LIFePo4 lifecycle with ~60% of discharge per cyle

[raktas]

Hi,
Sorry if this been covered elsewhere but I could not find this info...

Just took stats from CA for last year for my 48V 10AH LiFePo4 battery - 160 cycles, 900AH.
Doing simple math it gives around 5.6AH per charging cycle that is a bit less than 60% of battery capacity.
I cycle all year round. My comfort riding temp varies from up to +30*C at summer to -12*C in winter. Actually temp drops time to time below -25+*C here in winter but after few rides below -15*C I decided that -12*C is that minimum temp that I'm comfortable with. When garage temp is below +5*C I charge battery at home with timer delay (to let it warm up to ambient temp).
My usual route is home to work and back in total of 26km (16miles). Due terrain it takes me around 1.5-2Ah to go to work and around 3-4AH to return. I'm pedaling all the time. As for speed it's around 32kph (20mph) half of journey and much lowers speeds later.

Definitely I have too small battery to do two days per charge so I'm charging it every day.
Any thoughts how it will influence a total lifecycle of battery? To avoid discussions regards LiFePo4 lifecycle let's just assume that LiFePo4 battery lifecycle with 100% capacity is that promo 2000 cycles.
I'm not sure how they rate this - on 100% discharge, 90% discharge X discharge or it doesn't matter?

I'm thinking is it worth to change charging plan and charge at work as this is a bit more complicated than to do at home. In that case I could change charging pattern to something like this - work->home(2ah), home->work(2ah) -> work->home(4ah) and then charge. This is a bit risky as due voltage drop I will most likely consume more AH on last ride to work before charge but might work with some special battery saving techniques. And It's even more risky in winter...

 

[BVH]

I'm not an expert but I have read a lot here on ES and other forums and it seems to me the consensus is that there is no negative in using 80% or anything less than 80% Depth of Discharge. Many knowledgeable members indicate that all other factors being equal/unchanged, that the number of charging cycles you will get out of a LiFeP04 battery increase with lower DOD's. I routinely charge my two (run in parallel all the time) 48V/12Ah packs with between 7 and 8 Ah each, so a 60% to 70% DOD.

Think of it this way: (I am speculating a little here) Using your 48V/10Ah battery....

10 Ah capacity X's 80% (frequently recommended max DOD) = 8 Ah. 8 Ah X's 1500 re-charging cycles (some mfg's specs) = 12,000 Ah lifetime Ah provided by battery.

so if the battery is good for providing 12,000 lifetime Ah then:

12,000 lifetime Ah / 6 Ah (60% DOD) = 2000 charging cycles

 

[wesnewell]

The thing that is going to affect cycle life is not so much the DOD, but the discharge C rate. If you read the fine print for cycle life in most specs, you'll find that the 2000 cycles is based on <1C discharge rate, with most probably done at a .2-.5C discharge rate. I suspect the actual cycle life will be cut at least in half when discharging over 1C. AFAIK, a full cycle means 100% dod.

 

[999zip999]

It's up to your controller and how you start,hills and how you ride.

 

[raktas]

I've also somewhere read (I guess em3ev) that both charging and discharging LiIon (LiMn2O) to 90% might double your charging cycles but could not find really much info on LiFePo4. I'm also under same impression as BVH says but could not find any proof or deny of this.
I've read few PhD scientific research articles about LiFePo4 characteristics searching for lifecycle and temp influences on battery life but after that I had even more questions than answers.

As LiFePo4 lasts of so long I'm just curious has lot of people reached that end of 100%-90-80% DoD with natural cell degradation. As most of time it's a human error that leads to cell capacity drop (bad hands, overcharge, overdischarge, bad bms, bad charger, no bms and etc)
With my calculations I would rearch 2k cycles after 12 years. Plus I need to remember to mark my battery capacity time to time to have stats after 12 years...
That leads to another question what will kill battery first - age or cycle count. I really doubt that some rare used battery even stored in perfect condition (store charge as per spec, temp 20*C, even vacuum in out of space) let's say used 1 cycle per month will be in that 100% perfect condition after 7 or 10 years

Discharge. That's also excellent question. Another great marketing trick - to show you the numbers that you would like to see.
Just took a really random lifepo4 specs from google - and it has cycle curve:
http://www.batteryspace.com/prod-specs/5101_2.pdf - They rate discharge at 1C - it looks like it loose it's 100% capacity before you buy it on the very first cycle... LOL

As for myself - battery specs says it's 3C continous and 5c max. Of course I suspect that with continuous C3 discharge it will never reach 2000 cycles
My controller and motor is capable 25A (it says 30A but I haven't' seen it ever to go over 26A)

Well looking at CA readings during ride would say my normal discharge rate is below 1C except few hills where it gets to around 750w (15A).
AVG drain is about 300-400w and (with 50v thats 6-8A.) or .6-.8C discharge along with my own 100-200W of human power.

 

[dogman dan]

Charge it every day. It won't cost you twice as many of the theoretical 2000 cycles. When you charge could help, like plug it in to a timer, and have it charge in the early am. That way as much as possible of your storage time is less than full charged.

30c in the summer eh? You'll get about 4 years. Summer temps, and the time it spends fully charged at 30c won't be helping it last. In a warm climate, your battery will time out before you see 2000 cycles. But yeah, stored half charged in a cool place, it could last a lot longer. But your commuter will go out into that heat every day. You could put your battery in the fridge, but then it just under delivers capacity because it's cold.