I thought one was a 10Ah at 10c the other was 12Ah at 5c (or figures roughly similar) I suspect they are very similar weight. So the manufacturer has made a compromise in energy density to achieve C rating. Is one cell of worse 'quality' than the other, or simply different.