LiFePO4 vs Li-Ion

[103Alex1]

Hi.

Can anyone enlighten me about the relative merits of this :

48V20Ah LiFePO4 Shrink Tube EBike Battery Pack - BMSBATTERY

versus the much cheaper this :

48V20Ah Li-Ion Shrink Tube EBike Battery Pack - BMSBATTERY

 

[Jeremy]

Both batteries are actually Lithium Ion, so BMS Battery have inaccurately described them. The first uses Lithium Ferrous Phosphate in the electrodes, the second uses Lithium Cobalt Manganese. The differences between them focus on three main parameters, life, safety and energy density.

LiFePO4 lithium ion cells are considered to be slightly safer than LiCoMn, although to be honest that difference is often over-stated and there have been several incidences of LiFePO4 fires, so they aren't totally safe.

Generally LiFePO4 lithium ion cells have a longer cycle life than LiCoMn. This is highly dependent on cell quality though, so there's no guarantee that these particular no-name cells will exhibit that characteristic.

Finally, LiCoMn lithium ion cells have a higher energy density than LiFePO4 cells, so for a given capacity the battery pack will be lighter. In this case the LiCoMN pack weighs 7kg and the LiFePO4 pack weighs 9.7kg, a significant difference.

If you can handle the increased weight and would like a longer potential cycle life then LiFePO4 is probably the better choice. If you want the lightest weight and are prepared to accept a somewhat shorter cycle life then choose the cheaper and lighter LiCoMN pack.

 

[muckymits]

LiFePO4 lithium ion cells are considered to be slightly safer than LiCoMn, although to be honest that difference is often over-stated and there have been several incidences of LiFePO4 fires, so they aren't totally safe.

A few keep saying this, but the only ones I have found turned out to be the BMS.

 

[Jeremy]

Cell safety is one of those things that does seem to be over-stated, IMHO. Certainly early LiCoO2 lithium ion cells were prone to thermal runaway and posed quite serious fire risks if even slightly over-charged, although even then most of the fires seemed to be from a lack of any proper understanding of the need to manage cell voltage during charge. All lithium ion chemistries have moved on a fair way in terms of safety in the last few years, to the extent where some LiCoO2 cells can now tolerate significant amounts of gross over-charge without catastrophic failure or creating a fire risk.

The primary causes of fire and explosion do seem to be poor cell management, usually during charge, rather than cell chemistry. This seems clear when you look at the relatively high incidence of battery fires amongst those who are using them either without any form of charge/discharge control, or in home made battery packs where the standard of construction may not be that good.

If a pack is properly assembled, such that it is unlikely to suffer a shock or vibration induced failure, and has a reliable battery management system (unfortunately, many available on the DIY market seem to be fairly unreliable), then I suspect that the notional difference in safety between the various cell chemistries is fairly insignificant in practice.

 

[muckymits]

I charge mine with a RC charger and balance charge nearly every time, the worrying bit is discharge. Ive been thinking about a simple system to shut the bike down if a cell gets too low, some bits have just arrived (finally) from China, so will be having a go shortly.

 

[Jeremy]

I used to use (and still do on my electric boat battery) cell voltage discharge monitoring, but now all my bikes are fitted with discharge monitors, so I don't bother any more. The circuit I used was a simple one, with a TC54 across each cell, with the outputs from each driving an optoisolator. The optoisolator outputs were all wire OR'd to a shut down circuit, wired to the controller ebrake line.

This works well, but in practice I never once discharged a cell to the point where the cell LVC triggered. I decided I'd rather have a "fuel gauge" on the bike that showed how many Ah I had left, so designed a simple little circuit that senses the current draw (using the controller shunt) and subtracts the Ah used from a preset total and displays the remainder. Every time the pack is recharged I reset the total and so have a pretty good idea of how much battery capacity has been used. I've found this to be far more useful than the rather vague method of monitoring battery voltage and haven't once managed to run any of the battery packs low enough to get anywhere close to a cell being close to the point of no return.

Like you, I balance charge all the time, either with an RC charger or with a home made DC DC converter individual cell charger, so can be pretty sure that the cells are all properly in balance at every charge.

 

[jerrysimon]

Very interesting and that kind of all makes sense. I have however been using A123 cylindrical 2.3Ah cells in a small 2*(6s1p) config successfully for over 3 years now covering some 4500 miles and approaching 1000 charge cycles.

I don't use a BMS, though they have balance tabs on each of two 6s packs. They were balanced when built but for the first year I bulk charged them with a cheap lead acid 36v 1.6amp (cv/cc) charger. I lost a cell after a year of this abuse including running the pack flat most rides. I changed another cell this year, the same one I think. For an £80 DIY battery I think I have got my monies worth

In the last couple of years I always charge at work when the pack is about 50% full. I check balance and maybe balance them with my RC charger every three months or so if I remember.

Re the initial post, one of those batteries is heavier than my Brompton bike!

Regards

Jerry

 

[Kudoscycles]

Alex,your own threads show the major differences between LifePo4 and LiMno4...
LifePo4....9.7kg....2000 cycles
LiMno4....7.0kg.....800 cycles.
The decision to use LifePo4 on all our urban bicycles was one of the best decisions that we made when coming into the ebike industry....the battery type was originally developed for unmanned weather station usage,as such it basically had to look after itself,it is very user friendly,safe,cheap, and temperature tolerant,if you google 'LifePo4' it will giver a good account of its advantages,the primary disadvantage is its low energy density (ie its heavy),this tends to limit its capacity to 10Ah,after that its weight becomes unacceptable.
For this reason our latest range of performance ebikes will use LiMno4,for it seems crazy to spend a lot of effort to save a few grams when the battery is 1.7 kg heavier,however,we have kept the price low to compensate for the reduced cycle life.
The long cycle life on LiFePo4 is not just hype,some of our original sales are coming up to 2 years old with plus 500 cycles with no loss of performance.
One point about LifePo4 is the battery readout on the display works on voltage drop. There is almost no voltage drop of a LiFePo4 battery during usage until the power is almost exhausted,this means that the performance over the usage cycle is maintained but the exhaust point is sudden and unpredicatable-to my knowledge nobody has yet developed a battery usage display linked to LifePo4 which is progressive.
In contrast LiMn04 has a progressive voltage drop throughout the usage cycle,this means that the power does drop off progressively however the display gives a progressive readout.
Understanding the differences allows a choice between weight,range and lifecycle cost-if you run a Porsche you would expect it to perform better but cost more than a VW !!!
Hope that helps
Dave
Kudoscycles

 

[Jeremy]

Why is it that some dealers choose to turn any non-commercial discussion here into a strong advertising plug for their product?

It's something that used to happen a lot on another forum I frequented, one focussed on self-build (I'm currently just starting a house self-build). The net effect for me was to start keeping a list of all the vendors who butted in on non-sales discussion threads with thinly disguised adverts. I've used this list to make sure that I never do business with any of them, on a point of principle.

Oh, and for what it's worth, my battery "fuel gauge" (which is in the public domain) has been in use for a couple of years now and works just as accurately on LiFePO4 as any other cell chemistry. It gives an accurate display of the capacity remaining in any battery, just like the fuel gauge on a car or motorcycle.

 

[Old_Dave]

Just to add that details of Jeremy's fuel gauge can be seen in all its glory here..

http://www.pedelecs.co.uk/forum/electric-bicycles/7208-e-bike-fuel-gauge.html

and also here..

Endless-sphere.com • View topic - Simple ebike 'fuel gauge'

 

[Jeremy]

Thanks for digging that out! There's a newer version on my latest folder, with a better (and much cheaper) display. the component cost for the new version is around £10 for the display and ucontroller, plus it has some extra functionality, like turning off the bike after a few minutes with no motor current if you accidentally leave it on after use, to save running the battery flat. Here's a photo of the current cheaper version:

 

[BAH48]

"Why is it that some dealers choose to turn any non-commercial discussion here into a strong advertising plug for their product?"

Nonsense! He is just being helpful and I welcome the perspective of a dealer who clearly knows about batteries.

 

[103Alex1]

Thanks for the responses - I have a much better understanding now and the Kudos contribution was actually illuminating. Also very interested in that battery 'fuel gauge' display which seems to get round the perceived shortcoming of the LiFePO4.

The battery is just as important as the bike (more so for some people) so reckon it's well worthwhile getting to grips with the pros and cons of the market options.

 

[muckymits]

"Why is it that some dealers choose to turn any non-commercial discussion here into a strong advertising plug for their product?"

Nonsense! He is just being helpful and I welcome the perspective of a dealer who clearly knows about batteries.

Yes some dealers are really irritating (not so much Kudos)...but without them we would not have a forum

 

[Jeremy]

"Why is it that some dealers choose to turn any non-commercial discussion here into a strong advertising plug for their product?"

Nonsense! He is just being helpful and I welcome the perspective of a dealer who clearly knows about batteries.

Not quite nonsense. He was regurgitating points already made very clearly earlier in the thread, which frankly served no useful purpose. The only additional info seemed to me to be plugs for his own products and a bit of misinformation about being unable to display LiFePO4 capacity.

Add in that it's not just LiFePO4 cells that have a very flat voltage/state of charge curve, but that practically all newer chemistry lithium ion cells exhibit the same flat characteristic (it's only the older Li Ion chemistries that tended to have a slight voltage drop with SOC a little like SLA) and I find it hard to see what real content there was (other than an advert).

 

[Deleted member 4366]

.....the usage cycle is maintained but the exhaust point is sudden and unpredicatable-to my knowledge nobody has yet developed a battery usage display linked to LifePo4 which is progressive.

Yes they have. It's called a wattmeter. Most of them give a read-out for aH, so that you can see how many aH you've used. If you have a 10aH battery, and you've used 9.5aH, you know you won't be going much further with power.


You can get nice ones from Speedict that displays on an Android phone, or Cycle Analyst is a bit more expensive but has its own display, or you can get a cheap Chinese one one and do a remote shunt mod if you can solder.

 

[Jeremy]

Yes they have. It's called a wattmeter. Most of them give a read-out for aH, so that you can see how many aH you've used. If you have a 10aH battery, and you've used 9.5aH, you know you won't be going much further with power.


You can get nice ones from Speedict that displays on an Android phone, or Cycle Analyst is a bit more expensive but has its own display, or you can get a cheap Chinese one one and do a remote shunt mod if you can solder.

The only thing to watch with the simple wattmeters, like the one I used for the original remote shunt mod a while ago, is that they don't remember the Ah you used on the previous ride. For me this meant remembering the previous running total, or scribbling it down, as I tend to use one of my bikes for a lot of short rides (normally just down to the post office and back). It's the reason I decided to make something with non-volatile memory.

The CA has had that capability for years, AFAIK, and I believe the Speedict is the same. I have a feeling there are a few other similar systems out there too, as the other day I came across a display on a generic Chinese ebike that seemed to keep track of battery capacity used (at least it had a display that showed Ah remaining).

 

[jerrysimon]

You can get nice ones from Speedict that displays on an Android phone,...

Yes when it finally comes. I sent a failed one back in late October but the new version they promised me in exchange has again been delayed

Fingers crossed we get it before Christmas.

Jerry

 

[BAH48]

"I find it hard to see what real content there was (other than an advert). "

I needed to look up the LiMnO4 batteries as I hadn't come across them before. I welcome all input as I am new to all this.

 

[103Alex1]

He was regurgitating points already made very clearly earlier in the thread, which frankly served no useful purpose. The only additional info seemed to me to be plugs for his own products and a bit of misinformation about being unable to display LiFePO4 capacity.

Actually, to be fair, I had not noticed the point about life cycles on the BMS listings or earlier on the thread. It's a pretty fundamental point to understanding the trade-off on cost and weight of the 2 specific batteries linked. Not being a battery expert, I had also not appreciated (which I got from your posts) that Li-Ion being referred seems to be 'code' for manganese. Is that right ?

I guess it's natural that Kudos will reference their own experiences with battery performance reportage to provide experiential feedback. Think it's pretty clear I'm not looking to buy a ready-made bike from Kudos or anyone else by the links I posted originally so as a non-technical person in this area I found the points they highlighted helpful, partly for the content itself and partly for triggering subsequent discussions and such of those that I can understand as being relevant to me - basically the bottom line of what to get and options for monitoring in use whilst riding.

Kudos inadvertently triggered a discussion about battery capacity readings, which is extremely important to me, leading to on the allusions about what Speedicts can do coming up again. I haven't had time to get my head around any of that yet but seems this is something I'm going to most likely need and will have to look into properly. Battery capacity monitor, speedo and where fesible power controller all need to be sorted out effectively and space-efficiently for someone who wants the bike after building it to be user-friendly... 'tis being built primarily for post-build use rather than for the sake of the build itself (although that will be a great learning curve too).