Comparing (LiFePO4) to (NiMh) : Theory

LiFePO4 verses NiMh



Let's look into some of the issues associated with NiMh verses LiFePO4.

Gravimetric Energy Density:

NiMh : 60-120
LiFePO4 : 90-120
(about the same)

Load Current:

NiMh : 5C (peak) : continous is more like 3C : best 0.5C
LiFePO4 : 30C (peak) : continous is more like 15C : best 10C
(big advantage for LiFePO4)

Well you can see the chart...

One big issue is charging. Correct me if I'm wrong, but LiFePO4 exibits the same behavior that NiMh does when charged in that if it's in series it takes only one bad cell to trick the charger into believing that the charging is complete. In the worst case scenario you can get all kinds of weird cell reversal going on and it means that you need to do something like:

1. Use a Battery Management System even though you really are not likely to break the batteries, but if you don't and you have a fixed pack design then you can lose capacity if you don't charge balance.

2. Do something like I'm doing where you never solder your pack together and periodically disassemble your pack (in my case spring loaded cells) and inspect each cell individually to do a sort of crude balancing every once and a while.

As far as price, my numbers point to LiFePO4 costing twice as much to buy, but then when it comes to cycle life that equalizes and you arrive at about the same price.

What's the long term life span of LiFePO4 not counting the cycles?

How long could a LiFePO4 pack live if you DIDN'T ride very much?

Hypothetical Scenario

So in a hypothetical scenario someone might be tempted by the high discharge rates to buy fewer LiFePo4 cells (because it's twice the price of NiMh) but the result of such a decision is that while you get a powerful burst of speed for a short time your overall battery capacity is less than if you had bought twice as much NiMh. (though buying the NiMh will make your pack twice as heavy)

The key values are:

3C drain rate for NiMh
10C drain rate for LiFePO4

Depending on how long you plan to own the bike and use it you might choose to buy twice as much NiMh and get twice the range of LiFePO4, but half the life... (got that? :wink: )

The tradeoffs are:

1. Peak Power

2. Ride Range per each ride

3. Long term battery lifespan

4. Weight

" Gravimetric Energy Density:
NiMh : 60-120
LiFePO4 : 90-120
(about the same) "
Dont forget that only the very best (most expensive, like saft) nimh cells will compete with liFePO4. (and I'm not even sure they do in reality).
My NiMh from TMK batteries are towards the lower end of the scale, as are probably the majority of cells that are easily available, or already in ebike packs. In reality, LiFePO4 cells offer near double the energy density, or at least a 50% increase.
Also, as they sag a lot less than nimh (even taking into account double the AH od nimh), the overall efficiency is somewhat lower.

Jozzer said:

In reality, LiFePO4 cells offer near double the energy density, or at least a 50% increase.
Also, as they sag a lot less than nimh (even taking into account double the AH od nimh), the overall efficiency is somewhat lower.

Click to expand...

LiFePO4 are good batteries, this is true, and I do agree that there is some advantage in increased energy. I'm trying to discover a metric (or a set of metrics) that compares the two based on the type of riding you plan to do. What it seems like is that if you are willing with NiMh to carry around double the weight and have half the lifespan of LiFePO4 then for the same money you can have double (or roughly double) the range. Range is very important because it's the range that determines how fast you can build up the miles to pay back the batteries. If you only get 10 miles per charge and then have to wait until the next day to ride then that's slower payback than getting 20 miles per charge per day.

I'd be VERY surprised if a superior battery that is brand new is going to actually sell for less money than an older type. New things always cost more... it's just the way things are until a little time passes and the newer technologies come down in price.

Even with the short term bubble in nickel prices the cost effectiveness of nickel still seems to outperform the alternatives... except for lead of course, but then you're back to a lot of weight.

Lion (Li ion) still eats Life (LiFePO4) on an energy basis... (150-190) but you have the hassles of an unstable chemistry, so that's not good.

I'm seriously considering getting some LiFePO4 cells and trying the spring loaded tube solderless approach, so I'm not totally in favor of NiMh even now.

If entry level pricing were the only criteria then everyone would be riding a bike like mine... :wink:

SLA 3 x 38Ah x 12V... weight 86 lbs... cost $138

The argument for price with NiMH and LifePO4 packs if you were to looke on ebikes.ca website their NiMH for 36V 8~12AH varies between $200 ~ $385
Lets look at some prices of packs

(2) Foxx 36v10a Paks w/BMS & Cheetah Charger = $674 + Shipping (3 year warranty)

ebikes.ca 36V 8Ah Nexcell NiMH, High Rate (5C max) = $325
5.2 kg (11.46 lbs)

Lifebatt 36v 10AH with BMS / Charger + Shipping =$1005 (2 year warranty)

The only advantage is Lifebatt website say 2-3 days delivery which means they proberly have these packs on the shelf made up ready to go where as foxxpower claim 4-6 weeks so if you want your pack like now then lifebatt could be the only option

lets consider worked out to price per VA (or Watt as this is DC)


36v 8AH NiMH $325/36v/8 = $1.13 + charger

(2)36v 10AH LiFePO4 $674/2 = $337/36/10 = $0.94 including charger (fox)

36v 10AH LiFePO4 $1005/36/10 = $2.79 including charger (lifebatt)

36v 10AH LiFePO4 $420/36/10 =$1.16 + charger (iloveebikes)

36v 10AH LifePO4 $465/36/10 =$1.29 inc charger (iloveebikes)

It actually appears here that LiFePO4 is indeed CHEAPER depending on your supplier if lifebatt cells were guaranteed for at least 1500 cycles it would still be cheaper than NiMH

I am in the Process of ordering from fox just waiting for final shipping cost

Cell Level Comparison?

How about comparing "D" sized NiMh:

http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=325

...verses 26650 sized LiFePO4:

http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=3294

If you compare the cells themselves then you get a chart that's going to go something like this. The "Price Comparision" is sort of a weird blend where I factor the total price, capacity and lifespan to arrive at a number.

Price Comparision = ( Total Price / Cycle Count ) / Capacity Percentage*

*This is compared to my current bike.

It's probably not the best metric, but it's something that I have a good feel for since I know my bike very well by now.

The thing to notice is that the "peak current" of 60 amps or over is satisfied by all the options listed, but the LiFePO4 ends up with a lower range. For a "base battery" situation where you might have the option of a "base battery" and an "extra battery" the high discharge rates of the LiFePO4 make sense. You could then use another battery for your "extra" that had a higher energy but maybe a lower peak drain rate. If the "extra" battery was limited in it's current drain it could drain pretty much all the time and the "base battery" is mostly used at peak times. Anyway... I'm just thinking of interesting battery configurations.

There is no way Nihm are going to get 1000 cycles unless your only dipping into them like 10%. In addition even if you don't use them they have very bad self discharge so they have to be charged every 2 - 3 months. If you were to cycle the nimh to get 1000 cycles at 10% DOD then the LifePO4 would get over 2000 cycles. I don't believe the battery space cells do a very good job a representing the LifePO4 chemistry. Look at something like the a123's at $10 a cell and have 2.3 ah per cell. Here are tech specs: http://hybrids-plus.com/pmwiki/index.php?n=Ext.A123Cells

1where in the wiki did you find this chart.
I'd like to read the footnotes so perhaps they might clear up what appear to be a number of errors or perhaps just out of date.

2where does lipo fit into the chart.

3what gave you the idea for this thread.

4

safe said:

I'd be VERY surprised if a superior battery that is brand new is going to actually sell for less money than an older type. New things always cost more...

Click to expand...

So I take it you've given up hope on new whiz-bang 15Ah nimh D cells that sell for less than 10Ah cells on ebay? :wink:





1The nimh 5C peak rating may be for the previous generation of nimh tech because things move fast nowadays as Ferris would say. Look at the R/C nimh cells & the peak rating has always been 10C as standard, with some of the latest offerings claiming 12C.

2When price comparison shopping, I go by the 101 unit price. It's a realisticly affordable amount & about the number of cells you'll need anyway to build one large pack or two smaller ones with a few spares left.

3The metric you're looking for is a graph depicting comparisons of performance envelopes where specific power (speed) is plotted against specific energy (range). With that one picture all will be made clear & can then simply work out the price aspect after.

4Don't take <A HREF="http://en.wikipedia.org/wiki/Rechargeable_battery">charts</A> as gospel.

Toorbough ULL-Zeveigh said:

1The nimh 5C peak rating may be for the previous generation of nimh tech because things move fast nowadays as Ferris would say. Look at the R/C nimh cells & the peak rating has always been 10C as standard, with some of the latest offerings claiming 12C.

Click to expand...

Without exception, I found the reviews of high-drain NiMH cells over at rcgroups.com describe a very abbreviated cycle life at those amp drain levels -- like 12 - 50 useful cycles of 4500mah Sub-C's at a 10C drain rate.

My calculations above works out that LiFePO4 can be cheaper per 1V/1A than NiMH but still more expensive from PB (but thats is off topic)

But as I worked out two prices before for both technologies

36v 8AH NiMH $325/36v/8 = $1.13 + charger

(2)36v 10AH LiFePO4 $674/2 = $337/36/10 = $0.94 including charger (fox)

Ok I must Admit the NiMH 5C is the most expensive pack that Justin @ ebikes does and the pack from foxx batteries is the cheapest LiFePO4 pack retail, (I know I have posted larger packs before from a chinesse manufacture but these were trade prices to give us some idea of the cost suppliers are possibly paying.)

If anyone can see any errors in those two comparisons I would be glad to see where I have gone wrong

PS I am not trying to rubbish the NiMH from Justin (web site shows sold out on this line) but just a stock comparison not to sure what the *C rating for foxxbatteries are

xyster said:

Without exception, I found the reviews of high-drain NiMH cells over at rcgroups.com describe a very abbreviated cycle life at those amp drain levels -- like 12 - 50 useful cycles of 4500mah Sub-C's at a 10C drain rate.

Click to expand...

The same thing can be said across the board with any battery's peak rating regardless of chemistry. If you run LiFePO4 at its peak 60C continuously you would defiantly get fewer cycles as well. Understand I'm not knocking LiFePO4. I think you have to agree it's actually very nimh-like in it's characteristics moreso than what it has in common with the other lithium chemistries.

The point safe is making is selecting a proper operating point & sizing the battery accordingly. You don't want to operate at the peak point the way the R/C racers do. The peak rating just shows what the cell is capable of. People often mistake the peak spec for maximum capability & there's always the temptation to shave the pack size to minimum based on this spec to save money & weight.

And it's sort of my point as well. I think nimh makes a good fit for range, power & until recently, operating cost in typical (read legal) ebike requirements. Perhaps not ideal for going 60 mph but from where I sit it seems to be capable of that as well. The 20 mile range I get from my 8Ah nimh D-cells almost covers all my needs. I need range more than power & an extra 50% would be ideal which is why I'm trying to figure out how to best exploit the subC nimh capabilities. I hope to get that extra 50% in about the same size package I have now at a cost of about $0.90/Wh; 2 months ago it was $0.65/Wh when I first started looking. If the cycle life ends up biting me in the a$$, then it didn't cost a lot to try.

I'm not seduced by a pretty face & never buy something just because it's cool. The motor doesn't know the difference where the electrons are coming from

The same thing can be said across the board with any battery's peak rating regardless of chemistry. If you run LiFePO4 at its peak 60C continuously you would defiantly get fewer cycles as well. Understand I'm not knocking LiFePO4.

Click to expand...

But 10-50 times fewer cycles??? Many of the RC folks switched to LiFePO4 and LiMn from Li-Poly and NiMH because of the greatly increased cycle life at high drain rates. Avid RC flyers who were replacing their Lipo's or NiMH sub-C's every couple weeks are getting hundreds of cycles (and still counting last I checked) from their A123s and eMoli's. The new generation lithium cells are much heavier than Lipo's, and so Lipo is still preferred for competition flying.

Secondly, as you noted, you're comparing a max 60C drain rate to a max 10C drain rate -- so the new generation lithium cells are better by a factor of 3-6 there too. Drawing 45 amps from a 4.5ah 10C NiMH will shorten cycle from the hundreds to the tens of cycles; not so for LiFePO4.

LiFePO4 and LiMn are simply more tolerant of the heat produced at high drain rates than other chemistries.

I'm not seduced by a pretty face & never buy something just because it's cool.

Click to expand...

That's good -- much better to be seduced by a butt-ugly face and buy stuff nobody else wants.

xyster said:

That's good -- much better to be seduced by a butt-ugly face and buy stuff nobody else wants.

Click to expand...

uh, no. I won't be seduced by anything actually.
As I've repeated many times b4, the choice of which is best is determined by the application, which means running the numbers.

All I wanted to do was to correct a minor error in the chart for a proper comparison & not have it turn into splitting hairs over current capability.
I'm well versed with the implications of the peak specification on electrical components in general & how to apply them.

safe said:

With LiFePO4 you can have a "core" battery that is not so great in the range department, but the peak drain rate is good (10C) and so you can run really light and still have full power. Then you add an extra battery pack for the long rides where you tend to take it a little easier anyway. (that's more of a touring mindset)

So I see LiFePO4 as part of an overall solution.

Click to expand...

Same thing with the LiMn emoli cells. Congruent to your design philosophy, I'd like to build a lightweight 6-12ah 96 volt emoli "sports pack", then use my heavy, 33ah 80v lithium cobalt pack as the "touring pack". My boxes are quite modular, so switching will only take a minute.